2. Design Types and Working Principles

2.1 Standard design with side roll-crimping and front roll-crimping (flanging)

Scope: Standard contacts, normal ICT/FCT probes, applications with low current loading.

With the exception of only a few construction types, a spring contact probe consists of three components: a barrel (also called guide tube), a spiral-shaped pressure spring, and a plunger. The three parts are undetachably assembled by applying special rolling/crimping methods. The plunger, however, can travel a certain amount in longitudinal direction. In this assembly the pressure spring is biased with the result that the complete spring probe has a preload-force in its unactuated condition. The rated force of the spring contact will be achieved at the nominal travel which is reached after two thirds of the maximum possible plunger travel. The nominal travel is chosen to suit the design of the test adapter where the probe will be inserted. The last decades brought up sort of standards which are considered by the vast majority of international manufacturers. If one takes a closer look at spring contact probes of various series, certain differences are obvious from the external appearance: 

Some series show a creasing or necking slightly above the mid position of the barrel, some feature an inward roll up flanging at the top of the barrel. These differences stem from the two mainly used crimping methods for assembling the probes. Depending on whether the plunger has one or two guiding cylinder sections, that crimping is positioned alongside or at the top. We call it front rolling or side rolling.

When side rolling is used, the plunger has two guiding shafts with a constricted zone in between which defines the length of the total spring travel. This design occupies more space inside of the spring contact, so there is less room left for the spiral spring. The maximum spring force therefore is always smaller, compared with the front-rolled design with identical outer dimensions.

The front-rolled spring contact probe has only one guiding cylinder inside. Its plunger shaft, the springy part which protrudes from the barrel is thinner than the shaft of the side-rolled type. But higher forces can be achieved without increasing the housing. Both construction principles have a certain influence on the pointing accuracy of the contact tip. Details see chapter 7 of this Handbook.

2.2 The Bias-Plunger

Scope: Charging contacts for constant current loading, measurement contacts which require constantly low resistance.

For the aforementioned standard design it is easily understood that a sufficient air gap between plunger and barrel is required to ensure smooth travel of the plunger. Comparing this principle with a conventional combustion engine, this would mean cylinder with piston – but without piston ring. Under certain circumstances this small air gap could cause the pressure force between plunger and barrel to fall under the minimum value which is required to build up an optimum low-resistance contact. As a result, the spring contact probe could show discontinuities in the resistance value.

The so called bias plunger creates clear improvements in this respect. The term “bias” is used in the sense of tilted plunger bottom. The bottom of the plunger which faces the spiral spring is neither rectangular  flat, nor tapered, but tilted. The spring hits the tilted surface and pushes the plunger aside with a small force which leads to an excellent contact with the barrel. This design is somewhat more complex and thus more expensive to manufacture.

2.3 The Bias-Ball-Design

Scope: Battery contacts for application with high vibration and current loading.

To improve the effect of bias plunger even further, a ball is inserted between spring and plunger bottom. Ball and plunger form an ideal contact bridge between current carrying plunger and barrel. As the (correctly sized) ball is rolling along the travel during actuation it has a very low wear throughout the spring contact service life.

2.4 The Tubular Plunger

Scope: Short design, board-to-board-connection, SMD style battery contacts.

The probe designs described so far come to their limit when the total length of the contact probe should be as small as possible. The plunger guide shaft inside the barrel requires a certain minimum length. Perhaps a ball is added for optimised design. Then there doesn’t remain enough space for the spiral spring. A tubular plunger that takes up part of the spring provides a solution here. Specialized shape and position of the hole in the plunger can produce an effect similar to the bias plunger style. 

2.5 The Split-Plunger

Scope: High-current contacts with relative long stroke, test solutions for the automotive sector.

The plunger is split into two pieces. Not with a lateral, but an inclined cut. The two plunger halves contact via the inclined cut surfaces and are biased towards the barrel by the spring pressure. The principle is similar to the bias ball type, but even more effective as the contacting areas between plunger halves and barrel are much larger here. However, also much friction is generated, so a precise balancing between inclination angle and spring force must be met.

2.6 The Pass-Through-Plunger

Scope: Probing with the least possible and constant contact resistance and/or high continuous current load, also for small sized designs.

Contrary to all other designs where the plunger ends inside the contact probe sitting on the spring, this design features a plunger that reaches all through the spiral spring and sticks out at the bottom end of the probe. A highly flexible stranded wire will be plugged or soldered to that end to provide the connection. As the electric current flows only through a massive conductor, mostly made of copper alloy, this construction provides constantly low and easily calculable resistance. Only (small) disadvantage: during agitation of the probe the plunger rod is also moved – including the connected wire.

2.7 Externally Arranged Spiral Spring

Scope: Probing of high continuous current loading, short design.

Similarly to the design described in chapter 2.6, the signal is directly connected to the spring loaded plunger. As a consequence, no current flows through the housing and/or the spring. With this design the spring is externally put over the guiding shaft of the plunger. This optimises service life of the spring and allows for slightly increased spring force due to a larger spring diameter compared with an internally assembled spring.