Value Added Design Options

Forming

Flexible materials don’t guarantee that the circuit will functionreliably when bent or flexed. There are many factors that con- tribute to the reliability of a printed flex circuit and all of these factors must be taken into account during the design process to ensure that the finished circuit will function reliably.When designing a flex circuit, the designer must factor in all ofthe parameters that will have an impact on the circuit’s ability to bend or flex in the specific application. These include, but are not limited to: whether the application is static or dynamic, bend radii, dielectric thicknesses and type, foil weight, copper plating, overall circuit thickness, number of layers, and number of flexures.The tighter a bend radius becomes, the higher the probabilityof failure during flexing. Keeping the overall thickness of the flex circuit in a bend area to its minimum will increase reliability. The ratio of bend radius to thickness is one indicator of whether the design is going to be reliable or have a high probability of failure. If the bend radius is at least ten times the thickness of the material, there is a good chance that the circuit will func- tion reliably. If the calculated bend radius falls below ten to one, the design may be questionable. Formulas for calculating the minimum allowable bend radius for several circuit types can be found in IPC-2223.It is possible to design for much tighter bend radii, in a bend-to-install application which retains the formed shape of the cir- cuit. The circuit must be designed to withstand stretching along the outer bend and compression of materials on the inner bend. Stretching can tear covers or crack conductors while compression causes foil and cover wrinkling that can also lead to tears. These problems become more of a concern in applica- tions that require the circuit to be bent beyond a 90-degree angle. As the bend angle increases beyond 90 degrees, the damaging effects of stretching and compressing increase dra- matically. Any time that a reduced radii bend beyond 90 degrees is incorporated into a circuit design, the circuit should be bent one time only. On bends over 90 degrees, it is also advisable to constrain the circuit in the formed condition tokeep it from relaxing or being inadvertently reopened.The ideal circuit design would have no copper plating on theconductors in the forming or flexing area. Electrolytic copper has much lower ductility than that of rolled annealed copper, making it much more susceptible to fracturing when it is bent or flexed. Other types of plating, such as gold and/or nickel, should be avoided in the flexing area for the same reasons.Copper plating on the flexing conductors may be eliminated byusing pads-only plating or designing with pads-only layers on the external surfaces. Minco can factory form some flex circuits with radii all the waydown to a “crease”, dependent upon board design, to improve installation precision and repeatability within our customer’s assembly process. When designing a part for forming, it is important to avoidmechanical stressors in the bend zone. Stressors include pads, holes, components, and sharply angled conductors. These reflect the most common features problematic to forming.Forming imparts stress into circuits. Some designs will be bettersuited to this forming process than others. Minco encourages customers to discuss the intended usage of parts they want formed with our engineers to help determine suitability. Forms are likely to relax slightly over time. Tightly tolerancedforms are not normally held by flex circuits. Minco recom- mends specifications to read “reference only”or to describe the parts as “capable of attaining ‘x’dimensions”for both angular and linear aspects of forming. Formed parts are less capable of withstanding temperaturevariations than their un-formed counterparts. This is particularly true where the bend zones are exposed to solder reflow or high temperature sterilization procedures. To work around these lim- itations Minco can suggest optional designs or assembly processes. Please contact us to discuss your needs.

Population

Minco will also assemble hardware and electronic components onto your flex and rigid-flex circuits. Numerous connectors are available for flex termination, rangingfrom crimp connectors to nano-size SMT connectors or discrete pins.Heat-sinks, metal stiffeners, and plastic mounting frames mayalso be laminated, heat staked or glued to flex circuits.Flex-coil designs are an example of an embedded componentsupplied within flex, multilayer, and rigid-flex circuits. The most common electronic components are typically surfacemounted to the flex circuit using automated pick-and-place equipment. Design considerations for populating flex circuits differ slightly from rigid boards. Simple flex circuits usually need to be stiffened for surface mount components. Our design engi- neers will make recommendations for necessary design factors to meet your needs. Between our in-house capabilities and our vendor networkwe’re able to offer our customers bare parts, palletized and unpopulated, partially populated or fully populated. 22