Printed Circuit Design & Fab Online Magazine - Current Issue

Current Issue

Lenora Toscano

A rough topography aids reliability even while it complicates wet processing.

Read more: A ‘Promoter’ for Solder Mask Adhesion

From glob top to lamination, solutions for sealing parts in flex.

I need a flexible circuit with a temperature sensor on one end. I need this area to be as thin as possible and sealed against liquid ingress. How do I accomplish this?

Several ways will do what is described. Each has its own benefits and associated costs. I will cover each of them below.

SMT-mounted sensor with conformal coating. The most straightforward solution is to simply mount a standard surface mount NTC or other sensing device to the flex, then seal it with a conformal coat. The conformal coating is typically an epoxy like 3M 2216. This will seal the component and ruggedize the sensor’s solder joints to keep them from damage during assembly or subsequent handling (Figure 1). The downside to this option is the sensor and potting will protrude above the surface of the flex, and depending on the size of the component, this could be significant. Also, the potting is basically a “glob top” where a predetermined dose of epoxy is dispensed over the component and permitted to flow over the top to cover the component and surrounding area. This usually does not produce a perfect circle or oval footprint on the flex, so expect a fair amount of variation. But if there is space above and around the sensor, this is the easiest and least expensive option. Thinner conformal coatings may be used to seal the sensor (e.g., spray-on type), but those will not offer the mechanical protection of epoxy. Potting may also affect the sensor’s sensitivity and responsiveness.

Read more: Embedded Sensors in Flex Circuits

Meeting drawing requirements may be more complex than you think.

As a design is completed, the CAD data and drawing define all the important dimensions and requirements. It is the expectation that when the first shipment is received – and every one after that – the parts will meet all dimensional requirements. But has thought been given to how the manufacturer makes that happen?

It all starts with the CAD data the designer supplies. That data sets the standard. Now it is up to the manufacturer to create the tools to build the part. As it does, keep in mind that the manufacturer does not build a part, it builds a panel of parts. So unless the part is very large, the production panel will hold multiple parts. To accomplish this, the fabricators need to ensure all the features are aligned on all the layers across the entire panel.

As the number of circuit layers increases, so does the challenge of getting everything to align and finish at nominal. The first thing to consider is the materials used to make a circuit are not made of granite. They move in all dimensions, and different materials move differently. Manufacturers need to compensate for the movement. This is done with two techniques: alignment tooling and scaling.