Hi,
I want to pull a couple MN chips from old boards. Do they need any special care?
TIA,
Dan
Treat em' like any other no-longer-in-production chip that costs $10-$15 a pop! :evil: No, like any other IC, heat damage is your enemy. Use a good iron with a good, clean tip. Clean the solder side of the board real good. Cleaner means faster heat transfer, less time dwelling on the pins. Use a desoldering pump. If you can get heatsinks on the chip side of the pins that helps. Do one pin at a time and let it cool for a couple of minutes, then on to the next pin. All the usual anti-static precautions. I do this with every IC I want to work again.
I like to coat anything that needs quick soldering with a little liquid flux first. It makes the whole job MUCH easier.
(http://www.action-electronics.com/grc/gc104202.jpg)
This stuff is liquid gold. I always use it on every crimp connector that I solder (that's every one that I crimp). This stuff is also amazing for soldering braid that has a slight oxide coating on it.
Good luck,
-Peter
Buying heat sinks for chips is difficult, right? So how do you arrange to assure that a chip you are desoldering stays cool? What I do is to take a square of bathroom tissue (toilet paper) fold it up small, like a Chiclet, and wet it. Now apply it to the top surface of the chip before you flip the board over for desoldering. The damp paper will sink off a fair amount of heat (and even provide "audio" alarm if the heat build up is enough to boil the water and make steam).
I haven't tried liquid flux (and ought to, because I have a bottle of the stuff....and yes it IS liquid gold to me too). What I generally do is apply some solder to the pins first before I attempt to remove any solder. Older solder joints may often be corroded or otherwise dirty, which impedes heat transfer. The longer you have to keep heating the joint continuously to get the solder out, the greater the likelihood of chip damage. So, I apply a bit of solder to reflow the joint, let it cool off properly, and when I reflow the joint again and apply the solder-sucker, it all comes off easily. This is *especially* helpful if the board is double-sided and the chip is soldered on both sides of the pad.
A few other cautions:
1) Wait a little while in between doing consecutive pins. The heat is transferred not only to the solder but up the pin into the chip itself. You don't want the heat left over from the last pin to sum with heat generated from the current pin. If I'm trying to remove a number of chips from an old board, I'll do 2 non-adjacent pins on one side of a chip (e.g., 1 and 3 on an 8-pin DIP), 2 non-adjacent pins on another chip, and when I return to the first chip, do 2 nonadjacent pins on the *opposite* side of the chip (e.g., 5 and 7). I've never been professional enough to spring for cans of compressed air for instant cooling, but you may want to yourself.
2) Solder has a way of creating hard-to-see "webs" between the pin and pad. Struggling to remove a chip when there are such webs can often lead to pin breakage, which will render a chip just as unusable as if it were damaged by heat or static. So, what I do before removal from the component side is to grasp the end of each pin with some fine-tipped needle-nose pliers, and just give each pin a "wiggle" to break any solder webs present.
a beautiful idea.. heat gun adn the liquid flux.. never lost a chip yet
Thanks very much guys! For some reason I needed a little hand holding on this operation. The wet TP to keep the pins cool on the parts side of the PCB really added to my comfort level.
Dan
use a wad of paper for thermal transfer enhancer around and on the chip
put a stock of metal on that...[try to find something with a fair amount of thermal mass for this use]...tape or secure it with a rubberband or [___], put the whole thing in the freezer. desolder your chip using all the above recommendations [probably to a lesser degree]
If extreme cold is a concern, use the refrigerator, and screw frozen nuts instead on your threaded metal rod.
What degree of cold can these chips typically withstand in non-use conditions? I don't think a freezer would harm yer typical cct....am I wrong?
OT / [Whoops there goes another one] /The condensation should be allowed to dry from any cct board that has gotten condensation on it before powering up....try explaining that to the point of registering in a non-novice. !!
haha...
Dan, next time you use these chips, socket them!!! BBD's are expensive and hard to come by, you'll be glad you did.