The Impact of Material Selection on PCB Pin Durability
Surface-mount or through-hole pins on a PCB are used to create a variety of connections with signal or power (or both). They mate with female receptacles to create board-to-board connections or custom cable assemblies. As the pins used in these interfaces are repeatedly connected, disconnected and/or experience vibration, friction between pins and their receptacles can generate wear. This calls into question the durability of pin materials to ensure functionality over the lifetime in their envisioned application.
PCB connector manufacturers are well aware of the durability challenges involved in choosing PCB pin materials. The pin plating material will be the biggest determinant of reliability in mating, and designers should understand what material options they should consider when placing pins in a design.
PCB Pin Material Options
Individual pins and connectors, such as pin headers, are offered by the connector industry in a wide range of form factors for building interconnects. PCB pins provide specific functions in board-to-board connections, and they are available in multiple contact configurations; options include through hole, surface mount, wire termination and solderless press-fit.
During mating and contact, the pin materials will be subject to different forces depending on the contact interface. The durability of the surface plating material will play a significant role in determining the number of acceptable mating cycles for the interconnect. Pins could also be exposed to environmental contaminants; the ability for these pins to withstand exposure depends on the plating materials used.
For these contact pins, there are several common plating material options:
- Tin plating
- Tin-lead alloy
- Gold plating
The above options are most common due to the availability of materials, the solderability of these plating materials, and their durability. There is also the Restriction of Hazardous Substances (RoHS) Directive, which restricts lead, to consider when specifying the plating. Another reason these metals are used is because these surface platings are already used on exposed pads and traces in PCBs; it is a good practice to use similar metals whenever possible to reduce the risk of galvanic corrosion.
There are other plating options available, such as electro solder, palladium and silver. These alternatives may have higher costs than an inexpensive plating like tin but may be appealing for special applications. For now, let’s look at the particulars of these common platings as they are most frequently used in commercial and industrial products.
Tin Plating
Tin is not a noble metal, and it is the most inexpensive among the common pin plating options. Tin contains an oxide layer once exposed to air and it will corrode when exposed to acidic substances. However, it is easy to solder onto and will readily bond with inexpensive tin surface platings on PCBs, so it is a common connector option in many products that do not need to be rated for high cycle life and do not require corrosive resistance.
Another factor that arises in tin platings is the potential for tin whisker growth, which is a known failure mechanism in PCBs. To help mitigate the risk of tin whisker growth, a matte finish tin with whisker and oxide inhibitors should be selected.
Finally, in applications like aerospace and military, pure tin platings are not qualified for use in high-reliability products that must form solder joints, including pins soldered onto a PCB, as they are more prone to fracturing during vibration. Instead, tin-lead is preferred as it offers stronger solder joints due to its more ductile consistency and does not experience the whiskering problems of pure tin plating.
- Plating Thickness: Up to 300 microinches
- Under Plate Material: Nickel and/or copper
- Reliability: Suitable for a wide variety of consumer and commercial applications, not qualified for aerospace and military applications, suffers from whisker growth if a matte finish is not applied.
Tin-Lead Plating
Tin-lead plating on connector pins has many of the same properties as pure tin platings. The addition of lead gives three advantages: stronger solder joints, no whisker growth and easier soldering at lower temperatures. Aside from these aspects, the wear durability, contact resistance and corrosion resistance are all similar in capability to tin. However, the inclusion of lead would prohibit the plating finish to be used in RoHS compliant applications. • Thickness: up to 300 microinches • Under Plate Material: Nickel and/or copper • Reliability: Suitable for a wide variety of consumer, commercial and industrial applications, qualified for aerospace and military applications, no whisker growth. Cannot be used in RoHS compliant applications.
- Thickness: up to 300 microinches
- Under Plate Material: Nickel and/or copper
- Reliability: Suitable for a wide variety of consumer, commercial and industrial applications, qualified for aerospace and military applications, no whisker growth. Cannot be used in RoHS compliant applications.
Gold Plating
High reliability settings like mil-aero and industrial systems often make use of gold as a plating finish material. Gold is the most noble metal and is resistant to a wide variety of corrosive substances. It is excellent for low voltage and current applications due to its high conductivity. The coefficient of friction is low, which allows the mating cycle count to be high. Gold is ideal for harsh environments, and the most popular finish used in military, aerospace, medical and other mission critical applications. (10-50 u”)
- Thickness: less than 50 microinches
- Under Plate Material: Nickel and/or copper
- Reliability: Durable for high cycle life requirements, corrosion resistant, high conductivity, widely used in military, aerospace, industrial and medical industries.
Mill-Max: The Gold-Standard in PCB Pins
For most applications, tin or tin/lead plated pins and connectors are acceptable and are commonly found in many products. For higher reliability, including military and aerospace, gold is the preferred plating material. Consider these points before finalizing your pin selection and contact a reputable PCB pin manufacturer like Mill-Max for more information on reliability and durability.