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Desoldering/Soldering SMD's

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MrWho

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Hi Guys,

So I just started looking in to desoldering eeprom chips ( Motorola etc )

What is the safe temperature before they get fried ?

Is this technique good enough ...

1. Using masking tape - tape around eeprom to protect other smd's
2. Turn on Hot Air - let it preheat to 420C ?
3. add some liquid flux to the legs
4. with circular move - move hot air nozzle around the eeprom
4. lift the eeprom

What temperature ?
What nozzle something tiny or something like 8mm is good enaugh
What is the safe temperature ?
How long it should take before I can lift the eeprom ?

i'm wondering if I should maybe put a tiny heat sink on the eeprom to help cooling ?
Or maybe I should use chip quick for desoldering at lower temp ?
Or spray the IC smd with Freeze in a can to protect from overheating ? but Can't the cold temperature kill it as well ?

Thx
 
No, absolutely not. For either soldering or desoldering SMD's the board needs to be pre-heated. The only difference between desoldering and and a reflow soldering profile is any chips you don't want to be desoldered need to be shielded from the peak flow that brings the solder up into the fluid range, what are you using for masking tape?

**broken link removed**

That's a generic profile for that solder type. I'm not even sure where you got 420C perhaps you meant F? 420F is 210C which is closer to sane. You can find exact profiles for any solder you happen to be using simply by Googling the solder type and adding the words 'reflow profile' using the image search.
 
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Can I preheat them in regular small oven ?
What about other components and preheating ?

Its a lot IC's on the boards its car ECU.

I don't think I can desolder it at 220C ... unless i will sit on it forever and this would damage the IC's

Masking + tin foil as shield.
 
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No, not a regular oven, at least not safely, needs to be at least a forced airflow, even temperature is the key, look at the recommend times in the profile, these are pretty standard. You seem to be missing the point though, look at the above chart, you're supposed to get it happy at a highly elevated temperature IC safe that the solder is still solid at, then you hit it with peak heat to bring into the reflow range, then remove the heat and let it naturally cool to ambient (increased) airflow temperature. Then it's slowly allowed to cooled to human ambient over a few minutes to avoid thermal shocks.

Soldering can't be properly done by taking a cold board, slamming on heat and then rapidly cooling, this will create the worst possible solder joint fully of metal stress and internal fractures.
 
I guess I need to pick up something like this in Addition to my hot air soldering station ?

**broken link removed**
 
You don't want to subject the whole board to reflow temp just to repair one device. There is likely items like connector that cannot take the temp.

I use a butane gas torch with hot gas tip. Like Master Appliance UT-100 or Weller portasol PSI-100K.
https://www.alliedelec.com/search/productdetail.aspx?SKU=8611311
**broken link removed**

The hot gas butane torches have less gas velocity with higher temp then hot air guns that will blow small parts around.

Practice on some scrap boards. If you takes longer then a minute to remove a SMT I.C. then you are not using enough heating level and just doing damage. A cap or resistor only takes about 10 to 20 seconds to remove.
 
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RC, you missed the point, you don't heat the whole board to reflow temp, you pre-heat the general area to reduce the load on the reflow iron, and then apply a pulse of heat to the region you're attempting to reflow/desolder with the hot air tool. The whole board doesn't have to be pre-heated just the section surrounding the solder area. The pre-heat temperatures are safe for all but the most thermally sensitive devices. You can use a freegin hair dryer to preheat.... it reduces the wattage and total power required by the hot air unit by a large amount.

Figure the typical environment will be human habitable the air temperature is 72F, the point is you follow the curve, you pre-heat allow the heat to soak in an stabilize and then you pulse the higher power for the actual soldering event. I use a wattage controlled thermal gun to desolder parts from boards and as I've practiced I've determined the settings required for the pre-heat/desolder that cause everything on the board to jump off without harming anything where when I was just going in gun ho with a hot air gun and a room temperature board I was smoking chips and getting nothing off. The temperature profile for good desoldering/soldering is identified perfectly in the reflow profiles.
 
I mnight be doing it the crude way, but I've found the hot air attachment to my cheapy Gascat 60 (Antex) does the job nicely, altohugh its a bit small for wide SOIC28's. I have scraps of thick copper foil/sheet to mask around the chip in question, with the sides 'tapered down' a bit towards the chip so it doesn't immediately reflect the heat back up. After a few years of doing it with flux and desoldering braid, the hot air method is much more gentle on the board/chip/pads. I still use flux though!

A slow gentle circular motion over the top of the chip around 3/4" away (all depends on the size of your hot air nozzzle, and temp... mine isn't in any way temp regulated) for around 6-7 seconds, then an occasional 'nudge' with the end of a pair of tweezers. If you've got it slightly too hot the pads can easily be moved out of place. Once it's moving slightly, tweezers to gently move the IC off the pads (in the direction of the row of pins on one side) with it still flat on the board, and pull it off - the pins should be disconnected (in terms of solder) from the pads before lifting it, otherwise you could lift pads.

I pracitced a lot on old laptop motherboards, which are difficult because of the extensive copper power planes, which quickly draw heat away from the site you're trying to heat up. In which case, more air flow is needed. After that, two-sided boards become a breeze. I'm STILL surprised I haven't destroyed any chips/semi's yet. But at the start, I did damage some PCB's..
 
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My suggestion is one of these:- **broken link removed**

I run it at 300 - 320 deg C, and just play the hot air around the IC until it will move. As Blueteeth says, practice on some old boards.

To resolder, I make sure that the pads are reasonably clear of old solder, spread solder paste, place the component and use the hot air gun again. I try to get the whole IC hot so that it settles down nicely on the pads. If there is any bridging of legs with solder, I don't worry about it while using the hot air gun, but deal with it later with a soldering iron and a solder sucker.

For reflowing a whole board, which I only do when assembling and not when repairing, I use one of these:-
http://www.reflow-kit.com/rkuk/order_product_details.html?wg=1&p=16
with an earlier version of one of these:-
http://www.reflow-kit.com/rkuk/order_product_details.html?wg=1&p=242
I think that the main difference on the V2 is that it uses IEC connections rather than European plug on the back and corresponding socket on the front. It is really a bit too large to hang off a wall socket so I would think that the new shape is an improvement.
 
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