desoldering SMD DIP IC with a heat pad....

[fastline]

Trying to get a an SOIC chip off a board. So far this is not going well. I had to resort to running a wire under the pins and desoldering each and pulling them up. I certainly messed it up but toast anyway. Now I am realizing the heat pad in the middle is solder too! I cannot get to the back of the board.

I do have a desolder station and hot air equipment. I applied 300C hot air to the chip for several minutes and only succeeded in heatin the whole board up. What technique do I need? There are little exposed feet on the sides of the chip but my iron cannot pump enough watts in there to do any good. I think it is 40w.

 

[jpanhalt]

What is the chip number? I have soldered and de-soldered TO220 metal tabs to copper bus bars just using heat to the top of the tab.

John

 

[fastline]

This is an SO-36. The heat pad is the problem though, all pins are free.

 

[ronsimpson]

I have a hot air gun. In you case I would heat up the board from the back side. Cutting off the pins is fine. Often do that. If there was no heat sink pad I would blow hot air on the top side.

I also heat the entire board in a toaster oven. There is much on the internet about that.

 

[fastline]

The biggest problem here is I CANNOT get to the back of the board. There are components on the back that I risk unfusing. At this point, I just need to get the chip off.

What temp should I use for the hot air? 300C does not seem to be cutting it.

 

[jpanhalt]

SO-36 doesn't help. That just specifies the number of pins. What I was after was an image of how the center pad is connected to the "little exposed feet."

Getting it off, I agree about applying a lot of heat. My big iron is an 80W Weller with a chisel point.

Is it lead free solder? Do you have to remain lead free?

John

 

[fastline]

No, I don't need lead free. No idea what the factory used but I assume lead free as many places are going to that. Seems to cause more problems than it solves though.

CHip is an L9758

So is hot air out? Apply heat directly to the pad? I just tried again wtih 400C air with foil shielding. Stopped after about 1min of trying. Nothing started reflowing but board is getting too hot.

 

[Andrei_D12]

I bet he is talking about desoldering this chip! Well, I tried to recover some smaller chips (soic8, soic16) by using an ordinary letcon, using the following procedure. I put some solder on the tip ofthe letcon, then I rapidly crossed over the pins of one side, covering them with solder, so that the heat is uniformly distributed, and I was pulling up the chip from underside with a tweezer, so that the chip was ''flipped''. Then I desoldered the other side by the same method explained above... If you don't understand what I mean, then I will draw a sketch.

 

[jpanhalt]

OK, here's a bottom view of the chip showing the solder pad that has exposed tabs:

I would not use hot air, as that is not localized enough. I would use a high wattage iron with a wide tip directly on the chip or on both exposed ends. Melted solder on the tip will help. If you have some bismuth solder (very low melting), applying that to the end while heating may give enough of a lower-melting-point alloy right under the chip to help. ChipQuik (https://www.chipquik.com/store/ ) sells the stuff, and so do other sources. When you re-solder it, you may want to consider using bismuth-containing solder for that area.

John

 

[Andrei_D12]

This is what I was talking about. Carefully, and it's done!

 

[jpanhalt]

What the OP is talking about is not how to unsolder the pins, but rather a heat sink in the middle.

John

 

[fastline]

^^^ yep! Pins are already off. No choice but to destroy them without a very specialize device. I did put the bead of solder on the sides and realized it was not going to work. I only hope I did not screw up the pads.... I should have just cut the pins and focused on the heat sink. The sink pretty much makes the chip nearly impossible to remove without damage.

 

[jpanhalt]

A piece of heavy copper (>12 AWG) bent in a U and placed over the chip may help conduct heat to both tabs. Then use a really high wattage (like my 80W) iron to heat it. Although, I think that size iron directly on the chip back may also work and be simpler.

John

 

[Andrei_D12]

So there is a heatsink... I have rarely seen a chip like this on a pc board. Since this chip is mounted by a machine (not really a machine, but the components are put on the board with flux, then put into an oven,probably microwave because the metal heats very fast in this way,the other materials heating slower, and the jonts are soldered at the same time) I can't imagine any method for taking it out, but heating the pcb on the other side, right under the chip with a flame (i know it sounds really stupid)!
QUESTION:Are there components on the other side of the board?

 

[KeepItSimpleStupid]

Convection pre-heater: **broken link removed**

Remember to twist the IC. I'd count on destroying the part.

 

[fastline]

Resorted to this now.... Even pumping 150w into the sink, I cannot get it hot enough. Going to chase down a pencil torch now. This seems like flame heat territory. I don't know what is on the bottom of the board so preheating the entire board is probably not a good idea.

 

[ronsimpson]

This how I solder double sided PCBs. Remember the largest pads milt first.
I use a knife to reach under the part and act like a lever to pry up on the part. In the oven, when the solder milts, the knife will lift one end edge of the part. Then you can reach in and grab the part.
If you don't jar the board hard the bottom side pats will stay put.
Watch the video. At about four minutes into the heating process the solder will turn bright and flow on the small pads first.

I have put the part to remove on the bottom side and attached a weight to pull the part off.
Good luck.

 

[tunedwolf]

Motor driver chips often have those pesky heatsink tabs on the underside. You must pre-heat the board and at the same time raise the temperature of the part otherwise you will pull up the track. A generous amount of liquid flux in there won't do any harm either

 

[jpanhalt]

Resorted to this now.... Even pumping 150w into the sink, I cannot get it hot enough.

That is hard to believe. Do you have good thermal contact, like with molten solder between your soldering iron tip and the exposed metal? I would not rely on hot air for this.

Your pencil torch might work too, but I think you will run more risk of doing other damage that way.

John

 

[fastline]

I put a huge ball of solder on the gun, then put on the heat sink. While on the sink, the solder even touching the gun would not stay molten.

I can still control heat reasonably with a small torch. I plan to shield the rest of the board. Check in shortly.

Again, there is no way to preheat this unit. There components on the back or bottom and I cannot even get the board out of its Aluminum case. The thing was never meant to be serviced. If something comes loose on the bottom, I am up a creek bu the way I see it, it has to be done to do the repair so all I can do is try. Because there are so many components close to the chip (some resistors are .010" wide and only about .020" from the feet of the chip), my options are rather limited. Grinding it out at least got me right to the sink to apply heat.