Anyone any experience removing / replacing surface mount ic's using limited equipment? I potentially have four surface mount SOIC 74LS244 octal buffer drivers to change on a duff memory board. I've experimented removing a 40 pin SOIC PLA from an old serial interface using a paint stripper hot air gun, and that came off ok but whether the IC would survive I don't know - it distorted some nearby option jumpers!
AWEM
You can improve the focus of the hot air, by screening off the rest of the board with a few layers of tin foil with a small air gap between them.
If you don't need to save the device being removed, then cut down the side of its legs with a scalpel, remove the device sans legs, then flick off each leg with a normal iron.
The other option would be the Chip Quik desolder alloy:
Basically its a non eutectic solder alloy with a long "open" (i.e. liquid) time. You use a normal iron to reflow some of the alloy all over the existing connections, and it give you enough time to lift or move the chip before it cools and refreezes.
Nice demo:
Other related stuff discussed here:
On Apr 24, 1:36 pm, "Andrew Mawson" wrote:
Yes, I've tried lots of different ways. Do you want to re-use the devices that you remove? If not, then one method is to cut off the legs with a scalpel blade, taking great care not to put too much stress on the cut off legs as this can easily rip the pads off the pcb. Then remove the cut off legs by wiping the soldering iron bit along the row of legs and clean up the board. Another way is to lift each pin separately, starting at one end of a row. Use a sewing needle, (preferably with a handle attached) to lever the pins up while keeping the solder in the area molten. It is often useful to add lots of extra solder (with flux), then remove it and add some more. If this is done with tin/lead solder it replaces the old solder with new, less oxidised, lower melting point solder which is easier to work with. The bent leads can be reshaped and the component reused. Sometimes it is possible to lift one side of an ic at a time. Pick a suitable tool that hooks under one side of the ic and then flood the pins on that side with solder. Move the soldering iron backwards and forwards until the solder along the whole row is molten. Then lever that side of the ic up a little and slide some paper or Al foil under the legs to break the molten solder bridges between the legs and the pads. Then heat up the other side and lift the ic off. The slightly bent legs can be reshaped for reuse. Another technique is to make a rectangle of very thick copper wire (such as can be stripped from 2.5mm^2 mains cable) which sits over the pins. Flood the wire rectangle and the pins with solder. Then heat the whole lot up with a large soldering iron or hot air gun. The copper wire rectangle helps to keep all the joints at about the same temperature. The biggest risk with all the "simultaneous" removal methods is for most of the joints to melt leaving one or two pads still soldered down. Moving the IC then rips off the remaining pads.
Good magnification and bright light is essential. If you need to protect small components like SM capacitors from being blown away by a hot air gun, mask the area around the chip with adhesive kapton tape which resists soldering temperatures. Otherwise, Al foil might be suitable as a mask.
If you plan to reuse them, ensure the components are as dry as possible, perhaps by baking the board overnight before starting. Otherwise the steam generated from water absorbed in the plastic encapsulation might crack the package. (This has never happened to me, but manufacturers recommend baking new components before use if they have been exposed to moist air before reflow soldering.)
Use a larger soldering iron than you would for fine soldering. The main objective is to get as much heat as possible into the job without localised overheating. For this a large bit is much better than a small one.
John
In article , Andrew Mawson writes
Keep it simple, you don't have to use hot air.
You can cut the leads on the scrap devices to release the body then clean off the remains of the leads with a swipe of the soldering iron. For SOIC you probably want to use flush cutters (even a crappy 3quid pair from cpc). Do not overstress the pads by attempting to lever the IC body out of the way.
Clean the pads with solder braid ready for the replacement devices.
To replace, add a small (tiny) bit of solder to 2 corner pads then tack the corner pins of the device to these pads to hold the device while you solder the remaining pins.
Solder each pin in turn and remove any surplus (inc shorts) with solder braid. A wipe of flux on the pads before hand from a flux pen will make things flow a bit better.
All bits avail from CPC.
Or a bit of sticky tape to hold the chip in place. Tacking down the coreners can be fun until the chip is held... B-)
Before palcing the chip clean the pads and flux well, place and fix the chip (see above) then have a small amount of solder on the iron well fluxed and just run it along the row of pins. It'll flow and surface tension (provided you haven't too much solder on the iron) will prevent shorts forming.
YouTude videos are available...
Thanks to all respondents - some excellent advice there that I will act on!
(This is on a 'rare as hens teeth' memory board on a big 1993 CNC lathe)
AWEM
Well strangely, some pc shops that do repairs now operate at the chip level so many must have the gear to desolder and apply these chips, Maybe one could do your job.
Brian
That's what the solder balls on 2 of the corner pads are for (snipped), you hold the chip down and reflow the ball to hold the corner pin, rinse & repeat on the opposite corner and you have the device stable for the rest of the operation.
It works as you say (I forgot to mention the pad cleaning) but the trick is not to panic if you do get shorts, they will clean up with braid (juiced with flux before mopping). With practice it works all the way down to 0.5mm.
Indeed, what did we do before them :-)
Not for SOICs.
MBQ
A sharp scalpel is the best tool for this.
MBQ
That's certainly what I'd use for thin finer pitch leads but it would need a bit of care for SOIC I think, multiple passes and not to much pressure or you'll end up with a scalpel tip in your eye. Xacto modelling knife blade or carefully used stanley knife might be better.
Well, I do it frequently and all it takes is a single pass pressing down on a single lead at a time. The lead frame is quite soft material.
MBQ
Useful to know, ta.
The scalpel tip story comes from a former boss whose reckless use of same caused him to wear an eye patch for a fortnight, my how we laughed. Needless to say, those with a clue had never got close to breaking a blade.
at
The only think printed is the marking print and that's not too important.
It's highly unlikely that you will cause the copper to de-laminate using a scalpel to cut SOIC leads, unless you are particularly ham fisted or the board has been damaged by previous attempts applying too much heat.
MBQ
...
Some of the large SOIC packages do appear to be moisture sensitive. The small ones are not.
John
If one is trying to remove the IC in a destructive fashion, would a Dremel-type rotary tool cut through the pins (or the plastic part of the package) with less chance of damaging the copper pads? Or would the resulting debris be difficult to remove and tend to short the traces elsewhere on the board?
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