SMT Process Flow of FPC

Here we use the ordinary carrier board as an example to illustrate the Surface Mounted Technology (hereinafter referred to as the SMT ) points of flexible printed circuit (hereinafter referred to as the FPC). When using silica gel board or magnetic fixture, the fixing of FPC is much more convenient, no need to use tape, and the process points of printing, patching and welding are the same. 

1. FPC fixation:

Before performing SMT, it is first necessary to accurately fix the FPC on the carrier board. It is important to note that the shorter the storage time between printing, placement and soldering after the FPC is attached to the carrier, the better.

2. FPC solder paste printing:

FPC does not have any special requirements on the composition of the solder paste. The size and metal content of the solder ball are based on the fine pitch IC on the FPC. However, the FPC has high printing performance requirements for the solder paste, and the solder paste should have excellent properties. In terms of thixotropy, solder paste should be able to print off the mold easily and firmly adhere to the surface of the FPC, without the problem of poor mold release, clogging of the steel mesh, or collapse after printing. 

3. FPC patch:

According to the characteristics of the product, the number of components and the efficiency of the placement, it can be mounted by medium and high speed placement machines.

4. FPC reflow soldering:

A forced hot convection infrared reflow oven should be used so that the temperature on the FPC can be changed more evenly, reducing the occurrence of poor soldering.

1). Temperature curve test method:

A more secure method is to place two FPC-loaded carrier boards in front of and behind the test board according to the actual board spacing during the production process. At the same time, components are mounted on the FPC of the test carrier board, and the temperature is tested with high-temperature solder wire. The probe is soldered to the test point while the probe leads are secured to the carrier with high temperature resistant tape.

 

2) Setting of temperature curve:

In the furnace temperature debugging, because the temperature uniformity of the FPC is not good, it is better to use the temperature test curve of the preheat, soaking and reflow (as shown in the following figure).

SMT Reflow Temperature Test Curve

5. FPC inspection, testing and sub-boarding:

Since the carrier absorbs heat in the furnace, especially the aluminum carrier, the temperature is higher when it is discharged, so it is better to add a forced cooling fan at the outlet to help cool down quickly.

The removed FPC is visually inspected under a magnifying glass of 5 times or more, with emphasis on surface residual glue, discoloration, gold finger dipping tin, tin beads, IC pin blank welding, and soldering.

If it is a mass production of shaped FPC, it is recommended to make a special FPC stamping and splitting die for stamping and splitting, which can greatly improve the working efficiency. At the same time, the edge of the punched FPC is neat and beautiful, and the internal stress generated when punching the cutting board is very low. It can effectively avoid solder joint cracking.

 

Conclusion

In the assembly and soldering process of PCBA flexible electronics, the precise positioning and fixing of FPC is the key point. Followed by FPC pre-baking, printing, patching and reflow soldering. Meanwhile, strict production process management is also important. It is necessary to ensure that operators strictly implement each rule on the SOP. Engineers and IPQC should strengthen inspections, timely identify abnormal conditions of the production line, analyze the causes and take necessary measures to control the defect rate of the FPC SMT production line within dozens of PPMs.