Impact of Low Silver Paste on Area Array Joint Quality
This paper provides analysis to "fill in the gaps" concerning the implementation for a wide variety of lead-free solder alloys.
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Authored By:
Srinivasa Aravamudhan, Raiyo Aspandiar, Ph.D., Lilia May, Ph.D.,
Suddhasattwa Nad, Scott Mokler, and Dudi Amir
Intel Corporation
Hillsboro, OR USA
Summary
Reacting to increased silver metal costs in recent years,Personal Computer (PC) board assemblers are converting to low Ag SAC solder pastes, and transitioning away from the widely used SAC305 (Sn-3.0wt%Ag-0.5wt%Cu)composition. Previously, a few industry studies demonstrated similarities between low Ag SAC andSAC305 solder pastes in printability, slump, solder balling,and voiding [1-2]. However, no data has been published on the effect of using low Ag SAC solder paste on solder joint quality. Solder paste properties dictated by both flux composition and solder powder metallurgy are major determinants of solder joint defects such as head-on-pillow(HoP), non-wet open (NWO) and solder ball bridging(SBB) defects. In this study, flip chip ball grid array(FCBGA) packages were used to induce solder joint defects formation.
These packages were then assembled using low Ag SAC solder pastes as well as standard SAC305 solder pastes from various suppliers. To precipitate failures caused by the dynamic warpage of the packages, the solder paste printed volumes were maintained at the low end of acceptable ranges, and air atmosphere was employed in there flow oven. The surface mount technology (SMT)assembly solder joint yield was determined after reflow soldering from electrical testing of the daisy chain loops, as well as by failure analysis, which indicated the mode of solder joint defects. Analysis of the defects showed that the solder paste flux system was dominant over alloy metallurgy for determining solder joint yield. Moreover, to maximize solder joint yield when using low Ag SAC solder pastes, the peak reflow temperatures will need to be adjusted to higher levels than for that used for SAC305solder pastes.
Conclusions
Results from the study show that the selection of the right solder paste flux system is critical for low Ag or zero Ag solder alloy metallurgies to influence SMT process yields.Additionally, an increase in peak reflow temperature may be required to improve SMT yield and reduce solder joint defects. Due to the increase in liquidus temperature for low Ag and zero Ag, the minimum peak temperature should be increased based on the alloy used for the SMT assembly.However, an increase in minimum peak reflow temperature may result in an adverse impact to other components on the board that need additional evaluation.
Initially Published in the SMTA Proceedings
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