THE ROLE OF NICKEL IN SOLDER ALLOYS -PART 2. THE EFFECT OF NI ON THE INTEGRITY OF THE INTERFACIAL INTERMETALLIC IN SN-BASED/CU SUBSTRATE SOLDER JOINTS

摘要

In studying the effect of Ni on the behavior of the Sn-0.7wt%Cu solder it was found early on that nearly all the Ni ends up in the Sn-Cu intermetallic, substituting for Cu in the crystal structure so that phase can be characterized as (Cu,Ni)_6Sn_5. In earlier synchrotron X-ray diffraction (XRD) studies and differential scanning calorimetry (DSC) of sample powders it was found that the Ni suppresses the polymorphic transformation of this intermetallic from the hexagonal to the monoclinic form that is otherwise the stable phase at temperatures below 186°C. On the basis of the volume change associated with the polymorphic transformation it was hypothesized that in the constrained conditions of a solder joint the stresses generated could cause the cracking that is often observed in interfacial Cu_6Sn_5 when Ni is not present. In this paper we will provide an overview of the results of recent real time observations of the polymorphic transformation using high voltage transmission electron microscopy (HV-TEM) that have confirmed that the volume change does generate stress that could be sufficient to drive cracking of the interfacial intermetallic that could compromise the integrity of the joint.