Reliability of Sn-8 mass%Zn-3 mass%Bi lead-free solder and Zn behavior

Abstract

In order to evaluate the reliability of Sn-8 mass%Zn-3 mass%Bi (hereafter, Sn-8Zn-3Bi) solder, thermal shock tests (temperature range: from 233 to 353 K) and high temperature humidity tests (test condition: 353 K, 85% relative humidity) were conducted. The PCB (Printed Circuit Board) pads were finished by OSP (Organic Solderability Preservative), Sn and Ni/Au. The electric part for test was QFP (Quad Flat Package), and the lead was plated by Sn-3 mass%Bi. The joint strength was estimated by pull testing, and Zn-phase behavior in solder joint was examined. The pull strengths of Sn-8Zn-3Bi joints in the as-soldered state were measured to be about 16N, irrespective of the PCB pad coatings. After thermal shock testing (hereafter, TS test) up to 1000 cycles, the pull strengths decreased to around 14N. Meanwhile, after high temperature humidity testing (hereafter, HTH test) up to 1000h, the pull strengths deceased to around 6N, irrespective of the PCB pad coatings. The reason for the drastic decrease of pull strength after the HTH test was proven to be a crack that was observed along the zinc oxide-phase. The Zn-phase in the Sn-8Zn-3Bi solder changed from a separate acicular shape to a lined-up branched arrangement after HTH tests of 1000h. © 2005 The Japan Institute of Metals.