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Failure for mechanical reasons

SAQ


This is not just a question of reviewing the last unit, but of remembering what was said earlier! FR-4 is not ideal for mounting components for two reasons:

Being flexible, it will bend during assembly processes such as placement and depanelling, which can over-stress fragile components such as MLCs

There is usually a CTE mismatch between the board and the components mounted on it, which results in stress during temperature cycling, possibly leading to joint or component failure.

Where large, repeated temperature excursions are expected, the design should avoid specifying large, rigid components or potted modules, in favour of using parts with compliant leads, such as QFPs. Where this cannot be done, consideration should be given to choosing an alternative substrate material with a better CTE match.
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