GB2391116A

GB2391116A - Conductor paste for low temperature co-fired ceramic circuits

Info

Publication number: GB2391116A
Application number: GB0315957A
Authority: GB
Inventors: Masashi Fukaya; Yoshihiro Nakamura; Kazushige Onozumi; Satoru Adachi
Original assignee: SUMITOMO METAL; Tanaka Kikinzoku Kogyo KK
Current assignee: SUMITOMO METAL; Tanaka Kikinzoku Kogyo KK
Priority date: 2002-07-15
Filing date: 2003-07-08
Publication date: 2004-01-28
Anticipated expiration: 2023-07-08
Also published as: GB2391116B; GB0315957D0; DE10331811A1

Abstract

A conductor paste is disclosed which is used to print on a low-temperature fired ceramic green sheet (11) firable at or below 1000{C and contains, as a conductor powder, an Ag powder or an Ag alloy powder 0.005 to 0.050 wt. % Rh is added to the conductor powder, and the Ag powder has a mean grain diameter of primary grain ranging from 1.5 m to 4.5 m. The mean grain diameter of the primary grain is measured by a electron microscope observation method. The Ag powder has an average grain diameter of agglomerate ranging from 5.0 m to 12 m. The mean grain diameter of the agglomerate is measured by a centrifugal sedimentation. A shrinkage behavior of a printed conductor pattern 16,19 during firing is set so that a shrinkage factor ranges from 2.0% to 10.5% when the temperature is increased from 400{C to 700{C and so that a shrinkage factor ranges from 10.0% to 21.1% when the temperature is increased from 400{C to 900{C.

Description

GB 2391116 A continuation (72) Inventor(s): Masashi Fukaya Yoshihiro

Nakamura Kazushige Onozumi Satoru Adachi (74) Agent and/or Address for Service: M arks & Clerk 57-60 Lincoln's Inn Fields, LONDON,

WC2A 3LS, United Kingdom

23911 1 6

CONDUCTOR PASTE, METHO1:) OF PRINTING THE CONDUCTOR PASTE

AND METHOD OF FABRICATING CERAMIC CIRCUIT BOARD

This invention relates to a conductor paste used to print a conductor 5 on a low-temperature fired ceramic green sheet fired at or below 1000 C, a method of printing the conductor, and a method of fabricating a ceramic circuit board.

A multilayer ceramic circuit board has conventionally been widely used as a heat-resistant high density packaged circuit board. The 10 multilayer ceramic circuit board is generally fabricated by a green sheet lamination method in the following procedure. Firstly, via-holes for interlayer connection are formed in a plurality of ceramic green sheets by punching etc. and thereafter, the via holes of each ceramic green sheet are filled with a conductor paste by padding so that via conductors are formed.

15 Subsequently, a wiring pattern is screen-printed on each ceramic green sheet using the conductor paste. After lamination and thermo compression bonding of a plurality of the ceramic green sheets, each ceramic green sheet and printed conductor pattern are co-fired, whereby a multilayer ceramic circuit board is fabricated.

20 Currently actually used multilayer ceramic circuit boards are roughly classified into a high-temperature fired multilayer ceramic circuit board fired at or above 1 300 C, for example, alumina, and a low-temperature fired multilayer ceramic circuit board fired at or below 1000 C. The hightemperature fired multilayer ceramic circuit board has 25 the following disadvantage: firing is required to be carried out in a reducing atmosphere for the purpose of preventing conductors from oxidation with the use of high-melting metal paste such as W or Mo each having a relatively high electric resistance value as a co-fired conductor

paste. On the other hand, the low-temperature fired multilayer ceramic circuit board is advantageous in that co-firing can be carried out in the air using a conductor paste of Ag system such as Ag, Ag-Pd or Ag-Pt which are low-melting noble metals with a low electric resistance value and good 5 electric characteristic.

However, when the conductor of Ag system and the low-temperature firable ceramic are co-fired, shrinkage behaviors of the conductor and ceramic differ from each other to a large extent (shrinkage behavior of compared example 101 conductor in FIG. 3 corresponds to 10 shrinkage behavior of the conventional Ag conductor). The conventional Ag conductor paste starts shrinking with thermal decomposition of organic substance (binder resin, solvent etc.) when the atmospheric temperature has reached 300 C to 400 C after initiation of firing. However, the low-temperature fired ceramic green sheet contains glass as a principal 15 component. Accordingly, the low-temperature fired ceramic green sheet does not shrink so much until the temperature thereof reaches about 700 C (melting point) at which the glass component starts to melt. The low-temperature fired ceramic green sheet rapidly shrinks after the temperature thereof exceeds the melting point. The low-temperature 20 fired ceramic green sheet thus shows a peculiar shrinkage behavior as described above.

Accordingly, the difference between shrinkage factors of the Ag conductor and low-temperature firable ceramic is increased with increase in the temperature in a temperature range from 400 C to about 700 C.

25 The shrinkage factor difference is sometimes increased to or above 10o at about 700 C. When increased to such a large value, the shrinkage factor difference results in a large thermal stress at a junction of the Ag conductor and the low-temperature firable ceramic. The thermal stress

( warps the fired board and reduces a bonding strength of the junction. In the worst case, the junction is sometimes peeled, whereupon the quality and yield of the product are lowered.

Furthermore, disconnection occurs at a junction of the via conductor 5 and a wiring pattern when the difference is large between the shrinkage factor of the low-temperature fired ceramic green sheet and the shrinkage factor of a via conductor printed for cave filling in the via holes of the low-temperature fired ceramic green sheet. Accordingly, the conventional conductor paste for via-hole filling printing contains a larger 10 amount of glass frit or metal oxide (TiO2, SiO2 etc.) serving as firing retarder, so that the shrinkage behavior of the via conductor during firing is approximated to the shrinkage behavior of the low- temperature fired ceramic green sheet for the purpose of preventing disconnection of the via conductor. 15 However, when an amount of glass frit or metal oxide contained in the conductor paste is increased, an electric resistance value of the printed conductor pattern is also increased. Accordingly, it is desirable that a wiring pattern printed on the low-temperature fired ceramic green sheet should have a small wiring resistance value by the use of a 20 conductor paste containing a smaller amount of glass frit or metal oxide.

From this point of view, the via conductor and the wiring pattern are conventionally printed at two printing steps using conductor pastes having different compositions respectively. As a result, the production efficiency is lowered since the number of steps in the fabrication is increased.

25 Moreover, the production cost is increased since two types of conductor pastes need to be prepared.

Generally, a temperature at which a green sheet of the low-temperature firable ceramic starts to shrink during firing ranges from

( 650 C to 820 C, as shown in FIG. 3. On the other hand, an amount of shrinkage of the Ag conductor paste is small in a temperature range from 600 C to 900 C since the conventional Ag conductor paste shrinks in a temperature range from 400 C to 600 C to a large extent and the firing 5 then almost finishes. Accordingly, in the temperature range from 600 C to 900 C, the difference between shrinkage factors of the Ag conductor and low-temperature firable ceramic is increased with increase in the temperature, whereupon a large thermal stress occurs in the junction of the Ag conductor and the low-temperature firable ceramic. The thermal 10 stress warps the fired board and accordingly, necessary dimensional accuracy cannot be ensured in the fired board. Alternatively, the bonding strength is reduced in the junction of the conductor and the lowtemperature firable ceramic such that the junction becomes easy to be peeled. Consequently, the quality of the product and the reliability of 15 the product are reduced.

Therefore, an object of the present invention is to reduce warp of a fired substrate by approximating a shrinkage behavior of an Ag conductor paste to a shrinkage behavior of a low-temperature fired ceramic green sheet. 20 The present invention provides a conductor paste which is used to print on a low-temperature fired ceramic green sheet firable at or below 1000 C and mainly contains, as a conductor powder, an Ag powder or an Ag alloy powder both of which will hereinafter be referred to as "Ag powder," wherein 0.005 to 0.050 wt. % Rh is added relative to 100 wt. No 25 conductor powder, wherein the Ag powder has a mean grain diameter of primary grain ranging from 1.5 gum to 4.5 m, said mean grain diameter of the primary grain being measured by a electron microscope observation method, and the Ag powder has a mean grain diameter of an agglomerate

( ranging from 5.0,um to 12 m, said mean grain diameter of the agglomerate being measured by a centrifugal sedimentation, and wherein a shrinkage behavior of a printed conductor pattern during firing is set so that a shrinkage factor ranges from 2.0% to 10.5% until a temperature is 5 increased from 400 C to 700 C after thermal decomposition of the organic substance (binder resin, solvent, etc.) and so that a shrinkage factor ranges from 10.0o to 21.1% until a temperature is increased from 400 C to 900 C.

Generally, almost all the organic substance contained in a printed 10 conductor pattern vaporizes until a temperature of the printed conductor pattern is increased from a firing starting temperature to 400 C, whereupon the printed conductor pattern contains substantially only inorganic substance (conducting powder, Rh and other inorganic additive).

Accordingly, the shrinkage factor of the printed conductor pattern is not 15 affected by an amount of organic substance to be blended in the conductor paste. From this viewpoint, as a technique for limiting the shrinkage behavior of the printed conductor pattern, the invention provides proper range of the shrinkage factor in the case where the temperature is increased from 400 C to 700 C and proper range of the shrinkage factor in 20 the case where the temperature is increased from 400 C to 900 C.

Consequently, the printed conductor pattern can be limited to the shrinkage behavior thereof, which is not affected, by the amount of organic substance to be blended.

A first feature of the invention is the Ag conductor paste to which 25 0. 005 to 0.050 wt. To Rh is added. Since Rh has an effect of restraining firing of an Ag printed conductor pattern, the printed conductor pattern can be fired moderately in a temperature range from 600 C to 900 C.

Consequently, the shrinkage factor of the printed conductor pattern can be

( rendered smaller in the temperature range from 400 C to 900 C after thermal decomposition of the organic substance contained in the printed conductor pattern and particularly in the temperature range from 400 C to a yield point (about 700 C). Thus, a shrinkage behavior of the printed 5 conductor pattern can be approximated to a shrinkage behavior of the lowtemperature fired ceramic green sheet during firing without increase in an amount of glass frit or metal oxide as contrary to the prior art,

whereupon an amount of warp of the fired board can be reduced.

Moreover, since an amount of Rh added as a firing retarder is small, an 10 increase in the electric resistance value of the printed conductor pattern due to addition of Rh is also small, and accordingly, desirable electric characteristics of the printed conductor pattern can be maintained.

The glass component of the low-temperature fired ceramic green sheet softens and fluidizes in a temperature range from 600 C to 900 C 15 when the low-temperature fired ceramic green sheet and the printed conductor pattern are co-fired. However, the firing of the printed conductor pattern is restrained by the Rh, whereby the densification of the printed conductor pattern is retarded. Accordingly, since the glass component of the green sheet efficiently penetrates the printed conductor 20 pattern to diffuse into it, an improvement can be expected in the bonding strength between the printed conductor pattern and the fired board.

A second feature of the invention is that a proper range of the grain diameter of Ag powder composing the conductor paste is defined by a mean grain diameter of a primary grain and an agglomerate in order that 25 the via hole filling and wiring pattern printing may consist with the firing shrinkage restraining effect. More specifically, the grain diameter of the Ag powder is processed so that a mean grain diameter of the primary grain measured by the electron microscopy observation ranges from 1.5

( Em to 4.5 Elm and a mean grain diameter of the agglomerate measured by the centrifugal sedimentation ranges from 5.0,um to 12 pm. The primary grain refers to a minimum unit of the grain and cannot be divided more.

The agglomerate refers to a plurality of primary grains agglomerated into 5 an agglomerate or an aggregate of primary grains. The Ag powder used in the present invention has a relatively large difference between the mean grain diameter of the primary grain and the mean grain diameter of the agglomerate. The large difference improves the via hole filling capability.

By the firing shrinkage restraining effect of the agglomerate and 10 firing restraining effect by addition of Rh, the shrinkage factor can range from 2.0% to 10.5% until a temperature is increased from 400 C to 700 C and the shrinkage factor can range from 10.0% to 21.1 9to until a temperature is increased from 400 C to 900 C. Consequently, since the difference between the shrinkage behaviors of the printed conductor 15 pattern and the green sheet is effectively reduced, breakage can be prevented in a junction of a via conductor and a wiring pattern.

Moreover, when the mean grain diameter of the primary grain of the Ag powder ranges from 1.5 lam to 4.5 am and the mean grain diameter of the agglomerate ranges from 5.0 am to 12 Elm, the via hole filling printing 20 is compatible with the wiring pattern printing, so that both printing steps can be carried out concurrently. However, the conductor paste of the invention may be used when the via hole filling printing step and the wiring pattern printing step are carried out separately from each other.

The Ag powder blended in the conductor paste of the invention 25 preferably has a specific surface area ranging from 0.1 m2/g to 0.4 m2/g.

The specific surface area ranges from 0.1 m2/g to 0.4 m2/g when the mean grain diameter of the primary grain ranges from 1.5 lam to 4.5 An and the mean grain diameter of the agglomerate ranges from 5.0 Am to 12 m, as

shown in FIG. 10. Accordingly, the mean grain diameters of the primary grain and agglomerate can be confined to the proper ranges in the invention more reliably when an Ag powder whose specific surface area measured by the BET method ranges from 0.1 m2/g to 0.4 m2/g is used.

5 An organic substance (binder resin, solvent, etc.) contained in the conductor paste preferably has a loading ranging from 8 wt. % to 27 wt. % or in other words, an inorganic substance preferably has a loading ranging from 73 to 92 wt. No. When the organic and inorganic substances are within the respective ranges, a compounding ratio of the organic and 10 inorganic substances of the conductor paste becomes proper.

Consequently, the conductor paste has a viscosity suitable for both of the via hole filling printing step and the wiring pattern printing step.

Furthermore, a loss in the printed conductor pattern due to thermal decomposition of the organic substance can be reduced in a period from 15 the start of firing to the time when the temperature of the conductor is increased to about 400 C at which thermal decomposition of the organic substance starts. As a result, the electric characteristics of the printed conductor pattern can be stabilized.

An Ag content in an inorganic substance contained in the conductor 20 paste is preferably at or above 85 wt. %. Consequently, since an inorganic additive (for example, glass frit or metal oxide) other than Ag in the inorganic substance is not more than 15 wt. %, the electric resistance value of the printed conductor pattern can be prevented from being increased, whereupon the electric characteristics of the printed conductor 25 pattern can be stabilized.

The following relationship holds: 0.26sb/asO.73 where a is a shrinkage factor of the green sheet in % in a period when a

( temperature of the green sheet is increased from a firing shrinkage starting temperature to 900 C and b is a shrinkage factor of the conductor pattern in % in the period when the temperature of the green sheet is increased from the firing shrinkage starting temperature to 900 C.

5 According to the results of an experiment conducted by the inventors as will be described later, when the relationship, 0.26sb/asO.73, is held, the difference between shrinkage behaviors of the green sheet and the conductor pattern is rendered smaller than that in the prior art, in the

period when the temperature of the green sheet is increased from the 10 firing shrinkage starting temperature (generally, from 650 C to 820 C) to 900 C. Consequently, an amount of warp of the fired substrate can be reduced such that the dimensional accuracy of the substrate can be improved. Furthermore, a sufficient bonding strength can be ensured between the co-fired conductor pattern and the low-temperature fired 15 ceramic, whereupon the product quality and reliability of the lowtemperature fired ceramic circuit board can be improved.

A difference is preferably at or below 3.5 (%) between shrinkage factors of the green sheet and the conductor pattern in a period when a temperature of the green sheet is increased from a firing starting 20 temperature to 480 C. Almost all the organic substance (organic vehicle) contained in the green sheet and conductor pattern thermally decomposes and vaporizes until the temperatures of the green sheet and the conductor pattern are increased 480 C from the temperatures at which firing starts.

The green sheet and the conductor pattern shrink in the process of 25 decomposition and vaporization. Thus, warp of the fired substrate is increased and the bonding strength between the conductor pattern and the low-temperature fired ceramic is reduced when the shrinkage factors of the green sheet and the conductor pattern have a large difference caused

( by thermal decomposition of organic substance. According to the results of an experiment conducted by the inventors as will be described later, an amount of warp of the fired substrate can be reduced, whereby the dimensional accuracy of the substrate can be improved when a difference 5 is 3.5 (%) or below between shrinkage factors of the green sheet and the conductor pattern in a period when a temperature of the green sheet is increased from a firing starting temperature to 480 C. Furthermore, the bonding strength between the conductor pattern and the low-temperature fired ceramic can be improved.

10 in this case, too, the conductor paste used for printing a conductor pattern is preferably an Ag conductor paste mainly containing, as a conductor powder, an Ag powder and 0.005 to 0.050 wt. Rh (rhodium) is added relative to 100 wt. So conductor powder. In this case, since Rh is effective in restraining firing of the Ag conductor pattern, the firing of the 15 conductor pattern can be progressed moderately when a suitable amount of Rh is added. Consequently, the shrinkage factor of the conductor pattern can be rendered smaller than that in the prior art after start of

firing shrinkage and accordingly, the shrinkage behavior of the conductor pattern can be approximated to that of the green sheet during firing, 20 whereupon an amount of warp of the fired substrate can be reduced.

Moreover, since an amount of Rh added as the firing retarder is small, an increase in the electric resistance value of the conductor pattern is small and accordingly, a desired electric characteristic of the conductor pattern can be maintained.

25 The glass component contained in the green sheet softens and fluidizes in the temperature range from the temperature at which the green sheet starts shrinking to 900 C when the green sheet and the conductor pattern are co-fired. However, the firing of the printed conductor pattern

( is restrained by the Rh, whereby the densification of the printed conductor pattern is retarded. Accordingly, since the glass component of the green sheet efficiently penetrates the printed conductor pattern to diffuse into it, an improvement can be expected in the bonding strength between the 5 printed conductor pattern and the fired board.

The invention can be applied to any low-temperature fired ceramic, which is fired at 1000 C or below and serves as a material for a green sheet regardless of the composition thereof. For example, the green sheet may be made of a low-temperature fired ceramic comprising a 10 mixture of CaOSiOz-Al203-B203 glass and alumina. The low-temperature fired ceramic of this composition has a relative low firing shrinkage starting temperature of 730 C. Consequently, the shrinkage behavior can easily be adjusted into the foregoing proper range, and a remarkable firing restraining effect can be attained by addition of Rh.

15 The invention will be described, merely by way of example, with reference to the accompanying drawings, in which: FIG. 1 is a typical view of a low-temperature fired multilayer ceramic circuit board manufactured by a method of one embodiment in accordance with the present invention; 20 FIG. 2 is a flowchart showing a flow of manufacturing steps; FIG. 3 is a graph of shrinkage behavior during firing of a conventional Ag conductor paste and a low-temperature fired ceramic green sheet; FIG. 4 is a graph explaining a preferable range, more preferable 25 range and most preferable range of shrinkage factor during firing of the Ag conductor paste; FIG. 5 is a graph showing the relationship between an amount of Rh added to the conductor paste and an amount of warp;

( FIG. 6 is a graph showing the relationship between an amount of Rh added to the conductor paste and resistivity; FIG. 7 is a graph showing the relationship between an amount of Rh added to the conductor paste and bonding strength; 5 FIG. 8 is a graph showing the relationship between an amount of Rh added to the conductor paste and shrinkage factor at 700 C; FIG. 9 is a graph showing the relationship between an amount of Rh added to the conductor paste and shrinkage factor at 900C; FIG, 10 is a graph showing the relationship among mean grain 10 diameter of the primary grain and the agglomerate of Ag powder and specific surface area of Ag powder; FIG. 11 is a graph showing the relationship between mean grain diameter and an amount of warp of the primary grain of Ag powder; FIG. 12 is a graph showing the relationship between mean grain 15 diameter and an amount of warp of the agglomerate of Ag powder; FIG. 13 is a graph showing the relationship among an amount of organic substance blended, resistivity and an amount of warp of the conductor paste; FIGS. 14A and 14B are a plan view and a side view of a sample 20 substrate, explaining a method of measuring an amount of warp of the sample substrate, respectively; FIG. 15 is a graph showing shrinkage behaviors of the Ag conductor paste and the low-temperature fired ceramic green sheet of the present invention during firing; 25 FIG. 16 is a plan view of sample substrates of embodiments l to 9 and compared examples 1 to 3; FIG. 17 is a side view of the substrate, explaining a method of measuring an amount of warp of the sample substrate;

( FIG. 18 is a graph showing the relationship between an amount of Rh added to the conductor paste and an amount of warp; and FIG. 19 is a graph showing the relationship between an amount of Rh added to the conductor paste and an resistivity.

5 A first embodiment of the invention will be described with reference to the accompanying drawings. In the embodiment, a low-temperature fired ceramic green sheet 11 is formed into the shape of a tape by a doctor blade method using a low-temperature fired ceramic slurry. The lowtemperature fired ceramic used is a mixture of 50 to 65 wt. % or 10 preferably 60 wt. % CaO-SiO2-AlaO3-B203 glass and 50 to 35 wt. % or preferably 40 wt. % alumina. However, other low-temperature fired ceramic materials may be used. These materials include a mixture of MgO-SiO2Al203-B203 glass and alumina, a mixture of SiOz-B203 glass and alumina, a mixture of PbO-SiO2-B203 glass and alumina, and cordierite 15 crystallized glass, all of which can be fired at 800 C to 1000 C.

Subsequently, the tape-shaped low-temperature fired ceramic green sheet 11 is cut into pieces with predetermined dimensions and thereafter, via holes 12 and 13 are formed at predetermined positions in each green sheet 11 by punching. In the embodiment of FIG. 1, the via 20 hole 13 having a larger diameter is formed into a thermal via 15 for radiating heat from a mounted component 14, whereas the via hole 12 having a smaller diameter is formed into a via conductor 18 for connecting interlayer wiring patterns 16. A ratio of the diameter of via hole 12 or 13 to the thickness of green sheet 11 is set so as to range from 0.1 to 15 or 25 more preferably, from 0.2 to 10.

After the via holes 12 and 13 have been formed by punching, fabrication progresses to a concurrent printing step in which printing for filling the via holes 12 and 13 of each green sheet 11 and printing the

wiring pattern 16 are concurrently carried out using an Ag conductor paste of the following composition. The Ag conductor paste used in the concurrent printing step mainly contains an Ag powder as a conductor powder. The Ag conductor paste may contain another noble metal such 5 as Pd. Pt or Au powder as occasion demands. In this case, an alloy powder of Ag and another noble metal may be used. Thus, a conductor of Ag-Pd, Ag-Pt or Ag-Pd-Pt is made by adding Pd or Pt powder to Ag or alloying Ag and Pd and/or Pt. The conductor has an improved solder resistance etc. as compared with a conductor containing only Ag.

10 Furthermore, a firing restraining effect is also expected when the Pd or Pt powder is added to Ag powder. Accordingly, an amount of another noble metal relative to Ag should be determined in accordance with a required characteristic such as solder resistance.

A grain diameter of the Ag powder or Ag alloy powder is prepared 15 so that a mean grain diameter of a primary grain measured by an electron microscope observation method ranging from 1.5 1lm to 4.5 Urn or more preferably, from 1.7 Urn to 4.1 Urn and so that a mean grain diameter (accumulated 50% diameter) of an agglomerate measured by a centrifugal sedimentation method ranges from 5 0 lam to 12 m. The primary grain 20 herein refers to a minimum unit of grain and a grain, which cannot be further divided. The agglomerate is an aggregation of a plurality of primary grains agglomerated.

In the electron microscope observation method, an Ag powder is photographed by a scanning electron microscope (SEM), for example.

25 The primary grain whose whole grain appears in a predetermined range of a photograph taken has a longest portion. A diameter of the longest portion of the primary grain is measured, and a mean value is obtained as a mean grain diameter of the primary grain.

The aforesaid centrifugal sedimentation method is referred to as "light transmittance centrifugal sedimentation method," in which a centrifugal grain diameter distribution measuring device is used to measure grain diameter distribution of the agglomerate on the basis of an 5 increase in an amount of transmitted light due to sedimentation of grain under centrifugal force. A mean grain diameter of the agglomerate (accumulated 50% diameter) is obtained from the results of measurement.

The Ag powder preferably has a specific surface area ranging from 0.1 m2/g to 0.4 m2/g or more preferably, from 0.13 m2/g to 0.33 m2/g.

10 The specific surface area is an index for evaluating a degree of agglomeration and form of grain (for example, surface roughness and abundance of vacancy) of the primary grain. The specific surface area is measured by a BET method or the like.

The specific surface area ranges from 0.1 m2/g to 0.4 m2/g when 15 when the mean grain diameter of the primary grain ranges from 1.5 1lm to 4.5 am and the mean grain diameter of the agglomerate ranges from 5.0 Urn to 12 1lm, as shown in FIG. 10. Accordingly, the mean grain diameters of the primary grain and agglomerate can be confined to the proper ranges in the invention more reliably when an Ag powder whose specific surface 20 area measured by the BET method ranges from 0.1 m2/g to 0.4 m2/g or more preferably, from 0.13 m2/g to 0.33 m2/g is used.

Furthermore, the Ag conductor paste is added with 0.005 to 0.050 wt. % Rh, more preferably 0.005 to 0.040 wt. fib Rh, or most preferably 0.0075 to 0.030 wt. % Rh relative to 100 wt. % conductor powder. Only 25 Rh or an Rh compound may be used. When only Rh is added, an Rh powder is preferably used so that the specific surface area measured by the BET methodpreferably ranges from 50 m2/g to 150 m2/g or more preferably, from 80 m2/g to 140 m2/g. An Rh resinate such as sulfur-Ah

( compound containing cyclic terpene may be used as the Rh compound.

However, the Rh compound should not be limited to the Rh resinate.

When an Rh compound is used, an amount of Rh compound added is determined so that an Rh content of the Rh compound comes into the 5 aforesaid range.

An inorganic substance contained in the Ag conductor paste includes glass frit or metal oxide (for example, TiO2, siO2) for restraining firing as well as the above-mentioned conductor powder (Ag powder or another noble metal powder) and Rh. Regarding glass frit, the same glass (for 10 example, CaO-SiO2-AI203-B203 glass) as contained in the green sheet 11 co-fired with the printing conductor, However, the glass frit should not be limited to this.

An Ag content of the inorganic substance contained in the Ag conductor paste is preferably set to be not less than 85 wt. %, 15 Consequently, additive (glass frit or metal oxide) other than Ag contained in the Ag conductor is below 15 wt. %, so that an electric resistance value of the printed conductor pattern can be prevented from being increased and accordingly, the electric characteristic of the printed conductor pattern can be rendered stable.

20 An organic substance contained in the Ag conductor paste comprises a binder resin such as an ethylcellulose resin or acrylic resin, an organic solvent such as terpineol (TPO), butyl carbitol acetate (BCA), terpenol, ester-alcohol, and a plasticizer. A binder resin is dissolved into an organic solvent to which the plasticizer is added, to be formed into an 25 organic vehicle. The organic vehicle is mixed with the aforesaid inorganic substance to be made into an Ag conductor paste. An organic substance contained in the conductor paste preferably ranges from 8 wt. % to 27 wt. 56 or in other words, an inorganic substance preferably ranges

( from 73 to 92 wt. %. When the organic and inorganic substances are within the respective ranges, a compounding ratio of the organic and inorganic substances of the conductor paste becomes proper.

ConsequentlY, the conductor paste has a viscosity suitable for both of the 5 via hole filling printing step and the wiring pattern printing step.

Furthermore, a loss in the printed conductor pattern due to thermal decomposition of the organic substance can be reduced in a period from the start of firing to the time when the temperature of the conductor is increased to about 400 C at which thermal decomposition of the organic 10 substance starts. As a result, the electric characteristics of the printed conductor pattern can be stabilized. A viscosity improver such as metal soap may be added in order that the viscosity of the Ag conductor paste may be adjusted.

The Ag conductor paste is prepared so that a shrinkage behavior of 15 a printed conductor pattern during firing is set so that a shrinkage factor ranges from 2.0% to 10.5%, more preferably, 2.49to to 10.5%, and most preferably, 3.3% to 9.2% until a temperature is increased from 400 C to 700 C and so that a shrinkage factor ranges from 10.0% to 21.1%, more preferably, 11.8% to 21.1o, and most preferably, 13. 9% to 19.6% until a 20 temperature is increased from 400 C to 900 C, as shown in FIG. 4.

A thermo mechanical analysis (TMA) device may be used to measure a shrinkage factor of the printed conductor pattern. In this case, a pelletshaped sample made by repeatedly applying and drying a conductor paste is used. The sample is contained in an electric furnace 25 and a temperature thereof is increased at 10 C/min. from a room temperature to 900 C in the air. In the temperature increasing process, an amount of shrinkage of the sample is measured at times when the temperature reaches 400 C, 700 C and 900 C respectively. A shrinkage

( factor in a period of increase from 400 C to 700 C and a shrinkage factor in a period of increase from 400 C to 900 C are calculated on the basis of the result of the aforesaid measurement.

In the concurrent printing step, a screen mask (not shown) is set on 5 the green sheet 11. The screen mask is formed with a print pattern (not shown) for printing the via holes 12 and 13 and the wiring pattern 16.

The Ag conductor paste of the above-described composition is supplied onto the screen mask. A squeegee (not shown) is slid along an upper side of the screen mask so that printing for filling the via holes 12 and 13 of 10 each green sheet 11 and printing the wiring pattern 16 are concurrently carried out.

An inner layer wiring pattern 16 is printed on the upper side of a lowermost green sheet 11 concurrently with the filling of the via holes 12 and 13, and thereafter, a backside pattern 19 is printed on the underside of 15 the lowermost green sheet 11, The backside pattern 19 may be printed after the green sheets 11 are laminated or fired as will be described later.

On the other hand, a surface layer wiring pattern 16 is printed on the upper side of the uppermost green sheet 11 concurrently with the filling of the via holes 12 and 13. At this time, conductor patterns such as 20 component mounting land 20 and the like other than the conductor pattern are also printed concurrently. The surface layer wiring pattern 16 and the component mounting land 20 may be printed after the green sheets 11 are laminated or fired as will be described later.

Fabrication progresses to a laminating and thermal compression step 25 upon completion of the concurrent printing step. The green sheets 11 of the respective layers are laminated, and the laminate is then thermally compressed at 60 C to 150 C at 0.1 MPa to 30 MPa or preferably, 1 MPa to 10 MPa, for example thereby to be integrated together.

Fabrication further progresses to a firing step where the laminate of green sheets 11 is fired at a temperature increasing rate of 10 C/min.

The firing peak temperature ranges from 800 C to 1000 C (preferably, about 900 C). A holding time is 20 minutes. The firing is carried out in 5 the air. The laminate of the green sheets 11 fired concurrently with the inner and surface layer wiring patterns 16 and the via conductors 15 and 18, whereby a low-temperature fired multilayer ceramic circuit board is thus fabricated.

In the firing step, alumina green sheets are laminated on both sides 10 of the laminate of green sheets}1. The laminate is then fired at 800 C to 1000 C while being compressed. Subsequently, the remainder of alumina green sheet may be eliminated from each side of the fired substrate by a blasting process. This compression firing method is advantageous in that a dimensional accuracy of the substrate can be improved with a reduction 15 in shrinkage due to firing.

The inventors conducted experiments to study the composition of conductor paste, grain diameter of Ag powder and proper range of firing shrinkage behavior all of which realizes concurrent execution of via hole filling printing and wiring pattern printing. TABLES 1 to 4 show the 20 results of experiments. TABLES 1 to 3 show the data of embodiments 1 to 40 each embodying the present invention. TABLE 4 shows the data of compared examples 101 to 107.

ETABLE]

e n b u 1 i m L T C C c n du c t r R h s br sgtUaunc e Ag Pt Pa (vat. %) (urt. %) A 100.0 0.0 0.0 0.0050 15.0

2 A 100.0 0.0 0.0 0.0075 15.0

3 A 100.0 0.0 0.0 0.0100 15.0

4 B 100.0 0.0 0.0 0.0100 15.0

5 C 100.0 0.0 0.0 0.0100 15.0

6 A 100.0 0.0 0.0 0.0100 8.0

7 A 100.0 0.0 0.0 0.0100 10.0

8 A 100.0 0.0 0.0 0.0100 20.0

9 A 100.0 0.0 0.0 0.0100 23.0

10 A 100.0 0.0 0.0 0.0100 27.0

1 1 A 100.0 0.0 0.0 0.0200 15.0

12 A 100.0 0.0 0.0 0.0300 15.0

13 A 100.0 0.0 0.0 0.0400 15.0

14 A 100.0 0.0 0.0 0.0500 15.0

(C ontinued) A g powder FUh shrinkage e.mbeondt- onlna Y particle d I t surface area surface area 0 tor (a m) (a nD(n]/e3 (nI/83 7 93 9KHOl: 1 2. 8 7.4 0.20 118 9.8 17.8

2 2.8 7.4 0.20 118 8.5 17.3

3 2.8 7.4 0.20 118 8.4 17.1

4 2.8 7.4 0.20 118 8.4 16.8

5 2.8 7.4 0.20 118 8.4 16.8

6 2.8 7.4 0.20 118 7.1 15.2

7 2.8 7.4 _ 0.20 118 8.2 16.1

8 2.8 7.4 0.20 118 8.7 17.1

9 2.8 7.4 0.20 118 8.8 17.3

10 2.8 7.4 0.20 118 9.0 17.7

11 2.8 7.4 0.20 118 4.9 15.6

12 2.8 7.4 0.20 118 3.5 14.4

_ 13 -2.8 7.4 - -0.20 118 3.0 13.0

14 2.8 7.4 0.20 118 2.3 11.5

(Continued) bonding disconnection embed- resistivity warp strength rate note iment(flu cm) (/u nD (N) (%) 12.1 191 16 O amount of added Rh 2-2.1 125 17 0 t 32.1 113 16 0 LTCC

42.1 86 16 0 _ _ t 52.1 146 16 0 t 61.9 163 16 O organic substance 72.0 154 16 0 t 82.3 102 16 0 t 2.4 95 16 0 t 102.5 88 15 0 t 112.2 88 15 O amount of added Rh 122.3 95 15 0 t 132.4 103 15 0 t 142.5 137 15 0 t

( [TABLE 2]

conductor embodiment LTCC 'vat. %) Ag Pt Pd _ 15 A 99.1 0.9 0.0 _

16 A 95.0 0.0 5.0 _

17 A 100.0 0.0 0.0 _

18 A 100.0 0.0 0.0 _

19 A 100.0 0.0 0.0 _

20 A 100.0 0.0 00 _

_ 21 A 100.0 0.0 0.0 _

22 A 100.0 0.0 0.0 _

23 A 100.0 0.0 0.0

24 A 100.0 0.0 0.0

25 A 100.0 0.0 0.0

26 A 100.0 0.0 0.0 r 27 A 100.0 0.0 0.0 L

28 A 100.0 0.0 0.0

( (Continued) Ag powder mean grain mean grain specific e hod- diameterof diameter of ment p mary particle agglomerate surface area ( m) ( m) (trI/g) 15 2.8 7.4 0.20.

16 2.8 7.4 0.20.

17 1.50 5.0 0.40.

18 1.50 5.0 0.40

19 1.70 5.3 0.33

20 1.70 5.3 0.33

21 1.70 5.3 0.33

22 1.70 5.3 0.33

23 4.50 12.0 0.10

24 4.50 12.0 0.10

25 - 4.10 9.5 0.13

26 4.10 9.5 0.13

27 4.10 9.5 0.13

28 4.10 9.5 0.13

( (Continued) bonding disconnection embed- resistivity warp strength rate note iment (2 cm) (p nD (N) (X) 15 2.4 68 17 0 Pt 16 2 2 92 18 0 Pd 17 2.1 19816 O Ag.wliD smaller 18 2.1 12416 0 t 19 2.1 15316 0 t 20 2. 1 10916 0 t 21 2.4 10116 0 t 22 2.5 13916 0 t 23 2.5 18916 0 A8 ndih larder 24 2.1 5216 0 t 26 2.1 12016 0 t 26 2.1 97_ 16 0 t 27 2. 2 11116 0 t 28 - 2.4 12316 0 t

[TABLE 3]

conductor organic embodiment LTCC 'wt. X) Rh substance Ag Pt Pa (wt. %) (wt. %) 29 A 100.0 0.0 0.0 0.0050 15.0

30 A 100.0 0.0 0.0 0.0100 15.0

31 A 100.0 0.0 0.0 0.0500 15.0

32 A 100.0 0.0 0.0 0.0050 15.0

33 A 100.0 0.0 0.0 0.0100 15.0

34 A 100.0 0.0 0.0 0.0500 15.0

35 A 100.0 0.0 0.0 0.0050 15.0

36 A 100.0 0.0 0.0 0.0100 15.0

37 A 100.0 0.0 0.0 0.0500 15.0

38 A 100.0 0.0 0.0 0.0050 15.0

39 A 100.0 0.0 0.0 0.0100 15.0

40 A 100.0 0.0 0.0 0.0500 15.0

(Continued) A powder shrinkage embody diameter mean g specific spec c tor diameter of lament primary particle agglomerate s ce ea surface area (9 (p m) (p m) (nt/0 (nt/g3 700 C 900 C 29 4.50 12.0 0.10 80 9.0 18.5

30 4.50 12.0 0.10 80 6.5 16.6

31 4.50 12.0 0.10 80 2.3 10.7

32 1.50 5.0 0.40 80 10.0 20.5

33 1.50 5.0 0.40 80 9.5 19.3

34 1.50 5.0 0.40 80 2.7 12.6

_ 35 4.50 12.0 0.10 140 9.2 19.4

36 4.50 12.0 _ 0.10 140 6.2 16.0

37 4.50 12.0 0.10 140 2.0 10.0

38 1.50 5.0 0.40 140 9.6 20.1

39 1.50 5.0 0.40 140 8.8 18.5

40 1.50 5.0 0.40 140 2.3 10.4

(Continued) _ _ bonding disconnection embod- resistivity warp strength rate note iment (.cm) (p m) (N) (%) 29 2. 1 161 16 0 amount of added Rh 30 2.1 102 16 0 __

31 2.5 191 16 0 t l 32 2.1 198 16 0 t 33 - 2.1 124 16 0 t 34 2.5 183 17 t 35 2.1 173 16 0 t 36 2.1 131 16 0 t 37 2.5 189 17 0 t 38 2.1 195 16 0 t 39 2.1 128 17 0 t 40 2.5 193 17 0 t [TABLE 4]

conductor compared LTCC (w:. %) example Ag Pt Pd 101 A 100.0 0.0 0.0

102 A 100.0 0.0 0.0

103 A 100.0 0.0 0.0

104 A 100.0 0.0 0.0

105 A 100.0 0.0 0.0

106 A 99.1 0.9 0.0

107 A 95.0 0.0 5.0

/ (Continued) A g powder Rh shrinkage mean grain mean grain specific specific factor compared diameter of diameter of surface area surface area (%) agglomerate example primary partcl (p m) (no/) (/g) 700 C 900 C 10 1 2.8 7.4 0.20 118 15.3 19.4

10 2 - 4.3 9.7 0.11 118 1.8 11.0

_ 1 0 3 1.0 3.7 1.7 118 10.3 12.1

_ 1 04 4.8 13 0.09 118 1.7 14.5

0 5 2.8 7.4 0.2 22 11.3 18.6

06 2.8 - 7.4 0.20 118 6.3 17.2

10 7 2.8 7.4 0.20 118 6.5 14.6

(Continued) bondin disconnection compared resistinty warp strength rate note example (llocm) ( m) (N) (%) 101 _ 455 10 10 amount of added Rh = 1 02 3.0 325 15 0

10 3 peeling peeling peeling 100 10 4 2.6 175 15 20

= 1 0 2.9 282 11 10

1 0 6 2.7 339 9 30 Pt __ 1 0 7 3.8 274 12 10 Pd In TABLES 1 to 4, the lowtemperature fired ceramic is referred to 5 as "LTCC." Symbols A, B and C in the columns of LTCC have the compositions as shown in FIG. 5 respectively.

( [TABLE 5]

Composition of low-temperature fired ceramic green sheet (LTCC) glass blending ratio CaO A12 O 3 SiO2 B2 o3 the others yield point glass:Alz O3 LTCC (wt. %) (at.%) (wt.%) (wt.%) (wt.%) OC) _ A 27 4 60 8 1 720 60:40

B 19 11 54 15 1 680 65:35

C 2l l2 57 8 -2 73S 55:45 In TABLES 1 to 4, the mean grain diameter of primary grain of the 5 Ag powder was measured by the SEM observation method, whereas the mean grain diameter (accumulated 50% diameter) of the agglomerate was measured using a centrifugal automatic grain diameter distribution measuring device. Furthermore, the specific surface areas of Ag powder and Rh powder were measured by the BET method.

10The column of "700OC,, in the shrinkage factor refers to a shrinkage factor in a period when the temperature is increased from 400 C to 700 C.

The shrinkage factor will hereinafter be referred to as.700OC shrinkage factor." The column of ng00 C in the shrinkage factor refers to a shrinkage factor in a period when the temperature is increased from 400 C 15to 900 C. The shrinkage factor will hereinafter be referred to as n900 C shrinkage factory A thermomechanical analysis apparatus was used to measure the shrinkage factor.

Regarding each sample in TABLES 1 to 4, one thousand of via holes each having a diameter of 150 Em were formed in the LTCC green sheet.

20 The via hole filling printing and wiring pattern printing were concurrently carried out using the conductor paste of the composition described in the

( column of each embodiment or compared example, so that a via chain was fabricated. The via chain comprised a large number of via conductors serially connected by wiring patterns each having a width of 400 m.

Fifty thousand (50x1000=50000) of via chains were evaluated about 5 presence or absence of disconnection by an electrical inspection, whereby a disconnection rate was calculated, Furthermore, four LTCC green sheets each with a thickness of 0.3 mm were laminated. A test pattern was printed on the uppermost layer using the conductor paste described in the column of each embodiment or 10 compared example. The laminate was fired at 900 C to be fabricated into a sample substrate. The bonding strength of the test pattern and the substrate surface was measured. The four LTCC green sheets were used so that a sufficient strength was ensured for the substrate.

An amount of warp was measured in the following manner. Firstly, 15 four LTCC green sheets each with a thickness of 0.3 mm were laminated into a laminated sheet. The laminated sheet was cut into pieces each of which had a length of 30 mm, so that each piece served as a sample substrate. A conductor pattern 12 mm square was printed on an upper side of the sample substrate using the conductor paste of the composition 20 described in the column of each embodiment or compared example, as shown in FIG. 14A. Thereafter, the sample substrate was co-fired with the conductor pattern at 900 C. After the firing, the heights at central positions (I) and Q) were measured using a dial gauge, as shown in FIG. 14B, and the height at a middle position 3) of a ceramic exposed portion 25 between the two conductor patterns. The differences between the heights (I;) and (a) and the heights (a) and (a) were obtained each as an amount of warp. When peeling was found in the sample substrate, an amount of warp was determined 0.

( FIGS. 5 to 13 are graphs made by extracting parts of the measurement data of the embodiments and compared examples as shown in TABLES 1 to 4 respectively.

FIG. 5 is a graph showing the relationship between an amount of Rh 5 added to the conductor paste and an amount of warp. In the samples in which an amount of added Rh ranges from 0.005 wt. % to 0.050 wt. %, the firing restraining effect of the printed conductor pattern due to addition of Rh works to a suitable degree, whereupon the difference between shrinkage behaviors of printed conductor pattern and LTCC during firing 10 becomes smaller. Consequently, since an amount of warp of the fired substrate is not more than 200 urn, no problem of an amount of warp arises. When an amount of added Rh ranges from 0.005 wt. % to 0.040 wt. %, which range is a more preferable one, an amount of warp of the fired substrate becomes further smaller. Additionally, when an amount of 15 added Rh ranges from 0.0075 wt. % to 0.030 wt. %, which range is a most preferable one, an amount of warp becomes smallest.

On the other hand, in the sample in which an amount of added Rh is 0 as compared example 101 in TABLE 4, no firing restraining effect of Rh is obtained and accordingly, the difference between shrinkage behaviors of 20 printed conductor pattern and LTCC during firing becomes large.

Consequently, an amount of warp of the fired substrate is increased to 455 m, resulting in a problem.

FIG. 6 is a graph showing the relationship between an amount of Rh added to the conductor paste and resistivity. An amount of added Rh is 25 small in the samples in which the amount of added Rh ranges from 0.005 wt. % to 0.050 wt. %, which range is a preferable one. Accordingly, an increase in the resistivity due to addition of Rh is exceedingly small such that the resistivity is limited to 2.5 Q cm. Consequently, the electric

( characteristics can be maintained at desired values. On the other hand, in the sample in which an amount of added Rh exceeds 0.050 wt. No (compared examples 102 and 105 in TABLE 4), an adverse effect of increase in the resistivity due to addition of Rh appears. Consequently, 5 the resistivity is increased to about 3.0 Q cm, thereby deteriorating the electric characteristics.

FIG. 7 is a graph showing the relationship between an amount of Rh added to the conductor paste and bonding strength. The firing restraining effect of Rh restrains densification of the printed conductor pattern in the 10 samples in which the amount of added Rh ranges from 0.005 wt. % to 0.050 wt. %, which range is a preferable one. As a result, the glass component of LTCC efficiently penetrates and distributes into the inside of the printed conductor pattern, so that the bonding strength is improved between the printed conductor pattern and the fired substrate.

15 Consequently, since the bonding strength becomes 15N or more, a sufficient bonding strength can be ensured. On the other hand, in the sample in which an amount of added Rh is 0 as compared example 101 in TABLE 4, the densification of the printed conductor pattern progresses excessively during the firing such that an amount of glass component of 20 LTCC penetrating into the printed conductor pattern is reduced.

Consequently, the bonding strength is reduced to 10N thereby to become insufficient. FIG. is a graph showing the relationship between an amount of Rh added to the conductor paste and shrinkage factor at 700 C. The firing 25 restraining effect by addition of Rh works suitably when the mean grain diameter of the agglomerate of Ag powder ranges from 5.0 Em to Mom, which range is a preferable one, and an amount of added Rh ranges from 0.005 wt. % to 0.050 wt. %, which range is a preferable one. As a result,

the 700 C shrinkage factor ranges from 2.0 to 10.5, which range is a preferable one.

On the other hand, in the sample in which an amount of added Rh is O as compared example 101 in TABLE 4, no firing restraining effect of Rh 5 is obtained and accordingly, the 700 C shrinkage factor is increased to 15. 3%. Furthermore, in the sample in which an amount of added Rh is 0.10 wt. No as compared example 102 in TABLE 4, the amount of added Rh becomes excessively large such that the firing restraining effect works excessively. As a result, the 700 C shrinkage factor is below 2% and out 10 of the preferable range. Additionally, in the sample in which the mean grain diameter of the agglomerate of Ag powder is 13.0 m, which value is larger than the preferable range, too, the 700 C shrinkage factor is below 2% and out of the preferable range.

FIG. 9 is a graph showing the relationship between an amount of Rh 15 added to the conductor paste and shrinkage factor at 900 C. The firing restraining effect by addition of Rh works suitably when the mean grain diameter of the agglomerate of Ag powder ranges from 5.0 am to Mom, which range is a preferable one, and an amount of added Rh ranges from 0.005 wt. % to 0.050 wt. %, which range is a preferable one. As a result, 20 the 900 C shrinkage factor ranges from 10.0 to 21.1, which range is a preferable one.

FIG. 10 is a graph showing the relationship among mean grain diameter of the primary grain and the agglomerate of Ag powder and specific surface area of Ag powder. The graph also shows a theoretical 25 value in the case where it is supposed that the primary grain is spherical.

In the measurement data shown in FIG. 10, the mean grain diameter of the primary grain measured by the electron microscope observation method ranges from 1.5 Em to 4.5 1lm and the mean grain diameter of the

! agglomerate measured by the centrifugal sedimentation ranges from 5.0 Em to 12 m. In this range, the specific surface area measured by the BET method ranges from 0.1 m2/g to 0.4 m2/g. Accordingly, the mean grain diameters of the primary grain and agglomerate can reliably be put in 5 a preferable range when the Ag powder is used so that the specific surface area ranges from 0.1 m2/g to 0.4 m2/g (more preferably, from 0.13 m2/g to 0.33 m2/g). Furthermore, since the difference between the mean grain diameter of the primary grain and the theoretical value is relatively smaller, it is understood that the primary grain has an approximately 10 spherical shape.

FIG. 11 is a graph showing the relationship between mean grain diameter and an amount of warp of the primary grain of Ag powder. An amount of warp of the fired substrate is restrained so as to be not more than 200 1lm when an amount of added Rh ranges from 0.005 wt. % to 15 0.050 wt. %, which range is a preferable one and the mean grain diameter of the primary grain of the Ag powder ranges from 1.5 Urn to 4.5 m, which range is a preferable one. Consequently, no problem arises regarding the warp of the fired substrate.

FIG. 12 is a graph showing the relationship between mean grain 20 diameter and an amount of warp of the agglomerate of Ag powder. In the samples in which an amount of added Rh is O and 0.1 wt. %, an amount of warp of the fired substrate is 300 Em or more, resulting in a problem of an amount of warp of the fired substrate. However, in the samples in which an amount of added Rh ranges from 0.005 wt. 0 to 0.050 wt. %, which 25 range is a preferable one, an amount of warp of the fired substrate is restrained to 200 Em or below when the mean grain diameter of the agglomerate of the Ag powder is in a preferable range from 5.0 Em to 12 m. As a result, no problem arises regarding an amount of warp of the

( fired substrate.

When the mean grain diameters of the primary grain and agglomerate of the Ag powder are smaller than the preferable ranges respectively (compared example 103 in TABLE 4), a junction between the 5 printed conductor pattern and the fired substrate is peeled off, resulting in a failure in the firing. Furthermore, when the mean grain diameters of the primary grain and agglomerate of the Ag powder are larger than the preferable ranges respectively (compared example 104 in TABLE 4), the 700 C shrinkage factor becomes smaller than a preferable range.

10 Consequently, the difference of shrinkage factors of the printed conductor pattern and LTCC becomes larger, resulting in occurrence of disconnection. FIG. 13 is a graph showing the relationship among an amount of organic substance blended, resistivity and an amount of warp of the 15 conductor paste. In the samples in which an amount of blended organic substance is in a preferable range from 8 wt. So to 27 wt. %, an amount of warp of the fired substrate is restrained to 180 Em or below and moreover, the resistivity is restrained to 2.5 Q.cm. Consequently, desired electric characteristics can be maintained.

20 As shown in the following TABLE 6, preferable, more preferable and most preferable ranges can be obtained from the results of evaluation of resistivity, amount of warp and bonding strength of embodiments 1 to 40 in TABLES 1 to 3.

( [TABLE 6]

Ag content of amount of organic mean particle size inorganic added Rh substance of primary particle substance (wt%) (wt.%) (u m) 0.005 1.54.5

preferable range 0.050 more not less than 0.005 827 1.54.5 preferable range 85 wt.% 0.040 most not less than 0.0075 8271.74.1 preferable range 85 wt.% 0.030 (Continued) shrinkage factor shrinkage factor mean particle specific in temperature in temperature size of surface area increase period increase period agglomerate (nt/g) from 400 to Mom 400 to (it 700UC (%) 900.C (%) 5.012.0 0.0.4 2.010,5 10.0

preferable mage 21.1 more 5.012.0 0.10.4 2.410.5 11.8 preferable range 21. 1 most 6.012.0 0.13 3.39.2 13.9 preferable range 0.33 19.6 5 An Ag content in an inorganic substance contained in the conductor paste is preferably at or above 85 wt. %. Consequently, since an inorganic additive (for example, glass frit or metal oxide) other than Ag in the inorganic substance is not more than 15 wt. do, the electric resistance value of the printed conductor pattern can be prevented from being 10 increased, whereupon the electric characteristics of the printed conductor pattern can be stabilized.

The conductor paste of the invention is advantageous in that the

( printing for via hole filling and printing the wiring pattern can concurrently be carried out. However, the conductor paste of the invention may be used when the via hole filling printing step and the wiring pattern printing step are carried out separately from each other.

5 A second embodiment of the invention will be described. In the embodiment, as shown in FIG. 15, the shrinkage factor of the green sheet 11 and/or the shrinkage factor of the conductor pattern 16 is adjusted by, for example, an amount of added Rh so that the following relationship holds: 10 0.26sb/asO.73 where a is a shrinkage factor of the green sheet 11 in % in a period when a temperature of the green sheet is increased from a firing shrinkage starting temperature to 900 C and b is a shrinkage factor of the conductor pattern 16 in % in the period when the temperature of the green sheet is 15 increased from the firing shrinkage starting temperature to 900 C.

The firing shrinkage starting temperature is 730 C when the green sheet 11 is made of a low-temperature fired ceramic comprising a CaO-SiO2-Al203B203 glass and alumina. Generally, the low-temperature fired ceramic green sheet fired at 850 C to 900 C has a firing shrinkage 20 starting temperature ranging from 650 C to 820 C.

Furthermore, in the embodiment, the shrinkage factor of the green sheet 11 and/or the shrinkage factor of the conductor pattern 16 is adjusted by, for example, an amount of blended organic substance so that a difference c (%) is at or below 3.5 (%) between shrinkage factors of the 25 green sheet and the conductor pattern in a period when a temperature of the green sheet is increased from a firing starting temperature to 480 C.

A thermo mechanical analysis (TMA) device may be used to measure a shrinkage factor of the conductor pattern 16 or the green sheet

* 11. In the second embodiment, too, the conductor paste used for printingthe conductor pattern 16 is an Ag conductor paste basically having the same composition as that in the first embodiment. The Ag conductor 5 paste is added with 0.005 to 0.050 wt. No Rh. The low-temperature fired multilayer ceramic circuit board is fabricated using the aforesaid Ag conductor paste in the same fabricating steps as in the first embodiment.

The inventors conducted experiments to study proper ranges of a shrinkage factor parameter b/a, the aforesaid difference c and an amount 10 of added Rh. TABLE 7 and FIGS. 18 and 19 show the results of experiments. [TABLE 7]

( I CD I I I N 3513 11 51

_ _ _ __ cO 0 co u' u, O cat N N _ _ _

N O W W O N O

a en 0 Hi _ o, =-

gOS _ _ _= _ _ _ , N con N Cal Cal Cal N C q N N N Ed a _ _ -.E

O O O O O N O O O O O O..

O __ _ N _ _ _ _ O LO 14

-} O O, O, O' O. 00' O. O. O, O O O C

{D _ __

! O O O O O O O O O O O O

_ _ _ _ __ gn 0! O O O O O O O O O O O O

O _ _ O O O O _ Cii O _ O 1- _ _ _ _ -

O _ _ _....

1O _ O _ _ O O O O m _ _ _ O _ N

1O "a 0 0 _ _ _ _ _

p L' O N o K E a E:a a E E a E GE 0; a Si

The shrinkage factor parameter b/a is a ratio of the shrinkage factor b (%) of the conductor pattern 16 to the shrinkage factor a (%) of the green sheet in a period when a temperature of the green sheet is increased from a firing shrinkage starting temperature to 900 C. The 5 shrinkage factor parameter c is the difference between the shrinkage factor (%) in the period when the temperature of the green sheet is increased from a firing starting temperature to 480 C and the shrinkage factor (%) in a period when the temperature of the green sheet in the dried state to 480 C.

10 Regarding each of examples 41 to 49 and compared examples 201 to 203, the green sheets each of which had a thickness of 300 1lm were laminated into four layers. A wiring pattern (conductor pattern) having a line width of 400 am was printed meanderingly on the laminated green sheet as shown in FIG. 16. using the conductor paste having the 15 composition as shown in TABLE 7. Furthermore, four measurement strength measuring pads (conductor pattern) 2 mm square were printed on the green sheet. The used green sheet was made of a low-temperature fired ceramic comprising a mixture of 60 wt. % CaO-SiO2-Al2O3-B2O3 glass and 40 wt. % alumina. The firing shrinkage starting temperature of 20 this green sheet was 730 C. A sample substrate of each example and compared example was fired at 890 C in a conveyor furnace. Film thicknesses of the wiring pattern and measuring pad were measured after the firing. The measured film thicknesses ranged from 10 am to 14 1lm.

A conductor contained in the conductor paste used in each example and 25 compared example was an Ag conductor consisting of 100 wt. % Ag or Ag with a slight amount of Pt or Pd added. An Ag conductor paste made by blending a 23 wt. No organic vehicle into 100 wt. % Ag conductor powder was used. Furthermore, 2 wt. % glass frit was added to the conductor

( paste only in example 46.

A film thickness was measured at both ends of the wiring pattern as shown in FIG. 16. In the measurement of a warp amount of the fired substrate, heights were measured at a central position ó) and both ends 5 (a) and (a) respectively. A warp amount was obtained by subtracting a mean value of the heights (a) and (a) at both ends from the height At at the central position (see FIG. 17).

In examples 41 to 49 shown in TABLE 7, the shrinkage factor parameter b/a ranged from 0.26 to 0.73, whereas the shrinkage factor 10 parameter c ranged from 0.7% to 3.5%. An amount of added Rh ranged from 0.005 wt. % to 0.050 wt. %. Only in example 47, an Rh resinate was used for addition of Rh, whereas an Rh powder was used in the other examples and compared examples.

In examples 41 to 49, the resistivity ranged from 2.3 Ills em to 3 15 IlQ. cm and a warp amount of the fired substrate ranged from 23 Em to 87 Urn. A bonding strength between the pad and the substrate ranged from 16N to 21N.

On the other hand, in compared examples 201 to 203, a warp amount of the fired substrate was no less than 110 urn, which was a large 20 value. Since no Rh was added to the conductor paste particularly in compared example 201, the firing restraining effect of the conductor paste could not be obtained, whereupon the difference between shrinkage factors of the green sheet and the wiring pattern became excessively large. As a result, the shrinkage factor parameter b/a was 0.08, which 25 was a small value, and the shrinkage factor parameter c was 11.1, which was a large value, in compared example 201. Accordingly, a warp amount of the fired substrate was 125 m, which was a large value, whereupon a required dimensional accuracy of the fired substrate was not able to be

( ensured. Furthermore, the bonding strength between The pad and the substrate was ION, which was a small value, whereupon the pad tended to be easily peeled.

In compared example 202, an amount of Rh added to the conductor 5 paste was 0.001 wt. % which value was small. Accordingly, the firing restraining effect of the conductor paste due to addition of Rh lacks such that the difference between shrinkage factors of the green sheet and the wiring pattern could not be rendered sufficiently small. As a result, the shrinkage factor parameter b/a was 0.14 which value was small, whereas 10 the shrinkage factor parameter c was 8.1 which value was large.

Consequently, a warp amount of the fired substrate was 110 am which value was large, whereupon a sufficient dimensional accuracy of the fired substrate could not be ensure. Furthermore, the bonding strength between the pad and the substrate was 1 4N which value was small, 15 whereupon there was a possibility that the pad might be peeled off. This resulted in reductions in the product quality and reliability.

In compared example 203, the firing restraining effect of the conductor paste was expected since an amount of added Rh was 0.075 wt. fib. However, the shrinkage factor parameter b/a was 0.11, which 20 value was small and accordingly, a warp amount of the fired substrate was 110 Em which value was large, whereupon a sufficient dimensional accuracy of the fired substrate was not able to be ensured. Moreover, disadvantage (resistivity increase) due to addition of Rh appeared conspicuously such that the resistivity is increased to 9.0 cm which 25 value was large. As a result, the electric characteristics were reduced.

On the other hand, in examples 41 to 49, the shrinkage factor parameter b/a ranged from 0.26 to 0.73 and the shrinkage factor parameter c ranged from 0.7% to 3.5%. The amount of added Rh ranged

( from 0.005 wt. % to 0.050 wt. %. When the parameter b/a ranges from 0. 26 to 0.73, the difference between the shrinkage behaviors of the green sheet and the wiring pattern can be rendered small in a period when the temperature of the green sheet was increased from the firing shrinkage 5 starting temperature (730 C) to 900 C. Moreover, when the shrinkage factor parameter c ranges from 0.7% to 3.5%, the difference between the shrinkage behaviors of the green sheet and the wiring pattern can be rendered small even in a temperature range before start of firing shrinkage of the green sheet.

10 Furthermore, in examples 41 to 49, a suitable firing shrinkage restraining effect of the conductor paste can be achieved when an amount of added Rh ranges from 0.005 wt. So to 0.050 wt. %. Thus, with the abovedescribed optimization of the shrinkage factor parameters b/a and c, a warp amount of the fired substrate can be restrained to 84 An or below, 15 whereupon desirable dimensional accuracy of the substrate can be ensured, as shown in FIG. 18. Moreover, when an amount of added Rh ranges from 0.005 wt. % to 0.050 wt. % and is a small value, an resistivity of the wiring pattern is not so increased due to addition of Rh, whereupon the resistivity can be restrained to 5.3 IlQcm or below. Consequently, 20 desirable electric characteristics can be ensured. Moreover, the bonding strength between the pad and the substrate is not less than ION, whereupon the pad can be prevented from being peeled. Consequently, a good connection reliability can be ensured.

Although the embodiment of FIG. 1 is formed with the thermal via 15, 25 no such thermal via may be provided, instead. The structure of the multilayer ceramic circuit board may be changed. The number of low- temperature fired ceramic green sheets may also be changed. The above- described fabricating method may further include a printing step for

( printing a thick-film resistor paste on the low-temperature fired ceramic green sheet.

Claims

WE CLAIM:

1, A conductor paste which is used to print on a low-temperature fired ceramic green sheet firable at or below 1000 C and mainly contains, 5 as a conductor powder, an Ag powder or an Ag alloy powder both of which will hereinafter be referred to as "Ag powder," wherein 0.005 to 0.050 wt. % Rh is added relative to 100 wt. % conductor powder, wherein the Ag powder has a mean grain diameter of primary grain ranging from 1.5 Em to 4.5 Elm, said mean grain diameter of the primary grain being measured by 10 an electron microscope observation method, and the Ag powder has a mean grain diameter of an agglomerate ranging from 5.0 prn to 12 An, said mean grain diameter of the agglomerate being measured by a centrifugal sedimentation, and wherein a shrinkage behavior of a printed conductor pattern during firing is set so that a shrinkage factor ranges from 2. 09to to 15 10.5% until a temperature is increased from 400 C to 700 C and so that a shrinkage factor ranges from 10.0% to 21.1% until a temperature is increased from 400 C to 900 C.

2. A conductor paste according to claim 1, wherein the Ag powder 20 has a specific surface area ranging from 0.1 m2/g to 0.4 m2/g.

3. A conductor paste according to claim 1 or 2, wherein an organic substance ranges from 8 wt. % to 27 wt. %.

25 4. A conductor paste according to any one of claims 1 to 3, wherein an Ag content in an inorganic substance is set to be not less than 85 wt. %.

5. A method of printing a conductor paste on a low-temperature

( fired ceramic green sheet firable at or below 1000 C, wherein printing for filling a via hole of the low-temperature fired ceramic green sheet and printing a wiring pattern are concurrently carried out using the conductor paste defined by any one of claims l to 4.

6. A method of fabricating a ceramic circuit board, comprising: concurrently carrying out printing for filling a via hole of a lowtemperature fired ceramic green sheet firable at or below 1000 C and printing a wiring pattern using the conductor paste defined by any one of 10 claims 1 to 4; laminating a plurality of the low-temperature ceramic green sheets into a laminate after the concurrent printing; and firing the laminate at or below 1000 C.

15 7. A method of fabricating a low-temperature fired ceramic circuit board by co-firing a low-temperature fired ceramic green sheet firable at or below 1000 C and a conductor pattern printed on the green sheet using a conductor paste, wherein 0.26sb/asO.73 where a is a shrinkage factor of the green sheet in % in a period when a temperature of the green sheet is 20 increased from a firing shrinkage starting temperature to 900C and b is a shrinkage factor of the conductor pattern in % in the period when the temperature of the green sheet is increased from the firing shrinkage starting temperature to 900 C.

25 8. A method according to claim 7, wherein a difference is at or below 3.5o between shrinkage factors of the green sheet and the conductor pattern in a period when a temperature of the green sheet is increased from a firing starting temperature to 480 C.

r ( 9. A method according to claim 7 or 8, wherein the conductor paste used for printing a conductor pattern is an Ag conductor paste mainly containing, as a conductor powder, an Ag powder or an Ag alloy 5 powder, wherein 0.005 to 0.050 wt. % Rh is added relative to 100 wt. % conductor powder.

l O. A method according to any one of claims 7 to 9, wherein the green sheet is made of a low-temperature fired ceramic comprising a 10 mixture of CaO-SiOrAI203-B203 glass and alumina.

11. A conductor paste substantially as herein described with reference to the accompanying drawings.

15 12. A method of printing a conductor paste on a low-temperature fired ceramic green sheet, substantially as herein described with reference to the accompanying drawings.

13. A method of fabricating a ceramic circuit board substantially as 20 herein described with reference to the accompanying drawings.

14. A method of fabricating a low-temperature fired ceramic circuit board substantially as herein described with reference to the accompanying drawings.

f - Amendments to the claims have been filed as follows WE CLAIM:

i. A conductor nests.!.nich IS used to print O.'! a low-temperarure fired ceramic green sheet firable at or below 1000 C and mainly contains, 5 as a conductor powder, an AN powder or an An alloy powder both of which will hereinafter be referred to as "Ag powder," wherein 0.005 to 0.050 wt. Rh is added relative to 100 wt. % conductor powder, wherein the Ag powder has a mean grain diameter of primary grain ranging from 1.5 ilm to 4.5 m, said mean grain diameter of the primary grain being measured by 10 an electron microscope observation method, and the Ag powder has a mean grain diameter of an agglomerate ranging from 5.0,um to 12,um, said mean grain diameter of the agglomerate being measured by a centrifugal sedimentation, and wherein a shrinkage behavior of a printed conductor pattern during firing is set so that a shrinkage factor ranges from 2.0% to 15 10.5% until a temperature is increased from 400 C to 700 C and so that a shrinkage factor ranges from 10.0o to 21.1% until a temperature is increased from 400 C to 900 C.

3. A conductor paste according to claim 1 or 2, wherein an organic substance ranges from 8 wt. So to 27 wt. %.

2rj

4 A conductor paste according to any one of claims 1 to 3. wherein an Ag content in an inorganic substance is set to be not less than 85 wt. %.

5. A rl,ethvd Of prir.tir. a ce,..ductr paste a.. a!suT-tcll,pcra+ure

fired ceramic Free.-, sheet f;rable at or below 1.)CC, ushered printing foi-

filling a via hole of the low-temperature fired ceramic green sheet and ., _g u..g pa++ hi-= n.rronlr r^rrid no,,cina the conductor paste defined by any one of claims 1 to 4.

6. A method of fabricating a ceramic circuit board, comprising: concurrently carrying out printing for filling a 'via hole of a lowtemperature fired ceramic green sheet firable at or below 1000 C and printing a wiring pattern using the conductor paste defined by any one of claims l to 4; laminating a plurality of the low-temperature ceramic green sheets into a laminate after the concurrent printing; and firing the laminate at or below 1000 C.

7. A conductor paste substantially as herein described with reference to the accompanying drawin&'s.

s. A method of printing a conductor paste on a low-temperature fired ceramic green sheet, substantially as herein described with reference to the accompanying drawings.

9. A method of fabricating a ceramic circuit board substantially as herein described with reference to the accompanying drawings.

0. A method of fabricating a iow-remperaure fired cerarl-.ic circuit board substantially as herein described with reference to the accompanying drawings.