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By Mitch Holtzer, Cookson Electronics
SAC alloys became the industry-standard lead-free solders after RoHS went into effect. Technical and economic factors are reducing the silver content in SAC compositions, with disadvantages like slower wetting speeds, and advantages like minimized BGA voids. Solder choices are influenced by the desired results, cost, BOM, and other factors.
Tin/silver/copper (Sn/Ag/Cu SAC) alloys have become the industry standard for lead-free soldering since the implementation of RoHS in July 2006. Over the past three years, several technical and economic factors have shifted the overall consumption of SAC alloys from higher silver 4%, 3.7% and 3.5% to reduced silver offerings. SAC 305 remains the leading alloy for use in surface mount solder paste, but low-silver pastes are now commercially available.
Although these low-silver alloys have slower wetting speeds than SAC 305 and SAC 405, they have both technical and economic advantages over the higher-priced silver alloys.
Data has been used to show that BGA voiding is minimized when the BGA sphere alloy and the solder paste alloy have similar melting points. Low-silver paste and SAC 105 spheres produce the lowest combination of voids. It is believed that up to 40% of BGA packages now use lower-silver alloys, primarily for drop shock resistance and cost savings.
If voiding and cost are issues, it is suggested that a combination of paste and sphere with closely matched silver contents be evaluated.
Mitch Holtzer, global product manager, Alpha, Cookson Electronics, may be contacted at firstname.lastname@example.org.
For more on lead-free solder composition and performance, read:Melting and Freezing Characteristics of Common Lead-free Alloys;Solder Paste Basics: A Round-up;Lead-free System Reliability Avoid Likely Failures;Lead-free for High-reliability, High-temperature Applications.