Excessively large metal particles can block stencil apertures.

Solder paste consistency is very good lead-in to consistent printing yields, provided we remember the basics. When looking at process improvements, consider running simple trials on the paste product used or under evaluation. (Tests are detailed in J-STD-006 or IPC-TM-650, Test Methods Manual.) Solder paste balling, solder paste slump and particle size are well-documented by good suppliers and in international standards. Inconsistency, however, for whatever reason, can lead to blocked stencil apertures and defects.

Figure 1 shows a small sample of paste taken from a fresh pot of paste placed in a solvent to separate the metal particle. Clearly there is an issue here. The figure shows a range in size of spherical particles that may well be within specification. Review the standards and supplier documentation. There are samples of particle twining and many solder splashes, often found when the sample board is manually inspected, rather than relying only on AOI.

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.

Chris Hunt is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column appears monthly.