Specifying connectors for flexible circuits requires understanding of robust termination solutions.
By Mark Finstad
Flexible circuitry is becoming the interconnect solution of choice in applications requiring high reliability, high density, and low mass. Unfortunately, many design engineers unfamiliar with flexible circuitry struggle with the task of specifying reliable, cost-effective termination methods for flex circuits. Virtually any connector designed to be mounted to a rigid printed circuit board (PCB) can be mounted on a flex circuit. However, the material properties that make a flex circuit flexible will also make a flex circuit much more fragile than a rigid PCB. For that reason, careful consideration must be given to the specific application when designing the flex circuit and specifying connectors.
The first factor to consider is the amount of handling the circuit will experience during installation and service. If an application dictates that the circuit will be installed one time only, and will require little or no service during its life, a lower level of ruggedizing will be required to yield a reliable connection. On the other hand, if the application requires numerous plugging and unplugging operations during its service life, it will require a high degree of ruggedizing to make the design robust.
The second factor to consider is the overall thickness of the flex circuit. A single-layer flex circuit can be less than 0.005" thick. A circuit this thin is very fragile, and sensitive to excessive handling. Operators often use the flex circuit as a “handle” to decouple connectors during servicing. This can concentrate a considerable amount of force on the flex circuit, connector, and solder joints. For example, if a connector mounted to a flex circuit has 30 contacts, and each contact has a retention force of 2 ounces per contact, the total force required to de-mate the connector will be nearly 4 lbs. If an operator uses the thin flexible circuit as a handle to unplug the connectors, the 4-lb force needed to unplug the connectors will most likely damage the flex circuit. Naturally, a thicker, multi-layer flex circuit will have a much higher mechanical strength, and therefore will withstand more handling before problems occur.
The third factor to examine is the mounting method of the connectors to the flexible circuit. Most connectors can be purchased in either through-hole or surface-mount styles. The maximum force a surface-mount connector can withstand without damage is limited to the combined peel strength of the individual pads to which the connector is soldered. A through-hole-mounted connector typically will withstand more pull-out force than an equivalent surface-mount connector. The downside is that the through holes will consume considerably more board real estate than would the surface-mount version.
Several artwork features can be incorporated into the flex circuit to make connector terminations more robust. Through-hole pad sizes should be maximized to provide optimal solder joints. Conductors leading to pads in the termination areas should also be maximized as much as possible, and fillets should be placed at each point a conductor enters or exits a pad. Hold-down tabs should be used on both surface-mount and through-hole pads wherever possible to help anchor the pads in place (see Fig. 1).
 FIGURE 1. Hold-down tabs anchor through-hole pads in place in a multi-layer flex circuit. |
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Strength enhancement
Designers can add many mechanical features to enhance the strength of the termination areas. The most common method to ruggedize a flex-circuit connector is to laminate an epoxy/glass stiffener in the connector area. The stiffener is mounted to the same side as the connector in the case of a through-hole style, and on the opposite side of the connector for surface-mount style.
 FIGURE 2. The most common method used to ruggedize a flex-circuit connector is a laminated epoxy/glass stiffener in the connector area. |
In a through-hole connector, the stiffener makes the connector area more rigid, which distributes any pulling or twisting forces over the entire stiffened area (see Fig. 2). This will keep unwanted forces from being concentrated on a single pin, or group of pins. This method can be taken a step further by placing a bead of epoxy around the perimeter of the connector body to bond it to the stiffener. This essentially makes the connector body and stiffener act as one piece, so any outside forces are exerted on the connector body and stiffener, rather than on the solder joints. Of course, a thin flex circuit will still require care in handing to keep the flex circuit from tearing at the edge of the stiffener. An additional bead of semi-rigid epoxy can be applied along the edge of the stiffener which will reduce, but not eliminate, the possibility of tearing. A very thin flex circuit will always require extra care in handing, regardless of the design features that may have been incorporated to reduce the possibility of damage.
Potting in the form of rigid or semi-rigid epoxies is a common method of making a connector termination more rugged and reliable. A bead of epoxy applied over the soldered leads on a surface-mount connector will greatly increase the twisting and pulling forces that the connector will be able to withstand without exhibiting damage.
 FIGURE 3. Epoxy-potting lead used on surface-mount connectors (top) and lap-soldered connectors (bottom). |
For surface-mount connectors and lap-soldered connectors, the most effective method of enhancing the mechanical strength is to use epoxy-potted leads (see Fig. 3). Epoxy potting can also be used on through-hole connectors to make them more rugged. After the through-hole connector is soldered in place, epoxy can be used to fully encapsulate the solder joints. By potting all of the soldered leads together, any stress put on the connector will be spread uniformly over all of the leads. This will greatly reduce the chance that a single solder joint will experience excessive forces. The potting will also insulate the soldered leads and keep them from shorting to other conductors they may contact.
Flex configurations
In addition to surface-mount and through-hole configurations, many connectors also come in straight or right-angle versions. The right-angle version positions the connector contacts parallel to the flex circuit, and the straight version positions the contacts perpendicular to the flex circuit. The application should be scrutinized to determine which version will put the least stress on the soldered leads during plugging and unplugging operations. All of the strengthening features mentioned earlier can be used with either straight or right-angle connectors.
If the flex circuit will be exposed to particularly demanding mechanical forces, the designer may want to consider rigid-flex construction. Not to be confused with stiffened, multi-layer flex construction, the rigid-flex termination areas have hard board material laminated to both sides of the flex circuit prior to the copper plating operation. Then, the through holes are drilled and plated through both rigid and flexible materials. The resulting construction provides a deeper plated barrel, which makes for a much stronger solder joint. The hard board materials also make the termination area much stronger and more rigid. After the connector is soldered in place, potting around the perimeter of the connector body or over the solder joints can further strengthen the design.
Although rigid-flex construction offers superior mechanical strength in the termination areas, it does not come without cost. Rigid-flex construction can increase the cost of a flex circuit by as much as 50% over a comparable stiffened multi-layer. Even though reliability should never be sacrificed to save costs, it does not make sense to use a more expensive design if a less expensive design will be equally reliable.
A reputable flex-circuit manufacturer can be an excellent resource when making critical design decisions. The flex-circuit manufacturer can help the designer weigh circuit cost versus performance, allowing the designer to make informed decisions. With input early in the design process, the flex-circuit manufacturer can help guide the designer to create a reliable, cost-effective interconnect solution.
MARK FINSTAD is principal applications engineer for the flex circuit division of Minco, 7300 Commerce Lane, Fridley, MN 55432. Tel: (763) 571-3121; Email: Mark.Finstad@minco.com.
