Nowadays, more and more circuit boards use surface mount components. Compared with traditional packaging, it can reduce the total area of circuit boards, facilitate mass production and processing, and have high wiring density. The lead inductance of chip resistor and capacitor is greatly reduced, which has great advantages in high frequency circuit. The inconvenience of surface mounting components is not that it is conducive to manual welding. Therefore, the technical engineers of circuit board welding take the common PQFP package chip as an example to introduce the basic welding process of surface mount components in detail.
I. Special tools and raw materials needed
The welding tools must have a small soldering iron with 25W copper head, Jinqiao covered electrode, and a standard welding table with adjustable temperature and ESD maintenance. Pay attention to the tip of the soldering iron, and the total width of the top should not exceed mm. A sharp-pointed medical tweezers can be used to move and fix the immobile chip and check its power supply circuit. Also prepare fine covered electrode, soldering flux, isopropyl alcohol, etc. in advance. The purpose of flux application is mainly to improve the fluidity of solder, so that the solder can be pulled by soldering iron and encapsulated on the pins and solder layer with the help of surface tension. After welding, remove the flux on the board with ethanol.
Second, the welding process
1. The technical engineer of circuit board welding reminds readers to coat flux on the welding layer before welding, and solve it again with a soldering iron, so as to prevent poor tin plating on the welding layer or oxidation, which leads to poor welding, and the chip generally does not need to be solved.
2. Carefully put PQFP chip on PCB with medical tweezers, and be careful not to destroy the pins. Make it align with both ends of the solder layer, and ensure that the placement orientation of the chip is proper. Adjust the temperature of the soldering iron to more than 300 degrees Celsius, dip a small amount of solder on the tip of the welding head, press the pointed chip down with a special tool, add a small amount of flux on the pins at the two corners, and still press the chip down to weld the pins at the two corners, so that the chip is fixed and cannot be moved. After welding the top corner, check whether the part of the chip points again. If necessary, it is possible to adjust or disassemble and point to the position on the PCB again.
3. At the beginning of welding all pins, solder should be added to the tip of the soldering iron, and all pins should be coated with flux to keep the pins moist. Touch the tail end of each pin of the chip with the tip of the soldering iron until the solder injection pin is seen. When welding, the soldering iron tip should be processed in parallel with the soldered pins, so as to avoid overlapping of steel bars due to excessive soldering tin.
4. After welding all the pins, wet all the pins with flux to facilitate cleaning the solder. Remove unnecessary solder in necessary areas to remove all short-circuit faults and overlapping of reinforcing bars. Finally, use medical tweezers to check whether there is a virtual connection. After the inspection, remove the flux from the circuit board, dip the hard brush in ethanol and scrub carefully along the pin orientation until the flux subsides.
5. The technical engineer of circuit board welding reminds you that chip-mounted resistance-capacitance components are relatively easy to weld. You can put tin on a spot weld first, then put on one end of the component, pinch the component with medical tweezers, and weld one end. Then, see if it is aligned. If it has been adjusted, weld the other end. It takes a lot of practical activities to truly grasp welding skills.
