Typically inside every screw machine socket receptacle is a precision die stamping called a contact clip. Contact clips are usually stamped using beryllium copper (for conductivity) or beryllium nickel (for elevated heat applications). These stampings are press-fit into machined brass shells and function as the electrical and mechanical interface to a mating pin.
Contact clips are conical in shape, and feature contact fingers, sometimes 3, 4, 6 or 8 depending on the contact size and force rating. The function of these multi-finger contact clips is:
- To apply force to the mating pin and this includes the following:
- Initial insertion force: highest first force as the contact clip is plated and in a pristine state
- Second insertion force: fingers have been initially flexed, which is now the typical characteristic insertion force
- Normal force: the force required to hold the pin from falling out
- Extraction force: the force required to disengage the pin from the clip
Socket technology is one of those sciences that people either learn by working for a connector manufacturer, learning painfully from trial and error or through telepathic communication with aliens, as this topic is not taught in any college program.
Because contact clips are made of spring material, they have a pin diameter acceptance range. The material type and thickness determine the current capacity. Not knowing which contact clip to specify in your socket application is the difference between application success and failure.
As a general rule of thumb, select a contact clip with a pin acceptance range that your device lead falls somewhere in the middle of.
For a device with less than 10 leads, select a socket with a contact clip that will give you sufficient retention force to hold your device.
For a device with hundreds of leads, carefully look at the characteristic insertion and extraction forces to make sure you can engage and disengage the device.
Martin Houlroyd is Principal Engineer/Marketing Specialist at Preci-Dip. He has worked in the interconnect industry for 35 years. Email him at firstname.lastname@example.org.