Are the AMC7135 chips easy to damage when desoldering?

I recently desoldered some L7135 chips from the cheap driver boards when i wanted a lower current from them. I have recently tried soldering some of these onto multimode driver boards that have space for an extra one (dealextreme sku.7612) to try to drive a XP-G at 1400mA (with good heatsinking). However, according to my multimeter, the driver is still acting as though I had not added the extra chip, driving at ~1A on full mode. This was the case with 2 seperate drivers and 2 seperate de-soldered 7135 chips.

I am using a fully charged li-ion 18650 cell, and to check if it was a low voltage problem I very briefly connected a 5.5V source to the driver, yet the same current was drawn (5.5V was connected for literally 2 seconds while the reading was taken, the chips didn't even get warm to the touch). I have checked that the pins of the chip are connected to the equivalent pins of the already present chips and it all seems fine. Is it possible that I damaged the chips when I desoldered them initially? How resiliant are they? Only having one soldering iron i had to leave it in contact with each chip for ~15-20 seconds before all the pins solder melted and it could be removed. Are there any other checks I can do?

Thanks for any help.

To check if the chip's fried, solder some wire to an isolated chip, attach an LED and see if it lights up when you apply power. Just to confirm 100% that you've got connections (otherwise the wires will fall off) and that the board isn't doing something funny (which it might be).

I will say though, 15-20 seconds seems very excessive to me. Could you give a bit more detail for your technique? If you're letting the heat from the iron flow through the chip to melt the solder on the other pins that sounds like a great way of destroying them to me.

Th232 said:

To check if the chip's fried, solder some wire to an isolated chip, attach an LED and see if it lights up when you apply power. Just to confirm 100% that you've got connections (otherwise the wires will fall off) and that the board isn't doing something funny (which it might be).

I will say though, 15-20 seconds seems very excessive to me. Could you give a bit more detail for your technique? If you're letting the heat from the iron flow through the chip to melt the solder on the other pins that sounds like a great way of destroying them to me.

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Thanks for the reply. At the time I desoldered the chips, I was more interested in making the boards provide less current than in keeping the removed chips safe, so i was not careful with them. As I only had a single soldering iron i could only heat one place at a time so obviously removal was difficult.

I do have a few more boards left i could take some chips from, do you have any advice on the best way to do it? I may be able to borrow a friends soldering iron so would heating all areas at once be the best way to desolder these chips or do you have better advice for it. Since they are surface mounted with only a thin layer of solder underneath, I dont think either wick or a pump would help much.

SMT rework is best performed with two irons so you can heat both sides at the same time. If you only have one iron you need to quickly move the iron in rotation from pin to pin until the device comes free. Sometimes it helps to add lots of extra solder around the connections to retain the heat when you move the iron away.

Thanks for the tips, I'll give it a go once i get a second iron.

Agreed with what Mr Happy said, plus if you've got a selection of tips I'd get the broadest one possible, might be able to heat two pins at once that way.

I don't know if you've got access to one, but when I need to strip a board with no care for whatever's left, I break out a hot air gun (dedicated electronics or just one for stripping paint). I then hold the board component side down and heat from the top. The solder melts, that combined with a bit of shaking will make the components drop off. I find it's got less chance of overheating. Either that or hold the component I want with tweezers, have it component side up and let the board drop off. Maybe it's overkill for a 4 pin component, but on anything with more than 5 it's great.

I've been soldering electronics for 45 years, but I still class desoldering surface-mount chips as a very challenging tasks.

It's essential that you only use temperature-controlled soldering irons.

MikeAusC said:

It's essential that you only use temperature-controlled soldering irons.

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:twothumbs
But beware there are some stations that comes with a knob that controls power instead of temperature. These stations are made by big brands sold at entry level prices. They don't have a temperature sensor on the tip.

Temperature controlled(adjust) soldering irons don't have to be big and expensive. Here's a Japanese made Goot PX-201 that proves that they can be slightly chubbier than regular irons. There are also other makes and brands.

To safely remove a huge SMD chip with lots of pin say 5 x 5cm ( I doubt you find something like this in real life) I have a developed a special technique. It's rather simple.

There are 2 or more razor blades in a modern disposable plastic shaver. Dismantle the blades. They are thin, strong and very sharp. Heat a pin, slide the razor in between the pin and pad. This isolates the solder, move on to another pin. When you finish, you will be left with slightly raised pins due to the razor lifting.

Unfortunately the method above does not work with BGA chips and the likes. Those requires hotair or infrared systems.

That's a nice technique.:thumbsup:

Somehow though, I don't know what OHS will say if I start disassembling shavers at work.:laughing:

Th232 said:

That's a nice technique.:thumbsup:

Somehow though, I don't know what OHS will say if I start disassembling shavers at work.:laughing:

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If youre working at a place big enough that your worried about OHS, make them buy you a hot air pencil.

http://www.circuitspecialists.com/prod.itml/icOid/8010

I personally own this one, and for hobby stuff at home its fantastic. You could use this for putting down LED's onto MCPCB's too. Nice form factor, temerature regulated, adjustable flow, and replaceable tips for different packages.

These arent top of the line tools, if youre going to be doing it enough that you can convince the bossman to go bigger, you can eventually get up to this:
http://www.tequipment.net/WellerWR3000M.htmlhttp://www.tequipment.net/WellerWR3M.html I'd reccomend the WMRT tweezers too. Its a pair of tweezers that both parts of the pincer get hot. Makes for grabbing little SMD stuff a breeze. Between that, and the hot air, SMD rework is nothing. A put a nice fine tip on the WP-80 irons that come with that station and you'll be able to put down anything leaded (You WILL need a microscope if you get too small )

I do what Mr. Happy suggested and it wrks a bit quicker. I'm too cheap to afford a temp. controlled station, although if I see one for a great price, I'd probably snap it up.

Also, desoldering braid is good for getting that excess solder off after you've unmounted the chip.

What I do is I connect all 3 pins with one glob of solder. Then I start at the middle and heat the glob until the iron sinks and touches the middle pin under the solder. Once the solder glob is liquified, odds are that the solder under the chip is liquified too, but I usually apply steady shearing pressure to release the chip the moment that the solder allows for it.

What's great about temp controlled units, though, is that if you set it to a temperature lower than the max temp for the chip but hig enough to melt solder, you can spend as much time as you want reworking it