A major automotive-electronics test-equipment manufacturer needed to overcome several design challenges involving stackable, socketed pc test boards employed by users to field-configure the manufacturer's rear-end and engine test systems. Initially, the manufacturer specified relatively complex and costly conventional FR-4 multilayer test boards with plated through-holes and high pin counts. Assembly was time consuming, board rejects were too high, and bad boards had to be scrapped. Especially troublesome was an intermittent electrical "open" problem. This was due to solder from the preforms used on an adapter in the upper through-hole board running too far down the extended-length male pins that plug into a socket on a lower pc board.
To solve these problems, we developed a custom surface-mount, interstitial pin-grid-array (PGA) socket-adapter system (see the figure). This system employs a two-piece PGA connector consisting of a molded liquid crystal polymer (LCP) socket incorporating three-finger low-force, gold-plated beryllium copper contacts; and an FR-4 adapter with screw-machined, gold-plated brass pins. Both the socket and adapter use eutectic solder-ball terminals to provide secure joints.
The socket and adapter are reflow-soldered to the circuit board's surface-mount pads using solder balls. This alleviates the need for wave soldering, solves the electrical "open" problem previously caused by solder running down the through-hole pins, and permits boards to be reworked if necessary without scrapping them.
Using surface-mount PGA connectors to facilitate board-to-board connections lets the test-system manufacturer produce test boards with fewer layers, eliminate plated through-holes, simplify assembly, and reduce manufacturing costs. In addition, product reliability was enhanced and good-board yield increased by as much as 20% compared to the original production method.