EIPC Reliability Workshop, Tamworth, UK, September 22, 2016

Estimated reading time: 17 minutes

But how reliable was “reliable”? Micro-via constructions required additional testing over and above the standard IST cycling. Lewis made it clear that not all PCB designs were created equal, not all PCB manufactures were created equal, and some materials were naturally more robust than others. Not every PCB would achieve 500 IST cycles, indeed not every PCB would reach 200 IST cycles. Conformance testing alone was not a reliable way of predicting actual lifecycle, and it might be that conformance level testing could far exceed actual performance requirements. For this reason, in order to accurately assess the lifecycle capability of a design and to quantify the reliability of a PCB construction it was necessary to carry out an acceleration study, as previously referred to by Howard Swarbrick, which followed a similar process route to IST conformance testing but required a large number of dedicated IST coupons of identical design, split into three groups each of which was cycled to failure with a different upper temperature. Provided the failure mode could be confirmed as fatigue, rather than as a manufacturing defect, then the data could be used to predict the number of IST cycles needed to simulate the cumulative thermal stresses expected during the life of the product in its end operating environment.

Lewis concluded by reporting that since implementing IST, Airbus Defence and Space had experienced no test failures at assembly or equipment level that could be linked to the base PCB. IST had also enabled the direct comparison of dissimilar constructions for new product introduction and had been utilised as a risk reduction tool for current and future product.

The morning’s workshop programme provoked lively discussion and debate which continued through the lunch break, and the afternoon session commenced with Emma Hudson from Underwriters Laboratories expanding upon the partnership that UL had established with PWB Interconnect Solutions, referred to in Bill Birch’s presentation, to develop the UL IST Baseline Performance Programme. The objectives were vendor pre-qualification, technology and material characterisation, lot conformance, process and plating monitoring and trouble-shooting. The test programme included test vehicle design, reliability testing by IST, material robustness by DELAM, failure analysis, material analysis and database reference, with performance criteria driven by customers.

With the increasing demand for demonstrated reliability from all industry segments, IST was becoming an industry standard method for measuring PCB performance. But global industry performance standards were needed with representative test criteria based on specific industry product life cycles for consumer, computer, telecom, medical, automotive and aerospace sectors. And it was necessary for PCB performance standards to include standardised test vehicle design, a standardised test method, pre-conditioning based on the stresses experienced during the assembly process, and performance levels that correlated between quality and reliability specifications.

Based on data gathered by PWBIS over many years, a series of standardised test vehicles had been designed: cross-functional with 12- and 24-layer PTH coupons; automotive with 8-layer buried and microvia coupons; and 20-layer lead-free material reliability coupons for HDPUG. Under development was a standardized 4-, 6- and 8-layer flex test vehicle with sequential lamination coupons, for medical diagnostic equipment and implantable devices. Hudson discussed test coupon panel layouts, stressing that these were only relevant for the Baseline Performance Programme—standard IST testing would still require coupons to be included on the production panel. Test procedures were in accordance with IPC-TM-650 method 2.6.26.

She described the procedure for the IST Baseline Performance Programme: the client chose the technical level and the preconditioning temperature, and submitting between 16 and 30 coupons of each technical level to UL for testing, UL tested eight coupons per technical level and entering the results in the UL-IQ database. This entry effectively gave the client a “brand reference” supported by a Guide Card explaining the parameters and the test method used, which could give OEMs and ODMs an indication of the client’s capability. No follow-up services were proposed—this was presently seen to be a one-off exercise, but OEM feedback was being sought. As a point of clarification, she emphasised that there was no relationship between safety certification conducted by UL and any performance testing carried out—these were different tests looking at different aspects of the PCB and a failure in a performance test would have no bearing on a client’s UL safety certification.

Having made the clear distinction between reliability and safety, Emma Hudson then focused on UL safety standards and concluded the workshop programme with an extremely informative review of the consequences of the update of FR-4 classification for UL Recognised PCBs. As FR-4 had continued to evolve to meet industry and environmental needs, and halogen-free materials had been introduced, it had been necessary to split FR-4 laminates into two distinct groups because halogen-free FR-4 materials did not behave in the same way as traditional brominated FR-4 materials. So, as of June 2014, “FR-4” no longer officially existed as a UL/ANSI grade for laminates, and materials had been re-classified as either “FR-4.0” for brominated materials or “FR-4.1” for halogen-free. A small number of modified FR-4.0 and FR-4.1 materials did not fit with either classification and had to be treated as non-ANSI grades.

Hudson deftly guided her audience step-by-step through the hierarchy of procedures involved in creating a new UL Recognised PCB, updating an existing UL Recognised PCB, and adding a new FR-4.1 material to an existing UL Recognised PCB. She also clarified the route for adding a new solder resist to an existing UL Recognised PCB and decreasing the minimum build-up thickness of existing UL Recognised PCB, with examples of typical combinations and definitions and details of when and what additional testing might be required. In essence, the UL 796 PCB test requirements were the same for FR-4.0 and FR-4.1 as any other UL/ANSI grade, and additional FR-4.1 testing might be needed to make changes to existing “FR-4” board types based on historically conducted testing. Additional testing could be expected when adding solder resists to FR-4.1, as most were officially recognised for use in combination with FR-4.0 only, and when a resist was not officially recognised in combination with a UL/ANSI grade, additional 1.6 mm flame testing was necessary. Her efforts in helping to clarify a potentially formidable set of procedures were greatly appreciated by workshop delegates.

A full and highly enlightening day of presentations and discussions was rounded off with a tour of Amphenol Invotec’s factory in nearby Dosthill, an excellent example of a facility dedicated to technologically complex, small-batch, high-mix manufacture of high-reliability HDI and flex-rigid multilayer PCBs for defence and aerospace applications. 

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