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1. Introduction
All printed circuit boards (PCBs) have copper finishes on their surfaces. If left unprotected, the copper will oxidise and deteriorate. A number of protective finishes are available to the PCB manufacturer, notable among which are HASL, organic solder preservative (OSP) coatings such as Entec, ENIG, immersion silver (ImAg) and immersion tin (ImSn). The various materials each have their unique characteristics in terms of solderability. HASL is the dominant surface finish used in industry and provides good solder wettability. Its use is under restriction due to the use of lead as part of the alloy and the health and safety hazard it poses. OSP finishes offer excellent coplanarity and improved solderability properties. Careful handling, however, is needed as acidic fingerprints degrade the OSP and leave the copper susceptible to oxidation. ENIG has been used with great success on many boards, despite its high unit cost. It has a flat surface and excellent solderability. The main disadvantage is that the electroless nickel layer is brittle and has been found to break up during mechanical stress. ImAg is a recent addition to the list of PCB finishes and is gaining popularity in Asia and North America. During the soldering process, the silver is dissolved into the solder joint leaving a tin/lead/silver alloy which provides reliable joints for BGA packages. ImAg is considered a drop in replacement for HASL. The main concern is the issue with silver migration and reliability implications. ImSn is another alternative surface finish just like ImAg with many similar characteristics to its silver counterpart. There are many health and safety and oxidation issues, however, to consider with this surface finish. (Hetschel et al., 2009) investigated the oxidation behaviour of ImSn finish PCB after multiple reflow ageing. The tin oxide formation behaviour of ImSn was characterised using a number of techniques including X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The native oxide layer measured in this study was around 7 nm thick. The solder spread test conducted on ImSn showed significantly different results in “as received” condition as compared to those carried out under a nitrogen atmosphere.
To ensure good solder wettability in the assembly of electronic components, the PCB/component interface must be free of all contaminants, especially organics and oxides. Surface...