JPH1197823A - Formation of wiring with conductive paste - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a very fine and high density wiring on a ceramic substrate or a green sheet by screen printing conductive paste on the substrate or the green sheet, after that, reversing the substrate or the green sheet is reversed with the printed face side down, the printed conductive paste is heated and dried. SOLUTION: After screen printing, a ceramics substrate is reversed with a printed face side down. In this condition, the printed conductive paste is heated and dried. From heating and drying process, a solvent in the conductive paste is eliminated and in substance, the conductive paste loses fluidity. The paste also loses adhesiveness and therefore dust hardly attaches to the paste. It is preferable that the heating and drying process be conducted in a clean environment, in order to prevent duct from attaching to the paste during heating. As a result of this method, the sagging of paste can be prevented during dying, thereby forming a wiring of a fine pattern (width of wires, distance between wires, pitch) without causing short-circuiting due to the sagging of paste.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a wiring on a ceramic substrate or a green sheet using a conductive paste, and more particularly to a method for forming a fine and high-density wiring.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of miniaturization of electronic equipment, miniaturization of circuit boards on which electronic components are mounted has been required.
In order to reduce the size of a circuit board, it is indispensable to increase the density of inner and surface wirings on the board.

[0003] Wiring of a ceramic circuit board is generally formed by screen-printing a conductive paste obtained by adding a resin and a solvent to a metal powder. After printing, the substrate is heated (fired) to a high temperature to remove organic components such as resins and solvents by thermal decomposition and sinter the metal powder.
Wiring is formed on the substrate.

As a screen mask used for screen printing, a screen mask made of a thin metal plate is used in part, but in many cases, a mesh (made of metal such as stainless steel, but usually made of resin such as nylon or polyester) is used. A mesh screen mask on which a predetermined pattern is formed from a photosensitive emulsion on a mesh is used. In particular, a resin mesh screen has high elasticity and printing accuracy is hardly affected by irregularities on the surface of the substrate. Therefore, it is suitable for a ceramic substrate whose surface smoothness is not very high.

[0005] However, in screen printing using a resin mesh screen, formation of fine wiring becomes difficult due to clogging of the conductive paste at the screen openings. The mesh aperture ratio of the resin mesh screen is currently
The maximum is 60-70%, and a higher aperture ratio is impossible to maintain the screen strength. Therefore, it is difficult to prevent clogging by increasing the mesh opening ratio.

In order to avoid clogging, it is conceivable to use a conductor paste having a low viscosity for screen printing.
However, if the paste viscosity is low, the wiring tends to spread due to dripping after printing, and if the wiring interval is small, the wiring is short-circuited. Therefore, the development of a conductive paste having high thixotropy (high thixotropy conductive paste), which has a low viscosity during screen printing and has a high viscosity after printing, has been promoted.

[0007] Even if a mesh screen having a high aperture ratio is used using such a high thixotropy conductive paste, at present, when printing on a green sheet, the wiring pitch is about 100 μm, and when printing on a fired ceramic substrate afterwards, The limit is a wiring pitch of about 120 μm, and it is difficult to form finer wiring by the current screen printing method. However, circuit boards are getting smaller and smaller,
As a result, a wiring with a finer wiring pitch has been required.

Japanese Patent Laid-Open No. 2-3085 discloses a means for miniaturizing wiring.
No. 97 discloses that a fine wiring is formed by stretching a ceramic green sheet and then printing a circuit pattern, and heating or shrinking the green sheet during or before firing to shrink the green sheet. However, in this method, a difficult step of stretching the ceramic green sheet is added, and the shrinkage ratio of the stretched green sheet is not always constant, so that practical use is difficult.

Japanese Patent Application Laid-Open No. 57-60896 discloses that, in the production of a ceramic multilayer wiring board by a thick-film multilayer printing method, a conductive paste is printed on a release film instead of a substrate, and then heated and pressed on the substrate. A method for transcription is described. In this method, a release film is required for each layer of the wiring, which increases the cost. In addition, a transfer step using a heating press is added for each wiring, so that the manufacturing process becomes very complicated.

[0010]

It is known that sag during screen printing is more likely to occur on a substrate fired than on a green sheet. This is because the green sheet contains organic components such as a binder and a solvent, and the organic components in the conductor paste can penetrate to some extent. Therefore, when the ceramic multilayer substrate is manufactured by the green sheet multilayer laminating method, the inner layer wiring is printed on the green sheet, so that the conductive paste hardly sags and the formation of fine wiring is relatively easy. On the other hand, when a conductive paste is printed to form a surface wiring later on a fired ceramic substrate, sagging of the conductive paste is likely to occur, making it difficult to form fine wiring.

As means for preventing the conductive paste from dripping on the fired ceramic substrate, a resin is first applied to the substrate, and then the conductive paste is printed, and the resin applied during firing and the resin such as the resin in the conductive paste are removed. A method of forming a surface wiring by decomposing the components can be considered. However, this resin is eliminated by decomposition during firing, and is unnecessary for forming a circuit board. Considering the necessity of a coating facility, this leads to a high production cost.

According to the present invention, fine and high-density wiring is formed on a ceramic substrate or a green sheet from a conductive paste by screen printing without increasing the cost and adding a complicated process as compared with the conventional method. An object of the present invention is to provide a method for forming a wiring which can be performed.

[0013]

Means for Solving the Problems The inventors of the present invention screen-print a conductor paste on a fired ceramic substrate, turn the substrate over, and heat-dry the conductor paste printed with the printed surface facing down. It has been found that dripping of the printed conductor paste is effectively prevented, and the above problem can be solved. It has also been found that this sagging prevention effect can be obtained also when the conductive paste is printed on a ceramic green sheet.

Here, the present invention is characterized in that a conductor paste is screen-printed on a ceramic substrate or a green sheet, and then the substrate or the green sheet is inverted and the conductor paste printed with the printed surface facing down is heated and dried. This is a method of forming wiring.

When the conductive paste is printed on a fired substrate, it is necessary to fire the conductive paste after printing. This baking may be performed simultaneously with the above-mentioned heating and drying (that is, the baking is performed with the printing surface facing down, and the heating and drying are also performed during the baking), or may be performed in a step different from the heating and drying.

When the conductor paste is printed on the green sheet, a plurality of green sheets obtained by printing and heating and drying the conductor paste as described above are laminated according to a green sheet multilayer lamination method, and the laminate is fired at once. It is common to fire simultaneously with the conductor paste and the green sheet.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for forming a wiring according to the present invention will be described in detail. As described above, the formation of wiring according to the present invention can be applied to both fired ceramic substrates and unfired ceramic green sheets.

The type of the ceramic substrate is not particularly limited. A typical ceramic substrate is an alumina substrate, an aluminum nitride substrate, a glass ceramic substrate or the like, but other ceramic substrates can also be used. The ceramic substrate may be a multilayer wiring substrate having an inner layer conductor or via formed thereon, or may be a simple single-layer substrate, regardless of its form.

The green sheet is usually a single layer, but may also be a multilayer having an inner conductor. In general, a green sheet for a multi-layer substrate is formed with vias and filled with vias before printing the conductive paste, but this is not necessarily essential.

For the sake of simplicity, the following description focuses on printing on a ceramic substrate and supplements on printing on a green sheet as necessary.

The type of conductor paste used for screen printing is not particularly limited. Ag,
Ag-Pd, Au, Pt and other noble metal materials, Cu, Ni, W, Mo
There are relatively low-cost base metal materials such as -Mn, and any conductive paste can be used. The conductive paste is appropriately selected according to the sintering temperature of the ceramic substrate or green sheet.

For example, when printing on a ceramic substrate fired at a low temperature such as a glass ceramic substrate, a conductor paste requiring high temperature firing such as W paste or Mo-Mn paste is inappropriate. In the case of a ceramic substrate fired at a high temperature such as an alumina substrate or an aluminum nitride substrate, any conductive paste can be used.

On the other hand, when printing on a green sheet, a paste of a high melting point metal such as W or Mo-Mn must be used for a green sheet which is fired at a high temperature such as an alumina or aluminum nitride green sheet. In the case of a green sheet which is fired at a low temperature such as a glass ceramic green sheet, a noble metal paste or a Cu paste which is sintered at a low temperature is suitable.

As described above, the conductor paste to be used preferably has a low viscosity at the time of printing and has high thixotropy, which increases the viscosity after printing. Particularly preferred conductor paste is the above-mentioned highly thixotropic conductor paste.

In the present invention, since the sagging after printing can be prevented, the viscosity (viscosity at the time of flow in the case of thixotropic properties) is higher than before.
Conductor paste can be used. For this reason, clogging hardly occurs even with a mesh screen having the same aperture ratio, and printing can be performed without clogging even if the wiring width is small. Therefore, it is possible to print a finer wiring pattern than before.

The screen printing of the conductor paste can be carried out by any conventionally known method. For example, a mesh screen (especially made of resin) having a predetermined wiring pattern formed from a photosensitive emulsion is placed on a ceramic substrate, an appropriate amount of conductive paste is placed on the screen, and a paste is applied from a screen opening through a screen squeegee. Is extruded onto the substrate.

After the screen printing is completed, the ceramic substrate is turned over so that the printed surface faces downward. The printed conductor paste is heated and dried with the printed surface facing down. By this heating and drying, the solvent in the conductor paste is removed, and the fluidity of the conductor paste is substantially lost. In addition, as a result of the loss of the adhesiveness of the conductor paste, dust is less likely to adhere. This heating and drying is preferably performed in a clean environment so that dust does not adhere during heating.

Even if the conductor paste is heated and dried with the printed surface facing down according to the present invention with the printed surface facing down, if the time from printing to reversal of the substrate becomes longer, the printed conductor paste will sag before the reversal. Sometimes. This sagging is more likely to occur as the wiring spacing (space) is smaller and, if the wiring spacing is the same, as the wiring width is larger.
Therefore, particularly when the wiring interval is narrow, for example, 60 μm or less, the time from printing of the conductive paste to inversion of the substrate or green sheet is preferably within 30 seconds, more preferably within 15 seconds. If this time is within 15 seconds,
Wiring spacing 50μm or less, wiring pitch 100μ
Even if it is as small as m or less, short circuit due to sagging can be almost certainly prevented.

The heating conditions may be selected so as to achieve the above-mentioned functions, and differ depending on the boiling point of the solvent in the conductor paste. The heating temperature is usually 60-150 ° C, preferably
80 to 120 ° C, and the heating time depends on the temperature.
Preferably, it is about 30 minutes.

When heating is performed with the printed surface facing upward as in the prior art, the fluidity of the conductive paste temporarily increases during the heating, and the conductive paste easily spreads due to sagging. By heating with the printing surface facing down, this sagging is less likely to occur, and even for a fine wiring pattern with a narrow wiring interval, it is possible to prevent the wiring from shorting during heating and drying of the conductive paste, Fine and high-density surface wiring can be formed.

When printing on a fired ceramic substrate,
This heating and drying may be performed in a heating furnace for drying different from the firing furnace, or may be performed in the firing furnace as an initial stage of firing.

In the former case (that is, the heating and drying of the conductive paste and the firing are performed in separate furnaces), only the heating and drying may be performed with the printed surface facing downward. Since the dried conductive paste is less likely to sag, firing of the dried conductive paste may be performed with the printed surface facing upward, or may be performed with the printed surface facing downward.

In the latter case, there is no particular need to provide a heating and drying step, and this heating and drying is also performed during the step of heating the conductive paste to the firing temperature. Therefore,
Put the ceramic substrate into the firing furnace with the print side facing down,
Heat. At that time, if necessary, the above-mentioned heating and drying temperature may be held for a certain period of time, but since the amount of the conductor paste is generally small, it is not necessary to keep the temperature, and during the heating to the firing temperature, Heat drying is also completed.

In the case of printing on green sheets, usually, a plurality of printed green sheets are stacked and then fired at once, so that firing is not performed at this stage. Therefore, the green sheet on which the conductive paste is printed is inverted, and the printed surface is turned downward, and only heating and drying are performed. The heating conditions may be the same as described above. Of course, when a single-layer substrate is used, the heating and drying of the green sheet may be performed simultaneously with firing in a firing furnace.

The method of the present invention can be implemented by simply adding a means for inverting a ceramic substrate (or a green sheet) and a jig for supporting the inverted substrate in a dryer without touching a printing surface to a conventional apparatus. can do. Therefore, the manufacturing cost and the manufacturing efficiency hardly change from the conventional one.

Even in the case of a green sheet, it is easy to invert because it has a certain degree of rigidity. In order to hold the green sheet with the print surface facing downward, the green sheet may be suctioned from the upper surface side using a suction plate having suction holes. In order to prevent the green sheet dried in the suction state from peeling off from the suction plate, it is preferable to interpose a breathable sheet (eg, Japanese paper) between the suction plate and the green sheet.

[0037]

【Example】

(Example 1) A commercially available Cu paste (not high thixotropy) was applied onto an alumina substrate in a wiring pattern (wiring width: 75 μm, wiring interval: 75 μm, wiring pitch: 150 μm) shown in FIG. Mesh screen with mask
(Aperture ratio 60%) was used for screen printing.

Ten seconds after the completion of printing, the substrate was inverted so that the printed surface faced down, and the conductor paste was dried by heating at 100 ° C. for 10 minutes in an air atmosphere in a clean oven. After that, the conductor paste was fired by heating at 750 ° C for 30 minutes in a nitrogen atmosphere in a firing furnace with the printed surface facing up,
A surface wiring was formed on the substrate. The electric resistance of the obtained surface wiring was measured between the terminal 10 and the terminal 20 in FIG. 1 and the presence or absence of a short circuit was determined. As a result, no short circuit occurred.

For comparison, surface wiring was formed on an alumina substrate in exactly the same manner as described above, except that drying was performed with the printed surface up, without turning over the substrate. In this surface wiring, conduction between the wirings was confirmed by measuring the electric resistance, and a short circuit occurred. That is, by inverting the substrate after printing the conductor paste and drying with the printed surface down,
It was found that dripping of the conductive paste during drying was prevented, and fine wiring could be formed without causing a short circuit.

Example 2 A surface layer wiring was formed on an alumina substrate in the same manner as in Example 1, except that the wiring intervals of the wiring pattern were 75 μm, 100 μm, and 150 μm (the wiring width was 75 μm).
μm), and the time from the end of screen printing to the inversion of the substrate before drying was changed between 15 seconds and 120 seconds. Table 1 shows the results of the measurement of the presence or absence of a short circuit in the formed surface wiring in the same manner as in Example 1.

[0041]

[Table 1]

From Table 1, it can be seen that the narrower the wiring interval and the longer the time from screen printing to reversing the substrate,
It can be seen that a short circuit (accordingly, dripping of the conductor paste during drying) is likely to occur. If the time until the reversal is 30 seconds or less, short circuit due to sagging can be reliably prevented even if the wiring interval is as small as 75 μm.

Example 3 Using a commercially available high thixotropic Cu paste and a mesh screen having a mesh opening ratio of 60%,
In the same manner as in Example 1, a surface wiring was formed on an alumina substrate. However, the wiring pattern was made finer with a wiring width of 60 μm, a wiring interval of 40 μm, and a wiring pitch of 100 μm, and the substrate was inverted 15 seconds after the end of screen printing.

According to the present invention, the screen-printed conductor paste is dried face down to prevent sagging, so that fine surface wiring which does not show a short circuit by measuring electric resistance is formed on a ceramic substrate by retrofitting. We were able to.

Example 4 A high thixotropy Cu paste used in Example 3 was applied on a glass ceramic green sheet in the same wiring pattern as in Example 1 (however, the wiring width was 50%).
(μm, wiring interval 35 μm, wiring pitch 85 μm). Thereafter, the green sheet was inverted 15 seconds after printing, the upper surface of the green sheet was held by a suction plate with the printed surface facing down, and the conductor paste was heated and dried under the same conditions as in Example 1.

Thereafter, in order to measure the electric resistance of the wiring,
Leave this green sheet as a single layer in a nitrogen atmosphere at 750 ° C.
For 30 minutes to obtain a single-layer fired substrate. As a result of measuring the electric resistance of the wiring on the obtained substrate, no short circuit was confirmed.

On the other hand, for comparison, when a wiring was formed on the green sheet in the same manner as above except that the green sheet was not inverted, and baked, it was found that a short circuit occurred in the measurement of electric resistance. confirmed.

[0048]

According to the present invention, the conductor paste is dried by screen-printing the conductor paste on the ceramic substrate or the green sheet, and then inverting the substrate or sheet and drying the conductor paste with the printed surface facing downward. As a result, it is possible to form a wiring having a finer wiring pattern (wiring width, interval, pitch) than before, without causing a short circuit due to sagging.

The method of the present invention differs from the conventional wiring miniaturization method in that no additional steps are required and addition of new equipment is very small, so that the same manufacturing cost and manufacturing efficiency as those of the conventional method are maintained. The technical contribution in the electronics industry is remarkable in that it can satisfy the indispensable requirement for the miniaturization of the substrate for finer wiring.

[Brief description of the drawings]

FIG. 1 is a printed pattern of a conductive paste used in an embodiment.
(Wiring pattern).

Claims (2)

[Claims] 1. Wiring forming, wherein a conductor paste is screen-printed on a ceramic substrate or a green sheet, and then the substrate or the green sheet is turned over, and the conductor paste printed with the printed surface facing down is heated and dried. Method. 2. The method according to claim 1, wherein the time from printing of the conductive paste to inversion of the substrate or green sheet is within 30 seconds.