The data for most SMDs used in wave soldering came from experiments with the objective to find guidelines for the most dense packaging possible. Such guidelines are always a compromise. Sometimes the outcome of such a compromise is less favorable than expected.
The relative small pad dimensions for the SOT 23 (Philips gives 1.3 x 1.3 mm²) give a relative high risk for skipped joints. These skipped joints were never touched by the solder due to a combination of factors, such as flux encapsulation and outgassing together with the small solder pad dimension.
If the pad dimensions should be enlarged the skipping effect due to flux concentrations could be almost completely eliminated. For this reason, based on new investigations the pad dimensions for the SOT 23 will be considerably enlarged. Although no official data are available yet we have seen pad dimensions of 1.4 x 1.8 mm².
With pad dimensions of 1.4 x 1.8 mm² also the discussion for the preferred soldering direction for the SOT 23 is not relevant anymore.
A longer contact length e.g. by use of a 30 ° nozzle-angle might also improve the wetting behaviour on critical designs
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