TABLE OF CONTENTS Au overcoat on top of Ni has been used as an alternative to HASL for many years, due to the advantages of a flat surface, stability against environment, good shelf life and solderability, and reduced bridging at assembly. It is one of the favorite surface finishes for fine-pitch surface-mount technology (SMT) and ball grid array (BGA) packages. Ni serves as a solderable diffusion barrier and prevents copper migration into solder. Upon soldering and aging, Ni forms a Ni 3Sn 4 intermetallic layer with Sn-Pb or Sn-Ag alloys, although other intermetallics may also be formed, such as (Cu 1 -p-qAu pNi q) 6Sn 5 on top of (Ni 1 -yCu y) 3Sn 4 intermetallics when soldering with Sn-Ag-Cu alloys.34 Due to the lower coefficient of thermal expansion (CTE) of Ni than Cu (12.96 versus 16.56 ppm/ C), Ni can also stabilize plated-through-holes during thermal excursion.35 Although Ni forms a more stable solder joint interface than does Cu, it is prone to oxidation, and the Au overcoat serves as an oxidation barrier for the Ni underlayer.
If the Au layer is thin, it will quickly dissolve into solder and be removed from the interface, thus eliminating the concern about Au embrittlement. If the Au layer is not thin, Au embrittlement may be a concern. Au dissolves more rapidly into Sn than into eutectic Sn-Pb solder for the same amount of superheating.36 ,37 The same is true for eutectic...
