Printed Circuit Design & Fab Online Magazine - Current Issue

Flexperts

Nick Koop To avoid post-soldering delam, keep parts dry, and use plenty of adhesive.

I am experiencing flex circuit delamination. What is causing it?

Delamination occurs for a few different reasons. Very often, it happens during solder reflow during assembly. Usually this is due to moisture absorbed into the part prior to the soldering operation. Flexible materials absorb moisture, and during soldering that moisture can rapidly heat and expand, resulting in delamination. The first line of defense is prebaking prior to soldering. In a standard oven, just getting above the boiling point of water is good enough, so 225° to 250°F is plenty. Bake time can vary depending on circuit thickness, but two hours is a good start. Thick rigid-flex parts may need four to six hours. If parts are stacked in the oven, time may need to be added to ensure the parts in the middle of the stack dry out.

Read more: Causes and Cures for Flex Circuit Delamination

Mark Finstad Flex circuits can run 10+Gb/s signals, but many factors need to be met.

Read more: Maximizing High-Speed Performance in Flex Boards

Nick Koop Many flex designs perform well with panel plating for countless bend-to-install applications.

When copper-plating vias and through-holes, there are several process options in the PCB manufacturer’s toolbox. Typically, they fall into three buckets: panel plating, pattern plating and button plating.

Panel plating (FIGURE 1) means the entire panel surface and all the holes will be electrolytic copper plated to the full plating thickness requirement. The etch process will etch down through the base and plated copper, leaving a pattern with features comprised of both the base and plated copper. Pattern plating is accomplished by creating a pattern of all the circuitry on the two exposed layers with a plating resist, then plating up the pattern of the outer layers. After stripping the resist, the etch process will etch away the base copper between all the plated patterns, leaving a pattern with features comprised of both the base and plated copper. Panel and pattern plating essentially result in the same end-product. For this discussion, we will compare button plating and panel plating.

Read more: Should My Part Have Full Plating or Button Plating?

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