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Soldering, with Profanity

Hot diggety damn! I believe I’ve successfully soldered the Max II in its cruel 100-pin package with 0.5 mm pin spacing. At least, I’ve assembled enough of the board to connect it to a JTAG programmer, program an LED blinky routine to the Max II, and confirm that it works. There could still be all kinds of pins shorted or broken, of course, but at least I know I didn’t fry it completely.

Soldering that chip wasn’t fun. It took me about two hours just for that single TQFP 100. I used the drag soldering technique, where you goob tons of solder onto the pins and create pin-to-pin solder jumpers like crazy, then go back afterwards with solder wick to clean up the jumpers. At least that’s the idea.

I had the absolute worst time trying to wick away the jumpers. No matter what I did, the extra solder on the pins wouldn’t soak into the solder wick. I set the wick on top of the pins that had jumpers, then set my iron on the wick, and pressed down onto the sandwich. The solder underneath the wick would melt, but it wouldn’t go anywhere. When I removed the iron and wick, the jumpers remained right where they’d been, with nothing at all soaked into the wick. I applied flux everywhere, over and over, but it didn’t help.

This pattern went on for ages, and I got more and more upset. I started swearing at the board, using every profanity I could think of. I had to close the windows so the neighbors wouldn’t hear. After a while, I started singing random profanities to the tune of opera while I worked on the jumpers. I burned the soldermask off several of the traces, burned the board, and re-heated the same pins so many times that I was certain nothing was left inside the chip but melted slag. All in all, it was not a good time.

Eventually I stumbled onto a few techniques that helped a little, raising my wicking success rate from 0.1% to maybe 20%.

  • Some jumpers can be cleared simply by touching the iron briefly to the pins, without any wick.
  • Don’t stretch out the wick braid. Keep the strands pressed together, like stranded wire.
  • If a stubborn bit of solder refuses to be wicked away, add more solder. A huge glob is actually easier to remove than a tiny fleck.

Assuming the other as-yet-untested pins on the Max II are OK, then assembling the rest of the board should be cake.

Read 4 comments and join the conversation 

4 Comments so far

  1. @ndy - June 27th, 2011 1:48 am

    I find that if I get it first time then it’s nice and easy but once the board it dirty it takes *forever* to get it right.
    Of course, getting it right first time is all to do with confidence.

    I tin the pads before I put the chip on. To do this I paint the footprint with flux and rub a large iron and large bar of solder across the pads. I try to do it nice and quickly to get an even distribution.
    I fiddle with things until the pads are all even and nice and flat on the top.

    I then place the chip on the pads and just heat it with hot air until the chip sinks into the pads. If the hot air isn’t to hand I carefully push the corner pins down with a large iron and then run the iron along the board so that the flat bit of the iron makes contact with the end of each pin. You need to feed in a bit of solder as you do that to ensure that there’s good thermal conductivity between the iron and the pins.

    I then touch up pins as necessary to get them soldered down and maybe wick off the excess.

    If you’re finding that the wicking isn’t working then you might have the iron at the wrong temperature.
    Adding lots of solder is a good tip: I do that too. I also place the wick on top of the pins and then press the (tinned) soldering iron down on top and then pull the wick upwards and away which pulls the iron down the row of pins. You can exert a fair amount of force in the downward and upwards directions without breaking anything.

    I tend to use the solder for two things: actually soldering the pins and creating a good thermal contact between the iron and the thing I want to heat. The latter uses far far more solder than the former.

    Good luck with the rest: it really does get easier and you learn techniques to keep your hands steady and calm whilst shouting profanities at the top of your voice.

  2. Steve - June 27th, 2011 8:15 am

    Thanks for the suggestions! I believe you’re right that the key ingredient is practice.

    The iron is set to 350, and I’m using leaded solder with Radio Shack brand desoldering braid.

    I considered tinning the pads before placing the chip, but feared it would turn my 2D alignment puzzle into a 3D one if the tinned pads weren’t all the same height. Do you find that pre-tinning the pads makes a major difference? I don’t have a hot air setup, so it’s all iron work.

  3. @ndy - June 28th, 2011 1:42 am

    If you get the pad tinning exactly correct then you shouldn’t need to add any extra solder at all. Having said that, I almost always do end up having to touch up at least a few pins, especially when I’m not using hot air.

    You don’t need enough solder that the pins sink all the way into the solder: look at some professionally wave soldered boards and it looks like there’s hardly any solder there at all. The tops of the pins are often completely exposed.
    If you’re using the iron you’ll end up with more solder if only to get good enough thermal conduct between the iron and the bit of solder directly under the pin.

    The key to getting even tinning is lots of flux, a large iron, a large bar of solder and a bit of speed.

    I tend to pin the board down somehow, hold the solder in my left hand and and the iron in my right and then move both at the same time over all the pads, taking less than one second per side. Sometimes I tilt the board. If the flux and temperature are correct then the solder only goes where it’s supposed to. If your action was even then the pads all end up similar sizes. If you have soldermask (which you do) then you’re ahead of me: I tend to mask the local tracks out with masking tape.

    Experiment with the temperature until the solder no longer whiskers. When tinning the pads it should melt when you touch the iron to it and you should be using little enough that the surface tension holds the solder in place when you move the iron away. If I remember correct, if the solder is ‘beading’ too badly then the iron is too hot. Unfortunately I can’t remember what temperature I’ve got my iron set to right now. 🙁

    To avoid the 3D alignment puzzle you need all the pads to be relatively flat and all the same size. If you’re putting the pins down with an iron then you want a bit big enough to get as many pins as possible. It’s worth spending a while getting the tinning right as it reduces the time spent with the iron touching the part.

    I find that not tinning the pads makes it mildly challenging to get the pad, the solder and the pin all at the same temperature at the same time. With a tinned pad you can place the part on the pad and then rest the iron on its side, touching the board and the outside of the pad with the flat bit of the iron on the end of the pin and the whole lot heats up quickly at the same time.

    If you have hot air then you can get all the pins hot at once. This helps with the 2D alignment puzzle as, provided it’s roughly correct to start with, the surface tension of the tinned pads will pull the part into the correct alignment.

    Good luck. The practice is definitely worth it but I find that I do it infrequently enough that the skill fades just as quickly as it arrives in the first place.

  4. James - February 13th, 2013 1:52 pm

    This is a really old thread, but I can share a few tips. As has been mentioned, clean the board prior to soldering. A lot of times boards show up from fabrication with a bit of some sort of oily residue that will wreak havoc when trying to solder it. Just quick rinse with soap and warm water clears this up. Then the key is to use plenty of flux. When I do this stuff by hand, I use a Q-tip to wipe liquid flux over the pads, then tin them lightly if necessary by dragging a small bead of solder over them with a wedge tip iron. I apply a bit more flux, put the part in place, tack one corner pin, fine tweak the alignment and then solder the rest by pressing the flat side of the tip (after wiping on a damp sponge to clean it) down over a few pins at a time and dragging it toward the end of the pins. Occasionally there will be a bit too much solder that I have to wick away (dip the wick in flux first) but most of the time it just works. If things are still not cooperating, try a different brand of flux or different solder. Avoid the lead-free stuff, the stress of dealing with that junk it is a greater health risk than the lead.

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