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FPC reflow

Author:Luwis zeng Date:6/12/2018 9:38:21 AM
 A forced hot air convection infrared reflow soldering furnace should be used, so that the temperature on the FPC can be more evenly changed and the occurrence of poor welding can be reduced. If single-sided tape is used, because only the four sides of the FPC can be fixed, the middle portion of the FPC can be deformed under hot air, and the pad can be easily inclined. The molten tin (liquid tin at high temperature) can flow and generate air welding, continuous welding, Tin beads, the higher the rate of defects in the process.

1) Temperature curve test method:

Because of the different endothermic properties of the carrier board, the different types of components on the FPC, the different speeds at which the temperature rises during heating in the reflow process, and the amount of heat absorbed is also different. Therefore, the temperature profile of the reflow oven is carefully set to the quality of the soldering. Great influence. A more secure method is to place two FPC-loaded carrier boards before and after the test board according to the carrier board spacing during actual production. At the same time, components are mounted on the FPC of the test carrier board, and the test temperature is measured with a high-temperature solder wire. The probe is welded to the test point and the probe wire is fixed to the carrier plate with a high-temperature tape. Note that high-temperature tape cannot cover test points. The test points should be chosen near the solder joints on the sides of the carrier board and the QFP pins. Such test results can better reflect the real situation.

2) Temperature curve settings:

In the furnace temperature debugging, because the temperature uniformity of the FPC is not good, it is better to adopt the temperature curve of temperature increase/retention/reflow, so that the parameters in each temperature zone are easy to control, and the impact of FPC and components on thermal shock is small. some. According to experience, it is best to adjust the furnace temperature to the lower limit of the solder paste technical requirements. The reflow furnace wind speed generally adopts the minimum wind speed that the furnace can use. The stability of the reflow oven chain is better and there should be no jitter.

5. FPC inspection, testing and boarding:

Since the carrier plate absorbs heat in the furnace, especially the aluminum carrier plate, the temperature at the time of tapping is high, so it is better to add a forced cooling fan at the tap outlet to help reduce the temperature quickly. At the same time, the operator must wear heat-resistant gloves to avoid being burned by the high-temperature carrier board. When the FPC that has completed the welding is taken from the carrier plate, the force must be uniform and no brute force can be applied to prevent the FPC from being torn or creasing.

The removed FPC was visually inspected under a magnifying glass with a magnification of 5 times or more, and the problems such as surface adhesive residue, discoloration, tinned tin solder, tin beads, IC pin air soldering, and continuous soldering were inspected. Because the surface of the FPC cannot be very smooth, the erroneous rate of the AOI is very high. Therefore, the FPC generally does not cooperate with the AOI inspection. However, the FPC can complete the ICT and FCT tests by using a dedicated test fixture.

Due to the fact that FPC is mostly based on splicing plates, it may be necessary to do the slicing before testing ICT and FCT. Although blades, scissors, and other tools can be used to complete the slicing operation, the operating efficiency and the quality of work are low, and the scrap rate is high. If it is a large-volume production of shaped FPCs, it is recommended to make special FPC stamping dividing board molds for punching and dividing, which can greatly improve the working efficiency. At the same time, the punched out FPC edges are neat and beautiful, and the internal stress generated when punching the cutting board is very low. Can effectively avoid solder cracks.

In the PCBA flexible electronic assembly welding process, the precise positioning and fixing of the FPC is the key, and the key to fixing is to make a suitable carrier board. Followed by FPC pre-baking, printing, patch and reflow. Obviously, the difficulty of FPC SMT process is much higher than PCB hard board, so it is necessary to set precise process parameters. At the same time, strict production process management is equally important. It is necessary to ensure that the operator strictly enforces every rule on the SOP. Engineers and IPQCs should strengthen inspections, detect abnormal conditions on the production line, analyze the causes, and take necessary measures to control the defective rate of the FPC SMT production line within dozens of PPMs.

In the PCBA production process, a lot of machine equipments are needed to complete the assembly of one board. Often, the quality level of a factory's machinery and equipment directly determines the manufacturing capacity.

The basic equipment needed for PCBA production includes solder paste printers, chip mounters, reflow soldering, AOI detectors, component pincers, wave soldering, tin furnaces, plate washers, ICT test fixtures, and FCT test fixtures. Aging test stands, etc. Different sizes of PCBA processing plants will have different equipment.

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