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Soldering Surface Mount Components by Bacon, WA3WDR




 
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Author Topic: Soldering Surface Mount Components by Bacon, WA3WDR  (Read 8123 times)
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Steve - WB3HUZ
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« on: November 13, 2006, 12:47:38 AM »

OK, here's Old Bacon's Long, Old-Buzzard Transmission about surface-mount soldering.

My company has a PC board designer who solders the parts in for us too.  An interesting combination - anyway, he likes a pair of Elsyn temperature-controlled irons, and he tins a little blob of solder onto each pad, and he puts flux on them, and the component, and then he heats both ends of the component using the irons like chopsticks.  The solder and surface tension pulls the part to a nice center position easily.  AS for the soldering iron, I used to see Elsyn in Contact East, but not lately.  The less expensive 4449-TL Solomon units from MPJA ( www.mpja.com ) don't look like much, but they are just as good - only, watch their grounding!  They have a superior ground - IF YOU HOOK IT UP!!!  More on this below.

Flux is crucial; I use either 'no-clean' liquid rosin flux, or I use a relatively mild RMA (rosin mildly activated).  Either way, I clean it off with 99% isopropyl alcohol.  I use little the little pint bottles of 99% isopropyl alcohol from Safeway Supermarkets. Much cheaper than buying 99.4% anhydrous, and paying through the nose.  The no-clean flux is just about as good as the RMA flux, and it cleans off more easily.  I'm going to use up the RMA flux, though, because I got a gallon of it, and it's not that hard to clean off.

Our PC board designer / part installer uses water-soluble flux; the solder positively flashes at you, and you never run out of flux remover (tap water) - but wash it off quickly when you are done, because water-soluble flux is active when cold, like acid flux, and it rots the surface quick.  He uses a quart-sized air compressor to blow the residue out when he de-fluxes the boards, which is definitely a good idea with that kind of flux.  It would probably be a good idea with rosin flux too, but I don't bother.  I flood the parts, coax the alcohol out with a little piece of a paper towel, and I keep them warm with a heat gun as they dry.

63-37 or 62-2-36 (silver) solder are important, they flow best.  I recently learned why plumbers don't use electronic solder on plumbing - it flows too well, and drips out of the joint!  They want plasticy solder, so they used a 50-50 lead-tin mix so it would stay put.  But now they all use lead free solders, probably blended with the same idea in mind.  We, however, want free-flowing solder.

Radio Shack's little 0.15" diameter 62-2-36 electrical silver solder is great.

I use 63-37 or 62-2-36 solder, it hardly matters.  Maybe the silver is better for less leaching from capacitor leads.  Canadian Space Agency says that you should scrape off gold plating before soldering, because of leaching - but I don't, and it's no problem.

Use a temperature-controlled iron. These don't have to be real expensive.  I use a single iron (Solomon - 4449-TL at Marlon P. Jones & Assoc., Inc. www.mpja.com These irons are low-cost and reliable, and there is a tip selection, but watch their grounding!!!  I did experience some static damage because I didn't notice that they do not ground to the AC outlet ground, they have a ground terminal like a power supply.  That's probably a good idea, because those AC grounds can be awful.  But make sure you connect their terminals on the back of the base unit to ground, or you may be very sorry!  (This grounding arrangement makes for superior grounding, though.  Just pay attention to that with these babies!!!  Yeah, they only spec +/- 10 degree F control, while competitive irons will spec +/- 5 or 6 - but in the real world, you will move at least 25 to 50 degrees before you will notice any change in soldering performance.  +/- 10 degrees F is just fine.  Oh, and these irons will discolor immediately when heated.  Ignore it, and save money.  I really like these irons.

I normally use a single iron for my work, because two irons take up too much room in my crazy production environment.  But I have soldered with two, and it is a really nice way to do SMT work. I really worry about tip grounding with two irons; it is a good idea to add a ground jumper between the tips.

I like using a single iron, because mostly I do rework, and select-in-test part selection, because the designs are prototypes on nice boards with soldermask and silkscreen.  I like thick-film SMT resistors and pretty much any SMT ceramic you can find in Mouser or Digi-Key.  Some of the 10uF ceramics are too thinly laminated and they are too fragile, so use the bigger ones for a given voltage in the high-current portions in power circuits.

I remove and re-use SMT parts all the time.  Just be really gentle with them, and if they seem damaged, don't re-use them.  The ends can get ripped loose.

My boss always says "Flux! Flux!" - and he's right, a generous dose of good flux makes it MUCH nicer and easier.  I use liquid rosin flux,  MG Chemicals No-Clean Rosin, or Kester186 Rosin flux.  It doesn't seem too critical, any decent liquid rosin flux seems to work, but heavy RMA is harder to clean off after soldering.  I put it on the pads before I solder.  I tin one pad and solder it first.

I hold the parts with a wooden stick about 1/8" in diameter, with chisel points like a flat-blade screwdriver, or with narrow-tip tweezers, and solder them like W1RKW says.

Use solder-wick to remove excess solder.  If the solder doesn't like to flow into the wick, put a drop or two of flux on the wick.  Shoop!

Clean up after soldering.  I put 99% isopropyl alcohol into little bottles with #20 AWG needle applicators from Contact East, www.contacteast.com, lately they are www. Part Number 121-725.  I use these bottles for the liquid flux, too.  (Those darn brush-bottles always got left open and knocked over.  Arrggh!)  If the needle gets plugged with hardened flux, just heat it a little with the iron.

If the flux is stubborn, scrub it off with a wooden stick and a little torn off piece of a paper towel, or a Q-tip, to get the stubborn flux spots, then I shake it off the board and then flush with some more alcohol.

Always help the flux remover dry with a heat gun.  This is important, because the alcohol cools the board when it evaporates, and the cool surface collects cruddy condensation from the air.  The heat gun keeps the board above the dew point, so that doesn't happen.

Sometimes as a final wash, I use a small spritz of the horrible smelling carbon-tetrachloride/CFC type stuff; it really is a strong cleaner, but it's expensive.  Wipe it off with a paper towel or a Q-tip; otherwise the pollution will just redeposit when the solvent evaporates, and don't forget the heat gun either, so that goop in the air doesn't condense on the board as the solvent evaporates.

Get an anti-static work mat, but don't let them rip you off for it, and ground it.  Wear an anti-static wrist strap, and ground it too.  You can use gloves to keep fingerprints off the boards, and chemicals off our hands; that's nice.  I usually don't, unless I'm doing something really, really critical.

I build equipment that flies in space.  These techniques work fine. 

I don't know much about the lead-free solders yet.  I think they will be OK, they just require more heat, and they don't flow quite as well or look quite as nice.  They may work better with slightly different fluxes. What worries me with lead-free solder is the tin whiskers I am hearing about.  That stinks!!!

And now, to wrap it up: how to remove modern multi-pin ICs from PC boards.

I have removed several 68 pin ICs, and one 200 pin package IC, without damaging them, using special low-temperature solder.  Digi-Key ( www.digikey.com ),  Part Number SMD32-ND - it's really, really expensive, but it's worth it.  This stuff is not meant for use as solder, but it's great for removing this sort of part.  I think they sell this in smaller quantities; we do enough nutty jobs here that we sprung for the big tube of it.

Put flux on the leads and wick off the old solder fairly well, then put more flux on them and put a blob of this solder all over the pins.  Heat it up good, hotter than you would normally heat such things, but not red hot.  This solder melts at such a low temperature that it will stay melted for a real long time, allowing you to melt it all around the chip and finagle the chip off the board with minimum damage to all things concerned.  This was a real discovery!  The idea about the heat gun to melt the solder ought to work great with this stuff, and make it easier on everything. (Well, everything but the wallet.  This stuff is pricey!)  By the way there is such a thing as hot-air soldering tools, and you can do very neat work with them!

As I say, this stuff is really poor as solder, so wick it off good and clean up before soldering the replacement part onto the board.  It also comes in a kit with some vacuum-grippers that can help lift the chip off the board without making a big mess.

Tweezers - check out Contact East's selection. I like their SMD tweezers, especially Excelta Style 103 and Style 104, but their Erem Style 1 through 5 are fine and less expensive.

Unless you are very near-sighted, and you can just take your glasses off and work at ten inches or so, get a head mounted binocular magnifier - Contact East 407-029 or 407-030 are good.  Then you can see what you are doing, and scare burglars as well!!

Good luck!

  Bacon, WA3WDR
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W1RKW
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« Reply #1 on: November 14, 2006, 05:39:29 PM »

.....And now, to wrap it up: how to remove modern multi-pin ICs from PC boards.

I have removed several 68 pin ICs, and one 200 pin package IC, without damaging them, using special low-temperature solder.  Digi-Key ( www.digikey.com ),  Part Number SMD32-ND - it's really, really expensive, but it's worth it.  This stuff is not meant for use as solder, but it's great for removing this sort of part.  I think they sell this in smaller quantities; we do enough nutty jobs here that we sprung for the big tube of it.

Put flux on the leads and wick off the old solder fairly well, then put more flux on them and put a blob of this solder all over the pins.  Heat it up good, hotter than you would normally heat such things, but not red hot.  This solder melts at such a low temperature that it will stay melted for a real long time, allowing you to melt it all around the chip and finagle the chip off the board with minimum damage to all things concerned.  This was a real discovery!  The idea about the heat gun to melt the solder ought to work great with this stuff, and make it easier on everything. (Well, everything but the wallet.  This stuff is pricey!)  By the way there is such a thing as hot-air soldering tools, and you can do very neat work with them!

As I say, this stuff is really poor as solder, so wick it off good and clean up before soldering the replacement part onto the board.  It also comes in a kit with some vacuum-grippers that can help lift the chip off the board without making a big mess.....

  Bacon, WA3WDR


Here's a trick I've used for removing mega pin SMD IC's.  I find this doesn't heat stress the PC board as much as using desoldering wick.

With a piece of magnet wire, thread one end under the pins of the IC and anchor that end by soldering it to a near by  pad.  Hold the other end of the magnet wire and apply a small amount of tension to it and as you apply the tension to the wire heat up each pin on the IC.  The pin will pop up and you simply move down the side of the IC on each pin with the iron untill all pins are free.  Repeat for each side of the IC and the IC pops right off.  After the IC is removed simply clean up each pad with desoldering wick and you're ready to solder the next IC in place. 

A dental pick is a handy tool as well but I prefer the magnet wire method.

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Bob
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« Reply #2 on: November 14, 2006, 07:05:36 PM »

                  "   .....And now, to wrap it up: how to remove modern multi-pin ICs from PC boards."


                                                                            Dremel...
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