Flex PCBs are a wild card during assembly, but arrays may ease the process.

As you complete your new flex or rigid-flex design, thoughts naturally turn to the next steps. No circuit is complete until the components are placed and it is installed into the next higher-level assembly. Maybe the flex is like a cable and just has connectors on both ends. At the other end of the spectrum, it could be chock-full of BGAs, passive and active SMT components, and maybe even through-hole devices.

In any case, the next step is assembly. Will parts be placed manually one-by-one, or are pick-and-place or other automated processes planned? These considerations may drive how you want the parts delivered. In some cases, parts are fully singulated and shipped. This is the default unless something else is specified. In other cases, parts are arranged in an array and shipped in a pallet or sub-panel.

Generally speaking, shipping as single units is the lowest cost per part. This is because the manufacturer can optimize parts per panel without considering array borders, tooling holes and fiducials. Array layouts usually have fewer total parts per panel than individual part layouts. This is especially true of flex and rigid-flex, which often have appendages that complicate how parts may be laid out in the larger manufacturing panel for best utilization. From a total cost of ownership point of view, however, the array may result in the best total cost of PCB and assembly.

Read more: To Array or Not Array? That is the Question

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