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    Ways of Soldering SMT

    Date: 2018-07-12
    Times of browsing: 8


    1.1 Apply flux to the solder pad before soldering, and use a soldering iron to prevent the solder pad from being tinned or oxidized, resulting in poor soldering. The chip generally does not need to be processed.


    1.2 Carefully place the QFP chip on the PCB with tweezers, taking care not to damage the pins. Align it with the pad to ensure that the chip is placed in the correct orientation. Adjust the temperature of the soldering iron to more than 300 degrees Celsius, apply a small amount of solder to the tip of the soldering iron, use the tool to press down on the chip in the aligned position, and add a small amount of solder to the pins at the opposite corners. Press and hold the chip and solder the pins at the two diagonal positions to fix the chip and not move. Re-check the position of the chip after soldering the diagonal. Adjust or remove and realign the position on the PCB if necessary.


    1.3 When soldering all the leads, solder should be applied to the tip of the soldering iron and all pins should be soldered to keep the pins wet. Touch the end of each pin of the chip with a soldering iron tip until you see solder flowing into the pins. Keep the tip of the soldering iron in parallel with the soldered lead during soldering to prevent overlap due to excessive soldering.


    1.4 After soldering all the leads, wet all pins with flux to clean the solder. Remove excess solder where needed to eliminate any possible shorts and overlaps. Finally, use tweezers to check for solder joints. After the inspection is completed, remove the flux from the board. Dip the hard brush with alcohol and carefully wipe it along the lead until the flux disappears.


    1.5 SMD RC components are relatively easy to solder, you can first put tin on a solder joint, then put one end of the component, clamp the component with tweezers, after welding one end, then see if it is correct; Once it has been put on, it will be welded to another one. If the pin is very thin, you can add tin to the chip pin in the second step, then use the tweezers to clamp the core, smear at the table, remove the excess solder, the third step is not soldering on the soldering iron, soldering directly with a soldering iron . When we finish the welding work of a circuit board, we must check, repair and repair the solder joint quality on the circuit board.


    Solder joints that meet the following criteria are considered to be qualified solder joints:

    (1) The solder joint is in an arc shape (conical shape).

    (2) The whole solder joint should be complete, smooth, without pinholes, and without rosin stains.

    (3) If there are leads, pins, their exposed pins should be between 1-1.2MM.

    (4) The shape of the foot of the part shows that the dispersion of tin is good.

    (5) Solder will surround the entire tin position and the part feet.


    Solder joints that do not meet the above criteria are considered to be unqualified solder joints and require minor repairs.

    (1) Virtual soldering: It seems that the soldering is actually 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: The foot part is short-circuited between the foot and the foot by excess solder, and also includes residual dross to short the foot and the foot.

    (3) Offset: The pin is not in the specified pad area due to the inaccurate positioning of the device before soldering or the error caused by soldering.

    (4) Less tin: Less tin means that the tin point is too thin, and the copper skin of the part cannot be fully covered, which affects the connection fixing effect.

    (5) Multi-tin: The part foot is completely covered by tin, that is, the outer arc shape is formed, so that the shape and the pad position of the part cannot be seen, and it is impossible to determine whether the part and the pad are well tinned.

    (6) Tin ball and tin slag: Excess solder balls and tin slag are attached to the surface of the PCB, which may cause short-circuiting of the small pins.


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