Checking solder joint quality after wave soldering

Introduction
The solder joint quality is not only depending on the solder process but is also related to the joint design.
During the visual control of the soldered joints this design aspect should be taken into consideration.
A sound soldering process is only possible when the solderability of all components involved is in agreement with the demands for the process. This has to be ascertained on forehand and is no subject in this document.
This means that defects related to poor solderability will not be discussed.
Control criteria
1. If a PCB with PTHs is so designed that in a sound wave soldering process all PTHs can fill with solder, than these holes will be filled with solder during soldering. This means that during control of the component side one only has to check if the solder in these joints did at least wick-up to the hole rim. If so then this aspect is OK.
2. Next the solder side should be inspected.
Note: Here one should check if the joint contains the correct amount of solder. This amount is mainly depending on the lead diameter. Leads with a small diameter will due to the physical laws that are involved in the joint formation get a smaller solder cone.
3. In the joint the contours of the connected parts should be visible.
4. The wetting angle (contact angle) between the parts should be evenly shaped and possibly small.
Note: Due to the use of solder resist the solder at the joint has no possibility to drain off along the tracks. This means that often "fat" joint fillets can be formed as a result of that. Such joints do not need to be touched-up, as these are perfect joints.
5. The solder surface should be smooth and even.
Note: With lead-free solders a rough surface is often unavoidable due to the difference in solidification behaviour of the different metals in the solder alloy like copper and silver that can form with tin different eutectic combinations. These combinations will solidify at different temperatures and so create areas at the joint surface with different surface structures. At such surfaces even small cracks can be found either due to the natural 4% shrink of the solder during solidification, or as a result of the moving solder pad during the solder solidification stage. These solder pads can get a wedge shape during soldering, but will return to the flat shape again when the board cools down to ambient temperature. This mechanism can also result in pad-lifting, fillet-lifting or fillet-tearing.
6. Small blowholes at the joint surface are allowed and should be no reason for touch-up. They indicate however some defects in the metallization of the hole barrel, which may cause outgassing during soldering.