Compliant pins possess elasticity, enabling them to form a reliable solderless press-fit electrical connection with the substrate. When the pin is inserted in the plated through-hole (PTH), the compliant zone changes shape under the pressure, and the residual plastic forces create a retention force. Typically they are used for both power and signal contacts, and they have been deployed in telecom backplanes since the 1970s and in consumer electronics since the 1980s.

More recently, Molex Inc. has pioneered their use in the automotive sector. The company currently produces some 300 million compliant pins per month, representing some 30 percent of the company's current global pin production.

Compliant-pin press-fit technology offers numerous advantages. The connection is permanent and highly reliable, resistant to shock and vibration. Moreover, compliant-pin connectors can be used on both the soldered and component sides of the PCB, allowing double-sided PCB assembly (either SMT based or through-hole). They can also be used on both tin and tin/lead plated PCBs. A further advantage is that the integrity of the populated PCB can be tested prior to the addition of the connectors.

Soldering is not required, and this in itself offers several advantages. There is no thermal stress on the connector, allowing the use of standard plastic material for the connector. The cost of selective soldering is eliminated. The quality of the connector interface surface plating remains the same as when delivered, free of solder slugs, flux residue, bridging and cold spots. The hazardous substance lead is eliminated.

The most popular type of press-fit compliant pin is known as eye of the needle (EON). This pin type is relatively simple to manufacture, using a Molex proprietary alloy. It is the lowest cost Molex compliant pin, with all the benefits of being highly reliable and easy to install in final assembly. It requires only simple tooling, with minimum controls. Molex now has extensive field experience of EON connectors, including nine years in the automotive sector, and this has allowed Molex to develop automotive EON design guidelines based on lab data and existing designs.

Applications for EON compliant-pin connectors in the automotive sector include body electronics, powertrain, automotive telematics, infotainment, airbag control modules, the engine compartment, transmissions, and anti-lock braking systems (ABS).

Molex EON1.0, EON1.45, and EON1.80 connectors are currently being deployed in the automotive sector in sensor modules, transmission control modules, powertrain control modules, stitched power distribution boxes, and powertrain control modules. In all cases, the fully populated PCB can first be tested and then pressed onto compliant pins.