Influence of interfacial reaction on reliability of QFP joints with Sn-Ag based Pb free solders

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-SBi, Sn-3.5Ag-0.7Cu, Sn-3 .5Ag-2.5Bi-2.5In and Sn-37Pb solders. The strength and the microstructure o f 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-SBi solder significantly decreased with increasing holding time at 398 K, the strength of the other Sn-Ag based solder joints was comparable t o that with the Sn-37Pb solder before and after the high temperature exposu re 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)(6)Sn-5 in the joints with the A u/Pd/Ni plated Cu lead after the exposure at 398 K up to 7.2 Ms for all fiv e 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-5 Bi solder, in which the growth of the reaction layer deviated from the para bolic law and accelerated beyond the holding time of 1.8 Ms. The unusual gr owth of the reaction layer, which resulted from a liquid phase forming ahea d of the reaction layer and penetrating the grain boundaries of the reactio n products, caused the degradation in the strength of this solder joint. Th e 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 occ ur at the holding temperature above the Sn-Bi-Pb ternary eutectic point of about 370 K. The Sn-3Ag-SBi solder is, therefore, considered to be unsuitab le for assembling packages with the Sn-10Pb plated lead because of degradat ion in reliability during the high temperature exposure above 370 K.