Gold (Au) Embrittlement of Plastic Quad Flat(Pack), No-Lead (PQFN) Solder Joints and Mitigation Strategies
Authors: P. T. Vianco, T. Garcia, C.E. Jaramillo, B.M. McKenzie, and J. Reese Company: Sandia National Laboratories Date Published: 9/22/2019
Abstract: Gold (Au) embrittlement remains as a significant concern for the high-reliability electronics community. A study was undertaken to address this phenomenon for the solder joints belonging to a 48I/O plastic quad flat no-lead (PQFN) package. The PQFN had an electroplated Ni-Au surface finish on its bond pads. The PQFN solder joints were evaluated with respect to one of three conditions (two-letter designations): (a) as-fabricated (NT), (b) reflowed on an alumina plate with solder added using a printed paste pattern (DS); and (c) exposure of the bond pads to a selective soldering fountain (SW). The PQFNs were then attached to a PCB having an ENEPIG surface finish, using a 63Sn-37Pb (wt.%) solder paste and typical reflow profile. Selected segments of each test vehicle variant were aged at 70°C and 100°C for periods of up to 100 days. Seven (7) PQFNs were shear tested at a rate of 1.2 mm/min. Two packages were subjected to metallographic cross section and microanalysis. Each of the three solder joint categories exhibited a small decrease of strength between aging times of 25 and 50 days at 70°C and 100°C, respectively. The PQFN solder joints, which did not have Au mitigation (NT) applied to them, had shear strengths that continued to decline through the 100 day aging duration. Microanalysis showed that the NT joints experienced extensive solid-state diffusion and reaction activity between Au and Sn that generated large voids in the solder joint. The proposed mechanism was a combination of concentration gradients and unequal diffusion mass transfer that led to Kirkendall voids. Although the DS PQFN solder joints contained the equivalent amount of Au as the NT PQFN case, the absence of the concentration gradients mitigated the development of voids and accompanying loss of shear strength. This study illustrated that the secondary effects of a high Au content in the solder joints, when subjected to solid-state aging while in service, can jeopardize the reliability of the interconnections more so than simply the bulk embrittlement based on weight-percent Au concentration.