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Fill Up Through Holes in IoT Rigid-Flex and Flex Circuits

Requirements for Class III IoT devices are very specific. IoT applications falling into this category include mil/aero and medical electronics. In the assembly process for any type of PCB, either surface mount technology or through hole is used to place components onto an IoT rigid-flex or flex circuit.

As the name implies, surface mount is just that – components and devices are placed with their connections on the surface of the circuit board; through hole means tiny holes are drilled into the circuit board to insert component leads through them and thereby make electrical connections throughout the circuit board.

If through hole components are used for an IoT device’s circuit board, for example, a major mil/aero Class III requirement is to have the barrel of each through hole to be filled at least 75% or more solder. This is required so that the IoT device can sustain harsh environments and keep joints intact in all kinds of conditions. Fig. 1 shows an empty barrel, a filled barrel, and one that shows about 35% filled.

This Class III requirement is also known as the J Standard applied in mil/aero and similar demanding applications. In particular, IoTs targeted at aerospace applications must be extremely reliable. You not only have to prove through hole barrels are filled, but you also must have 100 % verification using x-ray or similar inspection and verification system. Here, there’s no human intervention, guessing, or judgment calls.