JPH03246990A - Formation of thick film conductor - Google Patents

Abstract

PURPOSE:To obtain a conductor whose bonding strength is large by a method wherein a first conductor paste whose glass frit content is high is printed on a substrate, a second conductor paste whose solderability is high is printed so as to be piled up on it and they are dried and baked. CONSTITUTION:A first conductor paste is printed on a ceramic substrate. It is desirable that the glass frit contained in the first conductor paste is at about 1.5 to 7 pts.wt. when the weight of a metal powder is set as 100 pts.wt. A second conductor paste is printed on the first conductor film. The second conductor paste contains 0.3 to 2 pts.wt. of glass so as to be bonded to the first conductor film. One to 4 pts.wt. of Bi2O3 are added in order to enhance the solderability. An extremely small amount of Bi2O3 which comes up to the surface of the conductor acts as a flux which makes the wetting property between a solder and a conductor good. Thereby, it is possible to form the conductor whose solderability is high and whose bonding strength is high with reference to the ceramic substrate, especially a substrate which has been baked at a low temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for forming a conductor having high adhesive strength and good solderability on a ceramic substrate, particularly a low temperature fired substrate.

[Prior Art] In order to meet the demands for higher speed and higher density circuits, ceramic thick film circuit boards, typified by hybrid ICs, are becoming increasingly multilayered.

Conventionally, multi-layering using alumina substrates involves printing a predetermined conductor circuit on a green sheet mainly made of alumina, stacking multiple layers and firing at a high temperature of 1500 to 1600°C. The materials are Mo, W
High melting point metals such as However, these metals have relatively high resistance, and alumina has a high dielectric constant, so they have not been able to fully meet the demand for higher speeds.

In order to solve this problem of increasing speed, many low-temperature firing multilayer substrates that are mainly made of glass-ceramics and can be fired at a low temperature of 900° C. or lower have been developed in recent years.

Such low-temperature fired substrates include Au, Ag, and Ag/P.
Since low resistance conductors such as d, Cu, etc. can be used, and the substrate itself has a low dielectric constant, signal propagation speed can be dramatically increased.

[Problem to be solved by the invention]

When making a multilayer ceramic circuit board using such a low-temperature fired substrate, first Au and Ag are deposited on a green sheet mainly made of glass-ceramics.

A paste containing metal powders such as Ag/Pd and Cu is printed, laminated, and fired. Form the necessary conductors and resistors on the surface of this multilayer board, overcoat it with glass,
By mounting various parts, it can be made into a hybrid IC. However, generally commercially available Au, A
When alumina substrate conductor pastes such as G, Ag/Pd, Ag/Pt, and Cu are used as surface conductors, they have extremely poor solder wettability, making it impossible to attach lead terminals, and their adhesive strength with the substrate is low. It could not be used as is.

Unlike alumina substrates, low-temperature fired substrates are difficult to form a strong bonding structure with the glass component in the conductor. In order to strengthen this bond, the surface conductor paste must contain a large amount of glass frit or additives, but this significantly deteriorates solder wetting. Thus, a conductor that achieves both bonding strength and solder wettability has not yet been obtained, which has been one of the reasons why it is difficult to put low-temperature fired substrates into practical use.

An object of the present invention is to provide a method for forming a surface conductor with high bonding strength and good solder wettability.

[Means to solve the problem]

In order to achieve the above object, the method of the present invention includes printing a first conductor paste containing additives highly reactive with the substrate and having a high glass frit content on the surface of the substrate, drying, and baking if desired, The method is characterized in that a surface conductor is formed by printing, drying, and baking a second conductive paste having high solder wettability on the dry film or baked film.

[For production]

The first or second conductive paste is Au, Ag, A
The first conductor paste may be made of either Ag/Pd or Ag/Pt, but Ag/Pd may be used as the first conductor paste.
It is preferable to use Ag or Ag/Pt for the second conductive paste.

The glass frit contained in the first conductor paste is Pb
General ones such as O-5iOz-B20i may be used, and Z
Crystallized glass such as nO-PbO-5iO□ may also be used. The content is 1.5 to 100 parts by weight of the metal powder.
About 7 parts by weight is desirable. If it is less than 1.5 parts by weight, sufficient strength with the substrate cannot be obtained, and if it exceeds 7 parts by weight, sintering of the conductive metal is inhibited and the resistance value increases significantly. Also,
Excess glass may ooze out of the conductor or impair the solder wettability of the second conductor.

Various additives can be used to improve adhesive strength. Zn and Cu are particularly effective, reacting with the substrate to form a spinel compound and forming a strong bonding layer. The content is Z
It is desirable that the total amount of one or both of n and Cu does not exceed 6 parts by weight. The content may be in the form of a metal, an oxide, or an organometallic compound. In addition, the common old 20. may be contained in an amount not exceeding 10 parts by weight.

The second conductor paste preferably contains 0.3 to 2 parts by weight of glass for bonding with the first conductor film. Glass may protrude on the surface of the first conductor film, and in this case, if the content of the second conductor is less than 0.3 parts by weight, it may peel off at the interface between the first conductor film and the 20th conductor film. This is because it will be put away. However, the inclusion of glass in an amount of 2 parts by weight or more is not preferable because it impairs the solder wettability of the surface. Also, the second
The conductor paste contains 1 to improve solder wettability.
It is desirable to add ~4 parts by weight of Biz03. A very small amount of BizO3 floating on the surface of the conductor acts as a flux to improve wetting of the solder and conductor.If it is less than 1 part by weight, it will be encapsulated in the conductor and absorbed by the first conductor film. Does not contribute to solder wetting of the second conductor film.4
If it exceeds the weight part, n12off of the second conductive film becomes excessive and the solder wettability deteriorates.

In addition, both the first conductive paste and the second conductive paste
When making a paste, it is preferable to include 12 to 50 parts by weight of an organic vehicle.

The method for forming the electrode may be screen printing or baking similar to a normal thick film process, and the baking may be performed by baking the first conductor and the second conductor individually or simultaneously.

〔Example〕

(a) Substrate used 5i0260 parts by weight, Pb025 parts by weight, 820310
A low-temperature fired substrate prepared by mixing glass powder and alumina powder consisting of 10 parts by weight and 10 parts by weight of CaO was used.

(b) Preparation of conductive paste A conductive metal powder, glass powder, and additives having the composition ratio shown in Table 1 are mixed with an organic vehicle prepared by dissolving 12 parts by weight of ethyl cellulose in 88 parts by weight of terpineol.
The mixture was sufficiently kneaded using three rolls to obtain a conductive paste.

Table G1: 45%ZnO-35%PbO-15%5iOz-
5%B20°G2: 55%PbO-35%5iOz-1
0% B203 (c) Evaluation method Using the two types of conductor pastes shown in Table 1, conductors were formed on a fired low-temperature fired ceramic substrate by simultaneous or separate firing, and the method shown below was applied. We conducted various evaluations. For comparison, Table 2 shows the results obtained by printing and baking once using only one type of conductor paste shown in Table 1 and a commercially available conductor paste.

■ Adhesive strength after heat aging Approximately 10μ thick film formed on a 2m x 2cm substrate
2Ag/62Sn/36Pb at about 230℃
Apply solder by dipping into solder for 5 seconds.

Subsequently, a Sn-plated copper wire is soldered to the conductor using a solder having the same composition, and the wire is left in a constant temperature bath at 150°C. 1
After 1,000 hours had elapsed, the copper wire was taken out and pulled at a speed of 80 min/sec, and the strength when the copper wire was peeled off from the substrate was defined as the adhesive strength.

■ Adhesive strength after heating and cooling cycles A sample piece to which copper wire was applied in the same manner as above was heated to 40℃ and
Each sample was exposed alternately to a constant temperature bath at 50'C for 30 minutes each, and after 1000 cycles, the adhesive strength was measured in the same manner as above.

■A substrate on which a conductive film with a solder spread rate of 10++ aaX10nnn and a thickness of about 10 μm has been formed is held at about 230 degrees on a hot plate.
Heat to C. When the temperature becomes constant, a diameter of 1 is placed on the conductor film.
A 2Ag/62Sn/36Pb solder pole of 2mm is placed on it and held for 10 seconds after the pole melts. After cooling,
The diameter of the solder pole that had melted and spread was measured, and the spreading rate was determined by the following calculation.

Solder spreading rate = (diameter of solder pole after melting 1) x
loo (%) From Table 2, the conductor film formed by the formation method of the present invention has better results in both adhesive strength and solder wettability after heat aging than when only one type of conductor is used. and was found to be very effective in improving them.

[Effects of the Invention] According to the present invention, a conductor having high solder wettability and high adhesive strength can be formed on a ceramic substrate, particularly on a low-temperature fired substrate.

Claims (1)

[Claims] A first conductor paste containing additives that are highly reactive with the substrate and having a high glass frit content is printed on the substrate, dried, and optionally further baked to impart high solder wettability to the dried film or baked film. 1. A method for forming a thick film conductor, comprising printing, drying, and baking a second conductor paste.