The effect of gold Au on the reliability of 0.65 mm pitch surface mount solder joints between plastic quad flat packs and CuNiAu FR4 printed circuit boards was investigated. CuNiAu is a desirable printed circuit board finish for multichip modules or printed circuit boards that would otherwise require a selective Au finish, for example for edge connectors or wire bondable parts. However, Au is known to embrittle solder when it is present in sufficiently high concentrations, creating a concern that solder joint fatigue life in service will also be adversely affected. This paper reports the results of mechanical shock, mechanical vibration and thermal cycling testing of fine pitch solder joints containing varying amounts of Au. Tests were performed on assoldered joints and on joints that had been heattreated to evolve the microstructure towards equilibrium. The tests were designed to accelerate inservice conditions in a typical industrial environment. Under these conditions, the Au concentrations tested did not promote solder joint failures. Microstructural characterisation of the distribution and morphology of the Au, Ni and CuSn intermetallics in the joint before and after accelerated testing was also performed. On the basis of these observations it is recommended that the Au concentration in solder joints between plastic quad flat packs and CuNiAu FR4 printed circuit boards not exceed 3.0 wt..
Circuit World – Emerald Publishing
Published: Mar 1, 1992
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