Replacing a Small Outline Integrated Circuit (SOIC)
In this example, I'll be replacing U177, the 74HCT174 real time clock address latch in an A4000D. This is a component that usually requires replacement if a leaking battery damages the RTC
Of course these same principles apply to all other SOICs, including ones with more or less pins and the wide body devices.
- If you are replacing a device which had been damaged by corrosion, you will need to refer to the RTC repair page for details of
cleaning the board and components.
Corrosion also makes it more difficult to work on equipment, because the surface of soldered joints becomes corroded/oxidised, which makes them difficult to heat with a soldering iron.
Fortunately repair using a SMD hot air tool is still fairly simple. Firstly, you will need to clean the worst of the corrosion away first using some cleaning alcohol and perhaps a fibreglass
Next apply a little SMD flux to the legs of the SOIC you're going to remove. This makes the solder a bit easier to work with.
Don't forget you can click on these images for a larger version.
Fit a small or medium (2-6mm inside diameter) single tube nozzle to the hot air rework tool. Set the heat to medium-low and the air speed to low.
You can see below I've used a heat setting of 3 and an air speed of 2. Switch the tool on and leave it to heat up for about a minute.
Hold the hot air tool in your left hand (if you are right handed). Position tip of the tool's nozzle about 10cm above the board and gently pre-heat the area for a
few seconds, this reduces stress on the circuit board and surrounding components.
Now hold the tip of the nozzle about 2cm above the SOIC we're removing. Move the nozzle about in a circular pattern above the body of the SOIC so the hot air evenly heats all of the solder
Important! Never hold the nozzle in one spot when it's close to the PCB! Keep
it moving all the time, otherwise the concentrated heat in one area can scorch the circuit board or cause it to blister.
The solder should suddenly melt in around 5 seconds. With a pair of fine tipped tweezers in your right hand (if you are right handed), gently keep touching the side
of the SOIC's body. When the solder melts, it'll move slightly.
Grab onto the body or one of the legs with the tweezers and lift the SOIC directly upwards. Avoid sliding it sideways, as the solder joints of surrounding components will probably be melted as
well. Touching other components will disturb their position. Remove the tool's nozzle away from the board immediately.
Turn off the hot air tool, or put it aside where the hot air isn't going to blow on anything like your workbench. Be careful as the metal nozzle is extremely
Using solder wick, clean all the old solder from the board's solder pads.
Using isopropyl alcohol, SMD flux cleaner or methylated spirits, clean the circuit board. Scrub it with an old
toothbrush and wipe it clean using a rag or cloth.
It's important that all surfaces to be soldered are smooth and clean for a reliable joint to be made. You want it looking like this:
- If there were signs of corrosion underneath the SOIC when you removed it (green/white powder), you will need to check tracks and vias (the small holes through the
PCB) to see if any are open circuit and repair them as necessary. Do this now and make sure track and via repairs don't stick up too high, or you'll have trouble soldering on the new SOIC. Most
repairs can be done using thin stands/hairs from a piece of fine electronic hook-up wire.
This diagram shows a cross section of the PCB and how it's made, as well as illustrating how to solder a wire to top and bottom side tracks in order to repair
an open circuit via.
- Next we're ready to fit the new SOIC. Apply a little SMD flux to the legs of the SOIC, and to the pads on the board. This makes the solder flow and attach itself to the metal much easier.
Get the solder paste syringe and squirt a bit out onto a piece of scrap paper to make sure you've got a nice clean flow. Draw a thin line down each row of solder pads as shown below.
You don't need as much solder paste as you might think, the amount shown here is about right. Too much paste and it'll bridge legs together when you heat it. Too little and you won't have a
reliable solder joint. If anything, it's better to have too little rather than too much. It's a lot easier to add a little more paste and heat it if you have to, as opposed to removing
Tip: store your solder paste in the refrigerator when you're not using it.
This slows it from drying out and it's still perfectly usable many months, even years past its expiry date.
- Locate pin 1 on the new SOIC. Some manufacturers put a small line or dot at one end to show this. The most reliable way to tell is by looking for the chamfered edge. All manufacturers do
this. You'll see one side top corner has a chamfer or bevel, while the other side is square. The chamfered edge indicates pin 1. Pins always count in an anti-clockwise circle. This diagram will
help to explain it:
The white silkscreen lines printed on the circuit board shows which end pin 1 goes towards. In the A4000D, this is shown as a small "V" in the line.
This is sometimes shown as a single dot or arrow near the pin 1 solder pad.
- Check again you've got it around the correct way and place the new SOIC onto the solder pads. Line it up by looking down on top of it, using tweezers or a spike to gently move the SOIC around
until it lines up perfectly in the centre of the pads.
Notice the chamfer on the SOIC right hand vertical edge in this picture:
- Using the tip of a small screwdriver in your left hand (if you're right handed), press down gently but firmly in the centre of the SOIC to hold it in place. In your right hand, use a hand
soldering iron to touch one of the corner legs, which will melt the solder paste.
Remove pressure from the screwdriver and check the SOIC is still centred on the solder pads.
- At this point you can use the hand soldering iron to solder the rest of the joints, or use the hot air rework tool. Hot air is preferred as it produces a much tidier job.
Turn on the tool again and allow it to heat up for a minute. Hold the nozzle about 5cm above the SOIC, heating it in a circular motion as before. The solder paste will get runnier and tend to
spread out, but ignore this. Slowly lower the nozzle until it's about 2cm from the SOIC, heating the legs in a circular pattern. You may need to press down on the SOIC using a screwdriver if
the air keeps blowing it away, but this usually means you're too close, or the air flow is too high.
Suddenly the solder paste will be begin to melt. The line of paste breaks up into separate pieces and attaches itself to the solder pads and legs of the SOIC.
Keep heating until all of the solder paste has turned into smooth shiny joints.
If you see some legs are shorted together or have not got enough solder, ignore this for the moment.
If it's sitting slightly crooked, remove a screwdriver if you're holding the SOIC down and heat the joints a little more. The SOIC will usually correct its alignment automatically due to the
tension of the molten solder.
- Remove the heat and let the board cool down momentarily.
Carefully inspect for shorts (legs bridged from excess solder). A bridge can usually be fixed by heating the pins with hot air, then drawing a fine metal spike (scriber) through the bridge,
between the legs. If it doesn't clear, wick some of the excess solder away using solder wick and a hand soldering iron.
- Using a fine metal spike (scriber), gently press against the side of each one of the SOIC legs. If they easily bend sideways, it means the joint has not soldered.
Apply a small dot of solder paste and use hot air or the hand soldering iron to heat the joint.
- Check again for shorts (bridges) and unsoldered joints. Here are some examples of good and bad solder joints.
Lastly, you may want to use isopropyl alcohol or flux cleaner to clean away the resin and soldering residue left over from the solder paste.
The job is now complete!
Questions, comments, errors? Email me.