Paste stuck in stencil apertures led to poor wetting during reflow.

The optical image in Figure 1 shows a side view of a chip-scale package with open joints. The gold pads did not wet during reflow soldering. Close examination showed the process was out of control, with poor solder paste printing, leading to pads not covered with paste. During process review, solder paste was found hanging up in the stencil apertures. With no paste, there also would not be any flux medium to aid reflow in an air environment.

It is possible that if the balls, pad and a nitrogen environment were used, reflow and wetting may have occurred, masking the root cause of poor printing. In fact, the problem is related to poor printing. This may be due to the design of the pad or stencil, poor printing parameters or blocked stencil apertures. The process needs to be reviewed and corrective action put in place. In-process inspection and training also may be beneficial.

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.  CA

Dr. Davide Di Maio is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); defectsdatabase@npl.co.uk. His column appears monthly.