RELIABILITY OF AREA ARRAY PRESSURE CONTACTS ON THE DTAB PACKAGE

Pressure contacts using noble metals have been in use for a long time. rhe contact metallurgy, the design, and the reliability are well unde rstood and proven in a variety of applications, including relays, conn ectors, sockets, and other separable contacts. Pressure contacts in VL SI package applications are rather new. The large number of contacts a t fine pitch and operation at high frequency and power present problem s in design, materials, and reliability not encountered in other appli cations. Demountable TAB (DTAB) is a VLSI package, developed and quali fied for high performance (>> 100 MHz), high pincount (> 400) ASIC's w ith higher power dissipation (approximately 40 W). It is a TAB-based p ackage that utilizes area array, gold-to-gold pressure contacts to con nect the tape to the printed circuit board, instead of the conventiona l reflowed solder joints. Extensive reliability testing has been used to optimize the design as well as to qualify the package for product a pplications. The formal tests have been extended beyond the industry s tandards to include system level tests, designed to stress the pressur e contact under conditions not expected from other equivalent packages . Testing during the development phase revealed failure mechanisms tha t relate to the mechanical design of the spring system that maintains the force on the contacts; materials relaxation over extended periods of time under severe environmental conditions; and creep deformation o f the printed circuit board under uneven distribution of clamping forc es. Testing has also revealed that this package produces very high rel iability sealed contacts, although thin gold of 5 muin thickness is us ed instead of the 50-muin thickness conventionally required by other a pplications. The test and testing methodology are discussed; the resul ts and the failure modes and analysis are presented. The design change s and materials used to eliminate the failures are described.