Connector Plating: What Works and What Doesn’t

Like most electronic components, the quality of countless sub-components and processes directly affects the quality and performance of the finished product. For PCB grade connectors, these factors include pin material, plastic type, quality of the molded plastic body, coplanarity of the tails, quality of the surface finish (plating), choosing the correct connector plating, manufacturing/assembly process (putting the pins into the plastic) and packaging, etc.

Connector plating is mission critical. Affects the life and quality of the terminal or socket; it affects corrosion resistance, conductivity, solderability, and of course cost.

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 wear resistance. Samtec's gold is alloyed with cobalt, which increases hardness.

Gold is a precious metal, which means it is not very responsive to the environment. Therefore, gold is also recommended for use in harsh environments as it does not produce oxides that may cause increased contact resistance.

Sometimes gold is a "must have" because as connectors become miniaturized, the contacts are too small to generate much normal force. Therefore, low normal force directs the need for gold plating.

The disadvantages of gold are primarily cost, followed by the porosity of the thinner plating, and some minor differences in solderability. Specifically, many customers "successfully" soldered these, but they did not solder to Au because the Au dissolved in the molten solder. They are soldered to nickel under Au. So you are technically correct that gold has poor solderability.

Why use tin plating?

Tin is a low-cost alternative to gold and offers 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 longer contact wipe areas to break through this oxide film.

Tin is usually preferred because it is 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 because it gives designers the best of both worlds. The contact area, the critical area for contact with the terminal pins and signal transmission, has the reliability of gold. The tail is soldered to the board for lower cost and improved tin solderability.

Can gold be welded?

This process should be considered very carefully, as the gold plating will dissolve 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 soldering is performed on the nickel base plate.

· Once the gold weight contribution to the solder joint is 3% to 5%, embrittlement becomes a problem.

What effect does gold plating thickness have on electronic contacts and connectors?

Gold prices are hovering near record highs, so manufacturers who use it are expected to want to cut costs wherever possible. Therefore, we are often asked whether the amount of gold can be reduced. It depends.

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 the areas where gold is needed.

As for thickness reduction, that 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 military and aerospace applications. 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 obtaining optimal performance from passive electronic components. Many factors must be considered when specifying thickness, including product application, environment and life expectancy.

For some parts, the greatest concern may be insertion and extraction force and life cycle testing. Regardless of the thickness used, nickel-hardened gold may hold up better than its softer pure gold counterpart. In this case, thickness is not as important as hardness. However, if life cycle testing is critical, a thicker gold plating of 0.000050 inches will last more cycles than 0.000030 inches.

For other components, weldability or porosity may be critical characteristics. When porosity is important, it is better to have a heavier gold plating or two layers of gold. However, higher gold thickness can have a negative impact on 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 high cycle, low force, good porosity and solderability are required, don't worry. If the connector is pre-tinned before being put into service, a gold deposit with 0.000050 inches of gold will provide all of the above. Pre-tinning will remove most of the gold and protect the nickel surface with a tin coating. Any remaining gold will be removed when it is finally soldered in its application.

Many contact parts mate only once in a relatively mild environment, making insertion/extraction and porosity less critical. This is another example of gold being 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 core of any thinning decision.