An alternative vehicle system which utilizes no resin for depositing an inorganic material onto a substrate. The vehicle system consists of mixtures of solvents having high intrinsic viscosity and good wetability providing a pseudoplastic rheology.

BACKGROUND OF THE INVENTION 
Silver-glass die attach adhesives are being used increasingly in the 
electronics industry to secure silicon based integrated circuit devices to 
their packages. These adhesives have both practical and economic 
advantages compared to the traditional Si/Au eutectic attachment methods. 
The Ag/glass adhesives typically consist of silver flake and glass 
particles suspended in an organic matrix or vehicle. The traditional 
vehicle system for a silver-glass paste composition has consisted of a 
resin binder dissolved in one or more solvents. See for example, U.S. Pat. 
No. 4,636,254, granted Jan. 13, 1987 and assigned to Quantum Materials, 
Inc. The subsequent processing (cure) of an Ag/glass paste results in the 
concurrent evolution of the organics, fusion of the glass, and sintering 
of the silver flakes. 
One essential requirement for proper performance of an Ag/glass paste is 
the substantial elimination of vehicle organics during cure. These organic 
materials of necessity must be capable of total (and non-carbonizing) 
volatilization or decomposition during the subsequent firing of the 
product to develop its adhesive properties. It is furthermore desirable 
that this organic burnout be essentially complete prior to the glass 
transition temperatures (T.sub.g) of the frit. The lead borate glasses 
traditionally used in the Ag/glass compositions are easily reduced by 
residual organics at such temperature and their adhesive properties are 
correspondingly diminished. Therefore, preferred solvents for these 
adhesives have boiling points well below 300 degrees C. Preferred resins 
that are considered to have adequate burnout characteristics include 
polyalkyl methacrylates and nitrocellulose (preferable .gtoreq.12.0% 
nitrogen by analysis). The methacrylate resins are usually fully 
decomposed to volatile compounds between 300.degree. and 400.degree. C. 
Resin binders have been used in the traditional silver-glass formulations 
by reason of the following properties they confer to the paste: 
1) They help keep the paste solids in suspension. 
2) Their presence helps retard solvent bleed from the paste after 
decomposition on a substrate. 
3) They impart some "green strength" to the paste during any drying step. 
There are certain disadvantages, however, that are associated with the use 
of these binders in a silver-glass die attach paste, as follows: 
1) Dissolved resin impedes the release of solvent vapors from the paste 
solids during processing. 
2) The presence of more than a fraction of one percent of these resins can 
reduce the thixotropic index of a paste and thus have a negative effect on 
dispensability. 
3) New, lower temperature processing compositions require that all organic 
residues be fully volatilized by 300.degree. C. 
Use of these resin polymers can also compromise product performance in 
other ways. The thermal coefficient of expansion for polymethyl 
methacrylate, for example, is more than an order of magnitude greater than 
that of the silicon die. It is believed that this thermal mismatch 
contributes to bond failure, particularly on large area devices. 
Nitrocellulose is an even less desirable resin alternative since its 
burnout is too rapid (resulting in partial delamination) and incomplete 
(giving rise to excessive carbon residue). 
SUMMARY OF THE INVENTION 
The present invention provides an alternative vehicle system which utilizes 
no resin. The vehicle system consists of mixtures of solvents having high 
intrinsic viscosity and good wetability, including some of the higher 
alcohols, ester solvents, higher aliphatic hydrocarbons, lower siloxane 
oligomers, and higher glycols and polyglycols. It has been found that such 
compositions, when properly formulated, can equal or out perform Ag/glass 
adhesives based on the traditional organic combinations. 
A preferred embodiment of my resinless die attach paste consists of a 
quantity of silver flake having a polymodal flake size distribution, a 
quantity of glass frit and a selected quantity of resinless vehicle of a 
type that results in a pseudoplastic rheology for the resulting mixture. 
As used herein, the term "quantity" is intended to broadly cover any 
desired quantity selected for use in the die attach paste. 
A further preferred embodiment of my resinless die attach paste consists of 
approximately 70-82 weight percent silver flake, approximately 8-18 weight 
percent lead borate or lead vanadate glass frit, glass frit made from 
oxides of lead, vanadium, bismuth, tellurium, silver or phosphorous, or 
any other suitable glass frit, having a glass transition temperature below 
400.degree. C., the quantities of silver flake and glass frit providing a 
metal-to-glass ratio of between about 4:1 and 7:1, and approximately 8-20 
weight percent liquid organic vehicle of a type that results in a 
pseudoplastic rheology and a thixotropic index of 7.5-15.0 for the 
resulting mixture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Unless otherwise specified, all percentages herein are given by weight. 
The disadvantages of resin based vehicle systems for silver glass 
compositions represent fundamental product capability limitations. It is 
submitted that more capable products could be achieved if the resin were 
simply eliminated. Such a change is reasonable on the basis of the 
following considerations: 
1) Solvent bleed is not necessarily harmful if the solvent wave front does 
not carry particles of silver or glass with it. 
2) Where solvent bleed is still considered to be a disadvantage for 
cosmetic reasons--several solvents and combinations thereof have been 
found that exhibit little if any solvent bleed or bare alumina or gold 
plate alumina surfaces. 
3) Current paste processing trends have shifted away from the use of a 
separate drying step in between dispense and firing of the paste. That is 
to say that the entire adhesive bonding step is performed in a single 
step. The new products, therefore, are not required to possess any green 
strength since there is no need for handling the parts during an 
intermediate (e.g. drying) step. 
The selection of fluids to act as an organic vehicle becomes much more 
critical when no resin is present. The most preferred resinless vehicles 
are those that have a pseudoplastic rheology. As used herein, the 
expression "resinless vehicle" or "resin-free vehicle" is understood to 
mean a vehicle consisting essentially of 100 percent volatile organic 
solvents. As used herein, the term "pseudoplastic rheology" refers to that 
property of a high viscosity paste-like material which will permit the 
material to flow when shear forces are exerted against it, but will permit 
the material to return to its prior high viscosity when the shear forces 
are eliminated. For example, some silver glass pastes will not flow out of 
a jar when it is opened and inverted until a spatula is inserted and 
moved. Once the spatula is removed, the paste will stay in place on the 
surface on which it has been smeared. 
Pseudoplastic behavior is generally recognized as a special case among 
materials showing thixotropic rheology. A thixotropic material 
demonstrates a continuous decrease in viscosity as a function of 
increasing shear rate. If the shear stress is removed the viscosity 
returns to a higher value. The recovery is not instantaneous, however, and 
there is a time delay before the material is restored to its original 
viscosity. The limiting case for this shear recovery hysterisis (where 
recovery time approaches zero) is the definition of pseudoplastic 
behavior. The compositions described in this invention possess 
pseudoplastic behavior according to all measurements available. One such 
measurement is the "thixotropic index". This thixotropic index is 
generated using a Brookfield RVT viscometer equipped with a "T-C" 
spindle. The thixotropic index is a unit-less number defined as the one 
(1) rpm viscosity divided by the twenty (20) rpm viscosity values. It has 
been observed that pastes possessing thixotropic indices between 7.5 and 
15.0 dispense well from automated die-bond equipment, with values between 
9.5 and 12.0 being most preferred. 
Fluids exhibiting pseudoplastic rheology, as a rule of thumb, often have 
melting points near room temperature. Another feature desirable for these 
fluids is that they be "good wetters". A good wetting solvent is one that 
has low surface tension or else has a molecular structure that resembles a 
surfactant. That is to say, some of the best wetting solvents are those 
that combine a hydrophilic polar group, such as a hydroxyl or ester 
function, with a hydrophobic tail (e.g. a long hydrocarbon fragment) all 
within the same molecule. Examples of these solvents include many of the 
higher alcohols such as 2-ethylhexanol, decanol, dodecanol, tetradecanol, 
terpineol, nopol, borneol, isoborneol, cyclohexanol, cyclohexyl methanol, 
2-cyclohexylethanol, cycloheptanol, cyclocanol, and cyclododecanol. Two 
ester solvents that have proven to be useful include those sold under the 
trademarks TEXANOL and TEXANOL ISOBUTYRATE. The composition of TEXANOL 
solvent is 2,2,4-trimethyl-1, 3-pentanediol monoisobutyrate. The 
composition of TEXANOL ISOBUTYRATE solvent is 2,2,4-trimethyl-1, 
3-pentanediol diisobutyrate. Fluids that are good wetters by virtue of 
their low surface tension include higher aliphatic hydrocarbons and lower 
siloxane oligomers. Useful aliphatic, cycloaliphatic and arene compounds 
include decane, undecane, dodecane, tetradecane, hexadecane, heptadecane, 
octadecane, cyclohexyl benzene, cyclododecane, adamantane, 
endo-tetrahydrodicyclopentadiene, and exo-tetrahydrodicyclopentadiene. 
Useful siloxane fluids include octamethylcyclotetrasiloxane, 
decametylcyclopentasiloxane, dodecamethylcyclohexasiloxane, 2.0 centistoke 
linear siloxane, and 5.0 centistoke linear siloxane. 
Other solvents include the higher glycols and polyglycols. Central to the 
performance characteristics of higher glycols and polyglycols is their 
long range order. Glycols and alcohols share the capacity to form hydrogen 
bonds. The former are distinguished by their difunctional nature and thus 
have the ability to form oligomeric molecular associations. Therefore, in 
a sense, the glycols may be considered to consist of numerous dynamic 
polymeric associations of individual glycol molecules. The weak hydrogen 
bonding forces in these compounds thus mimic the stronger covalent forces 
that link monomer units together in the resins, such as those mentioned 
above, but are much more readily broken. 
Examples of Ag/glass pastes formulated in accordance with the invention are 
set forth hereafter. These pastes include a quantity of silver flake, a 
quantity of glass frit and a selected quantity of resinless vehicle of a 
type that results in a pseudoplastic rheology for the resulting mixture. 
The examples should not be considered to limit the scope of the invention. 
Thus, although specified types of silver flake or glass frit may be set 
forth, it is submitted that other suitable silver flake preparations and 
glass frits may be used. Moreover, the relative quantities of silver 
flake, glass frit and resinless vehicle may be varied, yet still produce 
suitable die attach pastes. Unless otherwise specified, the relative 
quantities of silver flake, glass frit and resinless vehicle include all 
possible combinations and ratios deemed suitable for use in a die-attach 
paste. 
EXAMPLE I 
35.18%--Low Surface Area (0.3-0.5 m.sup.2 /g) Ag Flake 
35.18%--High Surface Area (0.8-1.1 m.sup.2 /g) Ag Flake 
17.64%--Lead borate glass 
6.00%--2, 3-Dimethyl-2, 3-butanediol 
6.00%--2-Methyl-2, 4-pentanediol 
This paste had a 10 rpm viscosity of 53.2.times.10.sup.3 centipoise and a 
thixotropic index of 10.7. 
EXAMPLE II 
34.78%--Low Surface Area (0.3-0.5 m.sup.2 /g) Ag Flake 
34.78%--High Surface Area (0.8-1.1 m.sup.2 /g) Ag Flake 
17.44%--Lead borate glass 
6.50%--2, 3-Dimethyl-2, 3-butanediol 
5.20%--2, 2-Dimethyl-1, 3-propanediol 
1.30%--Polypropylene glycol 4000 
This paste had a 10 rpm viscosity of 55.7.times.10.sup.3 centipoise and a 
thixotropic index of 9.9. 
In each of the above examples, a mixture of two different flake size types 
of silver is used. Thus, the paste composition can be said to have a 
"polymodal flake size distribution," or more specifically in the examples, 
a "bimodal flake size distribution". In terms of the organics, many of the 
potentially useful glycols are solids at room temperature. The invention 
takes advantage of the freezing point depression that such glycols exert 
upon each other when in admixture. Thus, it is possible to mix two solid 
glycols (using a stirring hotplate) and thereby produce a liquid vehicle 
that does not revert to the solid state. 
Advantages of a glycol-based resinless vehicle system include: 
1) The glycols used are pure and practically non-toxic; 
2) The resinless vehicle system imparts useful (pseudoplastic) rheological 
properties to the Ag/glass adhesive; 
3) The resinless vehicle system can be formulated for enhanced solvent 
release and therefore requires reduced cure time; 
4) The resinless vehicle system reduces or eliminates settling of the 
adhesive inorganics; 
5) Glycol blends can be formulated which have minimal solvent bleed; 
6) The requirement for organic burnout during cure is reduced or 
eliminated; and 
7) No stress can be induced via TCE mismatch between a resin and the 
bonding surface. 
A disadvantage seen with a resinless glycol system has been a decrease in 
viscosity stability. An interaction between the glycols and the lead 
borate glass has been identified as the origin of this problem. It has 
been found that pretreatment of the glass powder or frit with higher 
monofunctional alcohols or carboxylic acids stabilizes the adhesive 
viscosity. Presumably, these compounds form a monolayer on the glass which 
is not readily displaced by the glycols. Other means to passivate the 
glass include the deposition of a conformal silver coating (via 
traditional mirror plating techniques or decomposition of a silver 
organometallic) or silylation of the surface using a mono-alkoxy 
trialkysilane. 
It has been further discovered that a functioning Ag/glass adhesive can be 
made by using a single glycol. However, this material is inferior in its 
process performance. The preferred glycol-based embodiment consists of two 
or more glycols. To obtain an adhesive that will perform as a quick dry 
material it is important that one or both of the glycols have a boiling 
point equal to or less than 220 degrees C. An alternative composition 
consists of a mixture of glycols together with 1-15% of a polypropylene 
glycol. 
The latter composition is especially useful for applications in which a 
separate drying step is necessary in the use of the Ag/glass adhesive 
(e.g. for bonding large die to their packages). The polypropylene glycol 
in this case prevents "over-dry" of the adhesive which can lead to 
interface bond failures. 
Additional examples of Ag/glass pastes formulated with other solvents are 
set forth hereafter: 
EXAMPLE III 
______________________________________ 
Component Percentage of Composition 
______________________________________ 
Degussa-Metz 15ED Ag Flake 
35.98 
Degussa-Metz 26LV Ag Flake 
35.98 
Lead Borate Glass 18.04 
Resin-free vehicle 10.00 
______________________________________ 
The organic vehicle consists of 71.26 percent 
dodecamethylcyclohexasiloxane, 23.75 percent 5.0 centistoke linear 
siloxane fluid, and 4.99 percent nopol. All of the above ingredients were 
sheared on a three roll mill to obtain a homogeneous paste that had a 
Brookfield 10 rpm viscosity of 28.3.times.10.sup.3 centipoise and a 1/20 
rpm thixotropic index of 9.43. This paste could be processed at furnace 
ramp rate speeds as high as 110.degree. C. per minute without voiding. 
This was significantly faster than could be achieved (maximum 60.degree. 
C./min. with a control containing a low resin vehicle of similar 
evaporation rate). It was concluded that the absence of resin as well as 
the exceptionally low heats of vaporization of the silicone fluids were 
responsible for this superior process speed performance. 
EXAMPLE IV 
______________________________________ 
Component Percentage of Composition 
______________________________________ 
Degussa-Metz 5SB Ag Flake 
23.72 
Degussa-Metz 67 Ag Flake 
11.86 
Degussa-Metz 26LV Ag Flake 
23.72 
Degussa-Metz 50S Ag Flake 
11.86 
Lead Borate Glass 18.84 
TEXANOL 3.66 
Terpineol 3.67 
TEXANOL ISOBUTYRATE 
3.67 
______________________________________ 
This paste had a 10 rpm viscosity of 44.5.times.10.sup.3 centipoise and a 
thixotropic index of 10.6. 
This high solids paste (89%) was found to be particularly useful for 
processing large area die in relatively fast profiles. It was found, for 
example, that die as large as 600 mil.sup.2 could be bonded using a 
profile that required only 38 minutes from ambient to the end of the high 
temperature soak (at 390.degree. C.). The use of the low surface tension 
siloxane solvents did result in unusually high rates of solvent bleed on 
the substrate after paste dispense. However, by virtue of that low surface 
tension the paste solids were not carried by the solvent front (even the 
finest particles would not stay suspended at the solvent interface). 
EXAMPLE V 
______________________________________ 
Component Percentage of Composition 
______________________________________ 
Degussa-Metz 67 Ag Flake 
64.63 
Degussa-Metz 50S Ag Flake 
10.77 
Lead Vanadate Glass 
11.60 
THDCPD* 2.60 
Cyclohexylbenzene 5.20 
Cyclododecane 2.60 
Octadecane 2.60 
______________________________________ 
*Note: THDCPD = Tetrahydrodicyclopentadiene 
This paste was found to have a 10 rpm viscosity of 42.3.times.10.sup.3 
centipoise and a thixotropic index of 12.2. 
This composition was found to be especially advantageous for use with the 
reduction sensitive vanadium glasses. Pastes made with the traditional 
resin containing vehicles were found to be unacceptable because they 
caused undue reduction of the oxides in the glass and thus deteriorated 
the adhesive performance of the product. Even the alcohol and/or alcohol 
ester solvents alone, in the absence of resin, were found to be unusable 
since in the presence of these functional groups the viscosity of the 
paste was not stable. 
EXAMPLE VI 
______________________________________ 
Component Percentage of Composition 
______________________________________ 
Degussa-Metz 5SB Ag Flake 
24.00 
Degussa-Metz 67 Ag Flake 
12.00 
Degussa-Metz 26LV Ag Flake 
24.00 
Degussa-Metz 50S Ag Flake 
12.00 
Lead Borate 3-8 micrometer 
18.00 
Classified Glass Powder 
2,6-Dimethyl-4-heptanol 
5.50 
alpha-Terpineol 4.50 
______________________________________ 
The components above are first stirred together in a planetary mixer and 
then processed to yield a homogenous, smooth paste using a three-roll 
mill. The resulting paste has a 10 rpm (Brookfield) viscosity of 
37.5.+-.7.5.times.10.sup.3 centipoise at 25.+-.5.degree. C. and a 
Thixotropic Index (defined as the 1/20 rpm viscosity values) of 
9.25.+-.1.75. This paste is particularly suited for die attach of silicone 
dice as large as 700 mil.sup.2 in relatively fast profiles. The fired film 
of this composition yields a dense, void-free film. The above composition 
was also found to give excellent adhesion to bare ceramic or gold plate 
substrates at peak firing temperatures as low as 380.degree. C. The 
adhesion of this composition when fired at 395.degree. C. using 700 
mil.sup.2 silicon die was found to be 595 pounds force with a standard 
deviation of 71.4 pounds force. 
EXAMPLE VII 
______________________________________ 
Component Percentage of Composition 
______________________________________ 
Degussa-Metz 50S Ag Flake 
8.05 
Chemet EA 18 Ag Flake 
14.05 
Chemet EA 101 Ag Flake 
23.00 
Chemet AB001 Ag Flake 
34.50 
Lead Borate 3-6 micrometer 
11.90 
Classified Glass Powder 
2,6-Dimethyl-4-heptanol 
3.83 
alpha-Terpineol 4.67 
______________________________________ 
This composition is notable for its high metal to glass ratio of 
approximately 6.7:1. The adhesive bond produced by the composition was 
found to be especially resistant to degradation during temperature cycling 
tests (test condition =temperature cycling from -65.degree. to 
+150.degree. C., number of cycles=500). The adhesive was further found to 
form dense, void free adhesive bonds for silicon die as large as 700 
mil.sup.2 in relatively fast profiles. The paste was found to have a 10 
rpm viscosity of 45.+-.15.times.10.sup.3 centipoise and a Thixotropic 
Index of 9.25.+-.1.25. The average adhesion for twenty 700 mil.sup.2 
silicon test parts assembled with this paste was 521 pounds force with a 
standard deviation of 34 pounds force. 
EXAMPLE VIII 
______________________________________ 
Component Percentage of Composition 
______________________________________ 
Chemet EA 18 Ag Flake 
26.43 
Degussa-Metz 50S Ag Flake 
17.66 
Chemet EA 15 Ag Flake 
16.52 
Chemet EA 101 Ag Flake 
16.52 
VLSI Packaging Materials 
11.87 
C50 Glass 
Tetrahydrodicyclopentadiene 
3.67 
Tricyclodecane 1.83 
Cyclohexylbenzene 2.75 
Hexadecane 2.75 
______________________________________ 
The above resinless silver/glass adhesive composition was found to be 
particularly suited to low firing applications. It was found, for example, 
that this adhesive could be used to die attach silicon dice to bare 
alumina substrates at temperatures as low as 330.degree. C. It was also 
found to provide reliable adhesion to gold plated substrates when fired at 
temperatures as low as 350.degree. C. The glass component of this 
composition is a proprietary frit that contains oxides of lead, vanadium, 
bismuth and tellurium. This composition was tested using 400 mil.sup.2 die 
and a firing profile with a peak temperature of 350.degree. C. The average 
adhesion for ten test parts was 483 pounds force with a standard deviation 
of 50.6 pounds force. The average 10 rpm viscosity for six batches of the 
above composition was 42.3.times.10.sup.3 centipoise. The average value 
for thixotropic index for these same lots of material was 13.6 with a 
range from 13.3 to 14.6. The glass used in the above example had a glass 
transition temperature of approximately 250.+-.5.degree. C. The 
proprietary glass frit is available from VLSI Packaging Materials (1306 
Bordeaux Drive, Sunnyvale, Calif. 94089). 
EXAMPLE IX 
______________________________________ 
Component Percentage of Composition 
______________________________________ 
Degussa-Metz 67 Ag Flake 
58.19 
Degussa-Metz 50S Ag Flake 
9.77 
X37P Powdered Glass 
17.04 
Tetrahydrodicyclopentadiene 
3.00 
Cyclohexylbenzene 6.00 
Cyclododecane 3.00 
Octadecane 3.00 
______________________________________ 
This composition contains a glass formulation (X37P) that is described in 
U.S. Pat. No. 4,997,718 and available from VLSI Packaging Materials. The 
glass contains oxides of silver, lead and phosphorus and has a glass 
transition temperature T.sub.g of 165.degree. to 178.degree. C. (there was 
no evidence of any devitrification). 
Silicon dice were attached using the above adhesive. Ten bare backed 
silicon die that were 230.times.280 mils on a side were secured to alumina 
substrates in a profile with a 350.degree. C. peak firing temperature. The 
average tensile adhesion for these parts was 68.8 pounds force with a 
range from 57.4 to 79.5 and a standard deviation of 8.1 pounds force. Ten 
gold backed die with the same dimensions were also tested at the same 
time. The average adhesion for those parts was 68.8 pounds with a range of 
58.9 to 80.7 and a standard deviation of 7.1 pounds force. The same 
composition was used to attach 300 mil.sup.2 bare backed silicon die to 
alumina substrates using a peak firing temperature of 340.degree. C. The 
average tensile adhesion from that test was 65.3 pounds with a range of 56 
to 71 and a standard deviation of 4.5 pounds force. The 10 rpm viscosity 
for this paste was 45.+-.15.times.10.sup.3 centipoise at 25.+-.5.degree. 
C. The thixotropic index for this composition was 13.5.+-.1.5. While I 
have described several embodiments of my resinless vehicle system in some 
detail, it should be understood that my invention may be modified in both 
arrangement and detail. For example, its use is not restricted to Ag/glass 
paste adhesives for semiconductor die attachment. My invention may be 
utilized in almost any application where pseudoplastic rheology is desired 
for depositing an inorganic material onto a substrate. Therefore, the 
protection afforded my invention should only be limited in accordance with 
the spirit of the following claims and their equivalents: