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PCB Interconnects and Socket Technology

PCB Interconnects and Socket Technology

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Spring Pin Technology

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Junior Member level 3
Junior Member level 3
Aug 11, 2016
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The most popular machined interconnects on the market right now are precision machined spring pins. Not a new product, as it has been around for many years. There are two forms of spring pins: test probes and signal contacts. Test probes typically have hardened beryllium copper pistons with polygon cut tips for breaking through oxides on the PCB pad. Signal contacts manufactured of brass (but they can be beryllium copper for greater strength and extended durability.) Spring pin pistons are gold plated.

The function of spring pin contacts:
Product designers use spring pins to create PCB modules.

They can make a connection even when the mated PCB is misaligned. Spring pins adjust to uneven surfaces. A spring pin can mate either to a gold-plated PCB pad on a circuit board or a gold-plated machined pad connector. Depressing the spring pin piston near mid-stroke will give you the most tolerance flexibility.

Many spring pin applications are for a single cycle, e.g. assembling and sealing a PCB module.

A major advantage to spring pins is their ability to adjust to PCB stickup tolerances.

Spring Pin Construction:
Compression spring pins are miniature assemblies, consisting of many precision machined or cold-formed components. Spring pin termination is thru-hole, SMT, or solderless compression. Spring pin materials and plating are typically RoHs compliant. The internal compression spring is typically made of steel or beryllium copper.

Spring pin piston and shell must be machined or formed to precision tolerance to guarantee smooth gliding of the piston.

A test probe is a form of a spring pin with a polygon cut piston. Typically made of hardened beryllium copper for high-cycle durability. It may need to pierce through oxide layers on PCB pads. A standard spherical tipped spring pin for PCB mating is made of brass alloy.

Stainless steel springs operate up to 225C and beryllium copper springs operate up to 125C
The reason why the piston has a spherical tip is to prevent PCB surface debris from acting as a standoff (if the piston was flat tipped)

Right angle spring pin connectors can be SMT, positioning pegs on insulators for mechanical support.

First mate, last break is a method of indexing the PCB contact order by spring pins. Typically, the difference in heights is 0,5mm.

Factors affecting spring pin performance:
If your application will be subjected to high-shock and vibration, make sure your selected spring pin will not momentarily disconnect under these conditions.

When designing products with spring pins, take precaution and protect the spring piston from exposure to accidental side-load forces. One method is to protect the spring plungers with a shrouded entry.

Avoid fully depressing the spring piston such that you’re hitting the top of the shell. This may damage the spring pin barrel and possibly create piston binding.
Piston design affects the amount of characteristic contact resistance of the spring pin.

If the application will subject the piston to side-load forces then you should consider the material choice. Beryllium Copper has nearly 4x the yield strength of Brass.

Never make a spring pin connection at its maximum height, the forces are too light, and the contact resistance will be very high.

Do not mate the spring piston to a tin-plated surface, this can create a galvanic reaction between dissimilar metals.

Do not get any dielectric substance on the surface of the spring pin piston, it may clog the gliding surfaces and create piston binding.

Be careful when press-fitting spring pin shells such that they do not become compliant and bind the internal movement of the piston.

When a piston makes a crunchy sound as it is depressed into the shell, it can be due to piston biasing or poor plating coverage inside the shell.

Plating coverage inside the spring pin shell will depend on its construction.

Using spring pins to create a current divider is risky. If one spring pin overheats and fails, the resistance across the remaining spring pins will increase and overheat as well.

Optimizing PCB assembly:
Compression SMT spring pins allow new product engineers to create modular designs. The typical PCB pad land diameter is 0,25mm larger than the SMT base. If the piston is 1mm in diameter, its mating gold plated pad can be as large as the diameter of the spring pin base.

To reduce the above PCB height of spring pins, they can be designed with shells for embedding into PCB thickness.

Since spring pins can mate to a gold-plated PCB pad, there is no need for the cost of a mating connector.

An insulator shelf design can prevent over-compression of a spring pin

Packaging a single spring pin in a plastic sleeve not only can prevent accidental short-circuiting but supply a pick and place surface

When press-fitting a spring pin into an insulator, you must make sure the spring body barrel does not become compliant, this will create internal piston binding.

To help make precise placement onto a PCB, substitute solder tail or stub tail spring pins in the diagonal corners of an assembly.

Spring pins are the evolution of traditional pins and socket technology. Spring pins allow new product engineers to design easy-to-assemble modules. Misalignment, uneven surfaces, and tolerance stack-up which presents problems for a pin to socket mates are easily overcome by switching to spring pins.

Comments and suggestions are welcome.

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