Why use gold plating?
Gold is typically used in high reliability, low voltage or low current applications. Gold is used in high cycle applications because it is durable and has excellent abrasion resistance. Samtec's gold is alloyed with cobalt, which increases hardness.
Gold is a precious metal, which means it doesn't respond much to the environment. Therefore, gold is also recommended for harsh environments because it does not produce oxides that can lead to increased contact resistance.
Gold is sometimes a "must have" because as connectors are miniaturized, the contacts are too small to generate much normal force. Therefore, low normal force guides the need for gold plating.
The disadvantage of gold is primarily cost, followed by porosity of thinner plating, and some differences in solderability. Specifically, many customers "successfully" soldered these, but they did not solder to Au because Au dissolves in the molten solder. They are soldered to nickel under Au. So it's technically true that gold has poor solderability.
Why use tin plating?
Tin is a low-cost alternative to gold with excellent solderability. Unlike gold, tin is not a precious metal. Tin plating begins to oxidize the moment it is exposed to air. Therefore, tinned contact systems require greater normal force and a longer contact wipe area to break through this oxide film.
Tin is usually preferred because of its relatively low cost in applications with a small number of mating cycles with appropriate normal force.
What are the most popular plating options?
Selective gold tin plating is Samtec's most popular plating option as it gives designers the best of both worlds. The contact area, the critical area for contact and terminal pins and signal transmission, has the reliability of gold. The tails are soldered to the board for lower cost and improved solderability of tin.
Can gold be welded?
This process should be considered very carefully, as the gold plating dissolves in the solder, so the risk of solder bath contamination and gold embrittlement is real:
As mentioned before, solder paste will not solder to the gold board. The gold plate is dissolved in the molten solder and the soldering is done on the nickel base plate.
Embrittlement becomes a problem once the gold weight contributes 3 to 5 percent to the solder joint.
What effect does gold plating thickness have on electrical contacts and connectors?
Gold prices are hovering near all-time highs, so manufacturers that use it are expected to want to cut costs as much as possible. Therefore, we are often asked if the amount of gold can be reduced. It depends on the situation.
There are two ways to achieve this: either reduce the plating area or reduce the thickness of the gold deposit. In some cases, selective plating can be used to reduce areas that require gold.
As for reduced thickness, it depends on the application, many of which are governed by MIL specifications or ASTM specifications. This is especially true for gold-plated components used in the military and aerospace. Commercial and consumer products may be candidates for thinning, but the possibility of sacrificing performance and longevity must be considered.
Specifying and maintaining proper plating thickness is critical to getting the best performance from passive electronic components. Many factors must be considered when specifying thickness, including product application, environment, and expected life.
For some components, insertion and extraction forces and life cycle testing may be of greatest concern. Regardless of the thickness used, nickel-hardened gold may hold up better than its softer pure gold. In this case, thickness is not as important as hardness. However, if life cycle testing is critical, a thicker gold coating of 0.000050 inches will last more cycles than 0.000030 inches.
For other components, solderability or porosity may be key characteristics. When porosity is important, a heavier gold plating or two layers of gold is preferred. However, higher gold thicknesses can negatively impact solderability. More than 0.000050 inches of gold will cause the solder joint to become brittle, so less gold is best for it to bond properly to the nickel base plate.
If you need high cycles, low force, good porosity and solderability, don't worry. A gold deposit with 0.000050 inches of gold will provide all of the above if the connector is pre-tinned before being put into service. Pre-tinning will remove most of the gold and protect the nickel surface with a tin coating. Any remaining gold is removed when it is finally soldered in its application.
Many contact parts only mate once in a relatively mild environment, which makes insertion/removal and porosity less critical. This is another example where gold is enough.
So, can thinning save money? In some cases, yes. But we must remember that the main reasons for specifying gold for connector parts are corrosion resistance and electrical conductivity. These factors must be at the heart of any thinning decision.
