Question: We are attempting to hand solder through hole components in a very thick board, but are finding that the solder does not flow through to the other side of the board. Is there a method or technique I need to use to accomplish this?
Answer: Manual soldering of thick boards is a very delicate operation. Solder iron temperature, solder iron heat capacity, preheating of the board, are all issues which need to be considered when trying to fill these types of plated through holes.
More than likely, there is not enough heat or thermal capacity in the solder iron to heat the entire hole up to soldering temperatures, thereby the solder solidifies going up through the hole and that’s it, it doesn’t fill any further. One has to keep in mind the different thermal requirements when soldering such plated through holes especially through such a thick board with internal layers drawing heat away from the joint.
Once the solder solidifies in the hole the thermal requirements change drastically and more heat and energy are required to re-melt that solder to completely fill the hole.
My recommendations would be to preheat the board as the solder joint is being made. Use a solder iron and tip, which have plenty of energy to continuously supply heat to the solder joint and keep the solder molten, yet not hot enough to damage the laminate material and plated through hole inner layer connections. There are solder iron suppliers out there who can help you with this application.