Zero insertion force (ZIF) is a concept used in the design of IC sockets and electrical connectors invented to avoid problems caused by applying force upon insertion and extraction.
A normal integrated circuit (IC) socket requires the IC to be pushed into sprung contacts which then grip by friction. For an IC with hundreds of pins, the total insertion force can be very large (tens of newtons), leading to a danger of damage to the device or the circuit board. Also even with relatively small pin counts each extraction is fairly awkward and carries a significant risk of bending pins (particularly if the person performing the extraction hasn't had much practice or the board is crowded). Low insertion force (LIF) sockets reduce the issues of insertion and extraction but the lower the insertion force of a conventional socket, the less reliable the connection is likely to be.
With a ZIF socket, before the IC is inserted, a lever or slider on the side of the socket is moved, pushing all the sprung contacts apart so that the IC can be inserted with very little force (generally the weight of the IC itself is sufficient with no external downward force required). The lever is then moved back, allowing the contacts to close and grip the pins of the IC. ZIF sockets are much more expensive than standard IC sockets and also tend to take up a larger board area due to the space taken up by the mechanism. Therefore they are only used when there is a good reason to do so.
Large ZIF sockets are only commonly found mounted on PC motherboards (from about the mid 1990s forward). These CPU sockets are designed to support a particular range of CPUs, allowing computer retailers and consumers to assemble motherboard/CPU combinations based on individual budget and requirements. The rest of the electronics industry has largely abandoned sockets and moved to surface mount components soldered directly to the board.
Smaller ZIF sockets are commonly used in chip-testing and programming equipment, e.g., programming and testing on EEPROMs, Microcontrollers, etc.
Read more about Zero Insertion Force: Universal Test Sockets, Ball Grid Array Sockets, ZIF Wire-to-board Connectors, Hard Disk Drives
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