Chapter 17
Molecular Design of Epoxy Resins for Microelectronics Packaging
Downloaded by SUNY STONY BROOK on December 21, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch017
Masashi Kaji Research and Development Laboratories, Nippon Steel Chemical Company, Ltd., 46-80 Nakabaru, Sakinohama, Tobata-ku, Kitakyusyu-shi, Fukuoka 804, Japan
The relation between the structure and properties of epoxy resins were investigated to achieve the followings, (1) improved toughness, (2) low moisture absorption, (3) increased heat resistance, (4) low thermal expansion, and (5) decreased viscosity of epoxy resins for high filler loading. It was found that introduction of arigidgroup, such as 4, 4'biphenyl or 2, 6-naphthalene moiety, was an effective way to improve fracture toughness, and the naphthalene based resins were effective for lowering moisture absorption, increasing glass transition temperature, and lowering thermal expansion. Several epoxy resins of the bisphenol type with lower melting viscosity were synthesized and used as molding compounds for IC packaging. Crack resistance of the packages using these resins was markedly superior owing to highfillerloading due to their lower viscosity. Epoxy resins are extensively used as transfer molding compounds for plastic IC and LSI packaging. In this field, the package size has been steadily increasing while the thickness has been decreasing. Also the mounting method has changed from insertion mounting to surface mounting. These trends require development of new highperformance epoxy resins to overcome the problem of package cracking. Package cracking is due to moisture in the package (i). Moisture condensed between the bottom side of a die pad and a packaging material is expanded at the soldering temperatures of 215 to 260 "C. The resulting high pressure inside makes the package swell and start cracking at the edge of the die pad. The cracking mechanism indicates that there are several effective approaches for the base resin to overcome the problem of package cracking. They are: lowering of moisture content, increasing of toughness, increasing of grass transition temperature (Tg) in order to increase mechanical strength at soldering temperature, lowering of thermal expansion, and lowering of viscosity for high filler loading. 0097-6156/94/0579-0220$08.00/0 © 1994 American Chemical Society
In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
17. KAJI
Molecular Design of Epoxy Resins for Microelectronics Packaging
EXPERIMENTAL Phenol novolac was used as a curing agent and triphenylphosphine as a curing accelerator. Epoxy molding compounds obtained by compounding were molded at 150 *C for 3 minutes, followed by post curing at 180 for 16 hours. The conditions for characterization of the cured products are described in the previous paper (2). Cross-linking density was determined by using the kinetic theory of rubber elasticity (3).
Downloaded by SUNY STONY BROOK on December 21, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch017
RESULTS AND DISCUSSION Structural Modification of Epoxy Resins to Improve Toughness Although epoxy resins have good mechanical properties, their poorfractureresistance is one of their serious disadvantages. Resins having improved toughness show resistance against the yield caused by package swelling. There have been many studies on improving the fracture resistance of cured products by incorporation of elastomeric materials. Butadiene-acrylonitrile copolymers (4) or siloxane oligomers (5) were incorporated in the base resin in order to form a finely dispersed sea-island structure. But these methods often lead to lower Tg and deteriorate mechanical strength. Influences of cross-linking density andrigidnessof the resin backbone were investigated to improve the fracture toughness of the cured products without addition of elastomeric materials. Influence of Cross-linking Density. In general, thefracturetoughness of the cured product increases with decreasing cross-linking density of the network. However this causes Tg decrease. We can make a compromise between fracture toughness and Tg for the conventional resin systems (6). In order to increase Tg without increasing cross-linking density, bulky groups were incorporated into the resin structure (Fig. 1). An increase in the bulkiness increased Tg, which seemed to depend on the restricted molecular mobility of the resins. For the resin having thefluorenemoiety (No. 5 in Fig. 1), low values for tan δ were observed in the vicinity of 100 Ό by dynamic mechanical analysis (7), which were attributed to the restricted molecular mobility of the phenyl ring of the resin backbone. However, the fracture toughness decreased with increasing bulkiness (Fig. 2). The above compromise can still be reached by controlling cross-linking density based on the introduction of steric hindrance into the resin backbone. Influence of Rigidness of Resin Backbone. We examined next the effect of the rigidness of the resin backbone. Fig. 3 shows the relationship between cross-linking density andfracturetoughness for the bisphenol type resins. Thefracturetoughness of the cured product from the resin having biphenyi structure (No. 6) increased significantly without decreasing cross-linking density, with Tg maintained at more
In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by SUNY STONY BROOK on December 21, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch017
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POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS
than 170 °C. But, the cured product from the 2, 2'-disubstituted biphenyl isomer did not give higher fracture toughness, which gave a similar relationship as seen in the conventional resin systems. In the naphthalenediol based resin systems, an extraordinary behavior was observed (Fig. 4). The cured product from the 2, 6-isomer gave a significantly higher fracture toughness than those from the other isomers. The above results seem to indicate that the cured products from these resins having a symmetric and rigid backbone, such as 4, 4'-biphenyl and 2,6-naphthalene structure, have a different network structure compared with those from the conventional resin systems. For explaining these behaviors, we proposed a network model for the cured products (Fig. 5). The cured products from the resins having a symmetric and rigid backbone will have a larger free volumefractionthan those from the conventional resins, because the former have looser molecular packing due to steric hindrance resulted from their rigid backbone. The larger free volume fraction seem to cause large plastic deformation at the crack tip. Dynamic mechanical analyses revealed that the storage modulus of the cured product of the diglycidyl ether of 4, 4'-dihydroxybiphenyi was smaller than that from the 2, 2'biphenyl isomer at the glassy state. And the specific gravity of the cured product of diglycidyl ether of 4,4'-dihydroxybiphenyl was smaller than that from the 2, 2'biphenyl isomer which had a flexible bending backbone (Table I). These observations presumably could be explained by the largerfreevolumefractionof the cured products from the resins havingrigidbackbone.
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In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
17. KAJI
Molecular Design of Epoxy Resins for Microelectronics Packaging _ 1.4
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Downloaded by SUNY STONY BROOK on December 21, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch017
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a) A cured resin b) A cured resin having rigid having flexible backbone backbone Fig. 5. Network models of cured resins. • ; crosslinking point " i ; resin backbone
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Table I. Specific Gravities of Epoxy Cured Products No. epoxy specific gravity resin (g/cc) 4, 4'-isomer 1.18 2 2, 2'-isomer 1.20
Lowering of Moisture Absorption Moisture absorption is one of the most critical factors causing the package cracking. Packages are sometimes suppliedfromvenders in dry shipping containers, or they may be baked before soldering. These treatments have no industrial merit (8). As the amount of water absorbed in a package depends on the properties of the base resins used, lowering of moisture absorption is a strong demand for epoxy resins. Introduction of Polynuclear Aromatics. It is reported that moisture content increases with increasing cross-linking density for the o-cresol novolac epoxy resin system (9). For the conventional resin system, lowering of moisture absorption limit by decreasing cross-linking density would then leads to a decrease in Tg. The amount of water has a close relation to the content of the functional group, i.e., phenolic
In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by SUNY STONY BROOK on December 21, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch017
224
POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS
hydroxyl groups in a curing agent used (10). An increase in hydrophobicity of a resin is considered to be a good approach to decrease moisture absorption of a cured product. Introduction of condensed poly nuclear aromatic moieties, e.g., naphthalene and anthracene rings, and so on, is an effective approach to lower moisture absorption without decreasing Tg. Synthesis and properties of the naphthalene based epoxy resin are reported (11). This resin was prepared by replacing the o-cresol moiety with the 1naphthol moiety in the o-cresol novolac epoxy resin. Although the cured product showed improved heat and humidity resistance, the resin had a higher melting viscosity, which spoils moldability as a molding compound. No description about the relationship between the structure and properties of the cured product is given. Several kinds of the naphthalene based epoxy resins were synthesized through condensation reaction of naphthalene compounds with condensing agents in the presence of acid catalyst, followed by glycidyl etherification with epichlorohydrin (scheme 1), and the relationships between the structure and properties of the cured products were studied.
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