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Electronics => Repair => Topic started by: UnkaHeemee on April 07, 2023, 12:42:10 am

Title: Need help replacing an old IC on a 40 years old PCB
Post by: UnkaHeemee on April 07, 2023, 12:42:10 am
I have this 74LS74 chip that I want to replace in my C64 (https://en.wikipedia.org/wiki/Commodore_64). The plan is to remove this, install a socket and put in a new 74LS74 (https://secure.sayal.com/STORE2/View_SHOP.php?SKU=143946) in there. Here's the chip:

(https://i.ibb.co/ZcfYdWt/338870295-3582109898744981-4584287585523662763-n.jpg)

Instead of trying to desolder every pin and then trying to remove the entire chip, I was told that for these older boards, it might be safer to clip the individual legs off with pliers or something and then remove the legs one by one. Is that a sound strategy? I just want to make sure I won't be damaging tracers, pads or anything. Currently I have a soldering iron and one of those solder removing suction pump things (hand operated). 
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: james_s on April 07, 2023, 12:43:22 am
That's a common trick people used to use, it's really better to find someone with a proper desoldering tool though, I use a Hakko 808 for that sort of work and it makes it a piece of cake.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: fzabkar on April 07, 2023, 12:47:25 am
Snip the pins close to the body with flush cutters, then wet each pin with solder and lift them out with your iron. Heat each hole on one side of the PCB and suck out the solder from the other side.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: UnkaHeemee on April 07, 2023, 12:51:21 am
Quote from: james_s on April 07, 2023, 12:43:22 am
That's a common trick people used to use, it's really better to find someone with a proper desoldering tool though, I use a Hakko 808 for that sort of work and it makes it a piece of cake.

I wish! I see folks doing it with those guns and it looks so much easier.

Quote from: fzabkar on April 07, 2023, 12:47:25 am
Snip the pins close to the body with flush cutters, then wet each pin with solder and lift them out with your iron. Heat each hole on one side of the PCB and suck out the solder from the other side.


Are you asking me to apply solder and heat on one side and the pump on the other side only?
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: fzabkar on April 07, 2023, 01:08:35 am
Quote from: UnkaHeemee on April 07, 2023, 12:51:21 am
Quote from: fzabkar on April 07, 2023, 12:47:25 am
Snip the pins close to the body with flush cutters, then wet each pin with solder and lift them out with your iron. Heat each hole on one side of the PCB and suck out the solder from the other side.


Are you asking me to apply solder and heat on one side and the pump on the other side only?

Yes, that's the way I used to do it.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: Psi on April 07, 2023, 01:59:24 am
If you are super worried about damaging the pads from the cutters bending the pins as you cut then, then you can mount the PCB vertically and heat a pin with the iron from one side while cutting it from the other side. That way the solder is molten when you cut it and the pin has room to move outwards as the cut is done.

But this is usually not needed unless the PCB is super super fragile.
For example really old PCBs without plated through holes to provide an anchor between layers tend to be more prone to pads lifting as you twist things, and cutting the pins pushes the metal outwards which can lift pads on the opposite side to the twisting force.

Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: dobsonr741 on April 07, 2023, 02:09:41 am
Safer to snip pin by pin, then heat up one by one and pull each pin by a tweezer.

Then apply the solder sucker pump to free up the holes one by one. Sometimes you need to add some solder if the hole is half filled, so the a momentum of the bigger solder blob would  clean it out better.

The risk of using a desolder tool or hot air in one go is to overheat and delaminate pads, or pull out vias if not properly heated up all of them.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: SeanB on April 07, 2023, 05:43:21 am
I have had some fragile boards where I cut each pin, then took the new IC, and formed the leads to be close to the plastic case, then cut off the end, and soldered to the existing leads. Had to be done, as the original board was vapour phase reflowed, and they used double sided DIP loading, with each one staggered 0.1in from each other, to fit the number of packages they needed into the limited volume. You had to be really sure an IC was toast before replacing it. Another in the test bench, to avoid having to play in the wire wrapped and soldered underside (wire wrap and solder, just in case there was vibration) I simply cut the sides off the socket, and soldered the new turned pin socket to the stubs thus exposed. Then put the fusible link PROM into the socket, thankful that the original manufacturer had not soldered this particular IC into the socket, like the rest of the board had.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: Haenk on April 07, 2023, 05:52:09 am
As you don't need to salvage it - cut it.
That is certainly the safest way to not destroy anything.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: mariush on April 07, 2023, 05:56:45 am
Another option would be to get low melt solder and join all pins on one side with a line of low melt solder, and the same on the other side, and as the low melt solder is hot and liquid, gently pull from each end of the chip on the other side alternating between chip ends. 
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: djsb on April 07, 2023, 10:28:43 am
Assuming that you know that the chip is completely "Knackered" then your main priority is to protect the PCB pads from damage. If you keep the chip in place, it can act as a heat sink for excess heat from your soldering iron. My preference with desoldering is to use a flux pen and good quality desoldering braid. The braid can remove some of the heat also and therefore help protect the pads. If the pads are plated through holes, then you may have to actually ADD some solder again in order for the solder braid to suck up the solder. You need to use good quality braid like "GootWick" or "Techspray". If you cut the pins off the chip you remove the chance for the chip to absorb some of the heat and you are putting physical stress on the pads. Also make sure you are using a temperature controlled Iron and a bit that is the correct size.

HTH.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: Swake on April 07, 2023, 01:00:31 pm
Quote from: dobsonr741 on April 07, 2023, 02:09:41 am
Safer to snip pin by pin, then heat up one by one and pull each pin by a tweezer.

Then apply the solder sucker pump to free up the holes one by one. Sometimes you need to add some solder if the hole is half filled, so the a momentum of the bigger solder blob would  clean it out better.

The risk of using a desolder tool or hot air in one go is to overheat and delaminate pads, or pull out vias if not properly heated up all of them.

This. This is your best strategy with the tools and experience you have.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: rfclown on April 07, 2023, 01:25:38 pm
I did this same thing 40+ years ago when I was young and had a TRS-80 with a bad CMOS inverter. I was very nervous about it, but I followed the suggestion of cutting each pin with diagonal cutters. I only had a low end Radio Shack pencil soldering iron and a bulb solder sucker. I was able to do it. I installed an IC socket instead of directly soldering the new IC on the board. I didn't want to have to do it again.

The only thing I would do differently today would be to use solder wick to clean up the old solder instead of using a solder sucker. I don't think that I had ever used solder wick back then.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: wasedadoc on April 07, 2023, 01:33:14 pm
Many solder sucker pumps - the spring loaded type - have a recoil action. When you push the release button the plunger flies back to create the vacuum but the whole pump in your hand can move in the opposite direction and ram the nozzle on to the pad and cause possible damage. Do not rest the nozzle on the pad but leave a small gap and hold firmly.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: RoGeorge on April 07, 2023, 01:35:52 pm
Quote from: UnkaHeemee on April 07, 2023, 12:42:10 am
Currently I have a soldering iron and one of those solder removing suction pump things (hand operated).

No, clip the pins one by one, remove the IC, then desolder the pins one by one, without any pump.  Just heat the solder joint, and when it's melted, gently pull out the pin with a tweezers.  Use plenty of flux.  Then go to the next pin.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: tooki on April 07, 2023, 04:36:41 pm
Quote from: wasedadoc on April 07, 2023, 01:33:14 pm
Many solder sucker pumps - the spring loaded type - have a recoil action. When you push the release button the plunger flies back to create the vacuum but the whole pump in your hand can move in the opposite direction and ram the nozzle on to the pad and cause possible damage. Do not rest the nozzle on the pad but leave a small gap and hold firmly.
Or buy some silicone tube and cut a short length of it that you can slip over the tip of the sucker.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: UnkaHeemee on April 08, 2023, 02:48:42 am
Thanks for all your detailed helpful inputs, people!

The consensus so far seems like it'd be the best to clip the legs and remove them one by one.

The only concern about that shared by one of you is that the absence of the chip during the while process will transfer the excess heat to the board and a non-existing chip can't help soak up some of that.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: djsb on April 08, 2023, 05:01:25 am
Do you have a temperature controlled soldering iron? If not, it may be a good investment to buy one first. Then desolder your component.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: Gyro on April 08, 2023, 09:17:00 am
Quote from: UnkaHeemee on April 08, 2023, 02:48:42 am
...
The only concern about that shared by one of you is that the absence of the chip during the while process will transfer the excess heat to the board and a non-existing chip can't help soak up some of that.

That (aside from avoiding the problem of having to unsolder all of the pins at once) is sort of the point. It allows the solder to melt really quickly, before damage to the board occurs. You will also have tweezers or whatever grasping the IC end of the leg.

40 years isn't really that old in terms of PCB technology. They had long since mastered the technology of keeping the tracks glued down.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: nedrysoft on April 09, 2023, 03:36:55 am
Snip them, I don’t know whether it’s because they made them cheaply, badly or they’re just generally poor quality, but it’s common for people to take pads and/or traces off with a chip they are attempting to remove.

I’ve repaired loads of machines of this era and I just snip the legs despite owning a halo desoldrring gun, the gun is amazing but I rarely let it anywhere near any old computers since I know how easy stuff detaches from the laminate.

I’ve seen some try horror pictures where people have attempted to fix a machine.

Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: abdulbadii on April 10, 2023, 05:45:34 am
may worth try using the sharpest point iron tip which's been bent down, by use of pliers, at ~ 3 mm from the tip end, applied at min. 300 C depends on the total pad breadth connected, the larger that the higher temperature must be set

so may push down, expel the pins out as soldier's molten, urged to pull it of the other side if possible ie. on reaching halfway desoldering
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: wasedadoc on April 10, 2023, 12:17:24 pm
Quote from: abdulbadii on April 10, 2023, 05:45:34 am
..expel the pins out as soldier's molten..
If you are melting soldiers, you are surely doing something wrong!  :)
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: james_s on April 11, 2023, 11:35:34 pm
Quote from: wasedadoc on April 07, 2023, 01:33:14 pm
Many solder sucker pumps - the spring loaded type - have a recoil action. When you push the release button the plunger flies back to create the vacuum but the whole pump in your hand can move in the opposite direction and ram the nozzle on to the pad and cause possible damage. Do not rest the nozzle on the pad but leave a small gap and hold firmly.

I never had any luck at all with those things. I dunno if I'm just a total klutz or what but out of all the desoldering methods I've tried, those spring loaded suckers were the most useless. I could never figure out how to get adequate heat on the joint at the same time as I'm using the sucker. They probably work better for stuff like vacuum tube sockets and terminal strips I guess.

Once I bought a Hakko 808 I never looked back though. It was expensive but after using it the first time I wished I had bought it years earlier.
Title: Re: Need help replacing an old IC on a 40 years old PCB
Post by: nedrysoft on April 12, 2023, 09:58:30 am
Quote from: james_s on April 11, 2023, 11:35:34 pm
Quote from: wasedadoc on April 07, 2023, 01:33:14 pm
Many solder sucker pumps - the spring loaded type - have a recoil action. When you push the release button the plunger flies back to create the vacuum but the whole pump in your hand can move in the opposite direction and ram the nozzle on to the pad and cause possible damage. Do not rest the nozzle on the pad but leave a small gap and hold firmly.

I never had any luck at all with those things. I dunno if I'm just a total klutz or what but out of all the desoldering methods I've tried, those spring loaded suckers were the most useless. I could never figure out how to get adequate heat on the joint at the same time as I'm using the sucker. They probably work better for stuff like vacuum tube sockets and terminal strips I guess.

Once I bought a Hakko 808 I never looked back though. It was expensive but after using it the first time I wished I had bought it years earlier.

It’s not you, they’re just not very good and they’re risky on old computers because the tracks and pads can be fragile, hence why it’s best to cut them if you don’t need to salvage the part.

Somebody mentioned above getting some silicone tube to go over the nozzle, this makes them just easier to use since the silicone deforms and helps create a better suction and it stops the direct force of the pump on the pin/pad.

I use a Hakko desoldering gun because it’s a beast and it works every time, but before that I used the “engineer ss-01” which is the next best thing in my opinion, it uses a silicone sock and comes with a replacement length which you can cut to size, spoiler alert in 4 years I’ve replaced it once, once one end got worn I simply flipped it around and used the other end.

Eventually that end wore down, but I have 3 lengths of the tube that will outlive me, for sure now since I don’t use it unless I’m being lazy and it’s more convenient than firing up the hakko.