Testing technology - Testing of printed circuit boards (spring contact pins)

Reliable test methods are essential in electronics manufacturing. Spring contact pins serve as a precise interface for testing connections in electronic assemblies. But how exactly are spring contact pins used and which variants are available? In this article, you'll discover the many uses and different designs of these precise testing tools.

What are spring contact pins?

Spring contact pins are used for connection testing in printed circuit boards, electronic assemblies, or other connections in the electronics industry. They are flexible components that are particularly beneficial as connecting elements in applications that are subject to frequent vibration or shock. Spring contact pins are classified as testing devices. You can read more details about testing devices in our article about fixture design: testing devices for quality assurance.

The design is generally similar despite different forms of spring contact pins: They consist of the following precision-engineered components: Piston, housing or guide tube and spring. The spring ensures that the piston has a certain range of motion. Most of the time, this is about 2 to 12 mm. The housing and piston are typically made of brass, the spring is made of stainless steel or beryllium copper. Typically, the components are gold-plated for better conductivity.

In addition to spring contact pins, there are other versions of pins: Rotary contact pins, integrated contact pins, and test probes. On rotary contact pins, the piston rotates with the stroke movement. Oxide films, for example, are destroyed by this. This improves electrical conductivity and is particularly beneficial for oxidized or dirty test points. Test probes are measuring devices for manual inspection of connections. An overview of the common test procedures and requirements for the test technology in general can be found in this article.

Using Spring Contact Pins

Spring contact pins are a good solution for in-circuit testing to make temporary connections or for programming on printed circuit boards. Compared to connectors, spring pin contacts have a few advantages: They are less susceptible to vibration, manufacturing tolerances or shocks.

Spring contact pins are most commonly used in spring pin adapters to make electrical connections to the test points of a printed circuit board. This allows contact distances of up to 0.8 mm to be realized. The circuit board under test is positioned on the test adapter or vice versa, with the spring contacts pressing on precisely defined test points or solder pads on the circuit board. Spring contacts typically require only a small contact area, thus saving space. In addition, the springs can compensate for imperfections in offset stacks and are thus flexible. Depending on the complexity, several hundred spring contact pins can be used simultaneously, for example for measuring supply voltages, signal levels or component resistance, but also for short-circuit and continuity tests.

A common test procedure is the in-circuit test (ICT). All relevant connections of a printed circuit board are checked before they are installed in a housing.

The in-circuit test is an automated process that tests electronic components and their solder points for short circuits, breaks, incorrect readings, or polarity errors. Individual circuit boards and also entire circuit blocks can be checked.

In most cases, the assemblies are contacted by an adapter with a spring contact pin, but rigid needle adapters are also an option for reaching more-detailed structures.

During the test, the assembly is pressed against the adapters, for example, using a vacuum or compressed air. Spring contact switches are also sometimes used to detect mechanical conditions such as the presence or positioning of a component.

Spring contact pins are subject to natural wear due to frequent use. Spring contact pins are installed with contact receptacles to facilitate replacement, from which the worn spring contact pins can be easily pulled out and the new ones reinstalled. The contact receptacle assembly may look like this:

Another way to ensure a secure connection between the contact pin and pin clamp with connecting cable is to use shrink tubing for additional protection. Heat shrink tubing insulates and strengthens the connection and protects it against environmental influences such as moisture, dust, and mechanical stress. The assembly looks like this:

Misuses of spring contact pins

Due to their design, spring contact pins are not suited for full contraction, as this can lead to snagging. The spring contact pin should also always be subjected only to axial loads. The sleeve can be damaged if the load is lateral or oblique. A guide can be helpful to avoid lateral loads. It is also recommended to align the spring contact with the center of the contact on the other side.

Spring contact pin selection criteria

When selecting suited spring contact pins, note the following:

• suited for the pitch used on the printed circuit board (typically 2.54 mm (1/10 in.)), which results from the center-to-center distance of the individual component connections.
• Contact force, which depends on the contacting component. Insufficient force leads to instability, too much force can damage components.
• Stroke length, which indicates how far the piston can move during contact. Depending on the application, a limited installation height may be required.
• Material and coating that provide optimum conductivity and corrosion resistance, e.g., gold or nickel coatings.
• Intended application, e.g., ICT test, functional test

The head shape of the spring contact pins and also the final shape of the contact sleeves used are other important selection criteria. The standard head shape for spring contact pins is pointed for precise contact. Thanks to the tip, contamination or oxide films can also be pierced and a reliable connection can be made. The use of a waffle-shaped head is useful if contacts are uneven or the position of the connectors deviates. The waffle shape guarantees contact. Hollow tips in turn are suited for ball contacts and solder points. The MISUMI webshop offers a variety of spring contact pins with different head shapes.

While the head shape of the spring contact pin is crucial for the type of contact, the final shape of the associated contact sleeve determines the type of electrical connection. To provide a reliable connection to the line or assembly, a variety of end shapes are available. The following figure shows the most common variants and their respective applications.

• (A) Wrap connection: The wire is wrapped several times without insulation. The direct contact between the wire and pin ensures good electrical conductivity.
• (B) Solder and crimp design: These versatile socket sleeve ends allow both soldering and crimping. During soldering, the stripped wire is inserted into the sleeve and joined with solder by heating, creating a stable and conductive connection. During crimping, the wire is mechanically bonded to the sleeve by compression, which allows for a fast and reliable connection without generating heat.
• (C) Solder design, slightly curved at the end due to the machining method
• (D) For clamp fasteners: This end shape is specifically designed for use with clamp fasteners. In this case, the wire is mechanically secured by a screw or spring clamp, which allows a secure yet detachable connection.

Conditions for using spring contact pins

When using spring contact pins, some basic aspects should be considered. The operating temperature of spring contact pins is usually approximately 10–40 °C and a maximum humidity of 30%. The environment must be free of dust, corrosive gases, etc., as these can increase the contact resistance and lead to premature wear. The contact must always be fully established before switching on the electrical system to avoid micro-arcs and associated surface damage. In addition, the spring force should be adapted to the application – excessively high forces can damage test objects, while forces that are too low lead to unstable contacts. Regular cleaning of the contact surfaces also contributes to functional reliability and longevity.