PCB007 Magazine

PCB-Aug2016

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78 The PCB Magazine • August 2016 of the plastic strain that is found in the copper PTH barrel is not reversible. The PTH acts like a rivet, which resists this expansion, but the copper barrel is stressed and may crack, causing electrical failure. There is increasing strain on the barrel associated with a high-temperature excursion. Failure may oc- cur in a single cycle or may take place by ini- tiation and growth of a fatigue crack over the course of a number of cycles. For high-aspect- ratio through-holes subject to repeated thermal shocks from room temperat ure to solder reflow temperatures (220–250°C) during board fabri- cation (e.g., hot-air solder leveling) and assem- bly (reflow, wave soldering, rework), it is not uanheard of to encounter failures after 10 or fewer of these thermal cycles. While the Cof- fin-Manson model works fairly well in explain- ing failures, it does not take into account sev- eral other issues that may affect PTH reliability. These are: • Poor copper plating distribution in PTH • Poor leveling leading to thin areas of copper plating (Figure 1) • Glass fibers protruding from poor drilling (Figure 2) • Resin interactions with glass weave • Changes in copper grain structure from lot to lot • Rough plated copper • Copper ductility variations from lot to lot, board to board The majority of the issues listed above can be controlled while others depend on other fac- tors such as circuit board design, resin material properties and copper foil thickness, to mention a few. Process design, robust chemical processes and equipment will also influence the reliabil- ity of the PTH through various interactions. Protruding glass fibers (Figure 2) affect PTH reliability in several ways. As an example, poor drilling practices cause the glass fiber bundles to be ripped out from the hole wall. These fibers then can act as a high spot leading to poor plat- ing distribution within the hole wall. These ar- eas are stress points that can lead to PTH failure during thermal loading. Additionally, a critical concern with respect to PTH reliability is the wedge void. Obviously, there are a number of possibilities leading to this type of defect. First, one should note (although it is not exclusive) that the wedge originates at the B-stage side. The mechanism of the "wedge void" formation discussed below will illustrate BUILDING RELIABILITY INTO THE PCB, PART 2 Figure 1: Inadequate leveling leading to thin plating in the PTH (Source: IPC, Bannockburn, Illinois). Figure 2: Rough and/or poorly drilled PTH. Glass fiber protrusions are evident.

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