EIPC Summer Conference 2016, Day 2: Strategies to Maintain Profitability in the European PCB Industry

Estimated reading time: 23 minutes

Conductive anodic filamentation, CAF, is a failure mechanism that was first reported in the 1970s by Bell Laboratories, and has become a significant reliability concern with increasing circuit density and the rapid increase of the use of electronics in harsh environments and for high reliability and safety critical applications. Helmut Kroener, senior director OEM Marketing Europe at Isola Group, presented a paper on CAF experiences from an automotive product qualification. He explained the mechanism of CAF formation, an electrolytic corrosion effect which builds a conductive path along a glass filament - epoxy resin interface under conditions of high humidity and bias voltage, exacerbated by ionic contamination, and defined three typical time-dependent failure zones: infantile, where defects in the material led to copper migration after bias was applied, transition, where partial defects occurred and shorts could recover to isolation, and wear-out, where the true CAF capability of the material could be determined. Kroener showed many real examples, and demonstrated that in most cases the actual CAF filament was difficult to pinpoint.

Many factors could influence CAF, some due to the laminate manufacturer and some to the PCB fabricator. From the laminator’s point of view, the cleanliness of the glass fibres, the compatibility of the silane treatment, and the completeness of wetting of the glass by the resin were probably the most significant, and resin formulation was obviously another factor, particularly its thermal stability. To qualify a laminate for automotive applications involved extensive product and processing testing, and a separate qualification was required for each material category and each PCB factory location. CAF testing was one of the many requirements and the particular work reported by Kroener was part of the qualification programme for Isola 185 HR material at a tier-one automotive supplier, with PCBs manufactured at an Asian production location.

CAF testing was carried out in accordance with IPC TM 650 2.6.25 on six-layer HDI test coupons with blind and buried vias, preconditioned by 3 x 260°C reflow cycles. Test conditions were 85°C, 85% relative humidity, 100 volts, for 1000 hours. Material samples were supplied to the PCB fabricator for test vehicle manufacture and testing. CAF failures were reported by the PCB fabricator, all other tests having been successfully completed, and it was agreed with the OEM for the CAF testing to be repeated by an independent laboratory. Failures were again reported, and detailed failure analysis conducted. Agreed process improvements were made in material production and PCB manufacturing, and the test programme was repeated with successful results.

Kroener discussed in detail the failure analysis procedures, demonstrating the delicate microsectioning, microscopic and instrumental techniques used to identify the actual CAF defects, and the interpretation of the results. It was found that, of four failures reported, three had PCB manufacturing issues as the root cause and only one had been a true CAF issue. There was a clear need to understand the entire history, that both material production and PCB manufacture could impact the CAF end result, and that both had to be CAF-optimised to succeed. “A poor PCB manufacturer can turn a good CAF material into a bad one, but a good PCB manufacturer cannot turn a bad CAF material into a good one—both partners have to work at their respective processes to generate an optimum result and qualify for an OEM.”

Dr Despina Moschou, Research Fellow at the University of Southampton, gave conference delegates a fascinating glimpse into the field of microfluidics and biosensors with her presentation on Lab-on-PCB technology for bioanalytical applications.

Enormous progress had been made in the field of lab-on-a-chip (LOC) technology, a subset of micro-electro-mechanical systems (MEMS) devices, in which multiple laboratory functions had been integrated on a single chip to enable tasks such as automated medical diagnostic analysis. Microfluidics was the physics, manipulation and study of minute volumes of fluids. A microfluidic chip consisted of a set of micro-channels etched or moulded into glass, silicon or polymer, connected together in order to mix, pump, sort or otherwise handle the fluid, connected to the outside by inlets and outlets pierced through the chip. Integration of microfluidics and biosensors, the bringing together of these two technologies to enable the analysis of, for example, blood or saliva samples, was the subject of Dr. Moschou’s research.

Lab-on-PCB had been suggested as a solution in the 1990s, but the concept had been side-lined by easier microfluidic fabrication processes. More recently, PCBs had been recognised as potentially ideal integration platforms, particularly because the long-standing industrial infrastructure offered low-cost upscaling. University of Southampton was collaborating in the EPSRC-funded eμ-ELISA project, which aimed to develop low-cost, real-time detection devices by adapting well established PCB fabrication processes to produce bespoke functionalised electrodes coupled with micron-scale fluidic channels and chambers.

Dr. Moschou showed examples of sensors for DNA, lactate and glucose, and fluidic microvalves and micropumps made by PCB techniques, and demonstrated a 3-layer

PCB microfluidics device with reference electrodes on layer 1, sensing electrodes on layer 2 and microfluidics on layer 3, and also a 2-layer PCB sensing electrode structure. Prototype lab-on-PCB devices had now been successfully fabricated and the project was moving forward.

The session on solderable finishes and plating for PCBs was introduced and moderated by Paul Waldner. The first presentation was given by Chris Klok from MacDermid Enthone, who discussed the use of OSP finishes in automotive applications.

There had been interest in OSP in the automotive sector since 2010, the main driver being cost, along with concerns about whiskers, corrosion, electromigration and rework. Tier-one suppliers were driving the upcoming change, and the last challenge to be overcome was press-fit compatibility.

OSP was by far the most widely used solderable finish globally, but mostly in consumer electronics. Immersion silver was a popular finish in automotive, and immersion tin was gaining ground, but these metallic finishes were several times more expensive than OSP. Tier-one suppliers were moving away from immersion silver due to restrictions from several major European OEMs. HASL was gradually being phased out, and changes in soldering procedures favoured OSP. OSP was approved as solderable finish by the majority of tier-one suppliers of automotive electronics, but a generic specification was required for press-fit.

A German consortium of OEMs, tier-one suppliers, PCB manufacturers and press-fit pin manufacturers was doing research on press-fit capabilities on OSP-finished PCBs, carrying out optical analysis to IPC-A-610E, and mechanical and electrical testing and metallographic analysis to IEC-60352-5, with Eye of the Needle, Spring Shape and Cracking Zone pin types.

Insertion force measurements showed OSP to give similar results to immersion tin, and although a lot of qualification work remained to be done for different applications and pin types, and metallic finishes would still be required for specific applications and functionalities, press fit was no longer considered a road block for OSP in automotive and an increase in the use of OSP was to be expected

Still on the topic of preferred finishes for automotive electronics, Rick Nichols, Global Product Manager Final Finishing with Atotech, maintained that immersion tin was gaining market share, primarily due to the confidence of automotive OEMs. It offered maximum solder performance at a reasonable price and had corrosion resistance second to none. He discussed the practical and metallurgical aspects of soldering immersion tin and explained what defects could occur and how they could be avoided.

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