At a recent local SMTA chapter meeting, the technical committee asked for a paper on how to manage solder dross. Dross is oxidized tin, lead or silver (depending on the alloy used in the wave/machine soldering process), that forms on the surface of molten metal.
Given how mature wave soldering technology is, I was amazed at how many experienced electronics assembly professionals had never before considered how to manage solder dross, which if not managed, can affect reliability and performance of a PCB.
It is true that the use of wave soldering has declined precipitously over the last 15 years as surface mount technology has grown more popular due to its ability to enable smaller and higher functionality in electronic products (i.e. Smart phones, smart televisions, IoT devices, but wave soldering is still alive and well. Medical, military, aerospace, automotive and agricultural electronics still use wave soldering, therefore it is important for electronics assemblers to understand the value that can be recovered from solder dross.
Wave soldering generates dross when molten solder is exposed to oxygen. Dross formation can interfere with the wave soldering process by not providing a wettable material, and being incapable of forming an intermetallic joint between the plated through hole and the through hole component.
Therefore, removing dross from the wave soldering machine is critical to maintain through put and first pass yield.
But how can the value of this solder dross be recovered? The process of skimming dross from the wave solder pot always involves the collection of un-oxidized metal. There are several technologies that can separate most of the un-oxidized metal from the metal oxide (dross). However, dross (oxides) are typically entrapped in the recovered metal.
The most thorough way to recover metal from dross skimmings is to chemically reduce oxides back to their base metals. The basic chemical equation for reducing tin oxide to tin is shown below:
SnO2 + 2H2(g) = Sn + 2H2O
The process for reducing tin oxide to tin is one that is highly complex and rather delicate, as high pressure, heat and hydrogen can become an extremely explosive material. But when the process is perfected, a very large percentage of the oxide can be reduced to recoverable metal. Simple techniques can be used to separate metal from oxides but will not produce as much recoverable metal.
Maximizing the return received on metals and oxides skimmed from a wave soldering machine depends on the technology used to recover the metals. Electronics assembly professionals should use a well-developed and optimized chemical reduction process that will result in higher value from the material recovered from solder dross.