Reliability of Pb-free metallisations on electronic circuit boards

Aim
A very important question relating to the electronic industry is which metallisation(s) can be used in the future to enhance the performance of Pb-free soldering? This work aims to answer this question by studying the solderability of different Pb-free metallisations on printed circuit boards (PCBs) and evaluate the metallisations compatibility with Pb-free solder.
Background
Due to recent legislations concerning the toxicity of Pb; Pb-containing solders are being phased out of the electronics industry. Much work has already been carried out investigating the different Pb-free solder alloys available. The current situation is that there is a general acceptance as to which solder alloy is best suited for the electronics industry. However, the metallisation on boards and components also need changing from the traditional SnPb metallisations and little work has been
done so far to evaluate the compatibility and reliability of possible Pb-free metallisations.
Experimental work
Possible Pb-free alternatives include electroless Au over Ni, Cu OSP (Organic Solderability Preservatives), chemical Ag, chemical Sn and electroless Sn. Printed circuit boards that have been subjected to different storage atmospheres will be used to see if storage in protective gas, N2, can increase the shelf life. The solderability before and after storage will be evaluated using a wetting balance. The structure and composition of the surface will be studied with different methods (for example scanning electron microscope and GDOES). Pb-free solder will be reflowed on the surfaces and the microstructures investigated.