Issue link: https://iconnect007.uberflip.com/i/1496178
12 SMT007 MAGAZINE I APRIL 2023 December 2000 and adopted in February 2003 by the European Union (and then implemented by the U.S., Japan and other countries), much research and development efforts have been conducted by individual laboratories in the U.S. 1–8 , Japan 9, 10 , and consortia. One of the primary performance targets was to have a "drop-in" or "nearly drop-in" replacement for SnPb eutectic alloy so that the SMT manufacturing infrastructure, including PCB materials, reflow, and wave soldering processes could remain intact without being subject to disruptions. With comprehensive studies and thorough examinations of the potential of "logical" alloys, including perusing the entire periodic table, it was found that the most challenging property to be delivered was (and still is) to keep the melting temperature (liquidus temperature) of the lead-free alloy in the range of 175 o C to 195 o C, while meeting all other necessary properties and manufacturing requirements. Without delving into historical details and granularity, in order to keep the melting temp- erature low enough, one approach adopted was to add the element Bi to the Sn-based system. Phenomena A phenomenon termed as fillet-lifting was reported in the late 1990s and early 2000s, which refers to the partial separation (crack) of the solder fillet between the solder and the through-hole land on the PCB after the completion of wave soldering (Figure 1). It is worthwhile noting that this phenomenon has hardly been observed in surface mount solder joints; however, it repeatedly occurred with through-hole joints. It should also be noted that such fillet-lifting phenomena were evident immediately after processing (before being subjected to any accelerated reliability testing). Overall, key observations were: • Fillet-lifting were associated with through- hole solder joints after wave soldering • There was no detectible solder joint separation (crack) associated with SMT components • The solder joint crack often started from the far end of the through-hole joint (Figure 2) Figure 1: Fillet-lifting phenomenon.