The effects of surface finish on the reliability of lead free and tin lead chip scale package solder joints

Abstract

PurposeThe effects of thermal fatigue and printed circuit board (PCB) surface finish on the pull strength, failure modes and reliability of chip scale package (CSP) solder joints were investigated.Design/methodology/approachMechanical pull test, metallographic examination and electrical measurement were used. Tin lead (Sn‐Pb) and lead free (Sn‐Ag‐Cu) alloys were used with Au/Ni and organic solderability preservative (OSP) surface finishes.FindingsThe experimental results showed that the pull strength of the Sn‐Ag‐Cu/(Au/Ni) solder joint did not change noticeably with an increasing number of thermal cycles. However, the pull strength of the Sn‐Pb/(Au/Ni) solder joints drastically degraded and that of the Sn‐Ag‐Cu/OSP and Sn‐Pb/OSP solder joints slightly decreased during thermal cycling. For both Sn‐Ag‐Cu and Sn‐Pb alloys, the solder joint fracture of as‐soldered samples was the main failure mode when an Au/Ni surface finish was used. For the Sn‐Ag‐Cu/(Au/Ni) and Sn‐Ag‐Cu/OSP solder joints, the proportion of component trace tearing considerably decreased, whereas that of PCB trace tearing considerably increased, during thermal cycling. The Weibull lifetimes of the solder joints were increasingly longer in the order of Sn‐Pb/(Au/Ni), Sn‐Pb/OSP, Sn‐Ag‐Cu/OSP, and Sn‐Ag‐Cu/(Au/Ni).Research limitations/implicationsThis was not an exhaustive study and all of the findings are for lead free and tin lead CSP solder joints, which perhaps limits the usefulness of the results elsewhere.Practical implicationsA very useful source of information and impartial advice for engineers planning to conduct a switch from tin lead to lead free technology in their production lines.Originality/valueThis paper fulfils an identified information/resources need and offers practical help to an engineer starting out on an engineering development.