This article describes how to solder a quad flat pack component to a printed circuit board. The details of the process are covered, including chip placement, bent pin recovery, tacking, the soldering and solder bridge removal.
Get your chip ready by placing the component tray on your bench and opening up the packaging.
There are two things to do before picking up the chip. The first one is to make sure that you are not going to zap your chip with a lightning bolt out of your finger. Discharge yourself of any static charge build-up in your body by either wearing a grounded antistatic wrist strap, or by touching an exposed metal screw on the grounded case of a piece of equipment on your bench. Maybe your soldering iron base or a lab bench power supply.
The second thing to do is to look at where the pin 1 reference on the chip is. Rotate your circuit board so that when the chip is transferred over, it will be in the right position with pin 1 markers lined up.
Next, use the vacuum pickup tool to lift up the chip and transfer it over to the circuit board.
At this stage if you drop or bump the chip, you may have bent one or more of the legs. You can recover from bent pin legs using the following procedure.
Get a hypodermic needle and a fine pair of pliers. Make a small 90 degree bend on the tip of the hypodermic needle. The bend acts as a kind of hook or puller for the bent pin legs.
Next, put on your head magnifiers so that you can see what is going on. Hold down the chip and very carefully pull the bent leg back into position. Do this very slowly to avoid pulling it too far. Do this for all bent legs.
Place the chip down carefully on the board. It does not need to be too accurate at this stage.
Put a finger at opposite corners of the chip and carefully move it around so that the pads on all sides line up with the device legs. This takes some time to do, so be patient. It is important to line it up accurately at this stage.
Next, check all around the chip, by looking from the top, to make sure that all of the pins are nicely lined up with all of the pads. Do not bump the board otherwise the chip will move and you will have to realign again.
Now that the chip is in position, it is ready to be tacked down with a few solder blobs. To tack the chip down, start by putting a little blob of solder onto the chisel tip of your soldering iron.
Then very carefully touch the solder blob onto one of the corner pads. It does not matter if you get 2 or 3 pads at this stage. Do not let the actual soldering iron tip touch the legs – you just want some contact of the molten solder to the leg and the pad.
The objective at this stage is not to make a good solder joint but simply to get some solder onto the leg and pad to hold the chip down to the board.
Carefully rotate the board around and put a little solder blob on each of the other 3 corner pads.
The next step involves applying gel flux, so make sure that you are wearing powder free latex gloves to avoid getting flux on your hands.
Take the gel flux syringe and apply the flux to all the pads, all the way around the chip. Put on a reasonably good amount of flux. The chip is now ready for soldering.
There will be a lot of solder fumes during the soldering, so make sure that you are wearing a face respirator and have good ventilation in your work area.
Put some solder into the reservoir tip of the iron. You do not necessarily need to fill up the reservoir. As long as there is a small amount of solder there, the solder will flow out onto the pins and pads when the tip comes into contact with them.
Next, just run the iron slowly across the feet of the pins. Of course, do not press down too hard – just guide the iron along. It is OK to run the iron back along some of the pins if you feel that there may not have been solder coverage of some of the pads.
I like to wipe the tip of the iron on a damp sponge after every chip edge. This removes the dross or residue that builds up on the iron tip as you go along. Whenever you wipe the tip of the iron, you will need to reload the reservoir tip with some more solder before doing another edge.
If you can see that you have created some solder bridges on some of the pins, wipe the iron tip again. Put a little more flux just on those pins and then bring down the tip of the iron to suck away the excess solder from the joints.
After cleaning the flux residue from the board it is important to check all the way around the chip again for solder bridges. If there are any bridges, apply flux gel just to those pins and use the reservoir tip to suck away the excess solder. Once done, clean and inspect the board again.
That completes the description of the quad flat pack soldering process, including chip placement, bent pin recovery, tacking, the soldering and solder bridge removal.