PCB Assembly

Why did the SMT pad fall off easily when soldering the PCB boards?

I’m guessing that this was a hand-soldering operation. Most likely, the iron was too hot, or it was held on the pad too long, or both. If the latter, there may have been insufficient flux to enable good heat conduction into the joint, so the operator was forced to keep the iron in place longer than should have been necessary.

The epoxy in the PCB gets distinctly soft above the glass transition temperature(Tg). Guess what? All normal soldering operations happen above the glass transition temperature!

Item Temp.
Traditional FR4 boards Tg: 135C
modern boards for lead-free soldering processes Tg: 170C
Solder melting temperatures for eutectic Tin-Lead (Sn63) 183C
melting temperatures lead-free solders (SAC305) 217C
oldering iron temperature above the liquidus 130C

Facing this problem in soldering operation, all we need to do is making it so fast that there isn’t time for the epoxy to go soft, and that there is no time for the copper pad to lose all adhesion.

And, greater soldering technique should allow the flux to do the work of heat conduction. The soldering iron tip even does not, in fact, touch either component or PCB.

 

In real life, we almost all do allow such contact, but we take care not to exert pressure while doing so. Because undue pressure is what causes pads to fall off if the temperature has gotten too high!

 

In addition, tip condition may be the second factors. Tips, which are badly tinned, stop the heat transfer to the joint. Subsequently, many operators have to increase the iron temperature and press the iron into the board hard enough to do push-ups, causing SMT pad falling with ease.

#PCB Assembly #SMT PCB Assembly #PCB Manufacturing

Oliver Smith

Oliver is an experienced electronics engineer skilled in PCB design, analog circuits, embedded systems, and prototyping. His deep knowledge spans schematic capture, firmware coding, simulation, layout, testing, and troubleshooting. Oliver excels at taking projects from concept to mass production using his electrical design talents and mechanical aptitude.

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