SMD Soldering Guide Part II

Trying to reworking with an iron

Rework with just an iron = Bad

So you've neglected to correctly identify pin 1 of your IC and you've soldered the thing on completely backwards. If you're lucky, you've noticed this before powering the board and vaporizing traces. How do you get an SMD IC off a PCB? There are a few ways to remove an SMD component with just an iron. Pretty much all will destroy the component and potentially the PCB so these are just for informational purposes only: 1) Wick and flex. If you're trying to remove a resistor or cap with only two pins, wick away most of the solder on the pins. With hemostats, grip the piece, heat one pad and flex the component away from the molten pad. Now remove the iron - the pad will cool and the component should only be held on by one pin. Heat that pin and remove the component. This stresses the hell out of the last pad but it works. 2) Blob and swipe. If the 2-pin component is small enough (0603s especially), you can get a good blob of solder on the tip of your iron and heat both pads of the resistor or cap simultaneously.

Here's a breakout board. Let's remove the 0603 resistor near the Power LED

Add solder to both ends of the resistor

Push lightly side ways and the component should slide away after a split second.

You'll end up with the component on the tip of your iron.

3) Score and flex. If you're trying to remove a multi-pin device with leads, you can use an exacto knife to score the pins near the component housing.

Be careful not to push down too hard or the knife will go through the pin legs and scratch or cut traces on the PCB below.

Once you've got one side scored or cut, score the opposite side and flex the component upwards. Flex until the pins break.

Once the main component is gone, run the iron around the pads with a bit of solder on the tip releasing all the cut and broken pins from the pads and onto the tip of the iron.

You may need to clear jumpers between pads with wick before attempting to remount the IC.

This process is tricky and can only be done once (maybe twice) before the heat/re-heat causes the pads to lift from the PCB.

Here's what happens! With the best intentions, I still managed to completely kill the pad. It is no more. But! We can fix this. More on 'green' wiring later. 4) There are commercial products and/or tricks out there that I have not used and do not endorse but aid in the removal of a soldered IC. These products are composed primarily of an alloy that is added to the soldered leads of an IC. This alloy has a lower melting point than standard solder so as you heat this alloy with a standard iron, it stays molten for much longer. This allows you to move the iron around while the alloy stays molten, and once you have all the leads molten, you then pull out the IC. I'm not sure what the cost is, but this may be a cheaper method, initially, than a hot-air rework station.

5) Removing or adjusting leadless IC like a QFN is impossible with an iron. The PCB would be destroyed and the IC along with it. You need hot air!

Wonderful hot-air rework

Rework with hot-air = Good

So what is this thing? Hot air rework? It's really simple. You need to melt all the pins on an IC simultaneously to either remove it or to adjust it's position. Iron can't do it, but hot-air can!

The rework station is simple. It pumps air to a gun. The gun has a coil within it that gets very hot and heats the air as it passes by. Think hair dryer.

Be sure to checkout our hot-air rework videos on Page 8!

This one is in our assembly room and gets quite a bit of use.

Most guns come with a reasonably large nozzle - about 1cm in diameter. You then attach a nozzle to the end of the gun to limit where the air can go. By decreasing the nozzle size, the air flow increases. Now you have a pointed and very hot source of air. The air will heat up areas of the board, allowing pins to go molten giving you the ability to liquefy multiple pin connections! So let's go back to the multiple pin SMD part (like a SOT-223 LM1117 variable voltage regulator). Let's say it's blown due to reverse voltage or something (because this never happens) and need to replace it with a new regulator. Step 1 is to remove the IC completely from the PCB. Most people think you need a special nozzle for each different sized part. That fact is that we put the basic round nozzle on our unit and never take it off. The air will heat a relatively large area of the PCB.

Heating the component takes 5-10 seconds.

As long as you are dealing with less than 1 square inch or so, you can just move the nozzle around over the PCB until the IC and PCB reach the molten temperature (abount 425F?). Slightly push the component with tweezers to test if the component has been released. If not, keep heating. Your poking tool should stay to the side to keep from disrupting the air flow to the PCB. After 5-10 seconds, the IC will release and you'll have the IC in your tweezers and a very hot PCB!

Release!

Once you have the IC removed, let the PCB cool a bit, re-finish the pads with new solder then wick away the domed pads.

Dome one pad. Checkout all the scratches from the exacto knife technique. Very bad!

New device and cleaned pads

Tacking down the anchor pin

Solder the other pins and clean the anchor pin and you're done! Now what should we do if that exacto knife scratches accidentally cut through a good trace?

Green wire fixes

What do I do when...?

You are going to lift pads. Sometimes, there is nothing you can do to prevent it. If a pad is lifted, and you have access to the design files, you can usually find out where the pad was supposed to be connected to. In the case of this example, you can simply see a trace running from where the pad used to be, and where it connects to a via. Since the pad is now gone, you can jumper from the via directly to the pin on the IC, thus re-creating the electrical connection.

This is called 'green' wiring because historically, assembly houses used green wires to try to make it blend in with the board. Any kind of 'green' wiring is considered poor design on a commercial product, but is completely acceptable for saving protos!

Poor poor pad

Solder the IC down as it should be, but forget soldering the pin without the pad

Here is the 30AWG wire wrap wire and the very necessary hemostats.

Here is what I call 'wing-ing' the pin. This is also a good method when the pad is available but goes to the wrong place. Because of schematic errors, if the pad goes to the

wrong place, just wing the pin and solder a jumper from the winged pin to the correct place on your PCB.

Measure the correct length of wire. No point in making the jumper 2" if you only need 0.2". Use 30AWG wire strippers or adjustable wire strippers (set to a small aperture) to

strip the jumper. You'll notice I strip very little from the ends on the wire. This helps prevent jumpers from local pins to the exposed jumper wire.

Dome the via and pin where the wire is to attach.

And solder 'er down. The PCB is saved!

This green wire method is used for all sorts of errors. We often have to cut traces with the exacto knife and then green wire jumper. Get creative. I highly recommend working every bug out of a layout before running a revision. Even if it's ugly. It's really bad practice to find one layout bug, quickly re-run a PCB with the correction, and then find out there are a whole handful of other errors.

This is one of my favorite images from our history museum. This board was completely

messed up. But with a significant amount of work, we got the board functioning. By doing so, the next board revision worked! Had we not found all the trace problems, it

would have taken many more revisions. Always trouble shoot every function and feature of a design before settling down to revise a board.

Sneaky tricks for soldering leadless ICs

Now for a magic trick. If you've got a leadless component (like a QFN CP2102), you can solder the component onto a tight pitch footprint without solder paste or expensive machinery (or a hot plate). This requires some soldering and a hotair rework station.

Here is a CP2103 that was pulled from the USB board shown earlier.

Here's what the bottom looks like. Notice the uneven pads and gunk in the middle.

I call this 're-balling'. Here I simply run a blob of fresh solder around all the pads evening them out and clearing any jumpers that may exist. The hemostats are there only to hold the IC down, otherwise it would stick to the end of the iron. Note that you want to avoid

adding solder to the center pad.

If you do accidentally get a large blob of solder in the middle of the IC, you will want to clear it out with solder wick. Otherwise, if you try to hot-air solder the IC onto the PCB, the large blob in the middle of the IC will get squished out to the surrounding pads and

cause bad jumpers.

Here is the finished re-balling. It looks gunky, but it's actually pretty uniform. The balling helps add solder to pads so that when the IC meets the PCB with heat, there is something there to stick the two together. Normally you would solder the two together with an iron,

but this is not possible with this footprint and leadless IC.

This balling and hot-air soldering technique can be used on BGA devices as well! Scary, but true.

The new, clean board. Um, just ignore that this is a CP2102 board and we are throwing a CP2103 onto it...

Here you can see that I've added a similar balling to the PCB by swiping an iron with solder on the tip across all the pads. With a little work, you can even avoid jumpers

between pads. This micron layer of solder will help when the two objects come together.

Note again - I have not added solder to the center pad. This is a common misconception and problem. Almost all center pads are ground connections in addition to ground connections on exterior pads (so you don't even need the center pad connection). Soldering of the center pad is very rarely required! Don't even worry about it. If you do add solder to the center pad, you will need to wick it away. This is a large center pad very close to the exterior pads (bad design on my part). The center pad will very actively suck solder towards it if you get sloppy. If you're designing your own footprint for a device, be sure to make the center pad smaller and away from exterior pads.

What's the big deal? What happens if you get solder onto the center pad? If the center pad becomes domed in a big way, and you hot-air the IC onto this domed pad, one of two things will happen. More than likely, you will press the IC down. The domed solder will squish to the exterior pads shorting many things to the center pad. Or if you don't press down, the IC will float a couple hundred microns above the PCB leaving all sorts of open connections. Bad news all around. A little solder is not a big deal but try to leave the center pad as flat as possible.

Various customers have recommended adding vias to the center pad so that you can 'feed' solder from the back side. This is a decent approach, but widely not needed. If you're soldering with an iron only, this is an ok idea - but I've only seen 2 ICs, in my lifetime so

far, that required the center pad soldered to function. If you're prototyping, the center pad should be accounted for in your footprint, but don't lay awake at night worrying about soldering it. Finally, if you're using hot-air, more then likely there will be a minute amount of solder between the two pads, and even a mechanical connection may be enough of an electrical contact to take care of any center pad issue. In 6 words or less - don't worry about it.

Getting ready for some hot-air.

Here I've heated the PCB pads with hot-air for 5-10 seconds, placed the IC onto the pads and straight as possible, then held the IC in place while I remove the hot-air. The hot-air will BLOW away the IC if you don't hold it in place. Once the connections cool, take a look. More than likely, the IC is skewed like the image above - REALLY bad. All sorts

of pads are where they shouldn't be, but don't despair! Hot-air is your friend!

Yea. That's really boo-bad initial placement.

Slowly bring the gun back over the IC. Rotate the gun in a circle slowly heating the PCB while not exposing the IC to too much air force. I heated this board/IC for 5-10 seconds.

The IC righted itself and jumped into place! I did not touch the IC with tweezers or anything - only air. Really. I promise. Surface tension and hot-air are a fantastic thing. If

the footprint is good, the IC will center and right itself using the solder at hand.

Now for the non-faint of heart - if you get really good, try 'bumping' the IC with your tweezers/hemostats in one hand and the gun aimed in the other (keeping things molten).

If you bump lightly, the IC will literally slide slightly and then jump back into place once you remove your tool pressure. This is a good trick for trying to remove jumpers under

the IC and pads without removing the IC completely.

So what happens if I sit with the gun aimed at one spot for 30 seconds? Here we see black nastiness around the Power LED. If exposed to excess heat over time, the PCB will

delaminate, pop, smoke, hiss, smell horrible, and bubble up.

Bubble around the Power area.

Once the PCB delaminates, it's pretty much toast. Sometimes it'll work, other times traces will be broken from the stress. Don't worry about this - a little practice with the hot-air

and you'll be repairing and salvaging ICs, PCBs, and parts with ease!