You probably broke the through-hole plating in one or more of the holes. I usually use machined pin IC sockets so I can solder them on both sides to make sure the top and bottom layer connect.
it didnt occur to me that you had to solder the top part too. in the end i removed the cheap socket i had and soldered on both sides the chip directly on the board
Bull! I've replaced thousands of chips on Plated Through Hole PCBs in the last 20 years and only had a couple damaged PTH. I used the cheap Radio Shack desoldering iron on most of them, and sometimes a little solder wick with Kester RMA liquid flux to clean leads that were soldered to heavy traces on the top of the board. Its an acquired skill that takes time and practice on junk boards. BTW, I may still have a couple new VIC 20 boards in storage. I have a few of the ROM chips, and probably the video chips. I repaired hundreds of Commodore computers in the '80s and '90s including the VIC-20, followed by the C64s and C128s. I repaired over 500 Commodore computers and only scrapped a half dozen boards that weren't worth fixing. five were damaged by the owner trying to repair or modify the computer, and the other board was delaminating.
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Link to my "Computers for disabled Veterans" project website deleted
after threats were telephoned to my church.
Michael A. Terrell
Central Florida
That's fine for you, Michael - you and I have been soldering for decades and have refined our technique and tool selection in that time. From the O/P's question I gauge that he doesn't have anything like our experience.
I've never managed to do well at this, so I've given up and clip the leads so I can remove one leg at a time. Most of what I work on has been HP boards, and the holes are so tight that I can't get a leg to come completely loose by any amount of solder sucking. I have to pull them while hot.
I find that different manufacturers use widely varying hole sizes for their PTHs. That would make a big difference.
Does your desoldering iron allow you to heat up the whole chip at one time? I've often thought that I should make some modified tips for my Weller WTCP iron so I could do this. If I had that option, I'm sure it would be easy to remove complete ICs.
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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA
Jim, I remove all the solder, one pin at a time. After the pin cools, I put the side of the tip to the edge of the pin so the remaining solder will let go without heating the PTH and pull the pin to the center of the hole, where I let go an let it cool. Sometimes I have to take a small pick and press gently against the pin on the top side to free it from the wall of the PTH. if I have trouble pulling all the solder out of a hole, I add fresh 63/37 solder to the 80/20 that was used for wave soldering. It now has a lower melting point, and will come out easier.
If the pin is soldered to a large trace on top I either use a soldering iron to heat the top side and the desoldering iron to pull the solder out of the bottom, or I use solder wick on the top side first and finish from the bottom with the desoldering iron. Its a judgment call that you soon learn which way to go.
One tip: Use liquid RMA flux with the solder wick to reduce heat damage, and leave about 1/16" of the wick filled with solder when you clip off the used portion. The solid part allows you to transfer the heat to the pin, rather than the board so it flows into the braid a lot quicker.
Another thing, use a board holder to position the PTHs horizontally so you can get the solder out easier. When you do it from the top you don't always get it on the first try because gravity is trying to pull it back down. That allows an air leak and that's the end of the proper solder flow. A little practice on scrapped double sided (or more layers) will give you a feel for the way to do it.
I am trying to scrape up the money to buy a Fuji Finepix S5100 camera that does excellent close-up shots. If I do get it I will try to take a series of pictures and create a tutorial on CDROMs for those who are interested. I have plenty of boards to take example shots of, a decent workbench and good lighting, but none of my digital cameras will take a decent close-up shot.
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Link to my "Computers for disabled Veterans" project website deleted
after threats were telephoned to my church.
Michael A. Terrell
Central Florida
I find that the HP holes are so tight that I can't center the pin in both the top and bottom of the hole. I've tried a wooden tool, and often use toothpicks to clear the holes once the pins are out.
I do this too, but I didn't realize that the wave solder was non-eutectic. Why do they use the higher melting point solder?
I've never had much luck with solder wick and PTHs, but I've never had RMA flux. Where can one buy it?
That's another trick I never tried.
Thanks for the careful explanation.
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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA
80/20 solder is used because it goes to the solid state faster when it cools, and it reduces cold solder joints form differential expansion of the PC board, the solder and the component bodies and leads. The board temperature is slightly below the melting temp of the solder, so it sinks a little of the heat to help with the cooling as well. Sample boards are sent through the oven with one or more thermocouples attached to calibrate the temperature profile for that board.
Most large distributors carry it. "Rosin, Mildly Activated" which is usually just called RMA. Ersin, Kester, and most other brands of solder make liquid flux. Don't bother with the GC liquid flux. It is almost useless unless for this application. I have used Kester 197 RMA and
1544 Fully Activate Flux for more difficult jobs were the old solder was corroded, or the parts were old and hard to solder. The best way to buy it is by the quart for a small shop, and you should buy a quart of the thinner as well.
The RMA flux is a big help when you have to clean up old solder with cracks. I use a small plastic bottle with a hypodermic needle to apply the flux to a row of pins, apply a small fresh drop of solder to the tip and slide the tip from pin to pin through the flux to resolder all the leads on a chip or connector. This reflows all the leads and picks up the oxidized solder from the joints at the same time. Then wipe the tip on a barely damp sponge for the next row of pins. I have done boards with thousands of pins this way. it works with through hole or SMD, but both need the excess flux cleaned off when you finish. I found some spray cans of brake cleaner at a "Dollar Store" that are a mix of different alcohols that do a great job of removing the flux. Just do it with plenty of ventilation, and tilt the board so it runs off, but is there long enough to remove the flux.
No problem. I worked for four years in a manufacturing environment, and learned a lot of tricks to do the job right, and with the least chance of damaging the boards. It was a running joke when I took repaired boards to QC and had to show them every solder joint because they couldn't tell my hand soldering from the reflow oven.
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Link to my "Computers for disabled Veterans" project website deleted
after threats were telephoned to my church.
Michael A. Terrell
Central Florida
I'm having trouble understanding this part. Since the 80/20 solder solidifies at a higher temp than the 63/37, this means that the board with components must cool down over a larger delta T. Wouldn't this make any differential expansion problems worse?
And why would they heat the board to less than the melting temp? How do you get the solder to flow down a PTH that is at a temp below where the solder solidifies?
Thanks again,
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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA
It's a case of how experienced and capable you are in balancing the heat application and solder removal techniques. Newbies tend to make a mess of quite a few boards before the term "deft" would apply.
Furthermore, unless it is a particularly costly or NLA chip, I'd invariably sacrifice the chip to protect the board.
There is only a small difference in temperature, and the amount of solder that hits the bottom of the board in a wave solder machine is considerable, so the board's temperature is above the melting point as the excess runs off on the trailing edge of the wave. This limits the heat damage to the circuit board, but the faster cooling 80/20 solder doesn't give the components much time to move before it solidifies.
Have you ever seen a full blown wave solder machine? They can have several hundred pounds of molten solder ready to solder board after board as they move through the machine on the conveyor system. The boards touch each other all the way through the machine, and the solder wave is a continuos flow of liquid solder against the bottom of the boards.
There are cooling fans after the solder operation to reduce the temperature as the board moves towards the exit, but it takes a while for them to be cool enough to handle. I had a several boards that were sent through a wave solder machine that had a gap between boards and the top of the boards were covered with solder. They were frequency display boards for the Drake UV-3 radio, and they were setting the machine up for the run. Some of the boards had over a a pound of excess solder on them. One wasn't quite that bad, and I managed to clean it up with a vacuum desoldering iron and a regular solder iron and make it work. It wasn't really worth the time, I just did it for the challenge.
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Link to my "Computers for disabled Veterans" project website deleted
after threats were telephoned to my church.
Michael A. Terrell
Central Florida
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