Evaluation of Potential Printed Wiring Board Materials: Thermoplastic Polyimide + Polymer Liquid Crystal Blends
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Mat. Res. Soc. Symp. Proc. Vol. 515 0 1998 Materials Research Society
addition of a PLC also enables the control of the thermal expansivity. The expansivity differences between the chip and the printed wiring board (PWB) can result in fatigue failure of the joint over the service life 4. Numerous attempts have been made to solve the expansivity mismatch problem, typically using various non-conductive adhesives '. However, most of the materials studied were epoxybased, causing two new problems: moisture absorption and lack of reworkability of the finished
product
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Apart from PLCs, there is another class of materials of interest for the problem at hand: polyimides have found application in electronics and microelectronics, mainly due to their high temperature stability and good performance. Unlike other polar polymers, polyimides exhibit very low moisture absorption. The major limitation of polyimides has been their thermosetting nature, eliminating their reworkability. Following research at NASA Langley Research Center (LaRC), thermoplastic polyimides (TPIs) catering to varying needs of industry, from low glass transition temperature (Tg) polymers to high degradation resistance, semicrystalline, amorphous, and even liquid crystalline TPIs are currently known. Apart from high temperature stability and high performance, polyimides exhibit very good adhesion to metals, which is a desirable characteristic for the microelectronics applications. To take advantage of the useful properties of both TPIs and PLCs, we have decided to investigate TPI + PLC blends. The work reported here was performed in two phases. The first phase of the project was the determination of the composition range of interest. A semicrystalline TPI + a glass fiber reinforced PLC was used for this purpose. The thermo-irreversible crystallization of that TPI causes an undesirable change in the morphology, resulting in poor adhesion to metals. In the second stage, therefore, we have switched to the same TPI but in its amorphous form. To achieve better control of the expansivity of the blends, the PLC without the glass reinforcement was used. The film-forming capability of the blends as well as the effect of the PLC addition on the expansivity of the TPI have been studied. Experimental Materials The semicrystalline TPI (Aurum 450G) and the amorphous TPI (Aurum 500M) were obtained from Mitsui Toatsu Chemical Co./ MTC America, New York City. Aurum TPIs are known to be based on pyromellitic dianhydride and 4,4'-bis(3,3'-aminophenoxy)-biphenyl 7-9. The PLC used in blends with the semicrystalline TPI was Zenite 7130, from E. I. du Pont de Nemours & Co., Wilmington, DE, containing 30 % glass by weight. Zenite 7130 is described by the manufacturer as a "wholly aromatic polyester". After the evaluation of semicrystalline TPI + PLC blends, a narrower composition was chosen (0, 5, 10, 30% of PLC by weight of TPI) for further detailed analysis. In this phase, the amorphous TPI was blended with Zenite 7100, also from du Pont. This PLC does not contain gla
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