Printed Circuit Design & Fab Online Magazine - Current Issue

Flexperts

Mark Finstad Picking the material and designing the mold.

This is the second and final part of the series covering flex circuit over-molding concerns. This month we will discuss mold design, molding materials and molding procedures.

Mold design. While I am not an expert in the design of plastic-injection molds, I can share some experiences that will hopefully guide you when creating a mold. First, it is important to keep in mind that flexible circuits become very soft and malleable when heated. Also, keep in mind that molten plastic is hot! Now add in the fact that this very hot plastic is moving with high pressure across the flex circuit, which is becoming very soft and malleable.

See where this is going?

Read more: Over-Molding Plastic over Flex Circuits, Part 2

It can be done, but there’s always a learning curve.

Read more: Over-Molding Plastic over Flex Circuits

Nick Koop 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