A Possible Approach to Multilayer Manufacture to Enhance Soldeverability and Shelf Life

As miniaturisation reaches new levels with the advent of surface mount technology, the mixed technology of surface mount and through-hole brings into focus the need for soldermaskless printed circuit boards which can have considerable shelf life before assembly and which can satisfy the new requirements of NTL-STD-SOLD for optimum solderability. The ‘weak-kneed’ hole due to the tin-copper intermetallic encountered in hot air levelled boards needs to be eliminated. This paper describes a process based on cap layers for terminal and test pads, and a duplex alloy plating of tin, nickel and lead on the outer layers which has several advantages over the currently preferred solder mask over bare copper/selective solder coating process.