Reliability Testing and Failure Analysis: Lessons Learned


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However elegant and powerful the scanning electron microscope might be, the optical microscope remained one of the most valuable general-purpose inspection tools available, especially when used with an intelligent choice of bright-field, dark-field or cross-polarised illumination to reveal different features and structures. Dr. Anselm stressed the critical importance of proper sample preparation, declaring that microsectioning and cross-sectioning was an art-form, although simple precautions like thorough cleaning between successive grinding and polishing stages could help enormously in achieving good-quality sections. Micro-etching of copper-based structures with peroxide-ammonia revealed different grain structures and intermetallic boundaries. Brute-force techniques such as dye penetration testing offered a quick way of checking which joints had failed under a BGA, or which pads had cratered, as an alternative to laboriously microsectioning every row of interconnections.

To round off the morning workshop session, Dr. Anselm invited suggestions from the audience as to the root cause of a cracked capacitor on a PCB assembly. The customer had supplied a couple of badly focused photographs of the component in question and wanted to know what had happened and whether it had been a defective component. Typically, the customer did not tell the full story and it was clear that without further and better information no conclusion could be reached. Some help was at hand from a component supplier’s white paper on failure mode classification, which gave examples of defects and their causes and from the shape and position of the crack suggested that it had been induced by thermal stress. It was not until the actual assembly was made available for examination that it could be demonstrated that the failure was a result of clumsy rework of an adjacent component. “What do you tell the customer?”

The afternoon workshop focused on a discussion of the reliability of lead-free solders, surface finishes, and lead-free-compatible laminates and Dr. Anselm began by reviewing  the consequences of RoHS on PCB assembly, with particular reference to solder alloys, PCB processing temperatures, and PCB surface finishes.

Because RoHS restricted the use of lead except in some specific high-reliability applications, a wide range of lead-free solders had become commercially available, of which the most popular were tin-silver-copper (SAC) alloys: SAC105 containing 1% silver and 0.5% copper, SAC 305 containing 3% silver and 0.5% copper, and SAC 405 containing 4% silver and 0.5% copper. Apart from having higher melting points than eutectic tin-lead solder, SAC alloys showed significant differences in microstructure. A characteristic of tin was its anisotropic tetragonal crystal structure, which resulted in large differences in mechanical properties, depending on the lattice direction. Tin-lead alloys tended to have multiple small grains, which averaged-out these differences. But SAC alloys generally crystallised as a few large grains which, depending on their orientation, could have significant effects on reliability.

In general, SAC alloys were stronger than tin-lead, and their ductility varied with silver content--SAC105 was more ductile that SAC405--but there was not a direct correlation between strength and field reliability, it depended on the conditions. For example, SAC405 gave better results on thermal cycling whereas SAC105 gave better results on drop-shock.

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