Electrically conductive pastes

An electrically conductive paste is disclosed, which comprises (i) 60 to 90 wt % of metal fine powders comprising a silver fine powder, a composite fine powder of silver and copper and optionally either, a composite fine powder of silver and platinum or a platinum fine powder, or a composite fine powder of silver and palladium or a palladium fine powder, the copper content in the metal fine powders is 0.1 to 10 wt %, the platinum content, if present, is 0.2-10 wt % and the palladium content, if present, is 0.2-30 wt %, and (ii) 10 to 40 wt % of a vehicle component, such as terpineol, butylcarbitol, ethyl cellulose etc. The product can resist thermal shocks and has strong bonding strength having remarkably improved heat deterioration characteristics.

FIELD OF THE INVENTION 
This invention relates to pastes for cerdip substrate in particular, 
dotting pastes. 
BACKGROUND OF THE INVENTION 
In the recent years, electronic instruments have been remarkably made 
thinner and more compact, and their reliability has been greatly enhanced 
with the increase in the density of integration and their application has 
steadily expanded. A monolithic IC has undergone a rapid increase in the 
density and compactification, while also in the hybrid IC field, 
especially in industrial instruments such as control circuits for 
automobiles, power module devices etc., there has been made an effort 
toward large-sized hybrid IC having excellent heat resistance and thermal 
shock resistance. In the latest hybrid IC a ceramic substrate carries 
active components such as diodes, transistors, semiconductor IC etc. and 
also almost all electrical parts such as coils, transformers, condensers 
etc. Hybrid integrated circuits having a further increased degree of 
integration and greatly enhanced reliability have been developed. 
The hybrid IC is constructed by packaging individual parts or IC elements 
on a ceramic substrate on by applying thick film-forming techniques. A 
cerdip IC is generally made by fixing silicon IC chips on an alumina 
substrate comprising 91-96% Al.sub.2 O.sub.3 using a bonding paste, but it 
still needs even higher strength bond imparting better durability. 
In general, for the bonding method for cerdip, Au based pastes, solder, 
glass etc. have been used, as described in, for example, U.S. Pat. No. 
3,846,345. The Au based pastes have excellent electrical conductivity, are 
chemically stable, have the best bondability with Au wires, can easily be 
alloyed with Si, and are extremely good in bonding with substrate as 
described in U.S. Pat. No. 3,799,891. However, they have a disadvantage of 
being expensive. In order to eliminate this disadvantage, Ag--Pd based 
pastes have been developed which contains Ag instead of Au and also 
contained Pd so as to prevent the migration of Ag, as described in U.S. 
Pat. Nos. 3,929,491 and 3,918,980. 
These conventional pastes are those obtained by mixing metal powders with 
vitreous metal oxides and kneading using vehicles, and in bonding with 
alumina substrate they rely solely on the bonding force due to sintering 
of the glass frit. 
However, the glass frit is poor in thermal shock resistance and has a 
disadvantage that the bond strength is thermally deteriorated in a step of 
sintering substrate and packaging or by the change in surrounding 
temperature on use. Although attempts have been made to chemically combine 
it with alumina substrate by adding very small amounts of Cu etc. in order 
to enhance the bonding strength with the alumina substrate (See U.S. Pat. 
Nos. 3,929,491 and 3,918,980), it is difficult to greatly improve the heat 
deterioration characteristics as long as the glass frit is used. In other 
words, the mere addition of Cu fine powder causes separation of the Cu 
from other metal fine powders in the vehicle due to the difference in 
specific gravity, resulting in poor distribution on dotting, and therefore 
not only failing to give a uniform paste film but also giving a film 
having inadequate bonding strength because of poor diffusion on the 
alumina substrate. Further, the part where Cu is segregated during the 
course of firing undergoes localized oxidation and gets colored, and 
therefore a film having a uniform smooth surface cannot be obtained. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide dotting pastes for cerdip IC, in 
particular, convenient, economical fritless type dotting pastes having 
improved bonding strength between alumina substrate and silicone chips and 
excellent heat resistance and thermal shock resistance. 
Accordingly, this invention is an electrically conductive paste comprising 
(i) 60 to 90 wt % of metal fine powders which comprises a silver fine 
powder and a composite fine powder of silver and copper, wherein the 
copper content in the metal fine powders is 0.1 to 10 wt %, and (ii) 10 to 
40 wt % of a vehicle component. 
Another embodiment of this invention is an electrically conductive paste 
comprising (i) 60 to 90 wt % of metal fine powders which comprises a 
silver fine powder, a composite fine powder of silver and copper, and a 
composite fine powder of silver and platinum or a platinum fine powder, 
wherein the copper content in the metal fine powders is 0.1 to 10 wt % and 
the platinum content in the metal fine powders is 0.2 to 10 wt %, and (ii) 
10 to 40 wt % of a vehicle component. This exerts effects to prevent the 
migration of Ag and to improve wire bondability and solderability. 
Yet another embodiment of this invention is an electrically conductive 
paste comprising (i) 60 to 90 wt % of metal fine powders which comprises a 
silver fine powder, a composite fine powder of silver and copper, and a 
composite fine powder of silver and palladium or a palladium fine powder, 
wherein the copper content in the metal fine powders is 0.1 to 10 wt % and 
the palladium content in the metal fine powders is 0.2 to 30 wt %, and 
(ii) 10 to 40 wt % of a vehicle component. This is particularly excellent 
in prevention of the migration of Ag and has an effect to improve wire 
bondability and solderability. 
DETAILED DESCRIPTION OF THE INVENTION 
The silver fine powder which can be used in this invention generally has a 
particle size of 10 .mu.m or less, preferably an average particle size 
(D.sub.50) of 0.5 to 5 .mu.m. If it exceeds 10 .mu.m, the distribution in 
the vehicle is poor, and thus there is a risk of clogging of needle on 
dotting, and a smooth surface is not easily obtained on sintering. The 
silver fine powder can be obtained by the conventional reducing process or 
electrolytic process. 
In this invention, any composite powder of silver and copper may be used as 
long as the silver particles and the copper particles are combined with 
each other in the vehicle, and plated powders, coprecipitated powders, 
mechanical alloy powders etc. may be used. With respect to mechanical 
alloy, J. S. Benjamin has described on Metallurgical Transaction No. 1, 
1970, p. 2943. The composite powder can be easily obtained by a 
conventional manner. For example, the mechanical alloy powders are 
obtained by mixing and pulverizing silver and copper powders in a ball 
mill by high-speed, rotation, and they have the silver particles and the 
copper particles mechanically entangled and combined with each other and 
thus can maintain strong bonding between the silver particles and the 
copper particles without using any binder. The mechanical alloy powder is 
advantageously used in that a composite powder having a wide range of Cu 
content may freely be used. The composite powder generally has a particle 
size of 10 .mu.m or less, preferably an average particle size (D.sub.50) 
of 0.5 to 5 .mu.m. The copper content in the composite powder of silver 
and copper is preferably 20 to 95 wt %. If the copper content is less than 
20%, the film adhesion to substrate is not sufficient, whereas if it 
exceeds 95%, the effect of the composite powder is lost. Further, in order 
to facilitate dispersibility in the vehicle, the specific gravity value is 
preferably in the vicinity of the intermediate value between those of 
silver and copper. 
The copper content in the metal powders in the electrically conductive 
paste is 0.1 to 10 wt %, preferably 2 to 5 wt %. If the copper content is 
less than 0.1 wt %, the diffusion thereof on the alumina is inadequate and 
hence the bonding strength is not increased. On the contrary, if the 
copper content exceeds 10 wt %, the oxidation of copper is remarkable, 
exerting an adverse influence. 
The metal powder content in the electrically conductive paste should be 60 
to 90 wt % and preferably 75 to 82 wt %, otherwise paste viscosity 
increases so that easy handling cannot be obtained. 
The vehicle has functions to uniformly disperse the metal fine powders, to 
have appropriate viscosity and surface tension on use and to smoothly 
diffuse on the substrate surface. The vehicle used in this invention may 
be selected from conventional organic solvents such as terpineol, 
butylcarbitol, ethyl cellulose, butylcarbitol acetate, Texanol (trade name 
for 2,2,4-trimethylpentanediol-1,3-monoisobutyrate, produced by Eastman) 
etc. Further, 0.5 to 1.0 wt % of a surfactant may be added in order to 
improve compatibility with the metal powders and improve dispersibility. 
In the paste state, the viscosity is maintained high (generally 200 to 350 
Kcps) in order to avoid the separation and segregation of the metal fine 
powder particles, but on use, the viscosity is adjusted to 40 to 450 cps 
by diluting with a solvent. 
In the first embodiment of this invention, the gist of the invention 
resides in an electrically conductive paste which contains 60 to 90 wt % 
of metal fine powders comprising a silver fine powder and a composite fine 
powder of silver and copper, the copper content in the metal fine powders 
being 0.1 to 10 wt %, and the balance being a vehicle component. By 
composing the paste as described above, the obtained paste has thermal 
shock resistance and high strength bond having remarkably improved heat 
deterioration characteristics. Further, the paste according to this 
invention enjoys better dispersibility on dotting and provides an 
excellent surface film having smooth and uniform baking finish 
characteristics. 
In the second embodiment of this invention, the paste of the first 
embodiment further contains platinum, that is, an electrically conductive 
paste which contains 60 to 90 wt % of metal fine powders comprising a 
silver fine powder, a composite fine powder of silver and copper, and a 
composite fine powder of silver and platinum or a platinum fine powder, 
the copper content in the metal fine powders being 0.1 to 10 wt % and the 
platinum content being 0.2 to 10 wt %, and the balance being a behicle 
component. By composing the paste as described above, the paste not only 
has thermal shock resistance and high strength bond having remarkably 
improved heat deterioration characteristics but also has effects to 
prevent migration and to improve wire bondability, fine line resolution, 
solderability and electrical conductivity. Further, where a wire is to be 
connected to a cavity part, this paste has a great advantage that Al wires 
can be used. 
While platinum is chemically stable and hence is effective in improving the 
above-described characteristics even when it is singly incorporated, it is 
even more effective to use it as a composite powder with silver because 
they are uniformly dispersed in the vehicle. As the composite powder of 
silver and platinum, plated powders, coprecipitated powders, mechanical 
alloy powder etc. may be used. The platinum content in the composite 
powder is preferably 5 to 60 wt %. With the mechanical alloy powder, those 
having a high platinum content may easily be obtained. The composite 
powder generally has a particle size of 10 .mu.m or less, preferably an 
average particle size (D.sub.50) of about 5 .mu.m or less. 
The platinum content is 0.2 to 10 wt %, preferably 0.5 to 3.0 wt %, based 
on the metal particles in the paste. If the platinum content is less than 
0.2%, the effect by the addition cannot be obtained, whereas if it exceeds 
10%, the effect to cut the cost does not manifest. 
In the third embodiment of this invention, the paste of the first 
embodiment further contains palladium, that is, an electrically conductive 
paste which contains 60 to 90 wt % of metal fine powders comprising a 
silver fine powder, a composite fine powder of silver and copper, and a 
composite fine powder of silver and palladium or a palladium fine powder, 
the copper content in the metal fine powders being 0.1 to 10 wt % and the 
palladium content being 0.2 to 30 wt %, and the balance being a vehicle 
component. By composing the paste as described above, the obtained paste 
not only has thermal shock resistance and improved heat deterioration 
characteristics but also especially exerts a remarkable effect to prevent 
the migration of silver and also has effects to improve wire bondability 
and solderability and to obtain a uniform film having a smooth surface. 
Although it is well known that a paste incorporating palladium has an 
effect to prevent the migration of silver, the paste incorporating 
palladium singly has a disadvantage that palladium easily undergoes 
oxidation in the firing step and makes the surface roughness extremely 
coarse. For that reason, where palladium is to be added singly, it is 
necessary to use a fine powder having a particle size (maximum) of 5 .mu.m 
or less and preferably having an average particle size (D.sub.50) of 2 
.mu.m or less, more preferably 0.5 .mu.m or less. As one of the aspects of 
this invention, by using a powder obtained by compounding palladium and 
silver, it has been discovered that a film having extremely good surface 
conditions can be obtained while preventing the oxidation of palladium. 
As the composite powder of silver and palladium, coprecipitated powders, 
mechanical alloy powders, plated powders etc. may be used. The palladium 
content in the composite powder is suitably 10 to 40 wt %, preferably 20 
to 30 wt %, more preferably 25 to 30 wt %. The composite powder generally 
has a particle size of 10 .mu.m or less, preferably an average particle 
size (D.sub.50) of about 5 .mu.m or less. 
The palladium content is 0.2 to 30 wt %, preferably 0.5 to 10 wt %, more 
preferably 1 to 2 wt %, based on the metal particles in the paste. If the 
palladium content is less than 0.2%, the effect of palladium cannot 
manifest, whereas even if more than 30% is added, a marked improvement in 
the characteristics cannot be expected. 
The electrically conductive pastes of this invention can be dotted and 
fired on a base in a conventional manner. The firing condition is 
generally a 50.about.60 minute profile with a peak temperature of 
910.degree. to 930.degree. C. for 8 to 10 minutes.

This invention is more particularly described by the following examples, 
but the invention should not be construed as being limited thereto. 
EXAMPLE 1 
Pastes were prepared by kneading on a three roll mill using the metal 
powders set forth in Table 1, and as a vehicle, terpineol, butylcarbitol 
and ethyl cellulose, and a surfactant. The silver powder used was a 
commercial reducing powder having a purity of 99.9% and a particle size of 
1-4 .mu.m. The composite powder of silver and copper used was a mechanical 
alloy powder obtained by high-speed mixing and pulverization of 50% of a 
silver powder and 50% of a copper powder in a ball mill, or an electroless 
plated powder containing 20% copper. The composite powder was classified 
to obtain those having a particle size of 10 .mu.m or less. 
The vehicle component used was that obtained by mixing terpineol and 
butylcarbitol at a ratio of 1:1, and adding thereto ethyl cellulose and a 
non-ionic surfactant in amounts of 12 wt % and 2.5 wt %, respectively, 
based on the weight of the above mixture. The viscosity at that time was 
measured on a Brookfield viscometer using a No. 14 spindle, to obtain 
200.+-.50 Kcps. 
These metal powders and the vehicle were sufficiently kneaded using a three 
roll mill under the mixing condition set forth in Table 1 to obtain 
pastes. 
Thereafter, each paste was diluted with a 1:1 solution of butylcarbitol and 
terpineol used as a thinner to adjust the final viscosity to about 100 
cps, and used for dotting. 
The substrate used were of black alumina (92% Al.sub.2 O.sub.3 ; size: 
31.7.times.13.times.2 mm), and the cavity size was 
6.25.times.6.25.times.0.18 mm. The alumina substrate were used after 
washing with trichloren. On each cavity was fixed the viscosity-adjusted, 
diluted paste by dotting. 
The dotting device used was one manufactured by Iwashita Engineering Co., 
Japan. Each electrically conductive paste thus obtained was dotted on the 
substrate dried at 120.degree. C. for 20 minutes, and further fired in air 
in a thick film firing furnace, Model 4 MC manufactured by Watkins Johnson 
Co. The firing conditions were a 60-minute profile with a peak temperature 
of 910.degree. C. for 10 minutes. 
Each paste film surface thus obtained was observed and its surface 
roughness was measured by a surface roughness meter manufactured by Tokyo 
Seimitsu Co., Japan. Fifty samples were prepared using each paste for 
evaluation of the surface roughness. 
Further, using Au preforms of 2.5.times.2.5 mm.quadrature..times.25 .mu.m, 
silicon chips were bonded to the thick films obtained above at 450.degree. 
C. by a die attaching device manufactured by Westbond Co. The cerdip IC 
thus obtained were subjected to a bonding strength test and a heat 
resistance test. The results of these tests are given in Table 1. 
The bond strength was judged by die attach properties and a die push test. 
The die attach properties were determined by the time for scribing on 
bonding. In the die push test, a test piece after the heat resistance test 
was measured using a vertical bond tester manufactured by Engineered 
Technical Product Co. In Table 1, A means that none of the twenty test 
pieces showed film peal-off but die breakdown, B means that at least one 
of the twenty samples showed film peel-off, and C means that all of the 
twenty test pieces showed film peel-off. 
As the heat resistance test, a heat cycle test and a thermal shock test 
were conducted. The test conditions were according to MILL-STD 883B 
1010.2, CONDITION C for the heat cycle test, and MILL-STD 883 B 1011.2, 
CONDITION C for the thermal shock test. 
For comparison, using electrically conductive pastes obtained by merely 
mixing a copper powder with or without glass frit, tests similar to the 
above were conducted and the obtained results are also given in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Starting Material Formulation (wt %) 
Film Film Characteristics 
Metal Powder (Calculated Thickness 
Surface Surface 
Die Die Push Test 
Sam- 
as Pure Form) After Conditions 
Roughness 
Attach 
After After 
ple Cop- 
Powder Vehi- 
firing 
After Rmax Proper- 
Heat 
Thermal 
No. 
Silver 
per 
Form Frit cle (.mu.m) 
firing (.mu.m) 
ties Test Shock 
__________________________________________________________________________ 
Test 
1 74.92 
0.08 
Mechanical 
-- 25 13 Good 7 Good A A 
Alloy 
2 74.62 
0.38 
Mechanical 
-- 25 14 Good 6 Good A A 
Alloy 
3 72.75 
2.25 
Mechanical 
-- 25 14 Good 7 Good A A 
Alloy 
4 69.00 
6.00 
Mechanical 
-- 25 13 Good 6 Good A A 
Alloy 
5* 
75.00 
-- -- -- 25 14 Good 6 Poor C C 
6* 
74.25 
0.75 
Copper Sin- 
-- 25 12 Black Stripe 
8 Fair B C 
gle Element Pattern Appear- 
Powder ed on Surface 
7* 
71.25 
3.75 
Copper Sin- 
-- 25 16 Black Stripe 
9 Fair B B 
gle Element Pattern Appear- 
Powder ed on Surface 
8* 
74.96 
0.04 
Electroless 
-- 25 14 Good 7 Fair B B 
Plated 
Powder 
9* 
66.0 
9.00 
Mechanical 
-- 25 15 Good 8 Poor C C 
Alloy 
10* 
73.63 
0.37 
Copper Sin- 
1 25 14 Glass Migrated 
10 Fair C C 
gle Element 
(PbO-- onto Surface 
Powder based 
frit) 
__________________________________________________________________________ 
Note: *Comparative Sample 
As clear from the results of Table 1, the electrically conductive pastes 
obtained using the composite powder of silver and copper according to this 
invention showed extremely excellent effects, i.e., the surface after 
firing was extremely smooth, the bonding strength between the silicon 
chips and the alumina substrate was strong, and further, the bonding 
strength did not deteriorate even when subjected to the heat history. 
EXAMPLE 2 
Platinum-added electrically conductive pastes were prepared using a silver 
fine powder, a composite fine powder of silver and copper according to the 
mechanical alloy process and a vehicle similar to those used in Example 1 
by procedures similar to those in Example 1, and a bonding test was 
conducted. The platinum used was either a commercial fine powder of 0.5 to 
0.8 .mu.m, or a coprecipitated powder of silver and platinum at a ratio of 
85:15, which was used after classification to 5 .mu.m or less. The mixing 
conditions of the metal fine powders were as set forth in Table 2. The 
test conditions for the electrically conductive pastes were exactly the 
same as those in Example 1. The results of these tests are also given in 
Table 2. 
TABLE 2 
__________________________________________________________________________ 
Film 
Film Characteristics 
Thick- Die Push Test 
Starting Material Formulation (wt %) 
ness 
Surface 
Surface 
Die After 
After 
Sam- 
Metal Powder (Calculated as Pure Form) 
After 
Conditions 
Roughness 
Attach 
Heat 
Thermal 
ple 
Sil- 
Cop- 
Powder 
Plati- 
Powder Vehi- 
firing 
After Rmax Proper- 
Cycle 
Shock 
No. 
ver 
per 
Form num Form Frit 
cle (.mu.m) 
firing 
(.mu.m) 
ties Test 
Test 
__________________________________________________________________________ 
11 73.87 
0.38 
Mechanical 
0.75 
Platinum 
-- 25 15 Good 9 Good A A 
Alloy Single Ele- 
ment Powder 
12 73.87 
0.38 
Mechanical 
0.75 
Silver- 
-- 25 15 Good 7 Good A A 
Alloy Platinum Co- 
precipitated 
Powder 
__________________________________________________________________________ 
As clear from Table 2, it has been found that the electrically conductive 
pastes using the platinum powder or the silver-platinum composite powder 
were good in the baking finish conditions of the film and the bonding 
strength was not deteriorated by the heat history. 
Further, a bonding test of a wire to the cavity part was conducted using an 
alumina wire of 30 .mu.m in diameter ness, and the change in resistance 
after exposure to high temperature atmosphere was measured. This 
measurement was conducted by comparing the resistance after leaving in air 
at 300.degree. C. for 9 hours with the resistance before exposure. As a 
result, the resistance before exposure to high temperature atmosphere was 
205 m.OMEGA., and it became 210 m.OMEGA. after exposure to high 
temperature atmosphere. This resistance change was extremely small as 
compared with the case where the conventional product showed a change from 
240 m.OMEGA. to 1050 m.OMEGA., thus showing that the product of this 
invention is extremely thermally stable. 
Furthermore, since the product of this invention has a very low resistance 
and good bondability this invention also has a great advantage that the 
use of aluminum wires is possible. 
EXAMPLE 3 
Palladium-added electrically conductive pastes were prepared using a silver 
fine powder, a composite fine powder of silver and copper according to the 
mechanical alloy process and a vehicle similar to those in Example 1 by 
procedures similar to those in Example 1, and a bonding test was 
conducted. The palladium used was either a commercial fine powder of a 
particle size of 0.8 to 1.8 .mu.m or a coprecipitated powder of silver and 
palladium at a ratio by weight of 7:3, which was used after classification 
to 5 .mu.m or less. The mixing conditions of the metal fine powders were 
as set forth in Table 3. The test conditions for the electrically 
conductive pastes were exactly the same as those in Example 1. The results 
of these tests are also given in Table 3. 
TABLE 3 
__________________________________________________________________________ 
Film 
Film Characteristics 
Thick- Die Push Test 
Starting Material Formulation (wt %) 
ness 
Surface 
Surface 
Die After 
After 
Sam- 
Metal Powder (Calculated as Pure Form) 
After 
Conditions 
Roughness 
Attach 
Heat 
Thermal 
ple 
Sil- 
Cop- 
Powder 
Plati- 
Powder Vehi- 
firing 
After Rmax Proper- 
Cycle 
Shock 
No. 
ver 
per 
Form num Form Frit 
cle (.mu.m) 
firing 
(.mu.m) 
ties Test 
Test 
__________________________________________________________________________ 
13 73.87 
0.38 
Mechanical 
0.75 
Silver- 
-- 25 15 Good 6 Good A A 
Alloy Palladium 
Co- 
precipitated 
Powder 
14 73.87 
0.38 
Mechanical 
0.75 
Palladium 
-- 25 14 Good 7 Good A A 
Alloy Single Ele- 
ment 
Powder 
(D.sub.50 = 1.3 
.mu.m) 
15* 
73.87 
0.38 
Mechanical 
0.75 
Palladium 
-- 25 15 Black 10 Fair A A 
Alloy Single Ele- Stripe 
ment Pattern 
Powder Appeared 
(D.sub.50 = 5.3 on Surface 
.mu.m) 
__________________________________________________________________________ 
Note: *Comparative Sample 
As clear from Table 3, the paste prepared by singly mixing the palladium 
powder, the film conditions after sintering were poor and the die attach 
properties were also poor unless the particle size of palladium was made 
much smaller. On the contrary, where the composite powder of silver and 
palladium according to this invention was used, these disadvantages were 
solved. 
Further, when the film obtained by using this paste was left in atmosphere 
at a temperature of 60.degree. C. and a humidity of 95% for 100 hours, 
there was observed no migration of Ag. 
In addition, by procedures similar to those in Example 2, a bonding test 
was conducted using an aluminum wire of 30 .mu.m, in diameter, and the 
change in resistance was measured by exposing to high temperature 
atmosphere. The measuring conditions were also similar to those in Example 
2. 
As the result of the measurement, the resistance had been changed from 245 
m.OMEGA. to 270 m.OMEGA., from which it can be concluded that this is 
considerably thermally stable as compared with the conventional products. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.