Written by the United Test & Assembly Center Ltd (UTAC) in Singapore this paper explores the reliability of Lead versus Lead free soldered IC’s during drop testing.

Abstract

Portable electronic products such as cellular phones, PDAs, and MP3 units are increasingly designed for accidental drop. Repeated drop events can lead to solder joint failure and malfunction of these products. Hence board-level reliability drop testing is a useful way to characterize the drop durability of the printed circuit board with different soldered assemblies. Lead-free (or Pb-free) solders are replacing lead-based solders. Surface mounted electronic packages are getting smaller and with higher density (I/Os). In this study, Plastic Ball Grid Array (PBGA), Quad Flat No-lead (VQFN) and PQFP solder joint reliability characterization by drop impact testing was investigated for lead-based (62Sn-36Pb-2Ag) and lead-free (Sn-4Ag-0.5Cu) soldered assemblies.

The influence of different package types and the impact of PCB surface finishes for OSP and ENIG (electroless nickel immersion gold) were studied. The drop test results showed that lead- free solder joints with ENIG finish have weaker drop reliability performance than the case for OSP surface finish. The different solder alloy-to-surface finish type combination results in different intermetallics formed and contributed to different failure sites and mode of failure.

The formation of the brittle intermetallics (Cu-Ni-Sn) for Pb-free solder (Sn-4Ag-0.5Cu) with ENIG pad led to detrimental interfacial fracture of the PBGA solder joints. For both lead-based and Pb-free solders with copper-pad and OSP surface finish, the formation of Cu6Sn5 intermetallics resulted in a different failure site and mode. The failures migrated to the PCB copper traces and resin layers.

The small size (VQFN) package is most resistant to drop impact failures, due to it’s small size, weight and larger solder volume. The compliant leads for the PQFP is more resistant to drop failures compared to the PBGA solder joints.

Drop Impact Reliability Testing for Lead-Free and Leaded Soldered IC Packages