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ever ruin the PCB when changing ICs?

Discussion in 'Electronic Repair' started by komodore comrade, Jul 30, 2005.

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  1. hi people,

    i was replacing some 2114 RAM ICs on my commodore vic, (namely position
    UE5/6 and immediately over UD5/6 , on photo
    http://phantom.sannata.ru/museum/img/commodore_vic20/vic20_6.jpg )

    i soldered in 18 leg sockets to make future IC swaps easier.

    my problem is that something is wrong with one socket (UD6). if i put
    an old or new 2114 IC in it the VIC wont boot.

    if i leave it empty the vic will partially boot.


    i soldered in a new socket but did not help. what sort of solder work
    defects should i look out for in diagnosing whats wrong?

    thanks,
    Konrad
     
  2. James Sweet

    James Sweet Guest

    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.
     
  3. You probably broke the through-hole plating in one or more of the
    holes. I
    thanks james.

    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

    its working great now.

    thanks again
     
  4. budgie

    budgie Guest

    I presume you removed the original chip from the board one leg at a time? If
    you don't, damage is fairly much inevitable.
     

  5. 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.

    --
    Link to my "Computers for disabled Veterans" project website deleted
    after threats were telephoned to my church.

    Michael A. Terrell
    Central Florida
     
  6. budgie

    budgie Guest

    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.
     
  7. Jim Adney

    Jim Adney Guest

    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.

    -
     

  8. 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.

    --
    Link to my "Computers for disabled Veterans" project website deleted
    after threats were telephoned to my church.

    Michael A. Terrell
    Central Florida
     
  9. Jim Adney

    Jim Adney Guest

    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.

    -
     

  10. 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.


    --
    Link to my "Computers for disabled Veterans" project website deleted
    after threats were telephoned to my church.

    Michael A. Terrell
    Central Florida
     
  11. Jim Adney

    Jim Adney Guest

    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,

    -
     
  12. James Sweet

    James Sweet Guest


    I never do that, unless I'm absolutely positive that the chip is bad. I've
    had good luck just removing the solder and pulling the chip.
     
  13. budgie

    budgie Guest

    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.
     

  14. 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.

    --
    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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