Fixtures and special tooling are used to further assure surface mount (SM pad) stability and preciseness on a wearable or Internet of Things (IoT) PCB during the pick and place operation. The figure below shows a comparison between fixtures for conventional and flexible PCBs. These assembly steps are especially important since some wearable or IoT PCBs have different sizes and shapes and are based on a combination of rigid and flex PCB.
Each wearable/IoT PCB fixture needs to enclose the complete circuitry so that it’s co-planar or flat. This includes the periphery as well as the base line since the printing process is being performed.
A special fixture is needed to keep a flex circuit flat. Shown at bottom of photo is a conductive metal fixture with tight tolerances for wearable or IoT flex circuits compared to a regular fixture made of glass fiber-based material at the top of the photo.
Unfortunately, in some cases, inexperienced EMS providers or contract manufacturers (CMs) misjudge the requirements for such tooling and create aluminum tooling that absorbs most of the heat from a reflow oven at reflow stage of manufacturing and not enough heat is penetrated through the flex circuits to create sturdy solder joints.