Everyone loves traveling, but no one looks forward to packing. Even on weeklong trips, it’s rare for me to do any packing until the night before or even the morning of. Despite being an over-planner, I loathe arranging too many things into a small space. Fortunately, I travel light relative to most I know, and I’ve yet to have my last-minute packing tendencies return to haunt me.
The space issue isn’t just something I encounter on trips, however. PCB design, especially with the advent of HDI, faces numerous constraints relating to placement and routing and the overall industry march towards miniaturization. For some jobs, flex PCB technology holds the solution. With its ability to save valuable real estate through board-to-board connections and the added benefit of weight reductions, it endears itself well to many projects. While the flex manufacturing process differs from rigid, it is not so dissimilar as to confuse those familiar with the latter.
The Role of Material Properties in Flex Design
Flex and rigid-flex materials open up a new world of electronic design. Freeing boards from a rigid construct allows for greater viability when designing electronics with moving parts or irregular shapes. It can act as a stand-in for cable assemblies in multi-board designs. However, the design and manufacture of flex technology differ significantly from standard rigid materials. Due to variances in mechanical response to stress, shear, and reduced temperature profiles, manufacturers have devised alternative methods to process flex boards.
Not every adjustment to manufacturing should be considered a detriment: consider roll-to-roll processing. Unlike traditional rigid boards built and priced on a per-panel basis, roll-to-roll processing allows manufacturers to create flex electronics on sheets of material kilometers long. In essence, the roll-to-roll production yields the flex bare board equivalent. While there are some additional challenges relating to the fabrication of the flex board (like registration errors owing to tension mismatches between rolls), overall throughput can be several magnitudes greater than rigid board fabrication.
Changes in manufacturing arise from a difference in the substrate material between flex and rigid, and they are noteworthy. Flex traditionally focuses on polyester and polyimide, unlike the fiberglass-weave/epoxy resin mixture that provides form and rigidity. The most immediate difference between these two materials (and others that see less use) is the extremely high flexibility and tensile strength, which results in a circuit that can bend without damage to its structure. Additional manufacturing and environmental concerns differ somewhat between the two market-majority materials:
- Polyimide: Polyimide is the more favorable choice for most flex operations due to its better temperature resistance. It can successfully withstand soldering temperatures without damage or degradation. One major drawback is the material’s absorptivity. Care must be taken during the material’s initial processing and long-term installation.
- Polyester: The reduced melting point of polyester makes standard soldering processes inviable, although alternative methods are available. On the plus side, polyester is a much more economical material that is easy to work with.
An Overview of the Flex Manufacturing Process
Overall, the manufacture of flex circuits takes an “if it ain’t broke, don’t fix it” outlook relative to rigid techniques, with differences in processing arising where the material cannot adhere to established procedures. A quick rundown of the flex manufacturing steps:
- Laminates are cleaned of supplier anti-oxidant and anti-tarnish treatments before etching, which is replaced with a more suitable antioxidant coating that is more amenable to overall manufacturing.
- Artwork is applied to the laminate either by direct printing or photo imaging. Both result in the copper features of the design being transferred physically to the board-to-be.
- An etchant is applied, removing the extraneous copper (or other laminate material), leaving behind only the copper protected by the etch resist deposited in step 2. Typically this is performed in a bath that attacks the unprotected copper until the desired line widths are achieved. However, this common method can result in the undercutting of the copper features when trace lengths are especially narrow. Finer results from a more controlled etching process are available but at slow production speeds.
- The resist is removed before additional treatments of the cover-lay and cover-coat.
- Holes are perforated with a punch press since flex materials are ill-suited for drilling processes.
- Standard testing of the bare flex material for continuity, resistance, etc.
This method of removing excess material is known as subtractive processing. Though it is the dominant method due to its ease and production scalability, there are times when an additive process may be preferred. In this case, a plating stage occurs instead of the etching stage, where copper is deposited onto the laminate’s surface from a seed conductor layer before a final etch removal.
Flex PCB Technology Is a Lynchpin In Demanding PCBAs
Though flex may encounter some difficulties during manufacturing, the payoff is enormous. Designers and manufacturers can significantly reduce weight for crucial industries like aerospace, where mission payload targets are critical. Rigid-flex can optimize space in three dimensions by creating compact multi-board assemblies in small form factors. Mechanically, flex can outperform intricate cable assemblies and reduce interconnect strain and the possibility of intermittent connectivity failures.
Flex PCB technology continues to solve some of the most challenging issues in the industry, providing space- and weight-conscious solutions that standard rigid cannot provide. Though production may not be as straightforward, an experienced manufacturer can guide design with an eye toward prototyping and even mass production quantities. That’s where VSE comes in. We’re a team of engineers who build electronics for our customers and are committed to providing the finest quality in our PCBs. Together with our valued manufacturing partners, we’re eager to render excellent service and products to our customers.
If you are looking for a CM that prides itself on its care and attention to detail to ensure that each PCB assembly is built to the highest standards, look no further than VSE. Contact us today to learn more about partnering with us for your next project.