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CONNECTING THE DOTS

Working with Board-to-Board Connectors

by George Martin

Start ý Off the Beaten Path ý The Chosen Path ý Marking the Path ý The Final Stretch ý Sources and PDF

Every once in a while youýll need to add a daughterboard to a design. The daughterboard may have optional features that are not installed in every unit, or may have components that are intended for removal as a memory card application. I came across such a requirement in a recent design, and Iýd like to share what I found.

The first classic solution for board-to-board interconnection is the chassis with a backplane (I would put even the IBM PC into this category). The backplane can have either no active components or, as in the case of the PC, a ton of active components. So, the first solution to look at is the one that uses one-piece connectors mounted on the main logic board and accept cards with edge connectors built into their artwork. The connectors are relatively inexpensive, but the daughter card must have gold fingers to provide good connectivity. Also, the inexpensive versions of these connectors are spaced on 0.100ý centers, so there isnýt a dense connection.

The next classic solution uses the usual headers on 0.100ý centers, typically seen at the ends of ribbon cables. With a lot of manufacturers, good pricing, and availability, what more could you ask for? This product line is broad and you can find all sorts of oddball variations.

The basic setup would have a socket on the main board and right angle pins on the daughterboard. And, the daughterboard would connect at right angles to the main board. However, you can also find a socket that accepts the pins and provides clearance for the pins to go through the board. This can be used for card stacking, as in PC/104 systems, or for height adjustment. Another feature of this stacking approach is that the main board and daughterboard are mounted parallel to each other, making a smaller overall unit.

These connectors are also available with locking hardware for a more rugged application. And, some manufactures make these style connectors on 0.050ý centers, twice the density of the 0.100ý devices. One of the units I designed had three boards, each with 50-pin headers at the edge. I used ribbon cable with mating connectors for the backplane, which eliminated one printed circuit board and all the accompanying mechanical design of a classic backplane.

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