Advice on desoldering and soldering SMC

D

Deluded

[H]ard|Gawd
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Dec 31, 2009
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I didn't know where to put this, so here it is.

I have a car ECU that has gone bad. Problem is, it's VIN coded, so I can't exactly plop down an ebay ECU and have it start the car.

Soooo, the chip that's holding the VIN/immobilizer info is a four sided SMC. In theory, it should be possible to buy an ebay ECU, swap the chip holding the car VIN/immobilizer info on the PCB and have it start the car.

The chip in question looks vaguely like this. It doesn't quite have so many pins like the 100+ SMC, but not that few either.

I have access to a basic soldering kit and a heatgun, if that helps.

I've looked at various tutorials for desoldering and soldering SMC, but most of those were for SMC that had two sides. I'm aware that the principle for soldering for four sided SMC is exactly the same as two sides, but I'm not sure how to go about it.

Do I just apply solder to one side in one giant wave, then use a wick to remove all solder bridges?
 
That's actually an viable solution for desoldering...but can I use the same method to solder the chip back on?

http://www.youtube.com/watch?v=Dlb2duETdJU

In this video, this guy is removing the immobilizer chip and putting it in the new ECU. Is his way of removing the chip valid? He's also using a heat gun too.
 
There is no service that can flash the ecu?

I had the same issue with a newer motorcycle. Sent my ECU to a performance flash shop along with the donor ecu and they flashed it for cheap.
 
jmilcher said:
There is no service that can flash the ecu?

I had the same issue with a newer motorcycle. Sent my ECU to a performance flash shop along with the donor ecu and they flashed it for cheap.
Click to expand...

Not with my car.

1997 Mercedes E320... the ECU is married to the DAS (drive authorization system) which is tied to the immobilizer. If the ECU doesn't recognize the DAS (or vice versa), it won't start the car.

Programming won't fix it, as the chips become read only after the first 40 key cycles. This is why it's generally not possible to reuse a Mercedes ECU unless you can also take the DAS module and the ignition too.
 
You can solder chips back on with hot air but In my experiance it can be a bit hit and miss sometimes and I would certainly want to use a proper hot air desoldering tool with temperature control, not a paintstrippper. Personally I preffer to use an iron for soldering stuff back down.

If this is a one off on something relatively valuable (which it sounds like it is) you are probablly better off paying someone experianced to do it rather than DIYing. People who do this stuff frequently make it look a lot easier than it is.
 
Last edited:
Desoldering options:
-Hot air
-IR
-Solder wick

Soldering options:
-Drag soldering
-Hot air soldering
-IR soldering


Is it an off-the-shelf chip or a custom jobber?
 
If you don't have hot air soldering/desoldering equipment with temperature regulated heat (not just an ordinary hot air gun), use Chip Quik low temperature solder because nothing comes close to it in ease of use and low risk of damage. It's $1 per inch.

Demonstrations:

https://www.youtube.com/watch?v=UmD7F0--7Lc

https://www.youtube.com/watch?v=7kyaz4Zrd78

You'll still need copper desoldering braid (get the correct width -- too wide soaks up too much heat and prevents the solder from getting hot enough, too narrow gets saturated with solder too easily; probably 2mm - 3mm is about right) and rosin flux (liquid or paste).

Find out about your specific ECU because there could be a problem with a voltage regulator inside it, even if it was made with Japanese capacitors.
 
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