Case study on the validation of SAC305 and SnCu-based solders in SMT, wave and hand-soldering at the contract assembler level

Abstract

At the contractor level once a product is required to be soldered with lead-free solders all the processes must be assessed as to insure the same quality a customer has been accustomed to with a Sn63Pb37 process is achieved. The reflow, wave soldering and hand assembly processes must all be optimized carefully to insure good joint formation as per the appropriate class of electronics with new solder alloys and often new fluxes. The selection of soldering materials and fluxes are important as to insure high quality solder joints with leadfree solders which tend to wet slower than leaded solders but also the process equipment must be lead-free process compatible. Components must be lead-free and able to meet the thermal requirements of the process but also the MSL (moisture sensitivity limits) must be observed. Board finish must be lead-free and the PCB must be able to sustain higher process temperature cycles with no physical damage but also good solderability to enable subsequent soldering at the wave or hand assembly. Tin-silver-copper has received much publicity in recent years as the lead-free solder of choice. The IPC Solder Value Product Council as the preferred option endorsed Sn96.5Ag3.0Cu0.5 (SAC305) for SMT assembly and most assemblers have transitioned to this alloy for their solder paste requirements. The SAC305 alloy due to its 3.0% content of silver is expensive when compared to traditional leaded process for this reason many contract manufacturers and PCBA assemblers are opting for less costly options such as tin-copper based solders for wave, selective, handsoldering, dip tinning operations. In recent years tin-copper based solders with a variety of elemental additives have emerged which improve the overall properties and performance of tin-copper solders. Tin-copper solder without the incremental additions of certain elements is rarely used but the addition of nickel or nickel and bismuth as found for example in K100 and K100LD respectively do offer improvements in wetting, joint cosmetics and in some cases solder joint reliability. In this conversion SAC305 was used for the reflow process, tin-copper-nickel based solder K100 was used in the wave soldering operation and it was also used in solder wire form for hand assembly. This paper is a summary of the experience at a medium sized assembler in achieving the customer driven mandate to go lead-free and the maintenance of production yields and quality using both tin-silver-copper and tin-copper in the assembly of high end printer boards. Over 120,000 builds were achieved with a 99.6% first pass yield for the overall soldering process.