SMD desoldering tutorial?

Is

The absolute best and cheapest way (in my opinion) to desolder SMD chips is to use Chip-Quik. The "solder" they use has a VERY low melting point. Even after 30 seconds or so, it's still in a molten state which allows you to just pick up the chip with your hands.

More info here:

-Tim

it

Some fairly in-depth stuff at

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Correct address is cpemma at mexbro dot co dot uk

The following instructions will work for removing smd chips.

1.) Take an oven tray and cover with aluminum foil, place board on foil. 2.) Set oven to 300-350 degrees F. depends on solder type used. 3.) Place tray in oven for approx. 10-15 minutes 4.) Open oven door, grab a pair of tweezers and try lifting component. If component does not lift close oven door and try again every 2-3 minutes until successful.

That's it ...cheap too!

Remove "HeadFromButt", before replying by email.

The author of that page - Eduardo Barcellos - should consider giving proper credit to the original source of his idea

He presents the hot-air idea as if it were his own.

It is an unfortunate trend that students steal ideas from the internet and boldly present the information without due credit. A whole generation of people without morals would be the downfall of society.

Given your requirement is to avoid damage to the SMD components, I assume you need to re-use them?

As Tim H. posted, Chipquik is ideal for SMD removal by hand as it allows you to reflow solder at temperatures low enough to avoid heat damage to the board or component - the tricky part is removing and cleaning the component without bending any pins. Most of the SMD rework information available assumes devices will be discarded after removal.

I use the stainless steel strips orthodontists wrap around teeth. Coat the pins with copious amounts of paste flux and reflow the solder with Chipquik. The lowest temperature setting on my soldering station is ~325F, hotter than required but not hot enough to cause damage. Be generous with the Chipquik as this will make it easier to maintain molten material on all pins simultaneously. When the device is ready to lift, it will be floating in a puddle of flux and solder, now you can slide the stainless strip between the board and device to pick it up. Warming the board from the underside with a hair dryer while working facilitates the process. Where possible, slide the stainless strip under the device body first, then keep it parrallel to the row of pins as you move it under them to avoid bending. QFP devices are more difficult as there's no 'pinless' side from which to access the underside of the body, but trimming the stainless strip to a tapered point and starting at one corner works reasonably well - the key is to make sure the device is floating and avoid any sideways pressure on the pins. The stainless strip method has the advantage of leaving the site much cleaner than simply lifting the device with tweezers, so I use it when removing dead devices too.

The freshly removed device will be somewhat protected by a coating of Chipquik bridging all the pins, but still must be handled carefully. Next you need a clamp capable of holding the device body while leaving the pins accessible - mine came from the orthodontist. Hold the clamp at an angle to the edge of the workbench, reflow the Chipquik with the iron and tap the clamp against the edge of the bench to flick the pins clean. Results are best if you can flick all the material off a row of pins in one shot, so allow time for complete reflow before tapping the clamp. The Chipquik can be collected and re-used several times.

HTH

Sunny

So where did usbmicro get the idea? I'm supprised they don't include a small aquarium air flow valve so the air flow through the tubing can be adjusted for optimum air heating.

On Sun, 4 Jan 2004 10:14:44 -0800, Sunny wrote (in message ):

Is your brother-in-law a dentist? Where do you get the stainless steel strips? Is there a supplier you use? Or do you just ask your personal dentist?

Great ideas. Never having done this before, you have alerted me to some issues I need to be cautious about.

Thanks,

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DaveC
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On Sun, 04 Jan 2004 15:31:10 GMT, maxfoo Gave us:

Considering that 63/37 solder does not melt until it reaches nearly

500 degrees F, I'd say that you just described how to f*ck up a PCB.

No. Leave your head right where it is. Firmly planted up your ass for maximum punk ass foo.

On Sun, 04 Jan 2004 17:04:36 GMT, Gave us:

Sorry, dipshit, but the idea doesn't belong to the page you cited either.

What does that make them?

How do you know my brother-in-law's profession? :-)

I did get some used picks from him, but there's a surplus electronics store near me that stocks a variety of small surgical and dental tools, including the stainless strips. I have also used automotive feeler gauges, but it gets expensive buying full sets of those just to use the thinnest ones.

If at all possible, practice on junk hardware first. Each package type and location presents it's own challenges, and the first attempts often result in bent pins until you get a feel for the technique. Even slightly bent pins are almost impossible to straighten and greatly reduce the chances of a successful re-installation. If you must re-install a device with a small number of bent pins, IMHO it's best to leave them alone and resolder all the good pins, then push the point of a needle between soldered and bent pins to re-align them with their pads.

On Sun, 4 Jan 2004 12:22:52 -0800, Sunny wrote (in message ):

I need to preserve one set of ROMs from a PCB. This PCB can be trashed; I have no need for it afterward. The ROMs on another board can be trashed (they're defective), but the PCB needs to be preserved.

One job I need to be careful with the chips; the other, with the PCB.

Feeler gauges... hmm, I have several old sets of those lying around.

Thanks,

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It is not as bad as all that.

But I am still reluctant to cook boards or components that long.

--
John Popelish

HTH

On Sun, 4 Jan 2004 14:36:57 -0800, DaveC Gave us:

On the PCB with the chips that are trash, cut one lead at a time until all leads are cut. Remove the chip, then desolder one lead at a time from their pads. Carefully wick the pads (no pressure, just heat) clean. That PCB is now ready to accept the new (used) chips.

If they are j-lead (likely). If it is SOIC or fine pitch, the same thing goes, but it is a little harder to cut the pins.

Anyway, the point is that the chips to be trashed can be removed by cutting the pins, and then de-soldering the remaining pin segments from the board individually.

Half way there. You could look up a good contract Mfgr in your area, and ask what it would cost for removing two PLCC (j-lead) chips. If they are fine pitch or soic, the quote may be bit more. J-lead is easy though as hot air can pass under the chip.

They might want like $50 or $100 for the job. Is there no way to obtain a pair of pre-programmed chips?

On Sun, 04 Jan 2004 22:39:55 GMT, John Popelish Gave us:

Yes, but the chamber temp, and lead temp need to be settled in. I would go higher temp, faster settle.

Most reflow ovens, and wave solder machines only pre-het assemblies to nearly 350F. The reflow area, and wave area finish the job with the least component stress.

As long as the cycle is a slow ramp up, the parts should remain stable.

Of course, this does NOT include any electrolytics or other "wet" parts that heat can damage.

As an aside, we hd a CPU that looked like an old LCC in micro size.

There were no points to solder to on the chip edge. They were all underneath it.

Needless to say, this presents a problem for the proto first piece assy, as they are typically hand assembled.

I used our air bath and heat pencil to no avail. So, I broke out the big guns. Heat guns to be specific.

The whole key is the preheat. If the heat bath has the PCB assy up to a high starting temp, the reflow operation should be easy.

Ok. Hot PCB. Hot air gun. It works! We were worried though about the chip. I re-reflowed it back off with the heat gun, and replaced it on ther demo PCB.

It worked. It proved our layout to be defective, and proved that hot air does not a dead chip make.

If this guy can handle the slight risk, that does work. I held the chip with tweezers as I heated it. As soon as the solder reflowed, I removed the heat, so it couldn't have been heated much more than it should have been.

All heat was kept local to the part too. Except for the air bath, of course.

Anyway... it survived at least 4 cycles of that, and still works. It is a cpu with 8k of flash in it.

Very doable, unless he s a huge long term reliability issue with the heat damage potential. I would try to copy the chips after removal, and insert new chips.

On Sun, 04 Jan 2004 22:46:31 GMT, Robert Grizzard Gave us:

The difference between a chart and reality can be sharp.

I do not know of anyone that can successfully solder AT ALL at those temperatures. The settle time for an assy placed in an oven that barely makes said temperature is way too high for me.

I would ramp it up from a 250 F oven, into a 350F oven, and then use hot air for the removal. Much less stressful.

I would also STILL bet that full reflow on that many pins requires a temp were one can be sure that all pins are molten. Being right at or near the charted melt point temp is NOT that assured temp.

HTH

I had to replace an FPGA on a development board once. I used a $20 heat gun clamped in a partially-completed CNC machine I was building. It only takes a few seconds for the solder to melt when the heat gun is on its lowest setting. I also used the heat gun to solder on the new FPGA, then touched up a few pins with a soldering iron. Ideally there would be a heat shield to prevent heating up other components, but in this case it wasn't a problem. The pins heat up so much faster than the rest of the chip; all you need to do is keep the internal temperature of the device less than 450 degrees F. Here's a photo of the removed chip:

In that case my approach would be to remove the defective ROMS first.

With Chipquik and a temperature controlled iron, preferably at 300F but not more than 350F, there's virtually no possibility of damaging the PCB, so removing and cleaning the defective parts can be treated as a practice session prior to working on the parts you need to preserve and re-install.

If you plan to use the continuous flow method of reinstallation, practicing by installing one or two defective parts on the trash PCB first would also be a good plan - the temperature and solder volume need to be just right to avoid dry joints or bridges between pins, and bridges can be quite difficult to wick off without increasing temperature beyond the ideal. I wasn't aware cupped SMD flow tips were available for my station until I followed the links posted earlier, but I plan to order one tomorrow :-)

I have been posting to this thread under the assumption you are dealing with fine pitch (0.5mm) gull wing lead devices, SSOP or QFP/LQFP. I have no experience with using the techniques I've described on J-lead devices, and after looking at a couple I expect it might be difficult to flow Chipquik under the device to where much of the solder is.

Another post suggested cutting the leads to remove defective devices, however that's not an approach I'd use on fine pitch leads as there is a risk of damaging PCB pads due to mechanical stress.

The dental bands I use are .002" thick

DaveC wrote: (snip)

Rolls of stainless steel shim stock are not very expensive. You can cut the thinner sizes with sharp scissors.

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John Popelish