Chapter 35
Degradation of Brominated Epoxy Resin and Effects on Integrated-Circuit-Device Wirebonds
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M. Nakao, T. Nishioka, M . Shimizu, H. Tabata, and K. Ito Nitto Denko Corporation, 919 Fuke-cho, Kameyama-shi, Mie, Japan
The gases that are emitted when an encapsulant resin is exposed to a high-temperature were analyzed by GC-MS. The effect of these gases on IC devices were studied using a Teflon vessel. The result showed that large amounts of bromides and organic acids are emit ted from the encapsulant resin and that these gases are generated by the accelerated decompositions of the brominated epoxy resin by amine and silicone in the resin. The addition of certain ion trap ping agents to the resin was found to solve this problem and to increase the high-temperature durability several fold.
Semiconductor devices are being used in an ever increasing number of applica tions and under very severe environmental conditions. These applications have brought about the use of new evaluation methods. One such method is the high-temperature stress test, and many reports have been published discussing this topic (1-3). According to these reports, a semiconductor device fails during high temperature exposure when the brominated resin contained in a resin forms intermetallic alloys at the gold-aluminum interface, thereby increasing resistance (2). However, there are still unanswered questions about the mechanism leading to the generation of intermetallic alloys, and an explanation is yet to be pro vided for the difference in degradation rates among IC devices containing like amounts of brominated resin. Objectives The gases that are generated when an encapsulant resin is exposed to a high temperature were analyzed by GC-MS to disclose the cause for resin degradation and chip failure. This paper describes the cause for chip failure, and also presents a method for controlling the formation of intermetallic alloys. 0097-6156/89/0407-0421$06.00/0 ο 1989 American Chemical Society
Lupinski and Moore; Polymeric Materials for Electronics Packaging and Interconnection ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
422
POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION
Experimental and Results Corrosion of IC Devices by Decomposition Gases of Encapsulant Resin and Formation of Intermetallic Alloys. To study the effects on IC devices of gases that are generated by the thermal decomposition of encapsulant resin, encapsulant resin in powder form and wire-bonded IC devices were placed inside a Teflon vessel (Figure 1). The vessel was left standing for 20 hours in an oven heated to 225 ° C to observe the change in the IC device. The chip that was used is shown in Figure 2. Aluminum patterns were formed on a silicon substrate. No passivation film was formed. Corrosion of aluminum pads and formation of intermetallic alloys near gold balls were observed, indicating the strong effects of the discharged gases (Figure 3). Analysis of Thermal Decomposition Gases of Encapsulant Resin. The gases which were collected as shown in Figure 4 were analyzed by GC-MS (using model QP-1000 manufactured by Shimazu Seisakusho). The components are shown in Table I. The Table shows emission of large amounts of bromides and other highly corrosive gases which are know to degrade IC chips. The accelerated decomposition of brominated epoxy resin by other materials contained in the encapsulant resin was studied by GC-MS from the amount of methylbromide that was emitted from the resin. The Teflon vessel used in the previous experiment was again used to study the corrosion of the device. The results are shown in Table II. The results show that the decomposition of brominated epoxy resin is significantly accelerated by amine and silicone. Effects of Ion Trapping Agents. The effects of ion trapping agents were studied in experiments that used a Teflon vessel and gas analysis by GC-MS. Table III shows the results. The ion trapping agents that were used in this experiment were bismuth hydroxides. Actual encapsulant resins were exposed to high temperature tests to measure the generation of intermetallic alloys and the degradation of the device. The results are shown in Table IV. The generation of intermetallic alloys was measured by the cross section of the gold balls (Figure 5). The lifetime was determined from the time when an open failure in resistance was observed with the device. The results showed that the resin properties are greatly improved by the addition of ion trapping agents. Discussion Degradation of the Brominated Resin. From past studies, it is believed that the generation of intermetallic alloys is triggered by methylbromide (or hydrogen bromide) (1-3). The generation of these substances from brominated epoxy resin is discussed below. The thermal decomposition of the brominated epoxy resin depicted in Figure 6 occurs in the following manner (Figure 7). Because the reaction is a radical reaction, significant amounts of energy are required. However, if a nucleophilic reagent such as amine is present in this system, bromide ion easily separates and forms bromide complexes as shown in Figure 8. Furthermore, silicone undergoes an oxidation decomposition at high temperature, and generates formic acid and formaldehyde (Figure 9). Formic acid
Lupinski and Moore; Polymeric Materials for Electronics Packaging and Interconnection ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
35. NAKAO ET AL.
Degradation ofBrominated Epoxy Resin ' Lead Frame with Wire-Bonded Chip
[*-Teflon Vessel Teflon Mesh Sample Figure 1. Experimental setup.
CHIP SIZE 2X3.5MM AL PATTERN DISTANCE (A) 10/*n
(B) 20^m (C) 50//m AL PATTERN WIDTH 30μπλ AL PATTERN THICKNESS
1.2//m Figure 2. Test chip specimen.
(without Encapsulant Resin)
(with Encapsulant Resin)
Figure 3. Chip surface after high-temperature exposure test.
Lupinski and Moore; Polymeric Materials for Electronics Packaging and Interconnection ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
423
424
POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION
0ven:225'C X 2 0 h
ΐ>
G C -
MS
Sample Powder (5g)
Figure 4. Analysis of outgas from specimen with GC-MS.
Table.
I
C o m p o n e n t s of O u t g a s
from Molding C o m p o u n d ( d y Component C a r d o n Dioxide 1-Propene & other H y d r o c a r b o n s Methyl Chloride & other C l - C o m p l e x e s Methyl Bromide & other B r - C o m p l e x e s Acetaldehydc & other Aldehydes Formic Acid & other Organic A c i d s Methanol & other A l c o h o l s Methyl Formate & other Esters S:Strong
MrMedium
GC-MS)
Strength
W W W S s w
M M W:Weak
Lupinski and Moore; Polymeric Materials for Electronics Packaging and Interconnection ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
NAKAO E T Α Κ
Table, o
Degradation of Brominated Epoxy Resin
Effects of Ingredients in Molding Compounds on Degradation of B r - E p o x y
^ ^ ^ ^ ^ ^
RESULT
Effect of G a s
Outgassed CH,Br (ppm)
Br-Resin Br-Resin+ Epoxy-Resin Br-Resin+ Phenolic-Resin
Au-Ball
Al-pad
120
M
M
100
M
M
110
M
M
Br-Resin+Silicone
1200
S
s
B r - R e s i n + Amine
1800
S
s
Br—Resin+Filler
50
W
w
Br-Resin+Sb,0,
30
W
W
S:Strong
M:Medium
W:Weak
T a b l e , m Efîects of Ion Trapping Agent on the Degradation of B r - E p o x y Resin RESULT SAMPLE
Effect of Outg as
Outgassed CH,Br (ppm)
Au-Ball
Al-Pad
120
M
Br-Resin+Silicone
1200
S
S
B r - R e s i n - f Amine
1800
S
S
30
No
No
20
No
No
Br-Resin
B r - R e s i n + Silicone + X Br-Resin+Amine + X S:Strong
a b l e ,
iv
M:Medium
W:Wcak
X : i o n T r a p p i n g Agent
E f f e c t s of Ion T r a p p i n g on W i r e - ^ B o n d
^ - ^ R E S U L T SAMPLE^.
M
Agents
Degradation
Intermeta lies Layer Ratio (by a) (%) 100h 50h
Compound A
40
75
Compound Β
10%·-Fail Time (h) 200*C 225*C 95
960
70
100
70
340
C
0
0
310
>2000
Compound D
5
10
220
>2000
Compound
C o m p o u n d A:Silicone-less Resin C o m p o u n d B:Silicono-Modified Resin Compound C:Comp.A+X Compound
D:Comp.B+X
Lupinski and Moore; Polymeric Materials for Electronics Packaging and Interconnection ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
426
POLYMERS FOR ELECTRONICS PACKAGING AND
Intermetallic
INTERCONNECTION
Au-Ball
Layer
^Al-Pad b
*- H —
a
>
Si-Chip
Figure 5. Intermetallic layer ratio in the bonding pad area.
CH
CH,
CH ^
CH^ CH, ό
CH^ CH, ό
CH^ CH ό
Br
Br
2
3
2
Br
Figure 6. Br-epoxy resin.
RH
+Br«
R.
R-
-fO
ROO-
a
ROO.+RH ROOH R'Br
+R.
+HBr
ROOH+R. • RO ·
+ · OH
• R'-R
+Br ·
Figure 7. Br-epoxy degradation process.
...very fast.
Figure 8. Reaction acceleration mechanism of amine (nucleophilic reaction of amine).
Lupinski and Moore; Polymeric Materials for Electronics Packaging and Interconnection ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
35. N A K A O E T Α Ι ^
Degradation ofBrominated Epoxy Resin
427
that is generated in the reaction is a strong nucleophilic reagent, and accelerates the degradation of brominated epoxy resin and generation of methylbromide as shown in Figure 8. Intermetallic Alloy Formation Reaction. Intermetallic alloys such as A u ^ A l or A u A l are formed at the gold aluminum interface as early as when gold balls are wirebonded to aluminum pads. However, this is not a problem at room tem perature, because the rate of formation of the intermetallic alloys is very low at room temperature (4-6). But if bromide ions (Br") are present in this system, many different intermetallic alloys are formed very quickly following the reaction shown in Figure 10. The generation of intermetallic alloys increases the intera tomic distance ( A u , A l : 4.0 Angstroms; A u - A l complex : 6.0 Angstroms) (5). This increases the volume of the intermetallic alloys. The shear force generated by the expansion is transmitted to the gold ball/aluminum pad interface, generat ing the voids. Because aluminum bromide, which is formed by this reaction, has a low melting point, and the boiling point is near the storage temperature (Table V ) , the aluminum bromide volatilizes and diffuses immediately as it is generated. The above described reactions occur simultaneously, resulting in void growth. 2
Effect of Ion Trapping Agents. Ion trapping agents react with hydrogen bromide (Figure 11) and fix the bromide ion. Because the bismuth bromide that is formed in the above reaction has a higher melting point and a higher boiling point than the storage temperature, bromide ions cannot separate from the com plex. This reduces the formation of intermetallic alloys and extends the device's life.
i
i
I
I
CH,-Si-CH + 0 ,
- HCHO + HO-Si-CH
3
I Ο
3
I Ο
0
2
HCHO
• HCOOH Figure 9. Silicone oxidation mechanism.
AuAl
2
2A1 AuAI AuAl
2
4- 6 B r — A u + A I B r + 6 e 2
+ Au
— AuAl
+ Au
— 2AuAI
-f- Au
->Au AI
2Au AI + Au
— Au Al
2
e
2
2
5
2
Figure 10. Au/Al intermetallic alloy formation reaction.
Lupinski and Moore; Polymeric Materials for Electronics Packaging and Interconnection ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
428
POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION Table, v
M e l t i n g P o i n t a n d B o i l i n g Point of M e t a l B r o m i d e Metal Bromide
m.p.
b.p.
AIBr
3
97.5'C 225'C
BiBr
3
218'C 453*C
Bi(OH) + H B r - B i ( O H ) B r + H 0 3
2
2
Bi(0H)Br -f H B r — B i B r - f H 0 2
3
2
Figure 11. Ion trapping reaction.
Conclusion The study of high temperature exposure of encapsulant following points:
resin has disclosed the
1.
G C - M S analysis of the gases emitted during high temperature exposure of the encapsulant resin disclosed the emission of many bromides and organic acids.
2.
These gases are formed by the degradation o f the brominated epoxy resin.
3.
A m i n e and silicone contained in the encapsulant degradation of brominated epoxy resin.
4.
Bromide ions that separate from bromide compounds accelerate the degradation at the gold aluminum junction.
5.
The addition of certain inorganic ion trapping agents to the encapsulant resin improves the high temperature exposure durability of encapsulant resin.
resin
accelerated
the
Literature Cited 1. 2. 3. 4. 5. 6.
Gale, R. J. IEEE Int. Reliab. Phys. Symp. 1984, p 37. Blish II, R.C.; Parobek, L. IEEE Int. Reliab. Phys. Symp. 1983, p 142. Khan, M.M.; Fatemi, H. Proc. Int. Symp. Microelec. , 1986, p 420. Philofsky, E. Solid-State Electronics ; 1970, 13 , 1391. Tomioka, H.; Kitamura, H.; Ueda, S. J. Metal Fin. Soc. Jpn. 1987, 38 , 199. Plunkett, P. V.; Salporto, D. F. IEEE Proc. 32nd. Elec. Comp. Conf. 1982, p 421.
RECEIVED March 22, 1989
Lupinski and Moore; Polymeric Materials for Electronics Packaging and Interconnection ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
