SMD: Surface Mount Device Hand Soldering / Rework

• Personal Experiences
• Tools and supplies

Hand soldering suface mount components is done in an amazing variety of ways, and you will hear advice that seems to completely contradict itself.

Either:

• use the correct solder temperature. 63/37 solder melts at 361degrees farenheit.
   
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• You'll use a slightly higher temperature. Some say that 400F is perfect.
   
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• Others say crank the heat all the way up and leave it there.

Either:

• apply solder to the PCB and no solder when actually soldering down the chip. Don't try to solder high-density QFPs one lead at a time, lay a bead of flux down across the solder pads and apply the iron to fix the part. Check to make sure all the pins are actual in contact as differences in the height of solder originally on the pad can cause leads to float just above the pad. Use a thin, non-thermally conductive tool (sharpened stick) to push the pin down while applying head with the iron to correct this problem.
   
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• apply tons of solder. Don't worry about bridging yet. After you are done, use de-soldering tape to remove excess solder and the bridges. The de-soldering tape will not pull out solder between the pins and the PCB. Be very careful to ensure that all the bridges are removed. They can hide under the chip (especially when the traces extend under the chip and/or the entire area is fluxed) and can be so fine between the pins that they are invisible to the naked eye

In the end, the only thing to do is read up on what others have done, then try it out, and see what works for you.

Personal Experiences

According to Karl Lunt:

Several of us in the Seattle Robotics Society have been working with the 68hc12 variants, which use the same lead pitch as these chips. So far, soldering them by hand has been no serious problem, so I thought I'd pass along some observations...

You can do this either one of two ways. The first way requires a needle tip on your soldering iron, with the temperature control set to somewhere around 650 degrees. You will also need SMT solder, which is a low melting point solder in VERY fine diameter (as a guess, I'd say about 26 gauge or so).

Start by laying down a thin layer of solder flux on the pads. Place the chip on the pads (watch the alignment!) and gently press it in place. The flux should have just enough "stick" to it to hold the chip. Carefully solder one (and only one!) pin on one corner, then verify the chip's alignment. Now solder one (and only one!) pin on the opposite corner, then verify alignment again. If all is well, carefully solder each pin in order, working your way around the device.

NOTE: DO NOT apply solder to the pin! Use the iron to heat the pad, then flow the barest amount of solder from the pad under the pin. Watch for solder bridges; if you get any, use a very fine solder wick to remove the bridge.

Using this technique, I have soldered probably half a dozen 68hc12s with no failures. I have made a few poor connections that had to be reheated, however. Be sure to buzz out your work by probing with an ohmeter between the chip's pin and the associated pad.

The second method takes more confidence than skill, and is favored by several people in the club. Use a large tip soldering iron, say about 1/8th inch wide or more. Stick the chip onto the layout using the flux technique given above. Tack one lead in place at each of two opposite corners, to hold the chip in position. Coat a fair amount of solder onto the iron's tip then, in one smooth stroke, paint all the leads on one edge by wiping the iron's tip down the length of the chip. Repeat for all four edges.

Our surface-mount tech at work watched a trained assembler paint four 68hc12s onto PCBs in the space of a minute or two. He was stunned at how smooth the job went and what a beautiful job she did. I inspected the boards, and they looked nearly as good as those done by machine, and far better than mine (I use method one above). One of the guys in the club uses a wooden-handled soldering iron from the fifties, with a tip that looks like you could use it for body and fender work, but he does all of his high-pitch chips with it and does beautiful work.

Bottom line: Don't let the high-pitch scare you. You might want to practice on some junk PCBs to get started, but this type of solder work is well within the abilities of most hobbyists.

Mark D.Braunstein

I work with surface mount components and place them by hand all the time. For higher pin count parts, I would recommend that you use some kind of magnifier to better see what you're doing. A stereo microscope is really the best tool for the job but it's a bit out of reach for most people.

To hand solder parts, I use small diameter solder (0.015-0.020"), a small-tip iron, flux from a needle dispenser, solder wick, and tweezers. I wouldn't recommend paste unless you're using a hot air reflow tool. A standard soldering iron will cause the paste to form balls which roll around and could possibly end up underneath the part, shorting things out.

For boards that have already been tinned by the factory, I place a small amount of solder on one pad. Using the tweezers, pick up the part and place it on the board footprint. Check the alignment under the magnifier, then heat the one pad. Recheck the alignment. If the pins aren't lined up on all sides, reheat the pad and reposition the part. When everything is lined up, solder another pad on an opposing side and recheck the alignment. When everything is lined up, solder all of the pins. If you get too much solder on a pad, wick it off with solder wick.

William K. Borsum

I do a LOT of SMT work--including fine pitch (0.025" stuff) TSOP parts--including PICS.

Picked up an air dispenser for solder paste, tried that. Had a professional stencil made for a board--tried that. Consistent reflow was a serious problem. Quantity stuff gets farmed out to the Pro's these days. But for small lots--say ten boards or so, with maybe 50 components in a mix of 1206/0805/SOIC/TSOP, I have finally ended up doing it all by hand. You need--repeat NEED--a couple of things. They cost $$ but will pay for themselves faster than you can ever imagine: All of this stuff should be available from Instrument Engineers in San Diego at reasonable prices--Tell Jim Hoffman that Kelly sent you.

First a pair of binocular over the head and eyeglasses magnifiers--Harbor Freight sells a wonderful set for $5! Two powers via a second flip down lens. $30 for the same thing from any of the tool houses. Don't bother with the fluorescent lights with the big lenses--to cumbersome. Put it on your head and flip down when needed. If you can't see what you are doing.......

Second: a GOOD, HOT soldering iron with a small--but not too small--"angled screwdriver" style tip. I used Edsyn's 951SX (1/2 price sale at radioshack.com $75 as of 99/09/10) with the 95 watt heater and tips--and keep it turned all the way up.

Once upon a time I watched two professional painters do the door and window trim in our lab facility. They used EIGHT INCH WIDE BRUSHES, about two inches thick! NO JOKE! They had learned to feather the edges in holding the brushes and could go to a hair fine edge. And back to the paint bucket every other window. Try that with your tiny little brush.

The moral: In soldering the object is to get HEAT into the joint as rapidly as possible to melt and flow the solder completely--then get OFF. Thus minimizing total heat flow into the part itself. You can't do this with a teeny-tiny tip--or with a tip that is just over the melting point of the solder.

I use what are commonly called bent chisel or bent screw-driver points about 60 mils wide with a 90 watt temperature controlled (But set all the way up) iron designed for maximum heat transfer to the tip. Iron or gold plated so you never need to file them or do any major cleaning, and they don't contribute base metal to the solder.

Turn the tip sideways and its only 10-20 mils thick for those narrow pads, flat on for the bigger header pins, and with the long bent edge across multiple pins. All in one.

Third: a source of hot hair in a pencil fine stream. Again, Hakko 851 or equivalent. Pricey, but will pay for itself in the first few rework jobs you do. Can't emphasize this enough. I got a set of tips, but only use the smallest round one.

Lastly: at least one Precista 111SA tweezers. This little guy has an angled tip, with two flat surfaces that are grooved to grab and hold round diodes, chip resistors, and IC's very firmly with lots of control. I think I paid $40 or more for mine. Sorry, but fingers and pliers just won't work for production assembly of really small parts.

Also, Pamona makes a neat pair of tweezers with contacts on the ends and test leads. Plug it into your multi-meter and measure voltage drop across chip resistors with one hand. About $15.

Here is my procedure for really fine pitch stuff (25 Mil TSOP's):

0. First and Formost: ALL of our boards are made from 185 DegC GETEK material--will take total immersion in liquid solder--and I have yet to blister or otherwise damage a board thermally. Well worth the extra pennies per board. I will never ever have a board made of FR4 again. Also, be sure the board and the chips are DRY--bake out at 50 degC or so overnite--or see manufacturers recommendations. Keep chips in the original paks with dry-rite. Chips absorb moisture and will literally explode with too much heat too fast.

1. Get one of the water soluble flux pens from Mouser, and use it to coat the pads on the PCB. Then using a soldering iron and water soluble flux cored solder, flow solder across the pads. Get a blob on the tip of the iron and just drag it slowly across the pads. This will coat each pad with as much solder as it will hold. The flux keeps it all from bridging across adjacent pads. Takes a bit to get the knack, but is really easy.

2. flux the pads AGAIN--and the pins of the part.

3. If you have a solder pot--dip the well fluxed leads into the solder and shake off so the leads are tinned as well. This step is optional and works on difficult parts--but can leave all the leads blobbed together if you are not careful. Again--flux is the key. Sometimes I have to use Rosin flux here if the leads are dirty or not well tinned to begin with. Clean the Rosin flux off the parts before proceeding! Use chloroform, or if you can find it, a rosin saponifier. Mouser used to sell the stuff, but stopped with the last catalog.

4. Solder opposite corners of the part to the board to ensure alignment in the next step. flux again.

5. Using a pencil hot air system (mine is a HAKKO 815--about $700 from Fry's) reflow the solder on each side of the chip--2-5 seconds is enough. A slight pressure on the top of the part will let it seat down hard on the PCB. I have also used the bottom of a clothes iron, small electric frying pan or 700-watt lamp with heat reflector. Object is to get the pins and solder hot and melted together as quickly as possible and get off. Be sure to let the Hakko warm up for a few minutes to get maximum heat.

{an aside: I believe based on years of practical hands on experience that the hotter the heat source, the faster you get on and off the joint, and the lower the total heat transfer to the component--and the lower the probability of damage. Keeping an iron set just above the melting point of the solder is asking for thermal damage. Also, use the lowest melting point solder possible. i use a Kester Sn63Pb37 organic core solder. If you can get it with 2% silver, the liquidus drops another 5 degrees or so--but I've only seen it in bars}

6. Using a combination of Simple Green and VERY hot water, clean the board. Dry with compressed DRY air. Wash again with denatured alcohol or anhydrous rubbing alcohol (not the cheap stuff at the drug store), and again blow off. Bake dry at 50 degC or so. If you are near Mexico, get Cano Puro (99 % grain alcohol), and you can drink the stuff too--diluted)

7. Result is an extraordinarily professional looking PCB. Leaving flux on the board leads to corrosion and LEAKS. I've seen power consumption on a data logger drop from 35 to 10 uA just by cleaning and drying the board. I have done 100+ 16C57 chips an hour using this technique and a little planning.

Larger parts--like SOIC's with pads on 50 mil spacing:

Put a dab of solder on the #-1 pad for each chip. (this also provides a visual clue on chip orientation--dot on chip goes to solder blob) Using a pair of tweezers and a solder iron, locate the chip and slide pin one into the molten pool of solder to hold it in place. flux all the pins and pads Lay a piece of solder (again, I use the smallest diameter multi-core I can get--about 25 mil) across the ENDS of the pins so it is touch both the pins and the pads along one side of the chip--the side opposite pin 1. Touch the solder with a hot iron at the end of the solder. It will melt onto the pad and pin. Without moving the solder, do the next pin, and the next, till the side is done. Each time you touch, the end of the solder will be perfectly placed for the next joint. Repeat on the other side. And you are done. Clean as above.

Two pin and three pin parts--as above. Blob of solder on a pad--melt the solder and slide the correct pin into the solder till the part is located, then get the heat off. Solder the other pins.

If you have EVER tried to remove through hole parts, you know what a pain it is. I can get SMT resistors and multi-pin IC's on and OFF a board in seconds with NO damage to the board or part. Won't use through-hole passives or actives any may unless there is duress or SMT parts just aren't available. (pin headers are an exception--where the mechanical support is required for other purposes)

Jinx says:

One or two components are OK but a few dozen really tests your concentration and we all have those little involuntary twitches. Try this, worked for me. Use a piece of metal to rest the barrel of the iron on, sort of like a snooker cue rest. It doesn't have to have a notch cut in it of course, and make it smooth but not polished (slippery). I use a piece of 1/2" angle about 1" over the PCB like a bridge. Really does keep the tip steady and you can slide it in in a straight line at a controlled speed or lower it onto the pin.

Steve Wiseman says {ed: When trying to hand solder a part with pads which are under the part, and a PCB wih a pad layout that exactly matches the part; where the iron can't touch the pad since it is under the part}

If you can't extend the pads because the board's already made, my technique for things like these is to hover the part a small distance above the board. Solderblob one pad on the board, and the corresponding pad on the part. Tweezer the part into place, holding it level, but with the solderblobs touching. Melt the blobs, solder will flow, and the part will be supported by one pad. Be gentle at this point.

You then have gaps between the board and the part, into which you can melt solder. These parts are so light that they're unlikely to shake free from the slightly weaker solder joints. I've done this a lot recently, having laid out a bunch of boards for 2.5x3mm crystals, with no spare pad area. They'll be fine for reflow, but manual soldering requires this hovering tactic. Full success, and not terribly slow. It's obvious when it's not worked, and gives you room to get solderwick in if necessary.

Tools and Supplies

flux:

DONT TRY TO SKIP USING flux! Apply thick types with a hypodermic syringe and a thick needle or thinner stuff with a brush.  Mouser sells a flux pen. The secret to making this work is to use lots of flux. It's what keep the solder preferentially adhering to your PCB pads and IC pins instead of to each other. With thicker pastes, the boiling action can cause small balls of excess solder to fly off or bounce into other areas possibly causing shorts.

Solder:

Alpha Metals. There were three types of solder and two types of flux that were used most. The solders are classified by the types of flux contained in them. The are SMT Core, Cleanline 7000 (both no-clean), and Pure Core (core flux contains a type of acid - must be cleaned after use). The two types of liquid solder flux are 855 (used with the pure core solder - residue must be removed), and NR200 (can be cleaned or not, depending on application).

Magnifying glass:

Very bright lamp or lighted magnifying lamp. A 10x Hastings triplett is probably the most useful magnifier; 20x is pretty handy as well. If you've got lots of money, buy a Mantis.

A pair of binocular over the head and eyeglasses magnifiers--Harbor Freight sells a wonderful set for $5! Two powers via a second flip down lens. $30 for the same thing from any of the tool houses. Don't bother with the fluorescent lights with the big lenses--to cumbersome. Put it on your head and flip down when needed. If you can't see what you are doing...

Holders:

Precista 111SA tweezers. This little guy has an angled tip, with two flat surfaces that are grooved to grab and hold round diodes, chip resistors, and IC's very firmly with lots of control. May be about $40...

Dental Pic. Your local industrial liquidator is a good source.

Pamona makes a neat pair of tweezers with contacts on the ends and test leads. Plug it into your multi-meter and measure voltage drop across chip resistors with one hand. About $15.

Soldering:

Metcal SP200 soldering iron, which is a cheaper unit (around $300) and an SMT "hoof" tip ($25 ftp://ftp.metcal.com/pdfs/minihoof.pdf). (see www.metcal.com for an apps note with the "drag solder" style technique).

EDSYN model 951SX soldering stations (on sale at 1/2 price of $75 at radioshack.com as of 99/09/10).

Hakko 851 hot air pencil ($700 at Fry's) comes highly recommended.

Check out Rob Severson's UNDER $20 Hot Air Pencil! Almost as good as a pro-reflow pen and it sure beats a toaster oven!

Check out the MARVY EMBOSSING HEAT TOOL by Uchida It is a self contained hot air tool for crafters. It works great for surface mount device, as well as for flash remelting toner to remove pinholes. The best part is the amazing low cost: about $18! Can't build one for that.

Preheat:

Preheat the board to about 100 C (212 F) with an electric pancake griddle, keeping everything except the area being worked on covered with cardboard to avoid burns. (Commercial hot air soldering equipment uses preheat from 100 to 150 C AFIK) The other reason for preheat is to drastically reduce the amount of time hot air must be applied to complete the soldering operation, significantly reducing thermal stresses.

+See also:

• https://www.youtube.com/watch?v=5uiroWBkdFY Very nice demonstration of hand SMT soldering.

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