Finer powders may be more prone to solder balling.

This month we feature one recent problem of solder balls or fines.

Dip solder paste now commonly used in package-on-package (PoP) assembly has a lower metal content, more often designed for nitrogen reflow than air, and also has smaller solder particle size. The powder may be Type 5-6, as opposed to a stencil printing grade of Type 3-4, and hence more prone to solder balling during reflow. It is fairly uncommon for standard SMT solder paste to slump and solder ball like the example, unless it has been poorly stored.

It’s important for engineers to learn how to conduct solder balling and solder slump measurements of paste from different vendors. IPC and SMART Group specifications describe tests of this type using automated test equipment available from a range of suppliers. In simple terms, a sample of paste is printed on a non-solderable surface – in this case, a ceramic tile – and reflowed. The result observed should be a lack of spread during preheat, one single ball on the surface of the plate after reflow.

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.

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.