I like to think of myself as reasonably in shape. My exercise routine is challenging enough to push me, yet I can continually meet my moderate fitness goals. Rarely do I feel overwhelmed entirely – except after I visited a friend’s acrobatic gym. The prevailing mentality I’ve always had at the gym is that bodyweight exercises couldn’t form a rewarding workout – I instantly rethought this anecdote once I saw people balancing themselves with their hands on the edge of wooden chairs, legs pointed towards the ceiling.
Flexibility, including printed flex circuits, is often more complex than we’re willing to recognize. While flex circuits significantly reduce the size and challenges of wire harnesses, manufacturing is incredibly complicated. However, confined enclosures rely on a flex circuit prototype (or prototypes) to correctly incorporate circuit design into a minimal space without sacrificing functionality.
Flex Circuit Types
|Flex circuits contain a single conductive layer and a single or double-sided insulator with conductor access.
|As above, but with two conductive layers, a flexible insulating layer, and plated through holes.
|As above, but three or more conductive layers.
|As in Type 2, but with a flexible/rigid insulating layer—the plated through holes span flex/rigid material.
Staying Cost-Conscious with A Flex Circuit Prototype
Flex circuit prototypes differ noticeably from those of traditional rigid boards. Many aspects may appear similar from the design end, but as the mechanical properties expand the possibilities for printed circuit shapes and orientations, they greatly complicate manufacturing. Prototypes allow for a relaxation of tolerances, and design can be less than production-ready when testing and debugging are of greater concern than design for manufacturing (DFM). Still, producing flex circuits at any level of product development is a more intense undertaking than similar rigid designs.
As designs evolve toward production, per-unit manufacturing costs become more prominent. Early in development, per-unit costs are relaxed as the need for a testable, interactable device far outweighs any moderate financial concern. However, the design cost must be fully accounted for when production begins. Reviewing design requirements and feasibility through an economic lens will avoid over-tolerancing during the prototype stage:
- Panelization – Just as with rigid, manufacturing charges on a per-panel basis, not per-circuit. Careful design orientation will maximize the usable panel area and allow additional circuits per panel. The greater mechanical flexibility supports greater redesign possibilities; design teams should thoroughly consider
- Dimensioning – In general, printed circuits are far less reliant on tolerancing than their rigid counterparts. Loosening constraints is both a negative and a positive: printed circuit boards are akin to machined parts, whereas flex circuit technology cannot support such a high level of precision. However, this also means all non-essential dimensions need only be a reference to achieve a suitable level of design adherence.
- Feature size – Building on the dimensioning, smaller features that would be trivial to rigid boards can severely impact flex manufacturability. As the mechanical requirements of the flex circuit dominate the end product and its manufacturing process, it is often less expensive to increase flex circuit aspects like layer count than accept the corresponding yield reduction.
- Plated through-holes (PTHs) – Flex circuits diverge from rigid in that the stackup is not necessarily a constant throughout the design; this variance can be at the designer or manufacturer’s discretion to enhance producibility or a mechanical requirement of the circuit assembly. In either case, PTHs should only reside in like areas of the flex circuit where layer count and material construction are identical.
Dolls? How Flex Circuits Fit to Enclosures
Walking through flex circuit prototypes will be an even more iterative format than rigid boards due to reduced constraints: a flex material can bend, wrap, contort, and stretch to fill 3D space within an enclosure better. An enclosure for a rigid board will be more straightforward as it only needs the board dimensions, and the height or distance components extend past the board. Consider a rough guide to fitting flex circuit prototyping:
- Layout – DFM must adapt to the different properties of flex materials relative to rigid, such as elongating through-hole pads and adding tabs for better substrate grip or preventing conductor stacking that reduces flexibility and develops internal strain. Even the placement of features can have a more drastic effect on producibility due to the printed circuit’s thin profile and mechanical properties.
- Paper dolls – The first step to physically realizing the printed circuit is a layout with paper dolls – a 1:1 paper (or another similar cheap, flexible, and easily perforated material) stand-in for the circuit based on electronic CAD outputs. These drawings can come from production data like Gerber data or fabrication artwork. The purpose of the paper dolls is to ensure that when the printed circuits are folded/adhered to the surface of their enclosure, they fit as intended; any tearing or deformation could indicate a mismatch between the printed flex circuit and enclosure.
- Polyester mock-up – After finalizing the dimensions for the paper dolls, it’s necessary to re-fit the enclosure with polyester. Polyester is a standard printed circuit material and, therefore, will much closer simulate the final properties of the product (namely the stiffness – much more so than paper). Using a copy machine to transfer the outline to the polyester before cutting/trimming is the simplest method to put the design on polyester.
- Mechanical sample – The last mock-up, including component holes and outline features, will be materially indistinguishable from the flex circuit. No etching process of the copper layers will occur, but the inclusion of said layers will even closer simulate the final product. A thorough prototype revision process should require little (or no) modifications to the design at this point. As the ultimate pre-production stage, development teams should check and vet the design to ensure no further changes are necessary before finalizing orders.
Your Contract Manufacturer’s Experience Is More than a Flex
A flex circuit prototype requires many of the same steps as a rigid board, but the flex materials also cause a few changes to the standard process. The key to a successful flex manufacturing process is understanding where and how production constraints can relax to save money and accelerate the manufacturing schedule. Even for design teams well-versed in rigid manufacturing, the quirks of flex may require additional expertise to navigate successfully. We here at VSE bring over 40 years of PCBA experience, and our team of engineers is committed to building electronics for our customers. Across multiple industries, we pair with our valued manufacturing partners to help realize life-saving and life-changing electronic devices.