Successful symposium of Surface Finishes

Following an introduction to the aims and objectives of the Institute by Technical director Bill Wilkie, ICT Chairman Andrew Hall introduced papers by Frando van der Pas of Enthone and Nigel White of Atotech. Frando van der Pas addressed the specific topic of immersion silver, a lead-free solderable finish gaining in popularity, and described the limitations of existing processes as revealed by a survey of 64 fabricators and 29 assemblers. Principal areas of concern were galvanic attack, tarnishing and exposed copper, and particularly the phenomenon known as "champagne voiding" which can occur above the intermetallic layer at the interface of the solder joint and the substrate and adversely affects joint strength. Champagne voiding appeared to be associated with thick immersion silver deposits. Enthone had developed an entirely new immersion silver chemistry which yielded a very dense fine-grain deposit with high conductivity and tarnish resistance at thicknesses as low as 6-12 microinches, and the champagne voiding effect was very much reduced, as was the tendency to galvanic attack. The process, named AlphaStar, was stable and easily controllable. Nigel White took a broader approach, and comprehensively reviewed the benefits and limitations of a whole range of lead-free finishes: HASL, electroplated, electroless and immersion. The world trend was for the continuing decline of HASL and the increasing market share of OSP. The expected demise of ENIG had not happened and there remained a relatively steady demand, but the most significant increases were in immersion tin and immersion silver, each forecast to represent 17% of the world market by 2007. In Europe, immersion tin was predicted to show the highest growth. For specific applications, there was demand for universal finishes such as electroless-nickel/electroless-palladium/immersion-gold, which, although expensive, offered excellent solderability, aluminium and gold wire-bonding capability and a good contact surface. Combinations of finishes on the same surface, for example OSP for solder pads and ENIG for contacts, were increasingly specified for mobile phones, and processes such as Atotech's Aurotech SIT were a cost-effective option. It was clear that no single finish satisfied every requirement, and that there would remain an element of caution in the acceptance of new finishes for lead-free soldering applications until their reliability in the long-term had been demonstrated. Details of the presentations may be found on the ICT website www.instct.org