4. Materials and Productions Processes

4.1 The Barrel (Housing Tube)

As already described in chapter 2.1, a standard spring contact probe uses a barrel as housing for spring and plunger. This is not to be confused with the receptacle (German technical terms “Stifthülse” (=barrel) and “Steckhülse” (=receptacle) sound similar) which holds the entire spring contact probe and facilitates its replacement. The barrel guides plunger and spring and therefore its inside wall should be as smooth as possible. As aforementioned, there are designs with/without collar, barrels with open bottom end for pass-through-plungers and some specialty types.

Depending on production lot size and some technical preconditions, barrels are preferably manufactured using a deep-drawing process. A plate-shaped platelet is punched out of a sheet metal or a foil strip. In several consecutive steps it is reshaped into a tubular form. This method creates parts which are characterised by extra smooth surfaces and high stiffness and elasticity – even with very thin wall thickness. Typical raw material is nickelsilver (a silver-white glossy copper-nickel-zinc-alloy, also called Alpacca). CuBe (copper-beryllium alloy) or special bronze alloys are also used. Due to relatively high tooling expenditures for deep-drawing this production process is only suitable for standard series in relative high volume. Most of our deep-drawn parts use a nickel silver alloy which is very corrosion-resistant. Therefore it can be used without any further gold plating (resource-preserving economy variants). Moreover, the oxide of nickel silver is electrically conductive so it is ideally suitable for contacting elements.

Barrels with limiting collar or only very small production lots (custom made types or battery contacts) are machined, turned parts. Most common materials are nickel silver, brass, and certain bronze alloys – all in all again copper alloys. Due to machining, the surface quality is not as perfect as with deep-drawn parts. However, using latest technologies combined with the experience of our cutting machine operators, our quality results are almost equal to deep-drawn parts. The surface of barrels is normally galvanically gold plated over a nickel diffusion barrier. Silver plating is also common, especially in the case of high-current contacts.

4.2 The Plunger

The plungers of the various spring contact probes are not only available in various shapes and sizes but also made of different materials. The combination is always carefully chosen to find the optimum compromise between electrical conductivity and mechanical strength. Both factors are equally important for practical applications.

Copper based materials like CuBe or brass are perfect suited for electrical contacts, especially when finally gold or silver plated. Concerning wear resistance, these materials quickly fall back behind steel types – especially when it comes to aggressive head shapes like tips, crowns, dagger styles and similar forms. 

Therefore the simple rule of thumb perfectly works

  • Aggressive shapes: made of steel
  • Passive shapes: made of copper alloy

The individual selection of the suitable alloy depends on further aspects like machinability, current-carrying capacity, and others. Steel types are manufactured from quality steel with a total organic carbon content of 1.0 percent weight per weight. After hardening and tempering the manufactured parts show a Vickers hardness of approx. 600 to 800 HV5. Especially sharp-edged and spiky shapes thereby obtain a long durability. To improve electrical characteristics of steel plungers, the surface must be optimally refined. Passive shapes like serrated heads, conical points with 90 degrees, and even multipoint crown heads are machined from CuBe2  (Beryllium Copper) which can be thermally hardened. When hardened, copper beryllium alloys reach the highest hardness values of all copper alloys. Depending on the chosen hardening process, a hardness of 195 to 450 HV will be obtained. Flat or round types, or concave female cones are mostly machined from brass or phosphor bronze. Typically, the electrical conductivity has priority as the mechanical load is relatively low.

The surface finish is of particularly high importance for contacting plungers. Depending on the chosen raw material they are mostly nickel or gold plated. We use extra hard gold alloys which are accordingly long-lasting. In the area of high-current types – partly also for high voltage test probes – FIXTEST spring contact probes receive an additional dome, made of massive special silver alloy to counter spark erosion.

 

Other platings which are common in the test probe industry are electrolytic nickel, chem. nickel, rhodium, palladium, and palladium-cobalt. The layer build-up can be performed in several steps creating a mix of different layers. Every manufacturer has his own experience and recipes.

4.3 The Spring

The pressure spring not only contributes its name to the spring contact probe. There are other aspects why it is of vital importance. The selection of the spring material defines the spring force of the contact probe, its service lifetime, its operational temperature range, its electrical resistance and many more parameters. It is the same story as with the plunger: material selection always is sort of compromise. Different physical properties of various materials are competing. Strength and elasticity of spring steel allow for a maximum of spring force for a given size. However, spring steel can only be used up to a temperature of 120°C. And its conductivity is pretty bad. Surface coating with gold and/or silver improves conductivity, but from an electrical point of view, a steel spring always will stay behind a copper-beryllium alloy. Copper-beryllium is again back on stage and it can well be used to wind spiral springs. But from a mechanical, spring-force point of view, a copper-beryllium spring always will stay behind a steel spring. Copper-beryllium springs have an extended temperature range (up to approx. 200°C), compared with steel.

Stainless steel lies somewhere in between: strength and elasticity are higher compared with copper-beryllium, and lower compared with spring steel. Temperature range is higher than for both other competitors. Electrical properties, however, are pretty bad.

For very high temperatures special solutions are available: Springs made of rust-proof stainless steel 1.4571 withstand temperatures up to 300°C. And they are topped by Hasteloy C-4, withstanding temperatures up to 400°C.

Our decisive criterion: We check the planned operating temperature range for the spring probe, and then we check the required pressure force. Finally we can select the optimum spring material.

For more information on temperature range see chapter 6.