The influence of nickel/gold surface finish on the assembly quality and long term reliability of thermally enhanced BGA packages

Increasing numbers of chip carrier packages such as BGA and CSP use electroless nickel/immersion gold (ENi/IAu) surface finishes. However, the results of several studies attribute solder ball brittle failure to this surface finish. Brittle fractures have occurred in service and have been generated under various testing conditions. The typical failure mode is brittle interfacial fracture between the Ni plating and the intermetallic compound formed during soldering. Brittle interfacial failures are unpredictable and catastrophic but occur at relatively low defect levels. Discounting premature or infant mortality failures, ENi/LAu components can exhibit satisfactory long term reliability performance. This behavior is indicative of assembly quality or package quality defects, not true wear-out failures. This paper compares the results of quality and reliability testing of three sets of 560 I/O thermally enhanced BGA packages. Two sets were constructed with substrates using ENi/IAu surface finishes and the third is constructed with substrates using a typical electrolytic Ni/electrolytic Au surface finish. Ball shear tests, optical microscopy and SEM are performed on as-received and thermally preconditioned packages to evaluate package quality prior to assembly. Accelerated thermal cycling is used to evaluate long term reliability. Failure mode analysis is used to compare fracture modes in the ball shear and thermal cycled samples in the electroless and electrolytic packages. The results are discussed in terms of proposed mechanisms for brittle fracture and known characteristics of the thermally enhanced packages.