Maintain the right soldering profile to prevent caps from popping.

Through-hole and surface mount electrolytic capacitors can pop or blowout during operation or soldering in a SnPb or Pb-free process. Blowout in operation is related to the component and its use in a specific design that needs to be investigated, and is not necessarily an assembly issue. Poor control of soldering parameters may be contributing to failure, however, as seen in FIGURE 1.

Figure 1. Poor control of soldering parameters may lead to capacitor blowout.

Blowout has been seen where an attempt to copy a device got the electrolyte formulation wrong. A lot of computer manufacturers suffered failures in the field.
Today many producers do not perform the same level of component assessment as they did in the past. Also, there is less derating of parts in design.

Care needs to be taken with surface mount parts. Follow supplier guidelines. Check peak temperatures and confirm profile duration in forced convection and vapor phase soldering and rework; otherwise, the top of the parts will balloon (FIGURE 2). It is not uncommon to see the top of some capacitors expand on the top but the pressure relief points not rupture. The figure shows a part in which the relief points have opened, permitting pressure to escape without damage to the board.

Figure 2. Excessive exposure to high soldering temperature could force open pressure relief points.

These are typical defects shown in the National Physical Laboratory’s interactive assembly and soldering defects database. The database (http://defectsdatabase.npl.co.uk), available to all this publication’s readers, allows engineers to search and view countless defects and solutions, or to submit defects online.

Martin Wickham is a consultant at the National  Physical Laboratory (npl.co.uk); martin.wickham@npl.co.uk. His column appears monthly.

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