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Spring contacts are cylindrical, spring-loaded, compliant contact pins, which at their most basic, usually consist of some combination of a barrel, compression spring, and a plunger(s). They are quite similar in concept to the famous "Pogo Stick" toy, and are commonly referred to as "Pogo Pins". Pogo, as it applies to spring contacts, is a registered trademark.
Spring contacts are typically used in two ways. First, as a single or multi-pole connector for temporary electrical contact in a product or electronic system and secondly as a compliant test probe. Below are expanded overviews of each contact type:
Above, are the major components of a spring contact connector assembly. Each of the components requires proper engineering as it relates to design and materials, as well as precision manufacturing and assembly of the components.
Receptacle style test probes were largely developed in the mid-1960s to allow ease of replacement for worn or damaged test probes used in bare and loaded board bed of nails test fixtures. There are two major contemporary styles of test probe receptacles :
No receptacle style probes are typically captured in a connector body, or a test fixture, test socket, contactor. These assemblies can be "single-sided" or "double-sided". The difference being whether the probe features independently compressible plungers on each end, or a barrel serving as the tip on one end.
Below is more information on each major component of a spring contact assembly:
Barrel: Barrels are typically manufactured using screw machine processes, formed from tubing, or created from deep-draw processes. Barrel materials are commonly phosphorous bronze, BeCu, nickel silver, etc., and and plated to enhance conductivity and life.
Spring: The spring is the heart of a spring contact and provides compliance, and the contact force required to assure contact with the intended mating surface. Spring materials are typically:
Plunger: Plungers are typically manufactured from Brass, Stainless Steel, or BeCu (Beryllium Copper). Some manufacturers offer homogeneous materials, and there are several palladium and gold-based alloys used widely in wafer and package test probes. A heat-treat process is typically applied to plungers to improve durability. After heat treatment, plungers require a barrier plating, usually nickel, followed by plating with gold, silver, nickel, or other plating materials.
For example, below is an exploded view of a typical double-sided semiconductor test probe:
Unlike conventional pin and socket connectors, most spring contacts are designed to compress on the surface of a mating contact or a test target, vs. penetrating or inserting into it. There are exceptions to this of course, such as when probing a female header connector.
Spring loaded contacts and PCB test probes are generally of a relatively large size, while spring loaded test probes for semiconductor test are available with compressed heights of less than 1mm and pitches as tight as 150 microns (150µ= 0.15mm),. All styles are used in a very wide variety of applications, such as:
The use of a spring to offer compliance can result in a very high compliance-to-length ratio. Spring contacts can offer a full travel once mated, but typically operate at a suggested working travel. Within the compression and release range, a spring contact will have a range of optimal electrical compliance, and it's best to engage with a mating surface or test target in this range to achieve maximum performance. It's also best to operate a spring contact at rest vs. during any dynamic actuation.
The contact between the components of a spring contact is generally referred to as "biasing". There are many design enhancements to reduce the tolerances of the electrical performance characteristics and enable use in very challenging mating conditions, and even environments with vibration. Well biased designs allow the use of spring contacts in very high current and low voltage applications as well.
One of the biggest advantages of spring contact technology over other connector types, is tolerance absorption vs. a UUT or contact surface with a challenging topology, or other planarity challenges. In addition, the ability to articulate up to a 90° angle vs. a surface or target providing a very forgiving interconnect system.
A second great advantage of the technology is it's high insertion life. Spring contact probes generally allow cycling many times that of comparable contact systems, and are able to a constant contact force over large numbers of cycles.
Most spring contacts are based on coil spring designs, of a cylindrical geometry, housed inside of a barrel, however newer designs are coming to market with external springs, and "flat" components:
Spring probes, or connectors, can provide a low inductance, very short signal path while conserving valuable space adjacent to other contacts. These features combine to allow connector solutions for power & ground, analog, and digital applications with very good power and signal integrity. Coaxial, impedance matched "RF Probes" are available from many manufacturers for making contact through connectors, or directly to a PCB surface.
All of the features mentioned have resulted in spring contact usage in some of the most challenging and harsh environments and applications, with proven performance.
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