When connecting flex circuits to rigid boards, my favorite option is the pin and socket connector because they are so readily available. In particular, I prefer to use a surface mounted bottom entry receptacle on the flex circuit with either a through hole or surface mounted header on the rigid board.
This combination offers several advantages: First it simplifies the assembly process. If all of your other components on the flex are SMT devices you can solder all the components in just one pass. To use a through hole type you would have to run the flex first though the SMT line and then through the wave solder line to attach the connector.
Secondly, and more important in my opinion, by having the receptacle on top it gives a natural handle with which to disconnect the flex assembly from the rigid board. Many flex circuits are damaged when people try to disconnect them by pulling on the flex circuit instead of by taking hold of the connector.
Finally, since the flex circuit sits down flush against the rigid board it gives you a low profile assembly. It also provides a way to mechanically fasten the flex to the board with a screw if you need it.
Figures one and two show this type of connection used in a disk drive application. Figure one shows the header mounted on the header. Figure two has the flex assembly removed showing the header underneath.
Zif connectors are also a popular choice, because there is only one connector to buy. I particularly like the horizontal zif connectors placed on the edge of the rigid board. They give a nice low profile and by placing them at the edge of the board it is easy to get at the latch that secures the flex in the zif connector. Figure three shows the flex with the 1.0mm fingers that mate with the connector on the board. Figure four shows the flex inserted and secured in connector.
If you need a very fine pitch connector, Miraco Inc., a sister company of ACT in Nashua, New Hampshire make a family of fine pitch connectors for flex circuits. By using a unique arrangement of beryllium copper springs in a molded plastic housing, they can go down to 0.025" pitch and can mate with both sides of the rigid board at the same time.
Figure 5 shows an 80 position, .025" pitch connector used in a CD-ROM interface to interconnect a computer docking connector to the PCB assembly. This assembly was initially designed with the conductors of the flexible circuit exposed both sides for direct soldering to the printed circuit board, however during handling the conductors fractured at the transition between the circuit dielectric and the solder joint.
You can contact the delightful folks at Miraco at 603-665-9449. Tell them I said hello!
This article was published in the May/June 1997 issue of Flexible Circuits Engineering magazine. It has been edited slightly for this website.
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