Steps for determining the root cause of peeling problems.

One of the most common problems the industry faces is poor adhesion of conformal coating, causing bubbles and dewetting. FIGURE 1 shows peeling of the coating from the surface of large quad flat packages (QFP). This can happen to any user. If the problem is confined to just one type of device, it is safe to assume the cause is contamination on the IC plastic packaging, probably related to the molding process.

Figure 1. Conformal coatings are susceptible to peeling due to contamination or poor adhesion.

It’s fair to say the component supplier may offer no support on this issue, and the onus is on the assembler to put corrective action in place. Most coating suppliers have been faced with this problem over the years and may suggest cleaning, an adhesion promoting step, plasma treatment or a different coating material.
These may be better solutions, but always first check whether the assembly process itself is contributing to the process problem.

Take some of the plastic components and some samples that passed through assembly, including any rework and cleaning steps. Place these on a bare board and coat the QFP packages following the same coating parameters. It’s not difficult to perform a simple coating program. After curing, test the adhesion to determine if some other process is exacerbating the coating separation.

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. To complement the defect of the month, NPL features the Defect Video of the Month, presented online by Bob Willis. This describes over 20 different failure modes, many with video examples of the defect occurring in real time.

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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