The dynamic nature of flex circuits allows the designer a multitude of options. Compared to discrete wiring or use of ribbon cables, flex circuits offer customized repeatable routing path within the assembly. While this gives necessary dependability, the long life spans of flex circuits reduce service calls.
Flex circuit boards come in different types:
MIL-P-50884, IPC-6013 – Type 1 Single Layer: Only one flexible conductive layer with two insulating layers on each side.
MIL-P-50884, IPC-6013 – Type 2 Double Layer: Two flexible conductive layers separated from each other with an insulating layer. Outer layers may have exposed or covered pads. Connection between layers is provided with plated-through holes. Both sides may have covered vias, with stiffeners, pins, connectors, and components being optional.
MIL-P-50884, IPC-6013 – Type 3 Multi Layer: Three or more flexible conductive layers separated from each other with insulating layers. Outer layers may have exposed or covered pads. They may also have plated through holes to provide connection between layers, along with blind or buried vias. They may have optional components, connectors, pins, and stiffeners.
These boards also include High Density Interconnect (HDI) flexible circuits, offering increased options for design, layout, and construction over regular flexible circuits. They may have micro-vias and fine features for achieving high density, increased functionality, and smaller form factor.
MIL-P-50884, IPC-6013 – Type 4 Rigid Flex: Two or more conductive layers separated by either rigid or flexible insulation material in between each. Outer layers may have covered or exposed pads. Plated-through holes may extend through both flexible and rigid layers, but blind and buried vias are an exception.
MIL-P-50884, IPC-6013 – Type 2 Multi Layer, without Plated Through Holes: Two or more conductive layers separated by insulating layers between each. Through holes are present but not plated. Exposed pads or access holes may be available on either or both sides, with or without covers.
The conductor layer is usually made of copper of thickness varying from ¼ oz. (9 µm) to 10 oz. (356 µm). Different forms of copper may be used, including half-hard, rolled-annealed, and electro-deposited. The conductive layer may also be made of alternate materials such as beryllium copper, cupro-nickel (70/30 alloy), nickel, and silver epoxy.
Adhesive is used to bond the conductor layer to the rigid or flexible insulator. The adhesive is usually modified acrylic, with thickness ranging from ½ mil (12.5 µm) to 4 mil (100 µm). Depending on the application, the modified acrylic may be flame retardant type, pressure-sensitive type, or pre-impregnated material of FR-4 grade or polyimide with thickness up to 8 mil (200 µm).
Rigid or Rigid-Flex Substrate
The rigid substrate is usually FR-4 type, with thickness varying from 0.003” (0.08 µm) to 0.125” (3.18 µm). The rigid-flex substrate is generally Polyimide type, and its thickness may vary from 0.003” (0.08 µm) to 0.125” (3.18 µm).
The flexible insulator or substrate is usually a polyimide film such as Kapton, with thickness varying from ½ mil (12.5 µm) to 5 mil (125 µm).
Stiffeners may be required as reinforcement to allow connecting a part or extremity of the flex circuit with connectors, without incurring damage to the flex circuit. Materials such as copper, aluminum and other metals may be used with thicknesses depending on the application and stiffness required. Apart from metals, polyimide glass, FR-4, or polyimide material may also be used as stiffeners.
The increasing requirement for flexible circuits is demanding improved functionality and flexibility from flexible circuits. Use of flexible circuit technology is offering greater freedom of packaging geometry and significant reductions of interconnects, which may lead to potential via and plated hole reliability issues.
Typical construction methods of multi-layer flexible circuits, including rigid-flex circuits, use several layers of adhesives. As adhesives tend to expand faster than the laminate (nearly 10-20 times that of FR-4) with rise in temperature, vias in the rigid-flex area undergo tremendous amounts of stress, due to thermal cycles occurring during RoHS assembly, multiple cycles of assembly, and with higher system and operating temperature of components. The unequal expansion may result in cracks in the copper plating within via holes.
In rigid-flex PCB design, the adhesive can be a part of the copper clad flex laminate, the coverlay being used, or the material that bonds the rigid and flex layers together. To overcome the issue of via reliability, PCB manufacturers and material suppliers, including industry standard organizations, have developed solutions and drafted specifications to eliminate the use of adhesives in these areas.
This has led to the development of copper clad flex laminate construction without adhesives, where the copper directly attaches to the polyimide core. While this eliminates the requirement for an adhesive bond layer, the entire construction is much thinner, and has vastly improved reliability. Because no adhesives are present, the copper clad laminate can operate at much higher temperatures, offers higher copper peel strengths, and there is significant reduction in the thermal expansion stress on vias.
Earlier, rigid-flex designs had coverlays of adhesives covering the entire rigid area. The higher coefficient of thermal expansion of adhesives again put excessive stress on vias and plated through holes present in those areas.
Now, manufacturers use selective coverlay constructions that restrict its coverage to the exposed flex areas only, with only a limited interface within the rigid areas. The flex area does not have via and plated through holes.
Rather than use layers of flex adhesives, manufacturers are now laminating the rigid and flex layers into a final structure, using prepregs of high-temperature no-flow FR-4. The resulting structure is dimensionally as stable as a standard rigid PCB is. Refer to the IPC-2223C Sectional Design Standard for the above key elements for a reliable flexible printed circuit board design.