Welding method of SMD components in welding process
Welding method of SMD components in the welding process 2.1 Apply flux on the pad before soldering, and treat it with a soldering iron to avoid poor tinning or oxidation of the pad, resulting in poor soldering, and chips generally do not need to be processed .
Carefully place the QFP chip on the PCB with tweezers, taking care not to damage the pins. Align it with the pad, and make sure the chip is placed in the correct orientation. Adjust the temperature of the soldering iron to more than 300 degrees Celsius, dip the tip of the soldering iron with a small amount of solder, press down on the aligned chip with a tool, add a small amount of solder to the pins at the two diagonal positions, and still Hold down the chip and solder the pins on the two diagonal positions so that the chip is fixed and cannot be moved. After soldering the opposite corners, re-check whether the position of the chip is aligned. If necessary, it can be adjusted or removed and re-aligned on the PCB.
When you start soldering all the pins, you should put solder on the tip of your soldering iron and coat all the pins with solder to keep the pins wet. Touch the tip of the soldering iron to the end of each pin on the chip until you see solder flow into the pin. When soldering, keep the tip of the soldering iron parallel to the pin to be soldered to prevent lapping due to excessive solder.
After all the pins are soldered, wet all the pins with flux to clean the solder. Blot off excess solder where needed to eliminate any possible shorts and laps. Finally, use tweezers to check whether there is any false soldering. After the inspection is completed, remove the flux from the circuit board, dip a hard-bristled brush in alcohol and wipe it carefully along the pin direction until the flux disappears.
SMD resistance-capacitance components are relatively easy to solder. You can first put tin on a solder spot, then put one end of the component, and use tweezers to clamp the component. After soldering one end, check whether it is placed correctly; Put it right, and then solder the other end. If the pins are very thin, you can add tin to the pins of the chip in the second step, then clamp the core with tweezers, lightly knock on the edge of the table, and pier off the excess solder. In the third step, soldering iron does not need tinning, and soldering directly . After we finish the welding work of a circuit board, we must check, repair and repair the solder joint quality on the circuit board. meet the following standards
We consider solder joints as qualified solder joints:
(1) The solder joints are in an inner arc (conical shape).
(2) The overall solder joints should be perfect, smooth, free of pinholes and rosin stains.
(3) If there are leads and pins, the length of their exposed pins should be between 1-1.2MM.
(4) The appearance of the part feet shows that the tin has good fluidity.
(5) The soldering tin surrounds the entire tinning position and parts feet.
The solder joints that do not meet the above standards are considered as unqualified solder joints and need to be repaired again.
(1) False soldering: It seems that it is soldered but it is not soldered. The main reason is that the pads and pins are dirty, the flux is insufficient or the heating time is not enough.
(2) Short circuit: parts with feet are short-circuited by excess solder between the feet, including residual tin slag that short-circuits the feet.
(3) Offset: due to inaccurate positioning of the device before welding, or mistakes made during welding, the pins are not within the specified pad area.
(4) Less tin: Less tin means that the tin point is too thin to fully cover the copper skin of the part, which affects the connection and fixing effect.
(5) Too much tin: The part feet are completely covered by tin, that is, an outer arc is formed, so that the shape of the part and the pad position cannot be seen, and it is impossible to determine whether the parts and pads are well tinned.
(6) Solder balls and tin slag: Excess solder balls and tin slag attached to the surface of the PCB will cause small pins to short circuit.
