The differences need to know between the FPC assembly and Rigid PCB assembly
Ordinary SMT processing is basically the same, because the flexible circuit board, Rigid-Flex board and Rigid board all need the component placement and reflow soldering paste welding process. However,as to the Flexible PCBA and Rigid PCBA there is some details specially, if the extra details are not handled properly, then there will be tremendous problems.
Then, let’s move to the details as follow:
1. solder paste welding process
Just like the Rigid PCB assembly process, using the stencil and the solder paste to cover the Flexible Printed Circuit Board and Rigid-Flex PCB with solder paste. Many SMT workers are troubled by the size and the fragileness, differ from the Rigid PCB, the surface of Flexible PCB is not smooth, then need some fixtures and positioning holes to fix. Moreover, the FPC material is not dimensionally stable and will stretch or fold by 0.001 degrees per inch as a function of temperature and humidity changing. More interestingly, these stretching and wrinkling factors will result the FPC displacement in the X and Y directions. Due to this, the FPC assembly requires smaller carriers comparing to the Rigid PCBA.
2. SMT components placement
Under the trending of miniaturization of components, tiny components in the reflow process happens to some problems. If the line width of FPC is narrow, stretching and wrinkling will not be a significant problem, it need a smaller SMT carrier or additional Mark points. The carrier is not smooth also leads to displacements. SMT fixture is the main reason to keep the FPC SMT surface smooth.
3. reflow process
Before reflow soldering, the Flexible PCB must be dry, this is the main differece between the Flexible Circuit board SMT and Rigid Circuit board SMT. Besides the instability of the FPC materials, they are also hygroscopic and absorb moisture (up to 3% by weight) like sponges. Once the FPC absorbs moisture, it has to stop the flowing. Rigid PCB also has the same problem, but Rigid PCB has a high tolerance. Flex PCB require a preheat bake up to 225°-250 ° F, this process need to finish quickly within 1 hour. If it is not baked in time, then needs to be stored in a dry or nitrogen storage compartment.
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