Reliability Testing and Failure Analysis: Lessons Learned

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Ionic contamination of PCB assemblies was an ongoing area of concern, and ROSE-type tests gave limited information on the constituents of the contamination. Ion chromatography was a technique for separating and analysing complex mixtures of anions and cations so that individual species could be identified and their likely origin determined. Ion chromatography was very useful in investigating cleaning issues with low-standoff components and could be used on individual components if necessary.

The wetting balance, used in accordance with IPC J-STD-003, Solderability Tests for Printed Boards, continued to be an invaluable measuring tool for determination and investigation of soldering problems on PCBs and components.

Dr. Anselm turned the attention of the workshop to a discussion of the various microscopy techniques available and their applicability in defect investigation and failure analysis. Scanning acoustic microscopy was becoming a popular non-destructive analysis method, using ultrasound to detect changes in acoustic impedance in components and packages. Pulses of different frequencies were transmitted to the package through a fluid, and the intensity of reflections recorded and analysed to detect air-gaps, voids and delamination and to allow different interfaces to be examined and their mechanical integrity to be determined. Higher frequencies gave good resolution but low penetration and were suitable for examining flip-chips, whereas lower frequencies gave higher penetration at the expense of resolution and were more suited to BGA examination. White light interferometry was a useful technique for measuring the coplanarity of flip-chip bumps.

Scanning electron microscopy (SEM) was very widely used (and in some cases misused) for studying surface features and, in conjunction with energy dispersive X-ray spectroscopy (EDS), for providing rapid analysis of elemental composition. Dr. Anselm explained the principles of SEM and the differences between secondary electrons, backscattered electrons, and X-rays. Secondary electrons were emitted from atoms at the surface and produced an image of surface topography but could give confusing results in the interpretation of intermetallic areas if used on polished samples. Backscattered electrons were primary beam electrons reflected from atoms in the solid, and contrast in the image was determined by atomic number so it could show the distribution of different elements. Interaction of the primary beam with atoms in the sample caused the emission of X-rays whose energy was characteristic of the parent element. EDS worked by detecting and measuring X-ray energies to provide a means of elemental analysis.

An interesting case example involved the investigation of brittle failure of BGA joints in a laptop computer. The BGA finish was nickel-gold and the PCB finish copper-OSP, with tin-lead solder. But counter-intuitively, the brittle failure occurred at the component, rather than the PCB interface, and the incidence of failure appeared related to time above liquidus in the reflow operation. EDS analysis revealed a nickel-copper intermetallic at the component interface. Where had the copper come from? It had dissolved in the solder at the PCB surface, a longer time above liquidus had allowed it to diffuse into the bulk solder and then precipitate as a brittle intermetallic at the component interface. The problem was overcome by reducing the time above liquidus, using an adhesive to improve mechanical compliance, changing the PCB finish to nickel-gold and asking the component supplier to consider changing his under-bump metallisation from nickel-gold to copper--for reliability rather than cost purposes.



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