40

Poor Via Hole Fill
Via hole filling with solder has always been difficult during wave soldering which is one of the original reasons why design engineers introduced via hole capping or tenting. A via hole does not have a lead so has to rely on the pad wetting and then capillary action drawing the solder up into the hole. Normally the via hole pad size is very small, hence it is more difficult to wet. In the example the problem is exaggerated by the use of landless vias. The problem of filling vias is also made more difficult if the thickness of the resist coating is increased. The solder is deflected from the via just like the skipping seen on surface mount pads when the resist is too thick.