Rolling with solder balls: finer powders may be more prone to solder balling.

Author:Di Maio, Davide
Position:THE DEFECTS DATABASE
 
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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...

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