Publications

Microstructural evolution in electronic 63Sn-37Pb/Cu solder joints

Vianco, P.T.; Rejent, Jerome A.; Kilgo, Alice C.

The 63Sn-37Pb (wt.%, designated Sn-Pb) solder interconnections made to copper (Cu) pads were examined on two printed wiring assemblies (PWAs) that had been in the field for 17 years and subsequently exposed to an accelerated aging test environment. A qualitative assessment of the solder joints indicated that there was excellent solderabiliry of the pins and Cu pads. Void formation was minimal or did not occur at all. Manufacturing defects were limited to minor Cu pad lifting with cracks in the underlying epoxy resin and local areas of Cu barrel separation from the laminate hole wall. Both defects would not have influenced the effects of the accelerated aging environment. A quantitative analysis examined the intermetallic compound (IMC) layer thickness of selected components on the PWAs. The IMC thickness data indicated that the PWAs were exposed to considerably lower, cumulative temperatures inside the product assembly than were present outside as a result of the accelerated aging environment. The quantitative analysis also evaluated the Pb-rich phase particle size in both fillets and the hole region of the PWA solder joints. The Pb-rich phase size confirmed that the temperature environment at the PWA level was significantly less severe than that of the accelerated aging environment. The Pb-rich phase size data indicated that the solder joints were exposed to a limited degree of thermal mechanical fatigue (TMF) that likely originated from the nominal temperature fluctuations coupled with the thermal expansion of the encapsulant as well as large expansion of the circuit board laminate in the z-axis (through-thickness) direction. This study demonstrated the methodology by which, IMC thickness and Pb-rich phase size were used to assess the temperature/time conditions experienced at the Sn-Pb/Cu interconnection level versus the external environment. Copyright © 2006 ASM International®.