Surface Finish of PCBs and Solderability
In order to maintain the solderability of PCBs over a period of storage time, it is necessary to protect the copper surface mount pads with solderable surface finish. Up to now the most common finish is eutectic tin-lead alloy by hot air solder levelling (HASL) method because it has the most desirable properties of ideal PCB surface. Unfortunately this coating does not meet the requirement of soldering pads planarity – fundamental factor for fine – pitch, very large scale integration (VLSI) surface mount technology (SMT) and contains lead – one of the most toxic metals.
Alternatives to tin – lead (Sn-Pb) HASL finish may be co – planar, ecological coatings: electroless nickel/immersion gold (Ni/Au), electroless matt tin (Sn), organic solderability preservatives (OSP). All these lead – free finishes have been implemented in fabrication of PCB. OSP is not resistant against long – term, high temperature action. All of finish technologies have no negative influence on surface insulation resistance of PCBs. Shear strength of soldered joints is more or less comparable for all coatings. PCBs with all presented lead–free finishes are a good alternative to PCBs with Sn–Pb conventional finish for SMT applications.