Durability of Pin Plating Materials for PCB Connectors

Pin headers are one of the common components in PCBs, but they have a specifications that is not always considered: the plating material. The material you see in PCB pin headers, connectors, pogo pins, and through-hole posts is just a plating, and it is deposited over a base metal that provides the main structural properties. While it might seem like a minor detail, pin plating is a big determinant of reliability in pin connections, just as it is on the surface layer of a PCB.

For most applications, simple tin-plated connector pins are used and these will be acceptable from the reliability perspective. When higher mechanical reliability is needed, consider the plating options shown below. Many connector manufacturers offer these plating materials on their products, which includes off-the-shelf shrouded connectors and crimp contacts for cable assemblies.

Plating Materials for Electrical Connectors

Connector pins tend to have 5 different plating options: tin, tin-lead, gold, silver, or brass. There are also some connectors that use specialty plating materials, such as alloys containing nickel or palladium similar to ENIG/ENEPIG.

For base materials, the common option is brass or phosphor bronze, although a more rugged connector base material is stainless steel, which would be desired when higher strength connections are needed.

Now that we’ve covered material options, let’s look more in-depth at plating materials.

Tin Plating

This is the most common and cost-effective plating option, and it is commonly used in pin headers and connector leads, typically over brass base materials. Tin forms an oxide layer when exposed to air and it will corrode further when exposed to corrosive agents. Some of the benefits of tin plating:

• It is easy to solder onto with tin-based solder pastes or wires

• The plating is thick (100’s of microns) which allows many mating cycles

• Tin is among the lowest cost plating options

While tin connector pins can have a high mating cycle rating, the plating can still wear away due to friction, which is one reason the plating deposit may be very thick. Tin also has the potential to experience whisker formation, just as you would find in tin platings on PBCs. The whisker problem is less important on connector pins unless the pins are very dense.

Gold Plating

Gold plating is a more expensive option that offers some durability benefits over other platings. Gold plating is found on pin headers, connector pins and receptacles, and pogo pin/receptacle pairs. Although gold is a soft metal, it is also a noble metal which makes it more resistant to corrosion than tin.

• Low contact resistance in the mating area

• Thin plating (10’s of microns) over a nickel seed layer

• Low friction overcomes low hardness of gold to ensure many mating cycles

• Higher corrosion resistance than tin

You have probably used gold-plated pin headers like this strip from Harwin. [Product link on Octopart]

Palladium is sometimes used as an alternative to gold, although it is not common in commercial products. Some companies that offer plating services will provide palladium as an option that is much more durable than gold without excessive increase in cost.

Tin-Lead Plating

Because lead-free platings are not qualified for aerospace or other high-reliability requirements, tin-lead is normally used as a PCB plating, component lead plating, and in soldering. Tin-lead can form stronger solder joints and the presence of lead suppresses whisker growth in platings and solder joints.

• High reliability with easy soldering

• Thick plating (100’s of microns) and high hardness allowing many mating cycles

• Low cost plating option for many base materials (brass, stainless steel, etc.)

• Better corrosion resistance than lead-free platings

Despite inability to comply with the RoHS directive, tin-lead plating will be required in high-reliability applications such as aerospace until lead-free plating options can be shown to meet performance and qualification standards.

Silver Plating

Silver is less common but it does provide better compatibility with silvered solders to form similar intermetallics during soldering. It also offers the lowest contact resistance of native metal platings, so it can be used in applications with high current density where greater reliability is needed.

• Lowest possible contact resistance in the mating area

• Thin plating (10’s of microns) over base material or seed layer

• Low friction overcomes low hardness of silver to ensure many mating cycles

Just like silver surface plating on a PCB, silver connector leads can accumulate tarnish over time. If a silver-plated component requires soldering, then it must be done within a few months of receiving the board/components, otherwise tarnish will reduce the strength of the solder joint.

Example silvered crimp contact for a wire connection, available from Jameco

Selective Plating

Some pin headers and through-hole mounted pins could be selectively plated, where two different platings are used for different parts of the connector lead. For example, in pin headers, some products will have the gold plating only on the tip of the connector pins, while the solder point could be tin or tin-lead plating. This would give the reliability of tin-lead soldering at the connector base with the corrosion resistance of a gold plating on the exposed conductor pins.

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