Need help replacing an old IC on a 40 years old PCB - Page 1
Forum section: electronics / repair

author: UnkaHeemee
author_rank: Newbie
timestamp: April 07, 2023, 12:42:10 am
content: I have this 74LS74 chip that I want to replace in myC64. The plan is to remove this, install a socket and put ina new 74LS74in there. Here's the chip: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).

author: james_s
author_rank: Super Contributor
timestamp: April 07, 2023, 12:43:22 am
content: 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.

author: fzabkar
author_rank: Super Contributor
timestamp: April 07, 2023, 12:47:25 am
content: 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.

author: UnkaHeemee
author_rank: Newbie
timestamp: April 07, 2023, 12:51:21 am
content: Quote from: james_s on April 07, 2023, 12:43:22 amI 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 amAre you asking me to apply solder and heat on one side and the pump on the other side only?

author: fzabkar
author_rank: Super Contributor
timestamp: April 07, 2023, 01:08:35 am
content: Quote from: UnkaHeemee on April 07, 2023, 12:51:21 amYes, that's the way I used to do it.

author: Psi
author_rank: Super Contributor
timestamp: April 07, 2023, 01:59:24 am
content: 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.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: April 07, 2023, 02:09:41 am
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: April 07, 2023, 05:43:21 am
content: 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.

author: Haenk
author_rank: Super Contributor
timestamp: April 07, 2023, 05:52:09 am
content: As you don't need to salvage it - cut it.That is certainly the safest way to not destroy anything.

author: mariush
author_rank: Super Contributor
timestamp: April 07, 2023, 05:56:45 am
content: 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.

author: djsb
author_rank: Super Contributor
timestamp: April 07, 2023, 10:28:43 am
content: 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.

author: Swake
author_rank: Frequent Contributor
timestamp: April 07, 2023, 01:00:31 pm
content: Quote from: dobsonr741 on April 07, 2023, 02:09:41 amThis. This is your best strategy with the tools and experience you have.

author: rfclown
author_rank: Frequent Contributor
timestamp: April 07, 2023, 01:25:38 pm
content: 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.

author: wasedadoc
author_rank: Super Contributor
timestamp: April 07, 2023, 01:33:14 pm
content: 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.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 07, 2023, 01:35:52 pm
content: Quote from: UnkaHeemee on April 07, 2023, 12:42:10 amNo, 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.

author: tooki
author_rank: Super Contributor
timestamp: April 07, 2023, 04:36:41 pm
content: Quote from: wasedadoc on April 07, 2023, 01:33:14 pmOr buy some silicone tube and cut a short length of it that you can slip over the tip of the sucker.

author: UnkaHeemee
author_rank: Newbie
timestamp: April 08, 2023, 02:48:42 am
content: 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.

author: djsb
author_rank: Super Contributor
timestamp: April 08, 2023, 05:01:25 am
content: Do you have a temperature controlled soldering iron? If not, it may be a good investment to buy one first. Then desolder your component.

author: Gyro
author_rank: Super Contributor
timestamp: April 08, 2023, 09:17:00 am
content: Quote from: UnkaHeemee on April 08, 2023, 02:48:42 amThat (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.

author: nedrysoft
author_rank: Contributor
timestamp: April 09, 2023, 03:36:55 am
content: 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.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: April 10, 2023, 05:45:34 am
content: 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 setso 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

author: wasedadoc
author_rank: Super Contributor
timestamp: April 10, 2023, 12:17:24 pm
content: Quote from: abdulbadii on April 10, 2023, 05:45:34 amIf you are melting soldiers, you are surely doing something wrong!

author: james_s
author_rank: Super Contributor
timestamp: April 11, 2023, 11:35:34 pm
content: Quote from: wasedadoc on April 07, 2023, 01:33:14 pmI 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.

author: nedrysoft
author_rank: Contributor
timestamp: April 12, 2023, 09:58:30 am
content: Quote from: james_s on April 11, 2023, 11:35:34 pmIt’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.