Feature: Medical
• Te rigid-to-flex circuit boundaries. SMT components are typically attached using solder reflow. As such, the assembled components may need to be located on rigid PCB material to maintain solder joint integrity. SMT components soldered onto flex PCB material must be handled with care due to reduced strain relief. Even if the application involves relative freedom from physical disturbance, handling and next-level assembly must be considered for long-term reliability. Typical PCB stiffeners are implemented using FR4, polyamide,
polyimide and/or metal. In our CBT patch, we use a 4mm-thick polyimide for the flexible areas with a 12mm-thick polyimide for the stiffened areas. For stiffening purposes, the flexible finger circuits were reinforced with metal segments. Te CBT patch prototype is manufactured as a flat rigid-flex
assembly subsequently requiring two static bends. As shown in Figure 12, the circuit finger extending from the circular area to the TS4 temperature sensor needs two 90° bends for the final assembly. Brick-pattern metal segments are used to construct the TS4
flex finger design such that the fatigue imposed by the one-time static bends are mitigated. Figure 13 shows the staggered brick- pattern stiffeners as well as the neck-area reinforcement to mitigate mechanical stresses at the rigid-to-flex boundary. Furthermore, the
breaks in the brick pattern eliminate thermal conductivity along these metal paths. Tis design technique is also mimicked on the circuit finger extending from the circular circuit area to the connector, CN1. Other areas of consideration include avoiding 90° corners –
points of stress concentration – and the potential implementation of preform fabrication.
Manufacturing considerations To produce a robust and reliable design, it is recommended that the designer work closely with the PCB fab and assembly house. Design details of all electrical, thermal and mechanical aspects should be reviewed prior to first fabrication. In many instances, the fab/assembly house has alternative materials and techniques that can be implemented for design enhancement. During development of the CBT patch rigid-flex PCB assembly
process, several significant hurdles had to be overcome: Te solder reflow material used and the solder reflow profile. Originally, standard reflow solder was used, resulting in delamination; see Figure 14. Tis is particularly bad, given the fact that air pockets, acting as insulators, will influence the heat flow through the rigid- flex PCB. Tis was eventually resolved by using an alternative low-temperature eutectic solder. Te solder reflow profile had to be fine-tuned over several batches to achieve an acceptable yield.
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