A lecture on Soldering
1. What is soldering?
The photo shown above is the soldered seam magnified through a microscope, in a printed circuit board (PCB), on which is the Cu pattern. From the bottom to top are the PCB layer, Cu pattern layer and soldering layer. The alloy of tin and copper (metal compound) is formed between the tin solder and the pattern, as indicated in the red circle.
Soldering is often mistaken as adhesive work or ordinary welding. It is often thought that the job has been done as long as two parts have been jointed after being soldered.. However, actually the job has not been completed until the alloy layer is formed at the soldered spot.
Cohesion works when two parts are cemented as adhesive is hardened while welding works after the mother metals are fused and then hardened. But, soldering works through the formation of alloy layer.
Thus, the so called “soldering” is the technology of alloy formation.
Normally the alloy layer should be 3-9 um thick. Suitable thickness cannot be formed unless the heat is sufficient. However, if it is overheated, the alloy layer will become fragile due to its excessive thickness. So heating control is the key to good soldering.
The advantage of soldering is that the mother metals can be jointed without being fused as soldering can be done under a relatively low temperature, which is really good for jointing electronic parts.
In soldering, the fused tin solder flows to the joint by means of capillarity and wettability. Workmen involved in soldering can often hear words like “It has not been fully wetted yet…” The followings are explanation of “capillarity” and “wettability”.
As is known to everybody, if a dry cotton cloth is half dipped into water in a bucket with the other half out of the bucket, water will be creeping to the dry half and dripping to the ground outside the bucket.
Thanks to molecular attraction, liquid molecules tend to get as close as possible and reduce their surface area exposed to air.
So liquid will be siphoned into the narrow gaps in stiff shape or fiber shape if any.
In this same way, fused tin solder is siphoned flowing into the gaps among bound wires or between electronic parts.
It’s something like “being wet” or “being soaked”. The so called “in a wettable state” as shown in the photo on the left refers to the tin solder fused in thin and expanding state with a large contact area with the copper plate. Nevertheless the so called “in an unwettable state” as shown on the right refers to the tin solder fused in a water drip shape like a small ball with almost “next-to-nothing” contact area with the mother metal. We sometimes describe the fused tin solder as “has good wettability” or “do not have good wettability”. An alloy layer can only be formed when the soldering tin “has “good wettability”, that is to say, soldering will not work if soldering tin has “next-to-nothing” contact area with either part to be worked on.