FPC Assembly

Nowadays, technology is already very mature to rigid Printed circuit board, however, many problem may be faced by operator during SMT to FPC.
1 False soldering, short soldering,Tin beads.
2.The Reason is due to the hardness of  raw material of FPC(Flexible printed circuit) caused, FPC is out of shape when meet the hot air and high temperature in reflew machine. Lead the wave shape to Flexible pcb, and the slop to FPC pads, then cause the flow of molten tin (liquid tin under high temperature), which cause False soldering, short soldering,Tin beads.(The main reason is the hardness of FPC). So if make the hardness of FPC into the same situation as rigid printed circuit board, and all soldering problem to FPC can be solved. The condition is you should know the SMT process to PCB. And then improve the hardness of Flexible Printed circuit into with the same hardness as rigid PCB, and then FPC SMT can be realized. And some problem, like no fix hole, no mark point, or the FPC is over long, which are the fault of designer, and it is also hard to be solved even to rigid PCB SMT. And NG rate can be only reduced based on personal experience or device independence.

According to above comment, the main solution is to improve the hardness of FPC the same as hardness of rigid PCB.

B. How to realize stable soldering to FPC
How to get the stable FPC (flexible circuit board) to the rigid PCB welding effects depends on the degree of hardness.
Methods: using jigs(carrier, handling plate) to make FPC perform as rigid board during SMT.

Below is the instruction of Magnetic reflow soldering tray
The material
1. Choose imported alloy material: good flatness, stable performance under high temperature
2. High temperature resistant magnet (300 degrees) to ensure permanent magnet during reflow soldering process.
3.The special steel, with magnetic treatment and good elasticity, and no deformation under high temperature.
4.Performance
(1) Because the flatness to FPC surface cause from the steel pressure l, it can reduce the flow of solder tin during reflow, which can ensure the good quality of soldering and reduce the NG soldering at meanwhile.
(2) Ensure the life of tray when choosing tray material.   
(3) Easy and convenient operation with high stability. 
(4) Heat insulation, prevent the FPC from deformation
There’s no special requirement to the composition of solder paste of FPC, the size of the tin ball particles and the amount of metal content, etc will be subject to if there is narrow space of IC pins on FPC, BUT FPC has high performance requirements to the solder paste printing, solder paste should has good thixotropy, solder paste should be able to easily be printed , demoulding and can be firmly adhere to the surface of FPC, there will be no mold release lead the hole in stencil filled with tin or such as collapse after printing.

     Because FPC fixed on the pallet before SMT,  and the FPC with high temperature heat-resistant pressure-sensitive adhesive tapes for purpose of location, which make its planar inconsistencies, thus FPC printing surface is unlikely to be as smooth as rigid PCB and the same consistent on thickness and hardness, so operator should not use a metal scraper, and should use the polyurethane scraper which with hardness of 80-90 degree.

 Solder paste printing machine is best to be with the best optical positioning system, otherwise will have great influence to printing quality, FPC although fixed on the load board, however, there always be small space between FPC and the load board, this is the biggest difference as rigid PCB board, thus the equipment parameters’s setting will also has great influence to the printing effect.

    Printing stations is also the key location to prevent FPC from being smudgy, need to wear glove then can go ahead for operation, at meanwhile,  workstation should be kept clean, stencil should be frequently wiped to be clean, to prevent the solder paste to pollute the gold fingers and gold plated key button. 

Control of FPC SMT

According to the properties of the products, components quantity and efficiency of mounting, it is OK to adopt high-speed placement machine for SMT. Because every piece of FPC has positioning with fiducial MARK, therefore, there’s no big difference on surface mounting technology between FPC SMD mounting and rigid PCB SMD mounting. It is important to note that although the FPC are fixed on the load board, but its surface can’t be the same flatness like rigid PCB board surface, local space must  exist between FPC and load board, so, the height of suction nozzle’s dropping, compression of blowing should be exactly setted.  On other side, most bare FPC performance as panel; format, and the rate of good finished product relatively low, so it if normal that there’re part of pieces with not good quality, which request recognition function on BAD MARK to SMT machine, otherwise, when mounting FPC panel with the status of not all pieces are good, will show low efficiency.