A lecture on Soldering
7. The hardened surface shape after soldering
As shown in the photo on the left, the good soldering shines, with its shape like Mount Fujiyama, naturally sloping down outward. This profile is called “fillet”, which is a very often used term in soldering trade and supposed to be remembered.
New hands and workers who haven’t understood soldering correctly are prone to make mistakes shown on the left. The work also looks shining at the first sight, but seems it hasn’t been solidified yet, like a round drip of water.
Actually, the work in the photo on the right is done under-heated deliberately to serve as a negative example. The solder looks firmly solid as there is a lot of tin on it. But no fillet is shaped in it and as a result no good joint between the conductive wire and tin or that between Cu pattern and tin has been made, in another word, no alloy layer is fully formed in most cases. Should some electric products be worked in this way, heating or even flame may occur when the electric products have been put into use for some while due to the faults in electric conductivity.
The soldering with no fillet formed is unqualified, called “cold soldering”. If soldering is done correctly (both mother metal and tin solder are heated to about 250℃), a fillet will be formed automatically. So the formation of a fillet serves as a criterion in judging soldering quality.
7.1 Amount of tin solder
Amount of tin solder used has a lot to do with the formation of a fillet. Generally speaking, the profiles of conductive wires and terminals (of any parts) can be seen after being soldered. It might be thought that the more the tin solder is used, the more secure the joints would be. However, as was stated above, soldered parts joint firmly through an alloy layer of 3-9 um thickness. So even if more tin solder is used, it contributes little to the joint strength.
The profile of conductive wires and terminals of any parts should be seeable after being soldered. People would be surprised when they see the photo of the soldering sample, because tin solder used is less than expected. As is shown on the right, it’s hard to tell if the alloy layer was formed properly because the wire and the terminal are covered by too much tin solder. I personally think it is right to take the statement---- the soldering shall be regarded as unqualified if an inspector can not tell whether the soldering operation was done in a proper way----as a guideline.
7.2 Examples of unqualified soldering
Shown below are commonly seen unqualified soldering examples