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62 DESIGN007 MAGAZINE I MAY 2023 • (Flexible) bondply: A polyimide film, coated both sides with B-staged acrylic adhesive. is 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. ese layer types are found in typical flex- ible PCB stackups. e presence of these layer types is not accounted for in most current DFM analysis. is is one area where specific DFM analysis is required. Rigid DFM analysis can detect issues w ith a solder mask but cannot be used to detect trace corner s 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 manu- facturing issues or field failures. e fabrica- tion 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 com- mon source of squeeze-out that flows during lamination onto conductors, pads, cavities, fingers, or other features. is requires analy- sis that compares the size of coverlay exposure against the annular ring of adhesive below it. Missing Coverlay Exposures Like solder masks, coverlay is intended to cover exposed conductors on flexible layers. When button plating is used, vias on flexible layers must be exposed (non-tented) on the coverlay. Analysis must detect only tented vias in a bend area. I-Beaming When two opposing conductor layers of a flexible core have parallel traces, coincident trace segments on the layers may fracture when the PCB is bent. e analysis must ana- lyze only double-sided flexible cores having parallel traces on opposite sides. Book Binding In multi-layer flex, each flexible core bent on the same axis in the same loca- tion will result in what's known as book binding. e inner layer bends are at the small- est radius, while each successive outer layer bend requires a larger radius to prevent trace frac- ture. is requires the outer layers to be length- compensated before imaging. Partial Exposure Exposures created to facilitate interconnect between flex layers and adjacent shielding lay- ers is not correctly sized, resulting in poor or no connection. Analysis must compare exposure shape and location against its mated shielding layer conductive surface. Solder Mask Encroachment In some cases, both coverlay and solder mask are present on a flexible design. Because mask materials can be more brittle than cover- lay, there should be a maximum overlap of the two in or adjacent to bend areas. Analysis must detect maximum layer overlap between solder mask and adjacent coverlay layers. The fabrication process used to bond rigid and flexible layers also has its inter-layer dependencies to be managed.