QFP and Leaded Device Test Sockets
Peripheral leaded devices such as SOPs and QFPs introduce their own set of unique challenges into the testing environment. Because of these challenges, a common thought in the industry is that one could not use spring probe technology in leaded device test sockets. In the past we would have admitted that guaranteeing contact with QFPs was much more difficult than with QFNs or BGAs, but that has all changed with recent innovations at IDI. The primary challenge of leaded device testing is guaranteeing contact to the leads considering the large tolerance bands associated with these devices. In conventional alignment methodologies there are three ideas on how to align the device:
Corner-Body Alignment on leaded devices typically uses the corners of the package, where no leads extend, to guide the device into the socket pocket. Unfortunately these corners are often obscured with varying degrees of mold flash, making consistent alignment virtually impossible.
The process of Under-Body Alignment utilises the angled bottom of the package to guide the device into contact with the spring probes. While this may seem to be a feasible solution, it is usually quickly eliminated by a standard, but alarming angle tolerance of 0° to 12° on the bottom of the package body.
Lead Alignment is extremely difficult to use because of the significant positional, width, and length tolerances associated with the leads. There are two locations within the leads which one can attempt to align off of-the upper portion and the foot of the lead. While consistent, the upper portion of the lead often has residual edges from dam bars. By the time you get to the foot of the lead, the compounding tolerances are enormous.
We have developed a solution to each of these challenges while still meeting the strenuous requirements of production test. We utilise an individually combed floating nest which prealigns the device before ever contacting the spring probes. When entering the socket alignment pocket, the device first contacts a nest elevated above the probe tips where the leads are aligned in individually combed slots. The device then continues traveling to the spring probes which are individually aligned to specific leads.
This innovative design enables us to guarantee contact with each and every new device. This methodology has also proven to make Kelvin testing of leaded packages a truly viable option when partnered with our Offset Kelvin probe.