RoHS | December 10, 2006
RoHS PCB Specification -<br>What Has To Change?
SMART Group a technical trade association supporting the electronics manufacturing industry in Europe, focused on the practical realities and consequences of RoHS implementation from the standpoint of PCB specification and procurement at a workshop seminar held at Thame, in Oxfordshire UK, on 6th December 2006.
The event generated a remarkable level of interest from the UK electronics industry – a capacity audience of over 90 delegates, typically process engineers, quality engineers, development engineers and purchasing officers, paid keen attention to the presentations and participated actively in the open discussion sessions. Bob Willis, SMART Group's Technical Director and coordinator of the LEADOUT project introduced the proceedings by posing the questions: What are the key requirements when specifying PCBs for lead-free compatibility? What specifications should be defined? What problems have already been seen in manufacture? What criteria need to be considered during procurement and audit? What are the real issues concerning laminates in lead-free assembly? Alun Morgan, Managing Director of Isola UK, gave a plain-language explanation of the characteristics and properties of copper-clad laminates, starting from raw materials (which represent over 60% of the cost of the finished product), describing each stage of the manufacturing process and then comparing mechanical, physical and electrical properties of traditional dicyandiamide-cured with current novalak-cured FR4 systems. The popular misconception that higher glass-transition temperature equated to higher thermal stability was conclusively refuted, and the substantial improvement in thermal endurance offered by the epoxy-novalak chemistries in lead-free applications was clearly demonstrated. Because these materials were naturally harder and more chemically resistant, certain stages of the PCB manufacturing process, such as de-smearing and v-scoring, needed closer control for optimum results. Dennis Price, Technical Sales Director of Merlin Circuit Technology, comprehensively reviewed the available lead-free PCB finishes, then gave his own experiences from the point of view of a specialist high-technology quick-turn shop. 60% of Merlin's production was electroless nickel immersion gold, for which there remained a consistent demand. About 30% was hot air solder levelled (and the majority of this was tin-lead, for exempted medical and industrial control applications). The only other significant customer demand was for immersion silver. There was little interest in OSP finishes from Merlin's customers and, after a recent resurgence, immersion tin had rapidly lost popularity. Despite suppliers' claims that currently available proprietary chemistries carried no risk of tin whiskers, Dennis showed some alarming examples of whiskering on boards which had experienced low temperatures in transit to a customer in Scandinavia. Bob Willis recounted some practical experiences of lead-free soldering in manufacture, emphasising the need for the assembler to work closely with his PCB supplier to select the most appropriate finish for his particular application and to understand its characteristics and its effect on solderability. Using exceptionally clear close-up video, he illustrated the wetting behaviour of various finishes after a range of ageing cycles, with various solder pastes. It was generally acknowledged that the industry needed a simple and cost-effective solderability test to qualify boards in-process, and current IPC test methods had little relevance to surface-mount technology. The drop-coalescence test, as had been used in a number of NPL studies, showed very good correlation with wetting balance measurements, and test coupons could be straightforwardly incorporated into the off-cut areas of manufacturing panels. There was an enthusiastic response from the audience to Bob's suggestion that this test be considered as the basis of an industry standard. Len Pillinger, BSI's Certification Manager, succeeded in making a potentially boring subject entertaining and enlightening, in his presentation on specifications and the auditing of printed circuit suppliers. The BS9760 series of UK capability standards had been replaced by European CECC standards which themselves had subsequently been withdrawn, and IEC 62326 had now lost its initial momentum. Several of the traditional MIL-P standards had been phased out and IPC 6011 was becoming established as the internationally recognised capability standard, with IPC A-600 as its point of reference for workmanship. RoHS had significant impact on test methods, and a review of test criteria was being carried out within BSI with the objective of proposing meaningful parameters so that existing test methods could continue to be used. Without naming names, Len recounted a series of anecdotes from his experiences of auditing printed circuit manufacturing plants. In typical instances, people had made substantial investment in technology whilst overlooking simple things like handling procedures. And sales departments often did not properly appreciate the limits of capability of their factories when accepting orders. On the other hand, by not understanding the significance, relevance or details of the standards they were calling-up, purchasers of PCBs tended to get what they asked for, but not necessarily what they wanted. Dr Chris Hunt reported recent studies at National Physical Laboratory of conductive anodic filament (CAF) phenomena, in a context of increasing circuit density, increasing use of electronic assemblies in harsh operating environments and potential degradation of laminate properties during lead-free reflow processes. Testing had been carried out on a multilayer coupon with 6000 via holes, some in-line and some staggered, at various pitches and with various anti-pad annular ring clearances. Time to failure had been measured by change in resistance after thermal cycling, thermal shock and humidity, and precision micro-sectioning techniques together with EDAX analysis had been used to identify actual conductive filaments. Significant observations were that novalak-cured laminates showed far better CAF resistance than dicyandiamide-cured materials, whose CAF resistance was further reduced by lead-free soldering cycles. There was little difference in the performance of nominally similar laminates from different suppliers, and glass transition temperature had no significant influence on CAF resistance. Curiously, all other factors being equal, there was a distinct correlation between PCB fabricator and the incidence of CAF – another topic discussed at length during the lively question-and-answer session which concluded the day's proceedings.
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