AREA DISTRIBUTED SOLDERING OF FLEXIBLE AND RIGID PRINTED-CIRCUIT BOARDS

The drive for increasing both speed and functionality of circuit packs has created a demand for high I/O, high performance circuit pack to backplane interconnection. High I/O interconnection is usually achieved using pin and socket connectors. However, this interconnection strategy can exhibit poor electrical performance. Backplane pins and socket leads introduce lengths of unshielded conductors and consequently produce high crosstalk and uncontrolled impedance. Electrical performance can be improved by replacing lengthy socket leads with a flexible printed circuit. The flexible printed circuit provides a controlled impedance environment and reduces crosstalk by running signals in a microstrip configuration. With this approach, there remains the problem of attaching the flexible printed circuit to the circuit pack. Surface mount assembly of moderately high density connectors can require fine pitch assembly. An alternative to fine pitch assembly is area distributed soldering. Area distributed surface mount soldering is seldom considered as an assembly option since area distributed surface mount solder joints are typically difficult or impossible to inspect and repair. However, a novel design/process developed by the authors facilitates area distributed soldering of a flexible printed circuit to a circuit pack in an inspectable and repairable manner. A practical implementation of area distributed soldering that allows for inspection and repair required the development of a new solder joint design. In the new design, solder joints are formed between surface mount pads on a rigid printed circuit board and plated through holes on a flexible printed circuit. The joints are formed by heating both the flexible printed circuit and the printed circuit board to reflow temperatures and then allowing solder, which has been deposited on the surface mount pads, to rise through the plated through holes. Because solder rises up through the plated through holes to the exterior surface of the flexible printed circuit, spherical solder caps are clearly visible and inspectable. Solder joints are also easily repaired since the flexible printed circuit can be reheated to reflow one or all the solder joints. © 1990 IEEE