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PCB-Jan2017

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62 The PCB Magazine • January 2017 der-mask attack and excellent surface wetting by solder, whilst eliminating nitric acid, and ASPIS which had studied the mechanism of the black pad effect associated with the widely used elec- troless nickel/immersion gold finish (ENIG) and concluded that the acidic components in aqueous immersion gold chemistry caused corrosion at the grain boundaries of electroless nickel, which could be avoided by using ionic liquid chemistry. MACFEST had extended the knowledge gained from the ASPIS work to develop an ENIPIG finish, based on deep eutectic solvents, as a high-reliability alternative to ENIG, with improved functionality and reduced toxicity and environmental impact. Using a standard commercial phospho- rus electroless nickel as the base layer, a novel immersion process had been used for deposit- ing palladium, based on palladium chloride in Ethaline, operating at 80°C, to give a coating thickness of 70−100 nanometres in 20 minutes. This was then overplated with immersion gold at 50°C for 9−15 minutes from an Ethaline solu- tion containing gold as either gold chloride or sodium gold thiosulphate, which was both ac- id-free and cyanide-free. A study of the effect of the immersion coating on the electroless nickel substrate revealed no evidence of the type of corrosion effects seen on conventional ENIG, and hence no possibility of black pad failure. Initial solderability testing had shown rapid solder-substrate interaction, with consistent per- formance across multiple samples, for gold plating times of nine, 12 and 15 minutes. And there was no evidence of voiding or grain boundary attack. In a separate exercise, the solderability was independently benchmarked by Bob Willis in cooperation with Merlin Circuit Technology, and the results were reported by Tom Jones from Heriot-Watt University, studying at Merlin for an engineering doctorate. He described in de- tail the test vehicles supplied by Willis, each of which included 18 test coupons for solder spot and wetting balance measurements, with six rows of tracks for spot pattern, 22 paste dots and seven pads for wetting balance measurement. These were plated with ENIPIG from the process formulated at Leicester, and plating thicknesses measured by X-ray fluorescence against the rel - evant IPC specification. Prior to solderability testing, coupons were subjected separately to reflow oven heating and vapour phase heating, then left for four days. Reference samples had no thermal pre-conditioning. Jones showed a series of wetting balance and spot-wetting test results comparing the ENIPIG samples with ENIG, cop - per-OSP, immersion tin and HASL. On the wetting balance, ENIPIG performed well in terms of soldering force and wetting speed, and the spot wetting test demonstrated that ENIPIG was comparable to or better than ENIG that had been pre-conditioned under equivalent conditions. Separate test boards had been designed for wire-pull and ball-shear test- ing, but the results were not yet available. The MACFEST webinar prompted a very lively question-and-answer session, moderated by Bob Willis, and it was clear that there was considerable interest in following the progress of the project. In fact, a recording of the whole of the webinar, including the slides, the audio presentations and the Q&A session is available by clicking here. PCB Pete Starkey is based in the UK, and joined I-Connect007 as tech- nical editor in 2008. Starkey has more than 30 years of experience in the PCB industry, with a back- ground in process development, technical service and technical sales. To contact Starkey, click here. ENIPIG—NEXT GENERATION OF PCB SURFACE FINISH Tom Jones

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