Abstract
QFPs (quad flat packages) with Sn–10Pb plated Cu lead or Au/Pd/Ni plated Cu lead were reflow-soldered using Sn–3.5Ag, Sn–3Ag–5Bi, Sn–3.5Ag–0.7Cu, Sn–3.5Ag–2.5Bi–2.5In and Sn–37Pb solders. The strength and the microstructure of the solder joints were examined after an exposure test at 398 K . Although the strength of the solder joint with the Sn–10Pb plated Cu lead using the Sn–3Ag–5Bi solder significantly decreased with increasing holding time at 398 K, the strength of the other Sn–Ag based solder joints was comparable to that with the Sn–37Pb solder before and after the high temperature exposure test. The reaction layers formed at the interface between the solder and the Cu pad consisted of Cu6Sn5 and Cu3Sn in the joints with Sn–10Pb plated Cu lead and consisted of only (Cu, Ni, Pd)6Sn5 in the joints with the Au/Pd/Ni plated Cu lead after the exposure at 398 K up to 7.2 Ms for all five solders. The growth kinetics of the reaction layers obeyed the parabolic law except for the joint with the Sn–10Pb Plated Cu lead using the Sn–3Ag–5Bi solder, in which the growth of the reaction layer deviated from the parabolic law and accelerated beyond the holding time of 1.8 Ms. The unusual growth of the reaction layer, which resulted from a liquid phase forming ahead of the reaction layer and penetrating the grain boundaries of the reaction products, caused the degradation in the strength of this solder joint. The liquation was caused by the enrichment of Bi and Pb ahead of the reaction layer during the high temperature exposure to the extent where melting occur at the holding temperature above the Sn–Bi–Pb ternary eutectic point of about 370 K . The Sn–3Ag–5Bi solder is, therefore, considered to be unsuitable for assembling packages with the Sn–10Pb plated lead because of degradation in reliability during the high temperature exposure above 370 K.
