Rigid flex PCB circuits are among the most complex interconnection structures in production today. Having elements of rigid and flexible circuits technologies, these flex circuit constructions offer the best and the worst each technology has to offer. Rigid flex PCB circuits provide an excellent method for interconnecting complex electronic systems. They also are capable of offering cost and weight savings, as well as increases in reliability over conventional wiring harnesses. They also significantly reduce or even eliminate rework and repair. On the other hand, rigid flex PCB circuits represent demanding technical challenges and, like multilayer flexible circuits, are engineering-intensive and thus, expensive on an up-front basis. Even so, on a system level basis, rigid flex PCB circuits can provide a very cost-effective solution.
Although rigid flex PCB circuits are most often thought of in a military product context, more commercial applications are being developed, often with great success. In simplest terms, rigid flex circuits are hybridized constructions of rigid material laminated to flexible material and interconnected by means of plated through-holes. Like multilayer flex PCB, there is no such thing as a typical rigid flex circuit. Each construction offers its own unique challenges and requirements.
In its simplest form, a rigid flex PCB circuit may have just two conductive layers, one rigid and one flexible. In more complex constructions, there may be ten, twenty or more layers of flexible interconnects sandwiched between rigid outerlayers, totaling up to forty layers of circuits or more. Internall these constructions may include simpler constructions such as singleand double-sided flex circuits, which serve as interconnect tentacles.
Rigid flex PCB circuits evolved to solve weight and reliability challenges for military products and commonly served as formable backplanes for electronic system interconnec-.
tions and buses. These applications are still common and the designs and constructions are fairly straightforward, except that they often require the use of bookbinder construction, where each successive layer is lengthened in bend areas to mitigate the strain-related damage to the outer conductor layers or buckling of circuits that will occur if the technique is not used. The term comes from the requirement to keep pages flush on a closed book. it provides both physical and conceptual examples of bookbinder construction.
Rigid flex PCB circuits can be especially difficult to manufacture because of the requirement to mate materials of different composition and dimensional stability, hold them in register and reliably plate the through-holes.
Like multilayer flex circuit construction, there is no typical approach to building a rigid flex circuit construction. There are, in fact, a number of different standard and proprietary ways to build flexible circuits. As a result, the multilayer flex circuit manufacturing sequence is similar to the manufacturing sequences for rigid flex, except that rigid outerlayers are used and more pre-machining of rigid materials is required to freely access flexible elements of the finished product. Route and retain techniques are often used to facilitate assembly, especially when there are many circuits in a common panel. In the end, successful manufacture of rigid flex depends heavily on the tooling and mechanical fixtures used in fabrication.
It shows a variety of rigid flex circuits constructions.
– Flex PCB circuit with a stiffener (not a true rigid flex but presented for comparison).
– Simple rigid flex PCB with the flex circuit on the outside of the circuit sharing plated through-holes with the rigid material.
– Traditional rigid flex PCB with the flex circuit at or near the middle and the materials used for the flex portion.