Reliability of area array pressure contacts on the DTAB package

Pressure contacts using noble metals have been in use for a long time. The contact metallurgy, the design, and the reliability are well understood and proven in a variety of applications, including relays, connectors, sockets, and other separable contacts. Pressure contacts in VLSI package applications are rather new. The large number of contacts at fine pitch and operation at high frequency and power present problems in design, materials, and reliability not encountered in other applications. Demountable TAB (DTAB) is a VLSI package, developed and qualified for high performance (≫100 MHz), high pincount (>400) ASIC´s with higher power dissipation (~40 W). It is a TAB-based package that utilizes area array, gold-to-gold pressure contacts to connect the tape to the printed circuit board, instead of the conventional reflowed solder joints. Extensive reliability testing has been used to optimize the design as well as to qualify the package for product applications. The formal tests have been extended beyond the industry standards to include system level tests, designed to stress the pressure contact under conditions not expected from other equivalent packages. Testing during the development phase revealed failure mechanisms that 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 of the printed circuit board under uneven distribution of clamping forces. Testing has also revealed that this package produces very high reliability sealed contacts, although thin gold of 5 μin thickness is used instead of the 50-μin thickness conventionally required by other applications. The test and testing methodology are discussed; the results and the failure modes and analysis are presented. The design changes and materials used to eliminate the failures are described