It’s every assembler’s nightmare: you have completed soldering of parts onto a PCB, only to realize that the order requested Pb-free RoHS compliant assembly. That tin-lead solder you used will need to be stripped from the PCBA, and the board will have to be resoldered. How can you remove lead from SMD or through-hole pads and put the board through rework?
This involves 3 steps, with only two of the steps being simple:
- Desolder the parts from the PCBA by hand
- Strip leftover Pb-containing solder from the pads and holes
- Resolder parts onto the board
Desoldering is relatively simple and involves heating up the solder until melting, such as with a hot plate or heat gun. This could be done in a rework station, and the recovered parts would typically be discarded. Something like a BGA would need to be reworked in a more specialized rework station. In this article, we want to focus on step #2, where we remove the Pb-containing solder alloy from the surface layers of the PCB.
Methods to Remove Pb in Rework
In some cases, it is possible to remove Pb from PCBs without scrapping the bare board. In some boards, Pb reduction down to RoHS-compliant levels on the surface layer of a PCB can be difficult when certain surface platings are used. If ENIG is used as the surface plating, any bonded Pb alloy can have very high adhesion strength to the ENIG plating. Conversely, for something like OSP, the Pb alloy stripping process may ruin the surface treatment and create difficulty in further soldering onto the leftover pads.
Braided Wire With Flux
On smaller scale orders, or if a designer is working from home, the first method of attack is to remove the solder with a braided wire and flux. For any parts that need to be hand soldered, braided wire is needed to remove most of the solder from the parts and SMD pads on the PCB. Some flux will be used with the braided wire to encourage solder flow into the braid.
Use a braided soldering wick to remove the largest solder joints.
This will pull off most of the solder but will leave Pb bonded to the surface treatment on the exposed pads. This is a good way to get started, but further removal of solder down to RoHS-compliant levels will need further processing. After the initial removal with wicking, the simplest method to remove the remaining Pb-containing solder is to apply a Pb-free solder, allow it to set, and then remove it again with braided wire and flux. If you perform desolder/resolder three times, you will typically get the leftover Pb content below 0.1% on the SMD pad, which will comply with RoHS.
One standard method of removing and replating solder on SMD pads and holes in a PCB is with immersion dilution. In fact, when holes are infiltrated with solder, dilution is the only reliable method to fully remove the Pb-containing solder from the board.
Immersion dilution is a process where the PCB containing Pb solder is dipped into a molten pot of Pb-free solder. When the PCB is immersed, the Pb-containing solder will also melt and will flow away from the board (make sure flux is applied). Pb-free solder can then flow onto the pads and provide a thin coating onto pads/holes on the surface layers. From here, the Pb-free coated pads can be soldered in a standard process.
Grinding or Abrasives
In the extreme case, a grinder or abrasive can be used to remove solder from SMD pads. Note that this process will not help with solder that has infiltrated through-holes; immersion dilution will be the only process that can remove solder from through-holes. Grinding can be performed with a small hand grinding tool, such as a Dremel tool. Be careful with roughness; use a very light grinder to expose the copper, and once you start to see copper you should immediately stop grinding to avoid totally stripping the pad from the PCB surface.
A small hand grinder is best for removing Pb solder from the surface of a PCBA.
To Scrap or Not to Scrap?
In the worst case, where you can’t remove the Pb solder alloy from the surface layer, you will be forced to dispose of the boards and eat the scrap cost. If there is a tough plating (ENIG or similar Au-containing platings), this is probably the best option that will preserve operator time and sanity. In a low volume situation, this is very unfortunate and will require re-running the boards through fabrication. In a high volume situation, the scrap cost is usually manageable and should impact margin.
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