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Consider This: Termination of Flex Circuits

There are many possible connectors that are manufactured specifically for use with flexible circuits to connect from a flex circuit to a rigid PCB. Many use typical, military secure twist connectors which require reinforcement of the flex to solder to the connector pins (Figure 1). Also, a good choice is zero insertion force (ZIF) connectors for thin flex to rigid applications. Additionally, pierce type connectors are used (Figure 2). Direct copper fingers which plug into rigid header connectors can be used (Figure 3).

Many different types of connectors offer inexpensive and reliable methods to connect a flexible circuit to a rigid PCB. Consideration for flexibility always needs to be addressed. Examine the bend radius and possibly add cover coat reinforcement in higher stress areas. Almost any connector that can be utilized on a rigid PCB can also be installed on a flexible circuit. Using a through-hole style connector (Figure 1), the flexible circuit should have reinforcement in the connector area with either a FR-4 stiffener or a cover coat on the same side as the connector. However, when using an SMT type connector, a higher, stiffness reinforcement, such as thin FR-4, should be laminated to the opposite side of the flex circuit, away from the solder side.

There are many ways to terminate and connect flexible circuits to a rigid PCB. The most widely used technologies fall into four main categories: reinforced male finger contacts, unreinforced male finger contacts, standard through-hole and surface mount PCB connectors, and insulation displacement connectors. Unreinforced fingers are typically an extension of the copper traces with the insulation removed or thicker metallic fingers that are attached to the circuit traces. The copper fingers can be bent for angled installation or remain straight for a vertical installation. Depending on the size and copper weight of the copper fingers and clamping force of the connector, a secondary support method is suggested. Consider using a mechanical clamp-down to limit bending at the connector. As in all flex applications, bend radius and flexibility must be considered to ensure the copper traces are not bent, exceeding the crack radius or number of bend cycles.

The use of flex suggests that the connection may at some point be under stress. That stress must be accounted for, as failure to limit the bending will result in a stress concentration point between the connector frame on the rigid PCB and the flex circuit, resulting in intermittent or broken connections. Unreinforced fingers are typically used in applications which use the method of soldering the fingers directly into a plated through-hole or onto a pad. Unreinforced fingers are often used with a hot-bar soldering process to eliminate the need for a connector.

Reinforced copper, gold plated fingers are most used in ZIF or low insertion force (LIF) connector applications. When using reinforced fingers in a ZIF connector application, it is required to specify imperative circuit features on the drawing. Critical items, such as overall thickness in the finger areas, as well as circuit edge-to-finger tolerance is important as the flex must mate correctly with the narrow fingers in the connector. When the flex circuit is designed and manufactured properly, ZIF connectors can be an inexpensive and reliable option. An additional method for flex or silver-screened membrane to a rigid connection is through anisotropic Z-axis, conductive adhesive bonded directly to rigid PCB traces.

Insulation displacement connectors use barbs to pierce through the cover insulation and make contact with the copper conductor below (Figure 2). Many times, the bent-over barbs are soldered to the flex traces to improve reliability. Insulation displacement connectors are not as popular these days, moving more to standard through-hole and SMT connections.

PCB connectors utilizing through-hole and SMT connection methods are the most common and lowest cost method of connecting a flex circuit. By utilizing the more common varieties such as D-sub or micro-D, you can save money and be assured of a constant supply. You can design or use a custom PCB connector for your flexible circuitry. However, keep in mind custom connectors will probably be very expensive, with long lead times. Having a single supplier for a flex connector can prove to be fatal. The lead times on having a custom designed connector manufactured can be quite long, with two to four months being typical.

As experience has taught, many various connectors can have unexpected and exceedingly long lead times. It is always wise to check suppliers for alternative connectors and lead times. Many times connector designs change quickly. One should check whether the particular connector is stocked by other suppliers or if supply is limited because the connector has limited sales.

Typically in almost every design of a product, the flex circuit is the last part to be examined and designed into the system. It is not uncommon to hear stories that designers just thought of flex as an off-the-shelf item, as it is such a nondescript part. Many of the connector termination choices would have already been confirmed using typical rigid PCB connectors before the flex is even designed. To prevent future problems in assembly and parts procurement, it is vital to begin the design by locking down the interconnect technology that you plan to use. You need to roughly design the flex interconnection circuits, define the connectors, and look at stress and flexibility which all play an important part in the early design process. This is the time to define and set quality and reliability parameters. Whatever method you choose, your flex circuit provider can guide you in the selection process. 

This column originally appeared in the April 2021 issue of Design007 Magazine.