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Multilayer FPC and rigid flex pcb SMT chip patch difference

Author:Luwis Zeng Date:3/1/2018 4:45:43 AM
 LEAP ELECTRONIC today with everyone to understand, under the multi-layer PCB flexible circuit boards and rigid flex printed circuit of SMT chip where the difference. Ordinary SMT chip processing process is basically the same, because the flexible circuit boards, rigid boards all need to paste through the components and reflow solder paste welding process. However, there are some unique places for flex boards and rigid flex printed circuitboards, which can be extremely troublesome if these extra requirements can not be met in the production process.

1, solder paste welding process
Like hard board PCB processes, the solder paste is applied to the flex board and to rigid flex printed circuit through the operation of the stencil and solder paste printer. Many SMT workers by the size and vulnerability problems, unlike the rigid board, fpc surface is not flat, so you need to use some of the fixture and positioning holes to hold it. In addition, flexible circuit materials are not dimensionally stable and can extend or fold 0.001 degrees per inch with changes in temperature and humidity. More interestingly, these stretching and folding factors will cause the circuit board to shift in the X and Y directions. In view of this, flexible mounting often requires smaller carriers than rigid board SMT.

2, SMT components placement
In the current miniaturization of SMT components trend, small components in the reflow process can cause some problems. If the flexible wiring is small, extension and wrinkling will not be a significant problem, resulting in a smaller SMT carrier or additional Mark dots. The lack of overall flatness of the carrier will also result in displacement in the placement effect. SMT fixture is to maintain the SMT surface mount one of the leading factor.

3, reflow process
Prior to reflow, the flexible circuit must be dry, which is an important difference in the placement of the rigid flex printed circuit components. In addition to the dimensional instabilities of flexible materials, they are also hygroscopic and absorb water like sponges (3% upper limit of weight gain). Once the flexible circuit board absorbs moisture, it has to stop passing reflow. rigid board PCB also have the same problem, but with higher tolerance. The flex circuit needs to be preheated at ~ 225 ° to 250 ° F. This preheat baking must be done quickly in less than 1 hour. If not baked in time then it needs to be stored in a dry or nitrogen storage room.

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