Bonded Gold Fingers as a Low-Cost Alternative to Patterned Edgeboard Fingers for General PWB use

Abstract

The conventional means by which a plug-in printed wiring board is electrically connected to its mating backplane connector is by means of patterned and plated fingers at the edge of the printed wiring board (PWB). These fingers are usually an extension of the patterning of the board itself and are plated with a sequence of metallization layers to provide low-contact resistance between the mating parts of the connector over the life of the circuit pack. A conceptually new adhesively bonded finger connector that offers certain technical and economic advantages over the patterned finger connector for all conventional bonded and single sided PWBs is introduced. Two alternative methods are proposed for making connection between the fingers and the circuit pattern. The first, which provides for wave-soldered connection between the fingers and the PWB, handles that class of PWB that has output connections appearing on only one side of the PWB but requires gold edge fingers on both sides of the board. At least two types of PWB have this need. First, there is the bonded composite PWB, our first application. It uses a flexible circuit bonded to one side of a support member. Conventionally, the support member would be patterned only to provide gold fingers, and in addition, a connection means would be required between the support member and the flexible circuit. A second application is the single-sided PWB without plated-through holes that must be patterned on two sides merely to provide fingers on both sides of the PWB. Through-board connection would also be useful on those PWBs designed for a separable connector. While the through-board attachment method required for the above types of PWBs is useful for any PWB, it is probably not the best alternative for that large class of PWB that has been designed for two-sided patterning. For the conventional PWB, refiow soldering is probably the best electrical attachment method, because no PWB design changes are required to implement the proposal. Since the authors had need for gold fingers on both bonded and conventional PWBs, the authors cover both wave and reflow attachments.