Desoldering questions (hot air gun)

Thread Starter

larry109

Joined Oct 13, 2017
6
I'm super new to soldering. I just want to get the basics down at first. I picked up a temp-controlled hot air gun and iron station, some wick, lead, flux, desoldering pump, tweezers, etc... I got a few junk modems laying around so i decided to test out and try to desolder the chips.

Looks like a bunch of 8-pin EEPROM chips on them. I added some flux to the legs, set the temp to 350 celcius on the hot air gun, circled around the chip for about 5-10 seconds and the chip easily slid off. But on a different modem, I could not get the EEPROM chip off for the life of me. Had the hot air going for 3-5 minutes, even raised the temp to 400 and it just wouldn't budge.

Is there some sort of special solder on this modem that makes it nearly impossible to get the chip off? Is there a better technique?
 

AlbertHall

Joined Jun 4, 2014
12,345
The difference may be that solder containing lead used to be used - melting point about 185°C - but, as lead is a poisonous substance and is now not generally allowed in solder, a new lead free solder is used - melting point about 220°C. Also, maybe, one or more pins may be connected to a ground plane, a large area of copper on the board, which works as a heat sink and 'sucks' heat away from the soldered joint. If you are just 'messing about' then try higher temperatures until the chip does come loose.
 

Thread Starter

larry109

Joined Oct 13, 2017
6
The difference may be that solder containing lead used to be used - melting point about 185°C - but, as lead is a poisonous substance and is now not generally allowed in solder, a new lead free solder is used - melting point about 220°C. Also, maybe, one or more pins may be connected to a ground plane, a large area of copper on the board, which works as a heat sink and 'sucks' heat away from the soldered joint. If you are just 'messing about' then try higher temperatures until the chip does come loose.
Thanks for the super quick reply...so I finally got the chip off, but I had to use the iron to add solder to both sides and use the hot air gun at the same time...really crazy...but I did notice that when I finally removed it, there was a orange rectangle on the board that the chip was sitting on.....is that the ground plane you're referring to? On the modem with the chip that came off in 10 seconds, it was just the regular green color of the board underneath...

Also, won't the extended time of using the heat gun ruin the board or other components? How long could it possible take? 10 minutes with a hot air gun? Seems kind of crazy...what would you do to remove the chip without ruining it and the components around it?
 

Thread Starter

larry109

Joined Oct 13, 2017
6
Use a more appropriate nozzle so all of the pins are heated at the same time.
So a nozzle big enough that it can surround the entire chip at once, right? Won't that also focus the heat right dead center in the middle of the chip and ruin it?
 

dl324

Joined Mar 30, 2015
16,846
but I did notice that when I finally removed it, there was a orange rectangle on the board that the chip was sitting on.....is that the ground plane you're referring to?
It's probably adhesive used to hold components on boards that have components on both sides of the board. When the solder is melted, using the correct nozzle, the adhesive should also be melted so the component can be removed.

You should take care about heating PCB traces. They're also affixed with an adhesive.
 

philba

Joined Aug 17, 2017
959
And, one other possibility - an IC may have an exposed slug on the bottom that gets soldered down. Typical with high power ICs. You'll never get it off with hot air unless you heat the entire chip - thus damaging it (and probably the board as well).
 

Thread Starter

larry109

Joined Oct 13, 2017
6
And, one other possibility - an IC may have an exposed slug on the bottom that gets soldered down. Typical with high power ICs. You'll never get it off with hot air unless you heat the entire chip - thus damaging it (and probably the board as well).
What's the slug? The chip I was trying to remove is just a regular EEPROM 8-pin surface mount (not through-hole)...are there sometimes slugs on it?
 

philba

Joined Aug 17, 2017
959
Things like motor controllers, voltage regulators and SMPS ICs have a lot of current flow through them which generates heat. The slug allows good mechanical bonding to the copper via solder. There often are a lot of vias to conduct the heat to the other side of the board.
 

AlbertHall

Joined Jun 4, 2014
12,345
If you are trying to remove an IC without damaging the board it is probably better to cut the IC leads and then unsolder the remains of the legs from the board with a soldering iron and tweezers. The IC will not be reusable but the board and other components will be fine.
 

dendad

Joined Feb 20, 2016
4,452
Here is a motor driver that has the slug under it that is soldered to a large area of copper to help dissipate the heat.
L298-L298P-SOP20-font-b-motor-b-font-IC-Integrated-font-b-circuit-b-font-BR.jpg
Many power chips have these. I use an 8 pin SMT motor driver that will drive 3.5A at 40V and it has a slug to get rid of the feat. Not bad in an 8 pin SMT chip! www.allegromicro.com/~/media/Files/Datasheets/A4950-Datasheet.ashx
They are a bit harder to get off the board.
One way to help protect the other parts is to cover them with aluminum foil so the heat only goes to the part you are working on. Foil can be pushed down around the other parts to redirect the hot air away from them. But is is easy to get the board too hot if you crank the air temp up to far. More air at the required temp is better that too hot air with not enough flow.
 
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Thread Starter

larry109

Joined Oct 13, 2017
6
Here is a motor driver that has the slug under it that is soldered to a large area of copper to help dissipate the heat.
View attachment 137238
Many power chips have these. I use an 8 pin SMT motor driver that will drive 3.5A at 40V and it has a slug to get rid of the feat. Not bad in an 8 pin SMT chip! www.allegromicro.com/~/media/Files/Datasheets/A4950-Datasheet.ashx
They are a bit harder to get off the board.
One way to help protect the other parts is to cover them with aluminum foil so the heat only goes to the part you are working on. Foil can be pushed down around the other parts to redirect the hot air away from them. But is is easy to get the board too hot if you crank the air temp up to far. More air at the required temp is better that too hot air with not enough flow.
Interesting. I've never seen that slug thing before but good to know. So for a typical 8 pin small EEPROM SOIC chip, 350C with air at 3-4 (on a air pressure scale of 1-7) would be good?
 

dendad

Joined Feb 20, 2016
4,452
I don't have a calibration of the air on my hot air desoldering tool. And tend to run a bit hotter, maybe 375C. Have you some old PC motherboards or other ones you could practice on? That way you can figure out what settings to use with your device.
The bonus is you get a good stock of parts for later.
 

Tonyr1084

Joined Sep 24, 2015
7,853
Sometimes it helps to understand how these PCB's are assembled in the first place. As was mentioned before, sometimes components are held on with a spot of adhesive. This is usually done on boards that are populated on both sides. This way, when the board goes through the reflow oven (the oven that heats the board and solder up to melting temperature of the solder) the components don't fall away from the board on the bottom side. Components on the top side don't need adhesive because both gravity and surface tension of the solder holds the components in place. But on the underside the components can and will fall off if not adhered properly. So you may have encountered some adhesive.

Another post mentioned the slug. That's there to dissipate heat into the board. It's done where a heatsink is not needed but the additional heat flow into the board is sufficient to keep the chip cool enough under normal operation. It will take longer to remove such a part, as you have to get sufficient heat into the board to get that solder to reflow.

One more thing that has not been mentioned is that sometimes a PCB will have power and ground planes internally. They demand a lot of heat when soldering and desoldering. Often such a board goes through a pre-heating process thereby reducing the amount of time a board has to soak in high heat. Whether going through a reflow oven or on a workbench where a technician is hand soldering parts, sometimes a board has to be preheated so that solder can flow all the way through a board (typically in Through Hole Technology components).

So when you encounter difficulty removing a component it may just be adhesive or it could be a thermal slug. OR it could be that the board needs to be preheated to aid in re-instigating solder flow.
 

philba

Joined Aug 17, 2017
959
Good point about multilayer boards.

I've seen recommendations to preheat a board for desoldering. I don't do it because it requires some sort of heating rig though maybe a simple hotplate would suffice. The idea is to bring the board up to something like 150C for lead and 200C for lead-free (the high end of the pre-heat phase of the solder cycle - Tsmax). Then the hot air gun is used to bring the component leads up to the solder melting point.The theory is that this allows much shorter spot heating and less damage to adjacent components. It doesn't hurt to understand reflow solder profiles.

This is also how some people use hot air and solder paste to solder SMDs. especially find pitched stuff. Not sure how well it supports soldering slugs or BGAs.
 

joewales44

Joined Oct 8, 2017
218
i only build single sided boards so i use a piece of 1/4" thick aluminum on an old cookstove. works great and i've never delaminated a board. you just have to turn it to low when the solder starts to flow. the heat usually softens adhesive too.
 

philba

Joined Aug 17, 2017
959
i only build single sided boards so i use a piece of 1/4" thick aluminum on an old cookstove. works great and i've never delaminated a board. you just have to turn it to low when the solder starts to flow. the heat usually softens adhesive too.
Yeah, I have always wanted to build a reflow oven though I see people pretty successfully using uncontrolled frying pans and toaster ovens to do that. Your aluminum plate is in the same vein. Works pretty well, apparently.

Even if you put components on both sides, you could hand solder the second side after reflowing the most populated one.
 
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