Drilling disparate materials can lead to an assortment of problems.
In our last installment, Mark Finstad talked about some of the things that can go wrong in the manufacturing process. Learning near the end of the process of a problem that will impact delivery can be frustrating.
Once all the layers are laminated together, some risks still remain that need to be managed. For example …
Poor hole quality. Rigid-flex constructions bring an extra challenge to the drilling process. Beyond alignment issues caused by differential dimensional stability, it can be more difficult to create a cleanly drilled hole. To make a reliable plated through-hole, the hole must be cut cleanly through the material and prepared properly prior to plating. Since the rigid-flex stackup blends a variety of materials, it is possible to have drilling flaws such as excessive nailheading of the copper, dielectric smear over copper surfaces in the hole wall, and material tear out. Consider the differences of drilling a sheet of steel versus drilling through plexiglass. Cutting steel creates clean metal chips, while cutting plexiglass often looks like the material was melted rather than drilled. Drilling rigid-flex involves cutting materials that are very different in terms of hardness. If not done properly, the result can be holes of such poor quality that plating will not be successful.
Read more: The Nature of Processing Rigid-Flex, Part 2
Flat or folded, flex circuit designs provide superior impedance performance across a wide spectrum of signal speeds.
Question: Will impedance change when the flex portion of a rigid-flex is bent into place during assembly?
The short answer is no.
When a circuit is bent, the distance from a single-ended or differential pair to the reference planes remains constant, as does the distance between traces in a pair. There is no compression of the dielectric or movement of signal lines. As a result, the impedance remains constant as well. The performance as modeled in the flat condition will continue when bent.