Reliability of Pb-Free DIMMs Assemblies on OSP and Ni-Au PWB Finishes

摘要

Eutectic Sn-Pb solder became a de-facto standard in the electronic industry because of its unique combination of material properties and low cost. In recent years, environmental concerns have been raised regarding the use of Pb-containing solder in electronic products. The European community has a proposed Restriction on Hazardous Substances (RoHS) Directive that restricts the intentional use of Pb in electronic products as early as January 1, 2006. In addition, movement away from Pb-bearing solder is advancing due to competitive pressures in consumer electronics and concerns about the Pb in discarded electronic products. In this context, it is critical for Hewlett Packard (HP) and other electronics manufacturers to investigate the feasibility and the reliability of Pb-free products. In this study, Dual Inline Memory Modules (DIMM) have been assembled using a Pb-free manufacturing process. Boards with both organic solder preservative (OSP) and electroplated Ni-Au surface finishes were studied to evaluate the impact of PWB finish on solder joint reliability. The Pb-free solder paste used in the SMT process was 95.5Sn-3.9Ag-0.6Cu in a no-clean formulation. This study focused on Thin Small Outline Packages (TSOPs) with leads coated with 85Sn-15Pb to create "Pb-reduced" solder joints. The absence of delamination inside the packages was observed using acoustic microscopy, after the components were assembled at a peak temperature of 245°C. Pull testing was conducted on the leads after the attachment of the leaded components to investigate the mechanical resistance of the joints. A dye-penetrant analysis was performed after thermal cycling to assess the resistance of the Pb-reduced joints to thermal fatigue in comparison to Sn-Pb joints. Finally, one of the DIMMs assemblies was exposed to cold temperatures (-50°C for 6 months) to investigate the sensitivity of the Pb-reduced joints to tin pest. The data suggest that Pb-reduced DIMMs assembled with both OSP and electroplated Ni-Au PWB finishes are mechanically sound. Pull testing confirmed that the joints have excellent ambient temperature strength for both PWB finishes compared to the Sn-Pb control units. The resistance of the solder joints to thermal fatigue appears to be enhanced compared to the Sn-Pb control units. The best results were obtained with the electroplated Ni-Au surface finish. The kinetics of the tin pest phenomenon appear to be too slow to degrade the reliability of the joints during the storage and service life of the assembly.