Effectively transmitting electronic signals in an IoT product relies heavily on how device packaging, traces, routing and vias are designed in IoT rigid-flex or flex circuitry. Component or device packaging includes micro ball grid arrays (micro BGAs), quad-flat no-leads (QFN) and dual-flat no-leads (DFN). These electronic device packages are less than the size of a pencil eraser in some cases.
The IoT circuit designer creates signal traces and routing patterns on the rigid-flex or flex circuit. At the same time, he designs in via locations on the circuit. Vias provide the electronic connections between the different layers of a circuit board.
Today, the smaller electronic device packaging is characterized by extremely tight pitch of 0.25 – 0.20 millimeter (mm) and below. Pitch refers to the spacing between one lead or ball and the next one. There are literally hundreds of leads or tiny balls used to connect a packaged device to the flex circuit of an IoT product to conduct electronic signals.
The width or size of the 0.25 or 0.20 mm pitch is equivalent to the width of one strand of a thinly sliced human hair into multiple strands. That’s how tight pitch sizes are these days.
So, how does this tight pitch affect the IoT rigid-flex or flex circuit layout?
It affects the layout in the sense that when you are doing the fanning out of these traces through the micro devices, you have to make sure you are using the right thickness of the traces. You must have the right amount of spacing between those traces, as well.
In some cases, due to constraint requirements, you have to “neck down” the traces when you are routing between the micro package BGA balls or bumps and signals pass through those tiny balls or bumps. “Neck down” refers to trimming the width of a trace in between two locations of a BGA ball or a QFN bump.
This also means you have to be able to make sure that the current is passing through. This can become an issue if you are doing the length matching. The reason is typically when length matching of two traces is performed, not only the lengths have to be matched, but the width of these traces have to be matched within 5-10% of each other as well.
Traces in today’s technology dealing with three mils can be fabricated. However, producing two mil traces for an IoT rigid-flex or flex circuit requires an experienced fabrication house.
Avoid overly thin traces. Three mils could be a thin trace. But if it’s a smaller device, the recommendation is to not go any lower than two or even one mil, which most likely gets over-etched in the fabrication process during the etching stage of manufacturing.
You also have to keep in mind the tolerances that are involved with manufacturing processes. Sometimes, traces are over-etched and at other times, traces are under-etched if the tank chemistry is not properly maintained.
Now, via sizes range from eight to four mils. An eight mil via is a comfortable mechanical drill. A four mil via is at the edge where in some cases you may have to go to laser drilling. That will increase the fabrication cost and limit the fab shops that can do that. Device placement could be an issue due to finer via sizes, depending on the aspect ratio of the flex circuit. It could become a challenge using four mil vias when using thicker than 0.062″ thick board.
The idea is to keep the number of vias to a minimum because keeping the vias to a smaller number increases reliability. If it’s a smaller IoT device and you have too many vias, the space for trace routing is blocked going from one point to the next.
It’s best to minimize via size and reduce the number of vias, but still get signals across the circuitry and back.
It is also important to assure that the smallest via is small enough to carry the current. But it should be large enough to withstand flex circuitry bends and twists when it goes through mechanical stress of bending and twisting.
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