Issue link: https://iconnect007.uberflip.com/i/1359517
100 DESIGN007 MAGAZINE I APRIL 2021 circuit. Using a through-hole style connector (Figure 1), the flexible circuit should have rein- forcement 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 rein- forcement, such as thin FR-4, should be lami- nated to the opposite side of the flex circuit, away from the solder side. ere are many ways to terminate and con- nect flexible circuits to a rigid PCB. e most widely used technologies fall into four main categories: reinforced male finger contacts, unreinforced male finger contacts, standard through-hole and surface mount PCB con- nectors, and insulation displacement con- nectors. Unreinforced fingers are typically an extension of the copper traces with the insu- lation removed or thicker metallic fingers that are attached to the circuit traces. e cop- per 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 sug- gested. 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. e use of flex suggests that the connec- tion may at some point be under stress. at stress must be accounted for, as failure to limit the bending will result in a stress concentra- tion point between the connector frame on the rigid PCB and the flex circuit, resulting in intermittent or broken connections. Unrein- forced 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 oen 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 rein- forced fingers in a ZIF connector application, it is required to specify imperative circuit fea- tures 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 adhe- sive 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. Figure 3: Finger flex connector for insertion into connector on rigid PCB.