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Reid on Reliability: Cohesive Failures
September 18, 2013 |Estimated reading time: 1 minute
Since the advent of the European Union’s legislation for Restriction on Hazardous Substances (RoHS), lead has been banned from use in solder in surface finishes and pastes used in the PWB assembly process. The alternative metals and alloys to traditional tin/lead (Sn/Pb) solder required that the assembly temperatures be increased to achieve the higher melting point of the lead-free solders. The traditional assembly temperature reached a level of 230°C, but lead-free can require up to 260°C, although many assembly houses are using a more modest 245°C. Multiple exposures to the additional 15°C to 30°C has demonstrated a negative impact to the integrity of the FR-4 and halogen-free dielectric material used in PWB substrates. Quantification of material damage is now possible through a new technique that utilizes capacitance measurements to identify specific levels of bulk capacitance change that signify degradation within the resin system. This technique was employed to non-destructively identify both the locations within the construction and the magnitude of the change. Traditional microsectioning is completed to confirm the results of the capacitance testing.
When material damage occurs, it usually extends thermal cycles to failure, frequently giving false positive artifact results. Adhesive delamination and cohesive failures, however, can reduce the thermal cycles to failure. The crack may propagate starting in the epoxy and go through the copper in the barrels of the PTH, reducing the thermal cycle to failure, but this is a rare condition.Read the full column here.Editor's Note: This column originally appeared in the June 2013 issue of The PCB Design Magazine.