iisakkil@vipunen.hut.fi (Mika R Iisakkila) (05/29/91)
eric@zen.maths.uts.edu.au (Eric Lindsay) writes: > Does anyone have any hints and tips on > hand soldering stuff that small? (I made up a couple of ...little... > boards this weekend, and holding down chip resistors with a toothpick > while you solder them with a 40 watt iron didn't impress me one little > bit.) After spending 4 months building prototypes and repairing existing gadgets that were almost entirely SMD, I think I can give some advice on hand soldering :-) First of all, you'll need the thinnest solder wire you can find (you have probably noticed that already...). I found 0.5mm wire to be thin enough for soldering 44-pin PLCC-cased microprocessors and similar beasts. A small soldering iron with a needle-like tip is a must too. When you try to solder in a SMD resistor, transistor or some other small component, melt some solder on one pad on the PCB first (without the component). Then, hold the component at place with pincers and just warm the solder bubble until the component drops at place. After this, it is easy to solder the other contacts on the component. Try not to roast the component itself, SMD resistors easily crack and start malfunctioning in normal thermal stress during use. Larger components, like ICs, are a bit harder but they can be soldered with the same technique. Solder two pins at the opposite corners of the IC first to hold it at place while working on the other pins. It is very easy to make almost invisible short circuits while hand soldering (the machines make even more of them...). PLCC cases (with pins that bend underneath the case) can suck the offending solder bridge so far that it's impossible to find it without an ohmmeter. A clocksmith lupe (or better yet, a stereo microscope) is invaluable while debugging. Then, after you have found the short circuit, you'll notice that it's difficult to fix it while the component is in place (although solder removing wick or-whatever-it-is-in-English may help). Small components (less than 4 pins) can be removed by heating them directly with a soldering iron and _carefully_ pushing them with a small screwdriver. Excessive force will break the PCB because the board and the miniscule copper traces don't expand evenly with the heat. If you break the PCB, you're in big trouble. The components are also easily destroyed by the heat, so it's better not to reuse them unless you're short of cash. Larger components are impossible to remove with a soldering iron without ruining the PCB. The only way (that I know of) is to turn the board upside down and heat the pins of the bloody chip with a hot air blower until the chip falls off (don't push it, just shake the board gently). It's much easier than it sounds. The chips will survive the handling too, if you have an accurate blower and torch just the pins of the chip. Most of the SMD components are actually amazingly tough (they must be, to live through the IR oven in automatic soldering). I could tell you something on the availability of components in Finland too, but I guess that wouldn't be very useful to you... Have a steady hand - regards, Mika Iisakkila
adamd@rhi.hi.is (Adam David) (05/31/91)
In <IISAKKIL.91May28190249@vipunen.hut.fi> iisakkil@vipunen.hut.fi (Mika R Iisakkila) writes: >Larger components are impossible to remove with a soldering iron >without ruining the PCB. The only way (that I know of) is to turn the >board upside down and heat the pins of the bloody chip with a hot air >blower until the chip falls off There is another way that works for flat-pack ICs, though not for the PLCC types where the pins are bent under the chip. The only tools required are a watchmaker's screwdriver (really small flat one) and a small soldering iron. This method worked for me using a 15 Watt iron with the usual 45 degree bit. Work on each pin individually, heating the pad and not the pin itself. Wiggle the pin gently side to side with the screwdriver until it moves, remove the heat and keep the pin moving until the solder has cooled. Repeat the process with the rest of the pins, taking care not to heat the pins (pads) which have already been loosened. This takes around 10 to 15 minutes for a 100-pin chip. The screwdriver is made of black iron so the solder doesn't stick to it. The solder melts on each pad long before the board gets damaged by excess heat which would destroy the bonding between the copper and the board. Once three of the four sides are loose, it is possible to lift the chip very slightly on the side farthest from where it is still attached to the board. This makes the last side a little easier because when the last pin has been desoldered the chip just settles under gravity which helps it to seperate from the board. -- Adam David. (adamd@rhi.hi.is)
przemek@rrdstrad.nist.gov (Przemek Klosowski) (06/01/91)
Hello! I used razor blades/scalpels for things like that. I suppose it would also work for PLCC with leads tucked underneath the package. The razors are usually coated with some plastic, so the solder does not stick to them. I would put it across all pins on one side (side view) --------+ H <- soldering iron Flat +-- U // Pack | \ U // <- blade, at smaller angle than I could draw here --------| \---- // and then run the iron across the leads. Each time the blade will slide a little further under the pins. The heat transfer will be small, since per each pin it will average over time. -- przemek klosowski (przemek@ndcvx.cc.nd.edu) Physics Department University of Notre Dame IN 46556
look@s30.csrd.uiuc.edu (Stephen Look) (06/01/91)
We use a hot air gun that originally was used for plastics welding. Just
circle the device a few times while gently lifting a corner with a Xacto
knife and within a few seconds the chip will pop off. This causes no damage
to chip or board. This works extremely well with leadless smd parts.
--
Steve Look ka9szw Center for Supercomputing R & D
look@s30.csrd.uiuc.edu 305 Talbot Lab 104 S. Wright
(217) 244-5980 Urbana, IL 61801
"No, we don't have any CRAYs here, they are down the street..."