A lecture on Soldering
Ⅱ. Pb-free Soldering
6. Appearance of soldering surface
Observe the surface of Pb-free soldering tin after the job is done each time. You will get familiar with the appearance and before long, be able to identify the Pb-free soldering at one sight. It is featured by the following 2 distinctions:
- ① Glossless, grayish, and coarse
- ② Compared with eutectic solder, it presents no metal shining.
Furthermore, compared with eutectic soldering surface, Pb-free soldering surface gives the impression as if it were overheated. So the soldering quality can not be told by looking at the surface. It can be judged by the formation of fillet.
The photos above show both the eutectic soldering and Pb-free soldering, in identical profiles and with same tin quantities. It can be noticed that the soldering on the right presents grayish with no metal gloss. It can be judged that the Pb-free soldering is qualified as long as a fillet has been formed, though grayish and with no metal gloss.
6.1 Cold soldering easily occurring
Pb-free solder is sticky, and demanding for more iron thermal capacity, which contribute to cold soldering due to under-heat. It must be watched out.
The photo on the right shows a typical scenario of cold soldering easily occurring in Pb-free soldering operation. People who get used to eutectic soldering are prone to be involved in this scenario when Pb-free soldering is conducted with the same condition. Just a little change with wire solder and an iron head will come to the normal soldering operation as shown in the photo on the left. The way of change will be explained in section 7 “Improving the Performance”.
6.2 Concaves and air pockets
In most Pb-free soldering cases, concaves on the soldering surface or air pockets in hardened soldering tin could be formed, because pb-free soldering is more drastic regarding thermal expansion and shrinkage.
Modest concaves can be ignored, since they have no influence upon the soldering strength. But for the relatively big interior air pocket shown in the above photo, it should be watched out.
The flaws in the photo on the right all occurred in dip soldering. Manual operation won’t have the problem like that.