Challenges of DFM Analysis for Flex and Rigid-flex Design, Part 2

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(Editor’s note: This is the second of a three-part series. To read Part 1, click here.)

Flex and Rigid-Flex: Different Layer Types 

Everyday rigid FR-4 PCBs have a well-known layer stackup recipe: dielectrics, PCB conductor layers, plane layers, mask, and silkscreen (nomenclature or legend). More advanced layer types may include embedded or screened components or cavities with bonded bare die. 

Flex and rigid-flex stackups include those similar to rigid PCBs, such as dielectrics, conductor, mask, and silkscreen layers, but that is where the similarity ends. There are many additional layer types present for this genre of PCB. They include types like coverlay, adhesive, conductive film, conductive foil, conductive adhesive, bondply, and stiffener.

• Coverlay: An external polyimide film that encapsulates and protects flexible conductor layers.
• Adhesive: Bonding agent used to bond copper foil to a polyimide substrate or coverlay.
• Conductive film: Transparent but electrically conductive film found in LCDs, touchscreens, and other optical applications.
• Conductive foil: Thin sheet of copper or silver commonly applied to a flexible PCB as shielding.
• Conductive adhesive: Adhesive applied for electrical interconnect, thermal or structural bonding applications.
• (Flexible) bondply: A polyimide film, coated both sides with B-staged acrylic adhesive. This is essentially a coverlay with adhesive on both sides.
• Stiffener: Blank FR-4 or other material bonded to flexible materials to add stiffness in a specific area.

These layer types are found in typical flexible PCB stackups. The presence of these layer types is not accounted for in most current DFM analysis. This is one area where specific DFM analysis is required. Rigid DFM analysis can detect issues with a solder mask but cannot be used to detect trace corners in a bend area.

Inter-layer Dependencies

While less common in rigid PCBs, flexible PCBs have many inter-layer dependencies that, if not managed well, may lead to manufacturing issues or field failures. The fabrication process used to bond rigid and flexible layers also has its inter-layer dependencies to be managed. Here are a few examples of inter-layer potential issues.

Squeeze-out

Even with best design considerations and best manufacturing practices, adhesive can “squeeze out” and bleed into unwanted areas on adjacent layers. Coverlay adhesive is a common source of squeeze-out that flows during lamination onto conductors, pads, cavities, fingers, or other features. This requires analysis that compares the size of coverlay exposure against the annular ring of adhesive below it.

To read this entire article, which appeared in the May 2023 issue of Design007 Magazine, click here.

04/17/2023 | Mark Gallant, DownStream Technologies

Flex and rigid-flex PCB constructions are not new concepts. It has become commonplace as engineers look for alternative circuit packaging for ever-shrinking electronic products. A flat, one-sheet schematic for a straight ribbon cable is analogous to its physical flat substrate. A flat, multi-sheet schematic that details circuitry for a rigid-flex design bears little visual resemblance to its three-dimensional, variable material rigid-flex assembly. However, in both schematic examples, schematic-based analysis tools are applied equally. This same truth also applies to common FR-4-based two-layer or multilayer PCBs.

10/30/2022 | Andy Shaughnessy, Design007 Magazine

We recently conducted a roundtable with a team of printed electronic circuit experts from companies that run the gamut: John Lee and Kevin Miller of Insulectro, Mike Wagner of Butler Technologies, Tom Bianchi of Eastprint, and John Voultos of Sheldahl Flexible Technologies. In this third and final installment of the roundtable, these experts discuss some of the differences and similarities between PEC and traditional PCB processes, the future of printed electronic circuits, and why the best way to learn about this technology is through networking with veterans of this segment who are eager to share their expertise with the next generation.

07/21/2022 | Cody Stetzel, Cadence Design Systems

I have the tendency to try to replicate the delicacies I’ve ordered at restaurants in my own kitchen. One of my latest attempts at creating restaurant-worthy dishes was a Korean pancake that’s crispy on the outside but soft on the inside. With my amateur cooking skills, it proved to be an impossible task—I could either make only a hard pancake or one that was total fluff. While I’m still struggling to figure out the trick to bringing together the different textures of a Korean pancake, I’ve had more success in bringing together the hardboard elements and flexible PCB elements of a rigid-flex PCB. Compared to making Korean pancakes, striking the right balance of flexibility and rigidity on a rigid-flex PCB is easy if you abide by rigid-flex PCB design guidelines.