There are many connectors and termination options that are manufactured specifically for use with flexible circuits and flat flexible cables. Zero insertion force (ZIF) connectors (Figure 1) and insulation displacement connectors (Figure 2) are both good examples. They offer reliable and inexpensive ways to terminate a flexible circuit. At the same time virtually any connector that can be used on a rigid PCB can also be installed on a flexible circuit as long as certain design rules are followed. If you choose a through hole style connector the flexible circuit should be rigidized in the connector area with FR4 stiffener on the same side as the connector. However, if an SMT connector is going to be used, the rigidizing stiffener needs to be laminated to the side of the flex circuit opposite the connector.
There are many ways to terminate flexible circuits. The most popular methods fall into three main categories: supported and unsupported finger (male type) contacts, insulation displacement connectors and standard through-hole and surface mount PCB connectors.
Unsupported fingers can be either an extension of the copper conductors with the insulation removed or thicker metallic fingers that are attached to the circuit conductors via brazing or hi-temp soldering. The fingers can be bent 90 degrees for parallel installation or left straight for a perpendicular installation. In either case, the fingers will be extremely fragile pre-assembly AND after being soldered to their connections. Extreme handing care will be required, and it is advisable to encapsulate the soldered finger area of the assembly with an epoxy to make it more rugged after soldering. It is important that the epoxy completely encapsulates the fingers and at least a portion of the polyimide insulation. Failure to do this will result in a stress concentration point between the epoxy and polyimide and can result in broken fingers. Unsupported fingers are usually used in cases where it is desirable to solder the fingers directly into a plated through hole. Unsupported fingers can also be used with a hot-bar process to eliminate the need for a connector.
Supported fingers are most used in ZIF (Zero Insertion Force) connector applications, but they can also be used with an anisotropic (z-axis) adhesive or hot bar soldering to be bonded directly to a rigid PCB. When using supported fingers in conjunction with a ZIF connector, it is important to specify important circuit features on the drawing, such as overall thickness in each finger area (usually 275 to 325 um (.01 to .013 in) and circuit edge to finger tolerance. When the flex circuit is constructed properly, ZIF connectors can be an inexpensive and reliable way to provide termination. However, if the flex drawing fails to alert the flex manufacturer that a finger pattern will be used with a ZIF connector the result may be a circuit that will not make proper connection to the ZIF contacts. Since you should have already selected the exact ZIF P/N at the time of flex fabrication, it is a good practice to state on the drawing that the ZIF termination area must mate to P/NXXXXXX”.
Insulation displacement connectors use barbs to pierce through the cover insulation and make contact with the copper conductor below (Figure 2). While not uncommon, insulation displacement connectors are not employed nearly as much as standard through-hole and SMT connections.
Standard off-the-shelf PCB connectors of the through-hole and SMT variety are far and away the most common method of terminating a flex circuit. There are also many custom PCB connectors that can be installed on flexible circuitry. Keep in mind that if you choose a custom connector for your application, it will most likely be very expensive, and you may be stuck with a single supplier for the life of the program. This does not give you much leverage when you are looking for price concessions when your volumes increase. Also, the lead times on custom connectors can be very long (8 to 12 weeks is not uncommon).
What is the Benefit of Interconnect Methods?
In many cases, the flex circuit is the last component to be designed in the system, so most of the termination choices may have already been locked into options that mate with the other connectors previously designed into the system. This is why it is so important to start considering the interconnect methods (i.e., flex circuits connectors etc.) early in the design process. A few smart decisions early can be a significant cost savings, without sacrificing quality or reliability.
Flexible Circuit Technologies provides the industry’s most experienced Application Engineering support and is well positioned to guide you to success in getting to a cost-effective design that will perform flawlessly in your given application.
In addition to having the ability to produce the long format circuits, FCT also can provide flexible and rigid flex circuits, heaters, and an array of other products. We also can provide assembly including complete box builds at our EMS group InTFlex.
Contact us and work with our Applications Engineering team to get started.
