THE APPLICATION OF BI-BASED SOLDERS FOR LOW TEMPERATURE REFLOW TO REDUCE COST WHILE IMPROVING SMT YIELDS IN CLIENT COMPUTING SYSTEMS

摘要

The continued miniaturization of personal computing systems has a significant impact on the ability to surface mount high I/O density component devices with high yield. To ensure complete solder joint melting, typical SnAgCu (SAC) solder reflow temperatures peak in the 245 to 260C range. At these temperatures, the mismatch in Coefficient of Thermal Expansion (CTE) of the key constituents in the system, primarily the PCB and BGA components, results in dynamic warpage that leads to both bridging and open solder joint defects. The use of low temperature Bi-based solder paste reduces the peak reflow temperatures below 200C at which point The magnitude of the dynamic warpage is reduced and this improves SMT yield. Additionally, there are further positive by-products of low temperature reflow such as measurable energy savings, reduced carbon footprint and the opportunity to use lower cost components. Although Bi-based solders are common in adjacent markets such as flat screen TVs and appliances, it has been avoided in mobile computing applications due to the brittle nature of Bi alloys. This brittleness needs to be overcome before the use of such Bi-Sn low temperature solders can be extended to other market segments. These three aspects of the use of low temperature solder pastes for electronic assembly were evaluated in the present study. Firstly, the impact of lower FCBGA package warpage at the lower peak reflow temperatures on FCBGA package solder joints was determined by assemblying packages with both nominal and excessive warpage using BiSnAg and SAC305 solder pastes. Secondly, a practically study was undertaken to assess the extent of power use savings by using BiSn-based solder pastes instead of SAC305 solder paste. Thirdly, to improve the ductility of the brittle of Bi-Sn solders, some solder paste manufacturers added dopant elements to the alloy to modify their microstructure. The lower peak reflow temperatures with BiSn based solder paste resulted in significantly better solder joint yield for both the nominal and excessive warpage FCBGA packages, even though the SAC solder ball did not fully collapse at these low temperatures. The estimated cost savings of the low temperature operation was determined to be $168/oven/week or $8,749/oven/year. The ductile BiSn solder metallurgy compositions were found to be better in cold ball pull and component shock/drop exposures but still not up to par with the SAC solder joints.