31 August 2022Manufacturing / Production Technology, Hardware & Services
By Danny Boesing, Samtec
Like most electronic components, the quality of countless subcomponents and processes directly affects the quality and performance of the finished product. With PCB-level connectors, those factors include pin material, type of plastic, the quality of the moulded plastic bodies, coplanarity of the tails, the quality of the surface finish (plating), choosing the correct connector plating, the manufacturing/assembly process (putting the pins in the plastic), and packaging, amongst others.
Connector plating is mission critical. It affects the life and quality of the terminal or socket; it impacts corrosion resistance, conductivity, solderability, and of course, cost.
Why use gold plating?
Gold is generally specified for high reliability, low voltage, or low current applications. Gold is used in high-cycle applications because it is rugged and has excellent wear properties. Samtec’s gold is alloyed with cobalt, which increases the hardness.
Gold is a noble metal, which means it doesn’t react much to its environment. Gold is therefore also recommended for hostile environments because it will remain free of oxides which could cause an increase in contact resistance.
Sometimes, gold is a 'matter of necessity' in that as connectors are miniaturised, contacts are too small to generate much normal force. So, low normal forces guide the need for Au plating.
The disadvantages of gold are primarily cost, then porosity at thinner plating levels, and some nuances regarding solderability. Specifically, many customers solder these 'successfully', but they are not soldering to the Au, because the Au dissolves in the molten solder. They are soldering to the nickel under the Au. So, technically, it is correct to say Au has poor solderability.
Why use tin plating?
Tin is a lower-cost alternative to gold and has excellent solderability. Unlike gold, tin is not a noble metal. Tin plating starts to oxidise the moment it is exposed to air. Therefore, a tin-plated contact system requires greater normal forces and a longer contact wipe area to break through this oxide film.
Tin is often preferred due to its relatively low cost in applications with few mating cycles having the appropriate amount of normal force.
What is the most popular plating option?
Selective gold-tin plating is Samtec’s most popular plating option because it provides designers with the best of both worlds. The contact area, the critical area where the contact interfaces the terminal pin and the signal is transferred, has the reliability of gold. The tail, which is soldered to the board, has a lower cost and the improved solderability of tin.
Can gold be soldered?
This process should be considered very carefully because gold plating dissolves in solder, and therefore, the risk of solder bath contamination and gold embrittlement is real:
• As previously mentioned, solder paste does not solder to the gold plate. The gold plate dissolves into the molten solder and the soldering takes place with the nickel underplate.
• Embrittlement becomes a concern once the gold weight contribution to the solder joint is 3 to 5%.
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