ever ruin the PCB when changing ICs?

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

 

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.

 

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

 

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

 

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

 

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.

-

 

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

 

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.

-

 

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

 

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,

-

 

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.

 

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.

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

Michael A. Terrell
Central Florida