Doesn’t Happen Often
04/01/2007
Nearly a year after the presentation of “A Case for Socket Reuse,” by Paul Gaschke of IBM, at the 2006 Burn-in and Test Socket (BiTS) Workshop, I wondered whether compression-mount sockets (which enabled the socket reuse he described) were seeing more widespread acceptance and application by users, and being added to suppliers’ product portfolios. A few questions to a group of industry leading socket users and suppliers revealed the answer, and also captured the underlying technical and business drivers. Whether it was a simple “yes” or a more elaborate affirmation, the industry response was resoundingly universal.
Setting aside prevalent packaging trends, such as lead-free, stacked die, higher bandwidth, and increasing power/thermal loads, let’s take a look at a chief packaging trend driving compression-mount sockets - ever tighter input/output (I/O) pitch. Those sockets provide operational benefits of reusability. A tighter pitch enables more I/Os within a given area. Combined with increases in overall package size, the resulting package footprint may be more than 5000 I/Os across a 75- × 75-mm or larger package - and growing.
With tighter pitch, higher I/O counts, and a larger package, a prime consideration for socket engineers is the design and fabrication of a PCB to accommodate the socket for these packages.
Traditionally, sockets are mounted to a PCB with the contact pins set into the PCB’s plated thru holes (PTH) and then soldered. Wiring traces on the surface, and in the layers of the PCB, routes signals from the socket pins through the PCB to connect to other elements of the test or burn-in system (a space-transformation). As the pitch between I/Os becomes narrower and counts increase, more and smaller PTHs are needed, and with them, a commensurate increase in the number of signals to be routed from the socket through the PCB.
As PTH size and pitch shrink, hole drilling and plating - compounded by the growing number of I/Os - becomes more difficult and costly. Additionally, the increase in PTHs takes up more of the area within each layer of the PCB where signals would be routed. More layers can alleviate this, up to a point, and at an additional cost. On the other hand, a compression-mount socket contacts a surface pad on the PCB, thereby eliminating hole drilling and plating. Wiring channels within each internal layer of the PCB are freed up, potentially leading to fewer layers.
Instead of discarding an unusable socket along with the PCB because the socket was soldered in place, engineers can readily removed it from the PCB for servicing and/or replacement. Simple enough for a single-socket test board, but more compelling on multi-socketed burn-in boards (BiB) where the costly BiB would not be discarded for a single, or a few, “bad” socket positions.
The bad soldered sockets within a BiB usually cannot be removed, so they go unused while the remainder of the socket positions remain in service. Initially, it’s a minor loss of utilization, which grows significantly as more sockets go out of service, and eventually, the BiB is replaced. In contrast, with compression-mount sockets, full utilization of a BiB is achievable by removing and servicing or replacing individual sockets. Sockets are kept in spare-parts inventory with some un-socketed BiBs, instead of the more costly socketed BiBs.
It’s easy to see that once sockets are removable - with the “right” design - many components of the socket could be reused for a new application. Gaschke’s BiTS paper indicated that the cost-per-use for a socket continually drops for each reuse until overcome by refurbishing expenses; by his calculations, about 5 uses.
How about that? A packaging requirement becomes the stimulus for a socket style that raises utilization and lowers cost. Doesn’t happen often.
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Fred Taber, consultant and general chairman of the BiTS Workshop, may be contacted at BiTS Workshop, LLC, 34 Kuchler Drive, LaGrangeville, NY 12540; 845/226-7560; E-mail: [email protected].
Compression-mount Sockets and Contact Pins
Compression-mount sockets are attached to a PCB in a removable manner, such as with screws/nuts. Spring probes, elastomerics, and buckling beams are three types of contact pins for compression-mounted sockets. Each type exerts a spring force at both ends: to the PCB pad and to the package I/Os. Often the pins are pre-loaded to the PCB pad, thus ensuring sound electrical contact, while minimizing wear on the pads caused by the repetitive insertion and removal of packages to/from the socket.