Ask Helena & Leo

Question: How do I get a flow through on a 120 thick pcb with 3 oz copper layers. I saw your soldering tip #37. We have tried multiple applications but with no consistent success. Is there a special iron to use?

Answer: Soldering thick printed circuit boards or a board with thick copper foil, filling the holes with solder is based upon a few elements which need to be addressed.

1. Board design. Make sure there are heat relief areas where the copper layers intersect with the plated through hole. This will prevent the thick copper layers pulling the heat away from the plated through hole.
2. Flux. Make sure the flux covers all the areas to be soldered and prepares those areas. Is it the correct flux? Does it evaporate too quickly, and is it easily applied?
3. Temperature of the board. To fill the hole the holes they need to be heated above the melting point of the solder being used. If the solder solidifies when filling the hole, then it will be most difficult to get that particular solder to melt again so the hole can be filled.
4. Soldering tool or iron. The solder iron needs to have enough wattage to provide consistent heat over a period of time and that period of time is the time it takes to heat the board above the soldering temperature of the solder and keep it there until the solder has completely filled the hole.

If the incorrect solder iron is used or the technique is incorrect, improper heat will be applied to the plated through hole and it will not fill with solder.

In the solder tip, it was mentioned to preheat the board. Another avenue would be to place the board on a heat plate or above a preheater while it is being soldered as this would help in getting the board up to temperature prior to soldering.

The use of two heat sources is also acceptable, as this would be the use of two soldering irons to heat the hole prior to making the solder joint. Apply the irons to each side of the board, heating the barrel and then make the solder joint with the iron from one side of the board.

The board could also be processed through the wave solder system and the temperature of the board can be controlled through the preheaters in the board and the speed of the conveyor. Slowing the conveyor speed will also allow the solder to heat the board and allow the solder to flow up through the plated through holes.

If the product is a surface mount product, then solder paste could be applied to the component holes and if the components are able to withstand the thermal excursion of the surface mount profile, the boards can be processed through the surface mount oven and soldered in that fashion.

I would also recommend a high wattage soldering iron to do this work and these can be acquired through a variety of sources.

As a final note, once the solder starts to melt, keep the solder iron on the joint, do not remove and reapply, as once the iron is removed the temperature is lost and regaining the temp will be most difficult if not impossible, so do not remove the solder iron from the joint until the solder joint is completed.