SMT patch processing and repair basic process
The main nine processes of SMT patch repair
(1) Remove the components. Successful rework is first to remove the components from the fault location. Heat the solder joint to the melting point and carefully remove the component from the board. Heat control is a key factor in rework, and the solder must be completely melted to avoid damaging the pads when the components are removed. At the same time, it is necessary to prevent the PCB from being overheated and causing distortion of the PCB.
(2) The circuit board and components are heated. The advanced rework system uses a computer-controlled heating process that is as close as possible to the specifications given by the solder paste manufacturer and should be combined with top and bottom heating (shown in Figure 8-42). The bottom heating is used to raise the temperature of the PCB, while the top heating is used to heat the components. When the components are heated, some of the heat is conducted away from the reworked position. The bottom heating compensates for this heat and reduces the total heat required for the components at the top. In addition, the use of a large-area bottom heater eliminates PCB distortion caused by excessive local heating.
(3) Heating curve. The heating curve should be carefully set, preheated, and then reflowed. A good heating curve provides a sufficient but not excessive amount of warm-up time to activate the flux, which is too short or too low to do so. The correct reflow temperature and residence time above this temperature are very important. Too low or too short a time will result in insufficient wetting or open solder joints. Too high a temperature or too long a time may cause a short circuit or form an intermetallic compound. The most common method for designing the optimal heating curve is to place a thermocouple at the solder joint of the rework position. Predict the setting of an optimum temperature value, temperature rise rate and heating time, then start the test and measure the measured data. Record it, compare the result with the desired curve, and adjust according to the comparison. This test and adjustment process can be repeated multiple times until the desired effect is achieved.
(4) Take the components. Once the heating curve is set, the components can be prepared for removal. The rework system should ensure that this part of the process is as simple and repeatable as possible. The heating nozzle can be heated after aligning the components. Generally, start from the bottom, then lower the nozzle and component pipette to the top of the PCB and components, and start the top heating. At the end of the heating, a vacuum is created in the component suction tubes of many rework tools, and the suction tube is raised to lift the components from the plate. Absorbing components before the solder is completely melted can damage the pads on the board. Zero-force suction technology ensures that components are not removed before the solder liquefies.
(5) Pretreatment. This position needs to be pre-treated before changing the new component to the rework position. Pretreatment involves two steps to remove residual solder and add flux or solder paste.
1 Remove the solder. The removal of residual solder can be done manually or automatically. Manual tools include soldering irons and copper solder wires, but hand tools are difficult to use and are susceptible to damage to small CSPs and flip chip pads.
Automated solder removal tools can be used very safely for high-precision plate processing (Figure 8-43). Some cleaners are automated, non-contact systems that use hot gas to liquefy residual solder and then vacuum the molten solder into a replaceable filter. In the device. The automatic workbench of the cleaning system sweeps the circuit boards one after the other, removing the residual solder from all the pad arrays. The PCB and cleaner heating are controlled to provide a uniform process to avoid overheating of the PCB.
2 flux, solder paste. In mass production, the flux is generally immersed in the component, while in the rework process, the flux is directly brushed onto the PCB. Solder paste is rarely used for CSP and flip chip rework, as long as some flux is used. In the case of BGA rework, the method of solder paste application can use a template or a programmable dispenser. Many BGA rework systems provide a small form factor device to apply solder paste, which can be accomplished using a variety of alignment techniques, including component alignment optics.
Using templates on a PCB is very difficult and not very reliable. In order to place a template in the middle of adjacent components, the template must be small, and there is almost no space except for the small holes for applying the solder paste. Due to the small space, it is difficult to apply solder paste and achieve uniformity. effect. Device manufacturers recommend checking the pads more often and repeating the process as needed. There is a process that can replace the stencil coating solder paste, that is, the solder paste is directly applied to the component by the component printing table, so that it is not affected by adjacent components, and the device can also be used after applying the solder paste. As a component container, the components are automatically picked up in a standard process. Solder paste can also be spotted directly onto each pad by using a PCB height auto-detection technique and a rotating solder paste squeeze pump to precisely provide a consistent paste point.
(6) Replacement of components. After removing the components and pre-processing the PCB, new components can be mounted on the PCB. The developed heating curve should be carefully considered to avoid PCB distortion and achieve an ideal reflow effect. Temperature setting using automatic temperature profile development software can be a preferred technique.
(7) Component alignment. New components and PCBs must be properly aligned. For small-size pads and fine-pitch CSPs and flip-chip devices, the placement capability of the rework system must meet high requirements. The placement ability is determined by two factors: accuracy (deviation) and accuracy (repetition). A system may be reproducible, but not accurate enough. Only by fully understanding these two factors can you understand how the system works. Repeatability refers to the consistency of components placed at the same location, but good consistency does not necessarily mean placement at the desired location; deviation is the average offset measured at the placement location, a high-precision system is only small or fundamental There is no deviation placed, but this does not mean that the placement is very reproducible. The rework system must have both good repeatability and high accuracy to place the device in the correct position. The actual rework process must be considered when testing placement performance, including picking up components from component containers or trays, aligning, and placing components.
(8) Component placement. After the rework process is selected, the PCB is placed on the bench, the components are placed in the container, and then the PCB is positioned to align the pads with the leads or solder balls on the component. After the positioning is completed, the components are automatically placed on the PCB. The force feedback and programmable force control technology can ensure proper placement without damage to the precision components.
(9) Other process precautions. Small mass components may be blown and not aligned during convection heating. Some rework systems use a straw to position the components to prevent it from moving. This method requires a certain amount of thermal expansion when positioning components. . There is no surface tension when the components are aligned. This method makes it easy to place the BGA components too close to the PCB (short circuit) or too far away (open circuit). A good way to prevent components from moving during reflow is to reduce the convective heating flow. Some rework systems can be programmed to set the flow and reduce the flow as required by the process. Finally, the nozzle is automatically lowered to start heating. The automatic heating curve guarantees the optimum heating process and the system placement performance ensures accurate component alignment. The combination of placement capabilities and automated processes provides a complete and consistent rework process.
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