In previous blogs, we’ve discussed many of the specification requirements and process considerations involved with industrial plating. Today, we continue with solderability. This refers to how easy a metal is to solder to. The goal is to wet the substrate or metal to be soldered with an even coating of solder. As you could imagine, it’s an important factor for components used in the connector and other electronics industries.
When evaluating solderability, three key terms are used: wetting, non-wetting and de-wetting.
Wetting and non-wetting are easy to visualize since we’ve all seen water bead on a waxed car. The wax prevents the water from wetting the surface causing the water to bead. The beading of the water would be non-wetting. Most people might believe that the waxed car’s surface is very clean, but it’s the wax coating that prevents water from wetting the surface.
When evaluating solderability for electronics/connector applications, de-wetting and non-wetting are often confused. Here, materials such as copper or beryllium copper are plated with nickel and gold, tin, or tin lead for enhanced solderability. Each plated layer of metal serves a particular purpose. Nickel plating prevents copper oxidation and copper migration to the surface over time, or heat can act as a catalyst, but nickel easily becomes passive and needs to be activated to be soldered to. The top plating layer preserves the active nickel surface on the soldered side of the connector. It also provides conductivity to the side where electrical contact is made.
If properly applied, the plated part should have a smooth continuous flow of solder. However, stamped parts could have some edge pullback of the solder due to the surface tension of the sharp edge. This is to be expected and is acceptable. When solder doesn’t flow smoothly, de-wetting or non-wetting is occurring. It is important to differentiate between the two to determine the problem’s root cause.
In the case of gold plating, the part is non-wetting if gold is visible in the area where the solder pulled back. This would be caused by the surface having something on it that is preventing the solder from wetting it, similar to the effect of wax on a car. Re-cleaning may be all that is needed.
Years ago, the plater and incoming inspection at a connector house had to use an “R Type” flux when soldering. Often, this did not prepare or clean the part well enough to accept solder and failed otherwise good parts. Now the J-Standard requirements allow for rosin mildly active (“RMA type”) flux to be used. It is much more reliable and is consistent with what is used in pre-tinning the connector.
De-wetting occurs when the solder beads or pulls back and exposes a nickel-plated surface. When soldering gold, the gold is removed and the solder bonds with the nickel. If the nickel is passive, this solder will not adhere or wet the nickel underplate, resulting in de-wetting. This can only be remedied by removing the gold and activating the nickel surface.
A clean non-passive surface is critical to good solderability.