This article is
Copyright 'The Guru' 2005. The may not copy any portion of this
for your own web site or printed publication without my explicit
permission.
This page briefly
explains how to remove soldered-in ROMs, mostly for the
purpose of reading them for emulation.
First of all, let's get the terminology correct. Soldered-in
ROMs are regular DIP type chips, where the legs go through
the PCB and are soldered directly into the PCB. They are not
just sitting on top of the PCB, they are physically held between
the PCB layers with solder and the legs protrude out the other
side of the PCB. These are usually the chips you curse about
when you pick up a PCB and those pins stick into your finger and
draw blood ;-))
Sometimes they are only soldered on one side, but usually
they're soldered on both sides of the PCB, and most commonly
found on PCBs after 1985. The biggest problem with desoldering
them is later PCBs have the ground and +5v planes inside the
fiberglass PCB, usually having 4 or 6 layers where only 2 layers
are the circuit and the rest are for voltages. These planes are extremely large and suck the
heat away from the chip legs if you prod it with a soldering
iron, just like a heat sink does. Thus resulting in a BIG
problem when trying to remove the chip, particularly the legs
that touch these large power planes. You can heat it all day
with a soldering iron and suck away for days, you will never
remove the chip in one piece. Forget it!The
proper/professional process can be done several ways, but most
of them involve expensive desoldering equipment like these.....
OUCH! they even look expensive!
 
The method I will describe here involves very little cost and
not much equipment. But it does require some patience and skill. You could consider this to be 'The cheapskates
ROM removal tutorial #1' ;-)
The reason I'm doing this is due to several reasons. They
probably include lack of time to dump simple stuff, lack of
storage space to store old PCBs that can not be re-sold and
wanting to pass on some info to a future Guru-Apprentice, though
I have already done that I think ;-)
I've tried to be as
thorough as possible and I've added in a few Guru-tricks that
were lurking up my sleeve. This info may be updated from time to
time as I note more things that can be explained. However, this info
should get you started. If you have further questions on this,
feel free to contact me. |
|
First of all, you need to get
yourself some equipment. Most of these are simple hand tools or other homemade
bits and pieces. Note that not everything is used in this tutorial, but will be
needed later for the next one.... when you put the ROMs back and get that PCB
working again!
Check the pic......
The items above anti-clockwise from top left with approximate prices (in AUD) are....
- Hand-type solder sucker. $5
- Exacto knife, or other VERY sharp knife with break-off blade pieces. A
Stanley knife will also do. $5
- Tweezers (don't pinch your wife's/girlfriend's best tweezers, or if you
do, don't tell her I told you!). These ones have bent up ends on them and
are also useful for levering out chips from sockets. $2
- Fine sandpaper, preferably 300-400 grit. Really fine stuff like wet &
dry used in automotive spray-painting is too fine. One sheet costs about $1
- A large heatsink. This one shown came off a dead SSV main board. I knew
that trashed Eagle Shot Golf PCB RB sent me would come in handy for something
;-))
- Wire cutters/nippers. These have pointed ends for getting into tight
places and are strong enough to cut chip legs too, even ones soldered into
PCBs. Regular/fat wire cutters are useless. $5
- Pliers. Any type will do, fine-nosed ones like these are more useful in
this application. $5
- An assortment of sockets. Machine pin types are better, like the one
shown. You can cheap out with the regular dual-wipe ones if you must. They
range from $0.20c to about $1.00 for the largest 42 pin machine pin sockets.
- A good quality temperature-controlled soldering iron. Mine is a Hakko
936. Get whatever your budget will stretch to. Try to avoid getting those
ones that plug directly into the power from a lead connected directly to the
hand-piece and have no controls over temperature. Mine was about $150 IIRC.
Your mileage may vary.
- On top of the soldering iron is the key to your success. I call it the
'Chip-ClipTM'. It's simple to
make and will be explained later. Don't tell anyone, because this is one of
Guru's top secret inventions! ~$priceless!
For the special tool, you need some metal pallet strapping (a piece about 8
inches long is enough) and some tin-snips like
these.
- A digital multi-meter. I have a Fluke 12 like this one....
but any simple meter with a continuity tester will do.
- Not pictured....
- An old towel. Again, don't anger your wife/girlfriend by taking her
best towels! ~$2
- A pair of leather garden gloves. Usually you will only use one at
once. ~$2
- A common heat gun/paint stripper/hot air gun, like the one shown on
my equipment page. ~$20
- 0.8mm or 1.0mm fine solder. Not the fence-wire type! Think
thin. ~$2
- You will also need a dead/scrap PCB with a large soldered-in chip, like
a 6116 or 62256 or Z80 or similar. Anything will do, just make sure it's a
junk board!
Let's make the chip puller first. Grab that metal strapping. Cut a piece using
the
tin-snips to 3mm to 4mm wide by 160mm long (sorry I have only metric ruler here,
if you are an inches man, convert it :-)
Use the pliers to bend a 90 degree leg about 3mm long on one end. Do the same to
the other end, so both legs face inwards. Now bend it in the middle with a
gentle bend, so both legs are the same length. See the equipment pic above for a
quick refresher. Use some sandpaper/emery cloth, or a file to remove the sharp edges.
The Chip-Clip should sit under any chip and stand there by itself. Just bend it to
suit any chip. Make sure the distance between the 2 legs is set at about half the
length of the chip you need to pull out. This is so that the Chip-Clip will apply
pressure to the chip and hold it in place when you remove it..... like this.....
Grab that junk board with a Z80 or similar large chip. Find the location of the chip from the back of the
PCB.
Now ensure all the legs
are standing upright perpendicular to the PCB. If not, just bend them up with the pliers,
or tweezers or similar. This is very important. You'll see why soon....
Ok we're ready to begin. Before you start, make sure you are in a large room, or
outside as the fumes created by this method can be quite bad and give you a
headache or make you feel sick for a short time. As long as adequate ventilation
is provided, there won't be a problem.
Sit in a chair and make sure the heat gun is handy.
Put the towel across your knees and sit the junk board in it vertically, like
this....
Ensure the chip you need to access is near the top of the board as far away as
possible from your legs.
Hold the board with your knees tightly so it can not move. The towel protects
your legs from spiky chip pins and also from heat.
Grab the heat gun and select the high temp setting. Put on one glove in the hand
that you have free.
Heat up the board from 6 inches away and heat an area about 6 inches square
around the chip, moving the heat gun back and forth across the PCB... keep it
moving! This is very important or you will over heat the board and split the PCB layers,
de-laminating it. If you do that the board is usually scrap because you will
create dozens of broken tracks whereby the tracks from the top of the PCB no
longer go through the holes and to the bottom of the PCB. In other words, that
is bad news for your PCB. The board will also 'pop' when the layers split and it
will spit solder at you,
which will scare the crap out of you, believe me ;-)
Heat the back of the PCB (the solder side), not the front. If there are plastic connectors near the chip, try to direct the heat away from
it or shield the plastic connector with something, like a piece of thick
cardboard or the unused leather glove or similar. There are more advanced ways
to shield things but we can look at that later.
If there are wires on the back of the PCB, note where they are and write it
down! They will likely fall off as you heat it up. But if you know where they
were, you just put them back after :)
Heat it for about 3-4 minutes until you see the solder start to go dull or
slightly bubble.
Put the Chip-Clip under the chip and hold it. If it doesn't fit under, just hook
it onto the side of the chip and go to
the next step.
Now move the heat gun in to 2 inches away (never ever closer!) and focus it
directly onto the chip legs, keeping the heat gun moving still. If you didn't do
before, now put the Chip-Clip under the chip as the chip loosens.
It is very important that the chip puller be placed under the chip because the
heat can weaken the chip and it will break into many pieces if you do it the
wrong way. Don't even think about using pliers to remove it. Pliers only touch
the top sides of the chip and as you try to lift it out, the top of the chip
will break off into pieces.
As you heat it, lean the PCB over slightly so the top is facing up... at about
10 degrees off the vertical. Just twist your knees slightly and it will change
position (check the pic above to see what I mean)
Keep heating and apply gentle pulling pressure on the chip puller as
you heat it. After about 30 seconds or less the chip will come away very easily.
If you have to pull hard, it means you didn't straighten all the legs or the
solder is not fully melted. If you
try to pull out the chip with bent over legs, you can pull off the top of the
chip or break off the legs which will permanently damage it, so always ensure the legs are straight underneath
the PCB. If you try to pull out the chip and the solder is not fully melted, you
can lift tracks and cause all sorts of non-humorous problems. So be careful and
observe what you are doing.
At this point, the chip should be loose, but be very
careful or you can snap the chip in half because it is very, very hot now
(approx 600 degrees C). If you
bent all the legs upright as I said, the chip will almost fall out. But you pull
it out, don't let it fall. This is why we tilt the PCB over at 10 degrees. If
you let the PCB tilt over past vertical the other way, gravity will immediately
become your enemy and every part in that 6 inch square you heated will start heading for the ground.
Then you
will be in very serious trouble (but on the bright side, you'll have a whole mess of PCB parts for your junk
parts pile ;-)
If you made the Chip-Clip properly, it will hold your (very hot!) chip for
you like in the pic above.
At this stage turn off the heat gun and put it down in a safe place (it is very
hot now). Be careful not to place it across the power cord or on something that
will melt like the carpet, because if you do, your wife/girlfriend is going to kick
your ass, believe me.
Release the chip from the Chip-Clip by flexing it outwards and put the chip (upside
down) on the heatsink to cool. Don't even think about touching the chip, it's
currently about 600 degrees C. If you need to re-position it on the heatsink,
use the pliers or tweezers. Just let it cool on the heatsink, it will only take
about 1 minute.
Grab the PCB with your gloved hand and place it somewhere to cool. Do not jerk
the PCB or bang it or flex it.
Place it down with the component side upright, gently. Remember, many of the
parts are still loose around the chip. So if you knock it, most of them will
fall off the PCB and the board will almost certainly be scrap, or you'll have a
BIG job re-attaching them and remembering where they were! This is why you
lean the PCB down slightly so you can see the top of it as you heat it up......
gravity will hold the pieces in place. Don't attempt this on the moon for
obvious reasons ;-)
If some pieces did start to move, use the soldering iron to put them back in
place after it cools, or re-heat the board and push them back into place as the
solder re-melts using the tweezers.
Use the soldering iron to clean any large blobs of solder off the chip legs
by heating the solder blobs and wiping it away with the soldering iron. Clean
the tip after each wipe so you are not putting old solder back onto the chip. Then use the pliers to straighten the legs of the chip so they are all in line
and at 90 degrees to the body of the chip. Also straighten out any kinks in the
legs so they are all flat (like a new chip would be)
Cut a piece of sandpaper about 40mm long by 20mm wide, and bend it in half so it
is 20mm square.... this gives it some thickness. Curve it slightly so only the middle will touch anything,
using one hand, by holding it between your 1st and 2nd finger with your thumb on
top. Just like this....
This is so the sandpaper doesn't catch and bend a leg as you use it.
Allow the sand paper to protrude out slightly by 5mm or so from your fingers.
Use the fine sandpaper to polish the legs on the outside and on the inside of
each leg by running the sandpaper along all the legs at the same time, along the
full length of the chip (it's faster that way).
You do this by holding the chip in your other hand and resting the legs of it on top of your
1st finger, then holding it there with your thumb. Place your 1st finger on the inside of the chip and support the
legs while you polish it. Then turn the chip over and place your 1st finger on the outside and do the same to
the inside of the legs. If you accidentally bend the legs, stop and straighten it before you continue.
After that, just read the chip 3X as you would normally do.
But when you put the chip in your reader, loosen the lever slightly (hold it
back so there is less pressure on the chip, but still some pressure) and move
the chip sideways left/right a few times so that you scratch the insides of each
leg to remove any corrosion or built-up solder flux or solder. Remember, it's
very important to ensure the chip is in good contact with the reader's socket,
and the chip has just gone through hell so it can be dirty from the removal
process.
OK, before we get to putting the chip back in place, let's look at a more
advanced scenario.....
As you can see, there are 2 large soldered-in MaskROMs, but right near them is a
whole mess of other crap.... surface mounted RAM, surface mounted Quad Flat Pack
(Seta custom IC ST-0009), plastic chip sockets, and many other components. Even
worse, underneath the PCB are 2 large multi-pin plastic connectors used to
connect this PCB to the lower main PCB right near these ROMs, which TOTALLY
prevents the removal of the chips by heating the PCB from the other side. All of
these components and connectors can be easily damaged or moved if you heat the
PCB from the top to remove the ROMs. So we have a dilemma!
One solution is to shield the components so that when we heat the PCB, only the
MaskROMs are exposed, and this is the option we will take. Check the pics
below.....
and another view of it....
Here we have covered all of the parts near the chips with cardboard and placed a
small weight on top to prevent the cardboard from blowing away. You can probably
see that I'm using old business cards for this purpose and the weights are old
EPROMs stacked on top of each other (I have a plentiful supply of both ;-)
I've bent each edge of the business cards downwards with different lengths to
accomodate the different heights of the components so they are covered
completely and the weight effectively seals it against the PCB. If the positions
are a strange shape, just cut them to fit the area on the PCB. Add as many as
are required until all of the adjacent components are covered. You can
use any type of thick/dense cardboard that doesn't burn immediately, but it's
especially rewarding with business cards if you use cards from a dodgy car
salesman or insurance salesman for additional gratification ;-))
Attach the Chip-Clip and heat up the ROMs from above....
Make sure you watch to see that the shield(s) doesn't move. If it does, stop and
re-position it before continuing, adding more weight if required. Heat the same
way as mentioned above and eventually the chip will become loose and you simply
lift it off the PCB. Make sure you're wearing the glove when you do that,
because the Chip-Clip will be very hot!
Then put it on the heatsink to cool, and read it as usual.
Here's a view of the completed task, no more MaskROMs!
As you can see, all of the other fragile components are not damaged or moved.
Well, almost......
Apart from the toasted ROM label (which I have now removed, so no big deal
there!), this shows that one of the shields had allowed the hot air to get under
it and started to melt the socket. This is extremely minor and has not weakened
or damaged the connections from the ROM to it's socket. Remember the hot air was
about 600 degrees C, and plastic melts at temperatures well below that, so even
though the socket has started to bubble slightly, the shields have done
their job in preventing permanent damage to the surrounding components :-)
Next we'll look at how to properly clean up the PCB and put the chips back.
Until then, practice removing a few chips on your junk board. If you're feeling
REALLY adventurous, try to remove a 68000 CPU :-)))))
This article is Copyright 'The Guru' 2005. The may not copy
any portion of this for your own web site or printed publication without my
explicit permission. |
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