Method of encapsulating electronic devices

A method for encapsulating microelectronic devices is provided using a heat curable epoxy composition having a monomeric or polymeric diaryliodonium hexafluoroantimonate salt. Curable compositions are also provided as well as encapsulated microelectronic devices.

CROSS-REFERENCE TO RELATED APPLICATIONS 
Reference is made to copending applications of Crivello et al., Ser. No. 
064,433, filed June 22, 1987, and Ser. No. 103,156, filed Oct. 1, 1987 and 
H. Chao, Ser. No. Ser. No. 103,154, filed Oct. 1, 1987, filed concurrently 
herewith and assigned to the same assignee as the present invention and 
incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
Prior to the present invention, commercially available plastic encapsulants 
for electronic devices had several serious disadvantages. One 
disadvantage, for example, was that prior to use, the encapsulant required 
refrigeration, preferably to 4.degree. C. and protection from moisture 
during shipment and storage. Unless the encapsulant composition was 
refrigerated and protected from moisture, it did not provide suitable flow 
length for filling multicavity molds. Encapsulant molding powders are 
generally sealed inside several plastic bags and surrounded by dry ice 
before shipment by the manufacturer. As a result, the material must be 
allowed several hours to reach room temperature in the absence of moisture 
before it is used to encapsulate an electronic device to minimize a 
build-up of water in the powder due to atmospheric condensation. 
Although flow length during injection molding is an important consideration 
for qualifying commercially available curable polymeric materials for 
device encapsulation, another equally important requirement of the 
electronic system manufacturers is that after cure, the plastic 
encapsulated electronic device must have the ability to resist changes in 
humidity conditions over a wide temperature range. One way to test the 
resistance of plastic encapsulated electronic devices to high humidity 
over various temperature ranges, such as experienced in South East Asia, 
is by using the "HAST" test (highly accelerated stress test) as discussed 
by L. Gallace et al. in "Reliability of Plastic-Encapsulated Integrated 
Circuits and Moisture Environments", RCA Solid State Division, Somerville, 
N.J. 08876; and RCA Review. Vol. 45. (June 1984) pages 249-277. Another 
version of the HAST test is shown by K. Ogawa et al., Automatically 
Controlled 2-Vessel Pressure-Cooker Test--Equipment IEEE Transactions on 
Reliability, Vol. R-32, No. 2 (June 1983). The HAST test determines how 
long terminals of a plastic encapsulated device will survive while 
subjected to 18 volts during exposure to 85RH, 145.degree. C. and 2.7 
atmospheres of steam. The devices are periodically tested with a GenRad 
model 1731M/linear IC tester having internal diagnostics and system 
calibration. 
An additional consideration which must be addressed by encapsulant 
composition manufacturers for the electronic industry is the cure 
temperature required for commercially available encapsulants which are 
usually epoxy resin formulations. It is generally known that presently 
available encapsulants which are injection molded onto electronic devices 
require a temperature of at least 180.degree. C. Such temperatures often 
create excessive stresses at the interface of the cured plastic 
encapsulant and the electronic device upon cooling. 
In order to minimize the cracking of silicon chips resulting from 
180.degree. C. plastic encapsulation, silicone resin modified epoxy resins 
have been used as encapsulants, as shown by K. Kuwata et al., IEEE, 1985 
(18-22). 
An additional concern of the electronic systems manufacturers is that 
plastic encapsulated electronic devices requiring soldering often fail 
because the heat distortion temperature (HDT) of the cured encapsulant is 
often too low, i.e. between about 200.degree. C. to 250.degree. C. The 
molded encapsulant can experience a change in shape (distortion), when the 
device is dipped into a molten solder bath. Temperatures up to 290.degree. 
C. are sometimes unavoidable in instances where solder fluxes which are 
often used to facilitate metal contact can not be tolerated. 
It would be desirable, therefore, to provide less stressful encapsulating 
compositions curable at temperatures significantly below 180.degree. C., 
which have HDTs after cure exceeding 300.degree. C. Encapsulated devices 
fabricated from such encapsulants would have substantially reduced stress 
due to decreased thermal expansion. In addition, such encapsulated devices 
could be made with surface areas exceeding 1/8 sq.in., without cracking. 
It also would be desirable to have encapsulated electronic devices having 
greater than a 50% survival rate when subjected to HAST test conditions 
measuring reliability in moisture environments. 
The present invention is based on our discovery that if a catalytic amount 
of a compatible monomeric or polymeric diaryliodonium hexafluoroantimonate 
salt as defined hereinafter can be utilized in combination with an 
effective amount of a copper compound, as defined hereinafter, as a 
cocatalyst, and the resulting thermal initiator is employed in combination 
with a substantially chloride-free epoxy resin and a fused silica filler, 
superior silicon chip encapsulating compositions are provided. Many of the 
aforementioned requirements of the electronic systems industry can be 
satisfied. For example, the aforementioned diaryliodonium 
hexafluoroantimonate catalyst and copper compound cocatalyst has been 
found to provide encapsulating compositions which are shelf stable 
indefinitely at ambient temperatures and are curable at 150.degree. C. 
These encapsulants can provide sealed electronic devices having HDTs from 
about 250.degree. C. to 345.degree. C., and reduced incidence of cracking. 
The encapsulated devices also have been found to far exceed the 50% 
survival rate after a period of 80 hours under the aforedescribed HAST 
test conditions. In addition, the dynamic mechanical properties of the 
cured encapsulating composition of the present invention, in accordance 
with ASTM test D2236-81, show that devices encapsulated in accordance with 
the present invention have reduced likelihood of failure due to shape 
alteration during device performance as compared to commercially available 
devices encapsulated with plastic. 
STATEMENT OF THE INVENTION 
There is provided by the present invention, a method for making plastic 
encapsulated electronic devices capable of exceeding a 50% survival rate 
when subjected to a moisture-ladened environment at a temperature of about 
145.degree. C. for a period of at least 80 hours under highly accelerated 
stressed test conditions, which comprises encapsulating the electronic 
devices at a temperature of 120.degree. to 230.degree. C. at a pressure of 
400 to 1500 psi, with a heat curable composition comprising by weight, 
(A) 100 parts of an epoxy resin having less than 100 ppm of hydrolyzable 
chloride, 
(B) 10 to 1000 parts of fused silica filler, 
(C) a catalytic number of a compatible monomeric or polymeric 
diaryliodonium hexafluoroantimonate salt, and 
(D) up to an effective amount of a copper compound as a cocatalyst for (C). 
Epoxy resins which can be utilized in the heat curable compositions 
employed in the practice of the method of the present invention are, for 
example, Quatrex epoxy resins, such as Quatrex 2410 manufactured by the 
Dow Chemical Company, Midland, Mich. Additional epoxy resins which can be 
used are any epoxy resins having a hydrolyzable total chloride content of 
less than about 100 ppm. These epoxy resins can be in the form of a flaked 
solid epoxy novolak resin having a T.sub.g from about 
10.degree.-25.degree. C. Additional epoxy resins which can be used are 
shown, for example, in Plastic Focus, Vol. 16, No. 40 (Nov. 26, 1984) and 
certain substantially chloride-free epoxy resins shown by Shinohara et 
al., U.S. Pat. No. 4,358,552, incorporated herein by reference. 
Further examples of the epoxy resins which can be used in the practice of 
the present invention are shown in Chemical Week (Nov. 28, 1984) pages 
13-14. 
The fused silica filler which can be used in the practice of the present 
invention is preferably made by the fusion of .alpha. crystalline quartz. 
A description of .alpha. crystalline quartz can be found on pages 818-825 
of Vol. 20, Third Edition of Kirk Othmer Encyclopedia, which is 
incorporated herein by reference. Fused silica filler which can be 
utilized in the practice of the present invention also can be found in the 
Handbook of Fillers and Reinforcements for Plastics, Harry S. Katz et al., 
Van Nostrand Reinhold Company, New York (1978) pages 155-158, which is 
incorporated herein by reference. A preferred form of the fused silica 
filler is shown by the glass grain GP series manufactured by 
Harbison-Walker Refractories, North American Operations, Dresser 
Industries, Inc., One Gateway Center, Pittsburgh, Pa. 15222. Typical 
properties of the preferred fused silica which can be used in the practice 
of the present invention are as follows: 
______________________________________ 
SiO.sub.2 
99.6% 
Al.sub.2 O.sub.3 
0.2 
Fe.sub.2 O.sub.3 
0.05 
Na.sub.2 O 
0.005 
K.sub.2 O 
0.006 
______________________________________ 
Specific Gravity: 2.2 
Coefficient of Linear Thermal Expansion: 0.4.times.10.sup.-6 
in/in/.degree.F. 
Crystal Species: Amorphous 
Particle Size Distribution: (Sedigraph) 
______________________________________ 
% Finer Than 
Micron GP11I GP7I GP3I 
______________________________________ 
70 99 
60 98 
50 95 
40 90 99 99 
30 77 94 97 
20 58 77 91 
10 33 45 73 
6 15 28 57 
4 4 12 46 
2 1 5 29 
______________________________________ 
U.S. Screens (Cumulative % Retained) 
GP11I 
______________________________________ 
100 Mesh 0.1% 
200 Mesh 5.0 
325 Mesh 23.0 
Passing 325 Mesh 77.0 
______________________________________ 
High purity fused silica from tetraethyl orthosilicate using NH or OH as a 
gelation catalyst also can be used. The dried gels can be crushed into a 
powder. 
The diaryliodonium hexafluoroantimonate salts of the present invention are 
preferably compounds included within the formula, 
EQU [RIR.sup.1 ]+[SbF.sub.6 ].sup.-, (1) 
where R and R.sup.1 are selected from the same or different C.sub.(6-18) 
monovalent aromatic hydrocarbon radicals and C.sub.(6-18) monovalent 
aromatic hydrocarbon radicals substituted with one or more radicals 
substantially inert under encapsulation conditions. 
The diaryliodonium antimonate salts of the present invention, also include 
polymeric diaryliodonium salts comprising at least 0.01 mol percent of 
chemically combined divalent units selected from, 
##STR1## 
where R.sup.2 is a trivalent C.sub.(2-8) alkylene or branched alkylene 
group which can be substituted with radicals inert during encapsulation, 
R.sup.3, R.sup.5 and R.sup.6 are selected from C.sub.(6-14) divalent 
arylene groups which can be substituted with radicals inert during 
encapsulation, R.sup.7 is selected from trivalent C.sub.(6-14) aryl groups 
which can be substituted with radicals inert during encapsulation, R.sup.4 
and R.sup.8 are C.sub.(6-14) monovalent aryl groups which can be 
substituted with radicals inert during encapsulation, Q is an ester or 
amide linkage and Q.sup.1 is methylene or --O--, and when Q.sup.1 is 
methylene, R.sup.7 is substituted with --OR.sup.8, where R.sup.8 is 
C.sub.(1-14) monovalent hydrocarbon group, or monovalent hydrocarbon group 
substituted with radicals inert during encapsulation. 
Diaryliodonium antimonate salts which are included within formula (1) are, 
for example, 
##STR2## 
Among the polymeric diaryliodonium salts having chemically combined units 
of formula (2), there are included 
##STR3## 
where x is an integer having a value of at least 5. 
The preferred polymeric diaryliodonium salts shown by formulas (3) and (4) 
are, for example, 
##STR4## 
where y and z are integers having a value of at least 5. 
Monovalent aromatic hydrocarbon radicals which are included within R and 
R.sup.1 are, for example: phenyl, tolyl, xylyl, naphthyl, anthryl; 
substituted R and R.sup.1 radicals are, for example: halogenated 
C.sub.(6-18), monovalent aromatic hydrocarbon radicals such as 
chlorophenyl, bromoxylyl, chloronaphthyl; C.sub.(1-8) alkoxy radicals such 
as methoxy substituted phenyl, methoxy substituted tolyl. Radicals 
included within R.sup.2 are, for example, ethylidene, and propylidene. 
Radicals included within R.sup.4 and R.sup.5 are the same as R and 
R.sup.1. Radicals included within R.sup.3, R.sup.5, and R.sup.6 are 
divalent aromatic radicals such as phenylene, tolylene, xylylene and 
naphthylene. Radicals included with R.sup.7 are for example, 
##STR5## 
Copper compounds which can be utilized as a cocatalyst in combination with 
the onium salt of the present invention are preferably copper salts such 
as Cu(1) halides, for example Cu(1) chloride. Cu(2) salts, such as Cu(II) 
benzoate, Cu(II) acetate, Cu(II) stearate, Cu(II) gluconate, Cu(II) 
citrate. Copper chelates, such as copper acetylacetonate, are particularly 
preferred as a copper cocatalyst. 
An effective amount of onium salt of formulas (1-4) is that amount of onium 
salt sufficient to provide from about 0.01 to 100 mg of antimony, per gram 
of epoxy resin. In instances where a copper cocatalyst is found necessary 
to achieve effective results, there can be used from 0.01 to 100 mg of 
copper, per gram of antimony, of the onium salt catalyst. 
In the practice of the preferred form for making the encapsulation 
composition, the onium salt which hereinafter means the monomeric or 
polymeric onium salt of formulas (1-4) and copper cocatalyst is initially 
blended with the fused silica. There can be utilized during the blending 
of the onium salt and fused silica, an additive, such as carnauba wax at 
from 0.01 to 5%, based on the weight of encapsulating composition; 
additional additives such as antimony trioxide and carbon black also can 
be used. The various ingredients can be placed in a Henschel mixer to 
provide more intensive blending. 
The resulting onium salt/fused silica mixture then can be blended with the 
epoxy resin. A two-roll mill can be used in accordance with standard 
procedures to produce the encapsulation composition.

In order that those skilled in the art will be better able to practice the 
invention, the following examples are given by way of illustration and not 
by way of limitation. All parts are by weight. 
EXAMPLE 1 
An onium salt catalyzed encapsulant was made using Quatrex 2410 having less 
than 25 ppm of chloride and 6410 having less than 150 ppm of chloride. 
These are manufactured by the Dow Chemical Company. In addition, there 
were used GP11I and GP7I, fused quartz fillers manufactured by 
Harbison-Walker Refractories of Pittsburgh, Pa. The fused quartz fillers 
had average particle sizes of less than 40 microns and less than 90 
microns respectively. Copper acetylacetonate (Cu(AcAc).sub.2) was also 
employed in the composition which is as follows: 
______________________________________ 
Material Weight Percent 
Flammability Rating, UL 
1/8" V-O 1/16" V-O 
______________________________________ 
Quatrex 2410 27.63 27.38 
GP 11I 33.83 33.53 
GP 7I 34.32 34.01 
##STR6## .18 .18 
Cu(AcAc).sub.2 .13 .12 
Carnauba Wax .39 .34 
Quatrex 6410 1.93 2.39 
Antimony Oxide Grade KR 
1.45 1.91 
Carbon Black .14 .14 
100.00 100.00 
______________________________________ 
The above onium salt catalyzed encapsulant was prepared by mixing two 
kilograms of the above ingredients excluding the Quatrex 2410 and Quatrex 
6410 in a Henschel mixer for three minutes at 1800 rpm and then for three 
minutes at 3600 rpm. A portion of this master batch and the two epoxy 
resins were shaken in a plastic bag to provide approximately 500 grams of 
a preblend for two-roll milling. 
The two-roll mill was cleaned with a mixture of Quatrex 2410 and GP11I 
prior to milling the preblend with the front roller at 195.degree. F. and 
23 fpm and with the rear roller at 185.degree. F. and 30 fpm. The molten 
preblend mixture was removed from the rear roller, rolled over the doctur 
blade and folded several times. It was returned to the mill a total of six 
times in five minutes. After cooling to room temperature, the milled 
material was ground through a 1/16" diameter screen. The moldability 
characteristics of the resulting "onium salt encapsulant" was then 
compared to a commercially available encapsulating composition MP-101S of 
the Nitto Company of Osaka, Japan, as shown by the following: 
______________________________________ 
Onium Salt 
Encapsulant 
MP-101S 
______________________________________ 
Spiral Flow, 
inches @1000 psi* 
@150.degree. C.: 31 No Cure 
@180.degree. C.: 23 30 
Hot Plate Cure Time, 
seconds 
@150.degree. C.: 29 No Cure 
@180.degree. C.: 11 22 
Shore D Hot 
Hardness, - 2 min 80 No Cure 
90 sec 78 No Cure 
@150.degree. C. 60 sec 58 No Cure 
30 sec Cured Spiral 
No Cure 
2 min 85 85 
90 sec 85 80 
@180.degree. C. 60 sec 75 40 
30 sec 65 0 
Release Excellent 
Excellent 
Characteristics 
Lube Bloom Minimal Minimal 
Bleed None None 
Flash None None 
Flammability Rating: UL 1/8" V-O 
UL 1/8" V-O 
______________________________________ 
*ASTM method D3123; 0.12 to 0.14 inch cull thickness 
The above results show that spiral flow occurs at 150.degree. C. with the 
composition of the present invention, whereas the Nitto composition 
requires a cure temperature of 180.degree. C. 
Approximately 100 devices (Motorola LM-301 Operational Amplifiers) were 
encapsulated with the above onium salt resin and MP-101S. MP-101S was 
molded at 175.degree. C. and 1000 psi for 75 seconds, whereas the onium 
salt resin was molded at 150.degree. C. for 75 seconds. The transfer 
pressure was 800 psi and the transfer time was 12 seconds. The 
commercially available material was post-baked for 4 hours at 175.degree. 
C., while the onium salt resin was post-baked for 16 hours at 150.degree. 
C. The various encapsulated devices were tested in a linear IC tester to 
select samples of 10 units each for accelerated life testing in a pressure 
vessel at 131.degree. C., 85% relative humidity (RH) and 2.1 atm of steam. 
Each device had a bias of 30 volts applied. Periodically, the samples were 
taken from the pressure vessel and after cooling to room temperature, they 
were screened with the IC tester. A series of two trials of 10 devices 
each were performed in the pressure vessel to determine the failure 
pattern at 130.degree. C./85% RH/2.1 atm/30 V. The following results were 
obtained, where the numbers under Trial I and Trail II indicate the number 
of devices which failed out of the initial 10 devices used within the time 
shown. MP-101S is the commercially available encapsulant and onium salt is 
the composition of the present invention: 
______________________________________ 
MP-101S Onium Salt 
Time (hrs) 
Trial I Trial II Trial I 
Trial II 
______________________________________ 
50 0 0 0 1 
95 -- 6 0 0 
135 -- 3 0 0 
144 9 -- 1 0 
195 0 1 0 0 
305 0 -- 0 1 
430 1 -- 0 0 
470 1 0 
607 1 0 
649 1 1 
961 0 2 
1013 1 0 
1132 1 0 
1265 1 0 
______________________________________ 
The above results show that after 144 hours, 90% of the devices 
encapsulated with MP-101S failed, whereas only 10% of the onium salt 
encapsulated devices made in accordance with the practice of the present 
invention failed. It was found that 1013 hours were required to obtain a 
50% failure rate with the devices encapsulated with the onium salt 
composition of the present invention. 
EXAMPLE 2 
Additional devices having a single SiO.sub.2 passivation layer where 
encapsulated at 180.degree. C. with the commercial encapsulant MP-101S, 
and with the onium salt encapsulating composition of Example 1. It was 
found upon accelerated life-testing in accordance with the procedure of 
Example 1, that the devices encapsulated with the commercially available 
resin reached a 50% failure rate at 100 hours while the devices 
encapsulated with the onium salt resin of Example 1 experienced a 50% 
failure rate after 1,500 hours. Additional encapsulated commercial devices 
from National Semiconductor with dual passivating layers of SiO.sub.2 and 
Si.sub.3 N.sub.4 showed a failure rate of 50% after approximately 500 
hours. These results demonstrate that the onium salt encapsulating 
compositions of the present invention provide superior resistance to 
moisture and heat under accelerated life-testing conditions even in 
instances where the devices are protected by only a single passivating 
layer of silicon dioxide. 
EXAMPLE 3 
The procedure of Example 1 was repeated except that 
bis(p-t-butylphenyl)iodonium hexafluoroantimonate was used as the curing 
catalyst in place of the methoxyphenylphenyl iodonium hexafluoroantimonate 
of Example 1. Devices were encapsulated in accordance with the procedure 
of Example 1 and were tested at 131.degree. C./85% RH/2.1 atm and 30 
voltage bias. It was found that devices encapsulated with 
bis(p-t-butylphenyl)iodonium salt encapsulant provided encapsulated 
devices having a failure rate of only 20% after 620 hours of accelerated 
life testing. 
EXAMPLE 4 
Test bars were molded from the onium salt composition and commercially 
available encapsulating composition MP-101S of example 1. After a 
post-cure for 16 hours at 150.degree. C. and 180.degree. C., it was found 
that the average deflection temperature (HDT) of the onium salt 
composition was 310.degree. and 315.degree. C. while the average HDT of 
the MP-101S test bars was 241.degree. C. These results show that devices 
encapsulated with the onium salt composition of the present invention can 
be readily soldered at temperatures of up to 290.degree. C., and therefore 
do not require any fluxes which might tend to introduce contamination into 
the encapsulated devices. 
EXAMPLE 5 
The melt flow characteristics of the onium salt compositions of the present 
invention were compared to commercially available encapsulants after 
extended shelf periods at 25.degree. C. and 43.degree. C. under sealed 
conditions. 
It was found that commercially available encapsulating compositions were 
unsuitable as encapsulants after a shelf period of about 3 days at 
43.degree. C. An initial spiral flow of 33" under melt conditions dropped 
to about 18" after 3 days of storage. It was found that the onium salt 
composition of Example 1 exhibited an initial melt flow length of about 
25" which fell to an acceptable melt flow length of about 18" after about 
50 days at 43.degree. C. under sealed conditions. 
A melt flow comparison after a 25.degree. C. shelf period showed no change 
in melt flow length of the onium salt encapsulant after a period of over 
100 days. The commercially available encapsulant had an initial melt flow 
length of about 33", which fell to an unacceptable flow length of less 
than 18" after about 90 days of storage under sealed conditions at 
25.degree. C. These results indicate that the onium salt compositions of 
the present invention do not require refrigeration for an extended shelf 
period, while the commercial encapsulant composition must be refrigerated, 
particularly for use beyond one week under tropical conditions. 
EXAMPLE 6 
A series of encapsulating compositions utilizing onium salts in accordance 
with the practice of the present invention were molded at 150.degree. C. 
and 1000 psi for 90 seconds and post-baked for 16 hours at 150.degree. C. 
There were utilized from 0.05 part to 0.3 part of onium salts per 100 
parts of encapsulation composition prepared in accordance with Example 1. 
A comparison was made between diphenyl iodonium hexafluoroantimonate, 
diphenyl iodonium hexafluoroarsenate and diphenyl iodonium 
hexafluorophosphate. The various samples were molded at 180.degree. C. for 
three minutes at 1000 psi, using a 12-second transfer time. HDT's were run 
in accordance with ASTM D648 employing a 10-mil deflection at 264 psi with 
a bar 5".times.1/2".times.1/8". The flow in inches was measured in 
accordance with ASTM method D3123. The T.sub.g was measured in accordance 
with ASTM method D696 to determine whether the molded samples achieved the 
150.degree. C. minimum for thermal cycling determination. Shore D hot 
hardness was determined in accordance with ASTM method D2240 using an 
average of four readings on a 0.12 to 0.14" cull immediately after the 
completion of transfer molding. The following results were obtained: 
______________________________________ 
Catalyst Hardness 
(wt %) Flow(in) T.sub.g (.degree.C.) 
HDT(.degree.C.) 
(Shore D) 
______________________________________ 
SbF.sub.6.sup.- 
.05 43 87 124.2 40 
.1 25 138 332.0 82 
.2 17.5 187 312.3 92 
.3 17 202 316.6 96 
AsF.sub.6.sup.- 
.05 31 76 111.1 20 
.1 29 87 142.8 35 
.2 151/2 120 197.4 72 
.3 14 142 260.4 83 
PF.sub.6.sup.- 
.05 no cure cull not cured 
.1 90 80 104.1 cull not cured 
.2 33 120 212.2 25 
.4 14 170 308.7 56 
.6 11.5 185 313.9 74 
______________________________________ 
The above results show that the diphenyl iodonium hexafluoroantimonate salt 
provided a satisfactory flow length of about 18" with a T.sub.g exceeding 
150.degree. C. along with an HDT exceeding 300.degree. C. and a Shore D 
hardness exceeding 60 when utilized in a range of about 0.1 part to about 
0.3 part. The diphenyl iodonium hexafluoroantimonate encapsulant 
properties were sufficient to satisfy thermocycling requirements, and 
provided the ready removal of encapsulated devices from the mold without 
breaking in view of a Shore D hardness exceeding 60. In addition, because 
of the superior HDT, the antimonate encapsulated devices can be dipped 
into solder at temperatures sufficiently high to avoid the use of 
contaminating fluxes. On the other hand, the hexafluoroarsenate and 
hexafluorophosphate encapsulants did not provide a sufficient flow in 
combination with a satisfactory T.sub.g or HDT as well as a high Shore D 
hardness. 
EXAMPLE 7 
A mixture of 4 grams of o-cresol novolak (HT9490 of Ciba Geigy Company), 
14.2 grams (0.1 mole) of methyliodide, 13.82 grams (0.1 mole) of finely 
ground potassium carbonate and 100 ml of acetone was refluxed with 
stirring for about 12 hours. After cooling the reaction mixture, it was 
filtered and the filtrate was concentrated in vacuo. The residue was 
diluted with chloroform and then washed with water to remove the salt 
completely. An organic layer was separated, dried with magnesium sulfate 
and concentrated in vacuo to provide 4.0 grams of product. Based on method 
of preparation, the product was a methyl-capped orthocresol novolak. The 
material was found to be free of phenolic hydroxyl groups based on FT-IR 
spectra. 
A mixture of 4.0 grams of the above methyl-capped orthocresol novolak, 50 
ml of acetic acid, and 7.84 grams (0.02 mole) of phenyl iodosotosylate was 
stirred for 90 minutes at room temperature. A slightly exothermic reaction 
occurred upon mixing. After stirring for 90 minutes at room temperature, 
the reaction mixture was poured into 100 ml of water and then extracted 
wth methylene chloride. The organic layer was separated and dried over 
magnesium sulfate and concentrated in vacuo. Based on method of 
preparation, there was obtained a poly(diaryliodonium tosylate). 
The above poly(diaryliodonium tosylate) was dissolved in 30 ml of 
methylethylketone. There was added 5 dissolved in 30 ml of 
methylethylketone. There was added 5 grams (0.019 mole) of sodium 
hexafluoroantimonate, and the mixture was stirred at room temperature for 
one hour. The reaction mixture was then filtered through celite and the 
filtrate was concentrated in vacuo. The residue was diluted with methylene 
chloride and the solution was filtered again and then concentrated in 
vacuo. There was obtained 5.6 grams of a polymeric diaryl iodonium salt 
consisting essentially of chemically combined units of the formula 
##STR7## 
Elemental analysis of the polymer showed a 22.4% by weight of iodine and 
1.44% by weight of antimony based on the weight of polymer. 
A heat curable encapsulant composition was made by blending the encapsulant 
composition of Example 1, free of the iodonium salt and the copper salt, 
with 2% by weight of the above polymeric iodonium salt. The heat curable 
composition was found to form a hard film after 5 sec when stirred on a 
stainless steel hot plate at 155.degree. C. 
EXAMPLE 8 
There was added 25 ml of concentrated sulfuric acid drop-wise to a cooled 
mixture at 0.degree.-5.degree. C. of 40.85 grams (0.3 mole) of o-toluic 
acid, 25.0 grams (0.116 mole) of potassium iodate, 120 ml of methylene 
chloride, and 50 ml of acetic anhydride. After the addition of the 
sulfuric acid was completed, the reaction mixture was allowed to stir at 
room temperature for several days. Then 50 ml of water was added to 
dissolve the potassium bisulfate formed during the reaction. The resulting 
methylene chloride layer was separated, and an off-white product was 
collected by filtration. There was obtained 19.1 grams of product which, 
based on method of preparation, was 
4,4.varies.-dimethyl-3,3'-dicarboxydiphenyliodonium bisulfate. Examination 
of the proton NMR spectrum confirmed the identity of the product. 
A mixture of 19.1 grams of the above 
4,4'-dimethyl-3,3'-dicarboxydiphenyliodonium bisulfate and 70 ml of 
thionylchloride was heated to reflux and hydrogen chloride gas evolution 
was observed. Reflux was continued for 5 hours and excess thionyl chloride 
was removed by distillation under reduced pressure. There was obtained a 
yellow solid which was dried under nitrogen and recrystallized from 
methylene chloride/diethyl ether to provide 13 grams of a white product. 
Based on method of preparation, the product was the corresponding diacid 
chloride of 4,4'-dimethyl-3,3'-dicarboxydiphenyl iodonium bisulfate. 
There were added 0.81 g (0.0015 mole) 
4,4'-dimethyl-3,3'-dichlorocarbonyldiphenyl iodonium bisulfate and 0.31 g 
(0.0015 mole) phthaloyl chloride in 10 ml of methylene chloride to a 
rapidly stirred mixture consisting of 0.41 g (0.0035 mole) 
cis,trans-2,5-dimethylpiperazine in 20 ml methylene chloride, 1.25 ml of 
5% sodium laurylsulfate solution and 15 ml of ice water containing 0.25 g 
(0.0125 mole) of sodium hydroxide. The resulting mixture was allowed to 
stir for about 20 hours. The turbid reaction mixture was then poured into 
water. There was obtained 1.02 g of white waxy solid when the mixture was 
filtered. 
The above product was dissolved in 10 ml of dioxane and to it was added 1.0 
g of sodium hexafluoroantimonate in 5 ml of water. The mixture was heated 
to 60.degree.-70.degree. C. for 3 hours and then poured into water. There 
was obtained 0.5 g of polyamide containing iodonium hexafluoroantimonate 
groups. Elemental analysis showed the polyamide had 3.5% by weight of 
antimony. 
In accordance with the procedure of Example 7, a heat curable blend was 
prepared having about 3% by weight of the polymeric iodonium salt. A gel 
was obtained after about 3 minute cure on the 155.degree. C. hot plate. 
EXAMPLE 9 
The onium salt catalyzed encapsulant composition of Example 1 was evaluated 
for its thermal mechanical properties in accordance with ASTM D2236-81. 
For this dynamic mechanical test procedure, 1/8".times.2.5".times.0.5" 
test bars were molded at 180.degree. C. and 1000 psi with a 3 minute cycle 
and post-baked for 16 hours at 180.degree. C., which correspond exactly to 
the treatment to the encapsulated device. Similar test bars were prepared 
from commercially available MP101S and MP150SG encapsulants from the Nitto 
company for the same measurements. Forced oscillation of a cured bar was 
used to measure storage modulus G', loss modulus G", and tangent delta 
(TD). 
The 2.5" by 0.5" by 1/8" bars were heated at 2.5.degree. C. per minute 
while a sinusoidal deformation (0.05 percent strain) was applied to the 
top of the bar in a Rheometrics Model No. RDS-7700 spectrometer. The 
dynamic mechanical test procedure measures the T.sub.g and stiffness of 
the cured-post baked sample. Stiffness is expressed by tangent delta (TD) 
which is shown by the following expression, 
EQU TD=(G")/G' 
where G" is loss modulus corresponding to the energy dissipated in the 
sample, and G' is the energy stored in the sample. 
The following results were obtained where runs 1-4 show the commercially 
available test bars and 5-8 show onium salt test bars: 
______________________________________ 
Post-.sup.a 
Molding Cure Peak 
Temp. Temp. T.sub.g, .degree.C. 
Tangent 
Encapsulant 
.degree.C. 
.degree.C. 
DMA.sup.b 
delta 
______________________________________ 
MP1015 (1) 180 180 203 .246 
(2) 180 225 208 .203 
MP1505G (3) 180 180 191 .236 
(4) 180 225 200 .201 
Onium Salt 
(5) 180 180 186 .054 
(6) 180 225 252 .080 
(7) 150 150 188 .118 
(8) 150 225 246 .074 
Steel Bar 0.00 
______________________________________ 
.sup.a all samples were postcured for 16 hours 
.sup.b dynamic mechanical analysis with Rheometrics 
Post-baking of test bars 5 and 6 to 225.degree. C. raised the T.sub.g from 
186.degree. to 252.degree. C. This shows increased cross linking and 
higher resistance to shape alteration. 
Although the above examples are directed to only a few of the very many 
variables which can be used in the practice of the present invention, it 
should be understood that the present invention is directed to the use of 
a much broader variety of monomeric and polymeric iodonium salts, epoxy 
resins and fused silica as well as to the compositions used thereby as 
shown in the description preceding these examples.