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APRIL 2023 I SMT007 MAGAZINE 13 The following list outlines key findings in relation to a variety of lead-free compositions. Studies on fillet-lifting can be summarized as follows 9, 10 , where fillet-lifting occurred in: • More than 90% of through-hole joints with Sn3.4Ag>4.8Bi • More than 90% of through-hole joints with SnAg>7.5Bi • Severe occurrence in through-hole joints with various lead-free alloys (in the absence of Bi) when SnPb- coated components were used • Some through-hole joints with Sn3.5Ag0.5Cu1Zn • Some through-hole joints with Sn2.6Ag0.8Cu0.5Sb • 30% of through-hole joints with 96.5Sn3.5Ag • 0% of through-hole joints with 58Bi42Sn • 0% of through-hole joints with 63Sn37Pb • 0% of surface mount joint cracks with any alloys tested Factors and Causes Based on the observation that the fillet crack appears to initiate from the far end of the land from the barrel in conjunction with the finding that the crack occurs only with through- hole joints, fillet-lifting was attributed to the excessive stress generated during the cooling and solder solidification of an assembly. The contributors to this excessive stress may emanate from several sources: • Cooling rate • Maximum temperature gradient (solder pot temperature) • PCB construction (land design, board thickness) • Solder alloy composition (melting temperature, metallurgical pasty range, metallurgical phases) • Solder alloy (stress and strain behavior) • Wetting ability (intrinsic alloy wetting ability) • Wettability of PCB land In most practical cases, fillet-lifting was highly likely contributed to from more than one of the above factors. Metallurgically, the phenomenon was also considered a result of segregation during solidification and/or the formation of low melting Sn-Bi eutectic phase, when applicable. If or when low-melting SnBi eutectic or other low-melting phases are formed, low-melting phases may be a culprit. Nonetheless, low temperature Sn-Bi phase does not always form when Bi is just present. Fillet-lifting may or may not cause a circuit board failure or a product failure. However, even when there is no mechanical or electrical failure, the fillet-lifting phenomenon should be examined and remediated. Remediation and Prevention To alleviate the problem from the assembly operation (when PCB design is given and not subject to change), the following areas are to be considered: In process: • To lower the cooling rate • To avoid using high soldering temperature, if feasible Figure 2: The solder crack joint often started from the far end of the through-hole joint.