Have High Cu Dissolution Rates of SAC305/405 Alloys Forced a Change in the Lead Free Alloy used During PTH Processes?

ABSTRACT To date, the majority of the Electronics Manufacturing industry has implemented either SAC305 or 405 alloys to manufacture Pb-free (E.U. RoHS, compliant) products, for both the SMT and PTH card assembly processes. This original alloy composition choice, dating back to 1999, was based on early research into the metallurgy and reliability of the alloy/s and agreement amongst top companies involved in iNEMI, JEITA and within the European Union. A recent shift towards SAC305 has recently been observed within the industry due to its lower cost when compared to SAC405, based on lower silver content. Historically, reliability assessment efforts have focused on SMT solder joint connections as PTH solder joint connections were not typically a reliability concern due to their construction. Conceptually, it is easier from a process and supply chain point of view to use a single Pb-free alloy for both the SMT and PTH attach process. However, issues relating to high rates of copper (Cu) dissolution occurring during the PTH rework process using either SAC305 or 405 alloys may force a change in this concept. The high Cu dissolution rates experienced when using SAC305/405 may dictate a change in the Pb-free alloy used during the PTH rework process, in order for typical methods of rework (i.e. solder fountain) to continue to be used. However, making a change in the Pb-free alloy used for only the PTH rework process itself creates new questions which would need to be answered. For instance, what is the impact of reworking a SAC305/405 assembled connector using an alternate Pb-free alloy? Is changing the Pb-free alloy used within the primary attach process to match the PTH rework alloy the right solution? This leads to further questions relating to process controls and reliability of a final “mixed” Pb-free joint as well as the “pure” alternative Pb-free alloy selected which would need to be addressed. This paper discusses the work performed studying and comparing the Cu dissolution rates of various Pb-free alloys available on the market today. Although the use of a PCB with a nickel plated layer can reduce the occurrence of Cu dissolution, all experiments in this study were performed on an OSP finished board. Finishes such as OSP which do not have a nickel later represent the worst case scenario with respect to Cu dissolution. An OEM'server product was used as the test vehicle throughout this study. A total of six Pb-free alloys and a eutectic tin/lead (Sn-Pb) control alloy were included in the evaluation. Specifically, two binary eutectic and four ternary “near eutectic” Pb-free alloys were included. Each of the “alternative Pb-free alloys” studied include varying levels of certain elemental additives. Common additives included in some of these alloys are, nickel (Ni), germanium (Ge), bismuth (Bi) and antimony (Sb). This paper also includes a brief metallurgical analysis into the effects of adding each of these above additives. In addition, both time zero analysis and ATC (0-100°C) thermal reliability analysis of the Sn-Cu + Ni solder vs. SAC405 will also be discussed. Finally, the manufacturing impact when altering the Pb-free PTH alloy will be briefly discussed, including process control, contamination, cost and supply chain considerations. There is enough data to indicate that several alternative Pb-free alloys available on the market today are suitable replacements to SAC305/405 for PTH solder fountain rework, allowing up to a 2X rework process. In addition, the cost of these alloys warrants further study into also replacing the wave solder alloy as a cost reduction to SAC305/405.

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