Workshop video #01: Sjaak solders a QFN chip, attempts the BGA challenge

We took our cameras on the road and visited Sjaak’s workshop this week. He showed us how he solders QFN chips with a bit of hot air and normal drag-soldering. We also surprised him with a challenge to solder a tiny BGA chip, see how it turns out.

Here’s some links mentioned in the video:

We aim to post some sort of workshop video every Tuesday and Thursday. Later this week we’ll be back in our lab with a look at Bus Pirate firmware release v6, and we’ll solder a few new prototypes too.

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  1. Damn! I was pretty sure that Sjaak was able to solder that tiny guy!

    Well, maybe that can be soldered with electric skillet method. I’ve started using it a few days ago but it is really good for SMD soldering.

    1. I think with enough time we would have got it, but we ran out of daylight. Pietja did it twice though, maybe we can visit his workshop too :)

      1. Workshop is a big word, is just a simple bedroom with a desk in it a huge pc monitor 2 PS3’s and a soldering station :p

      2. That is the keyword: no workshop ;)

        Looking back at the vid i forgot to flux the bga. that would remove any oxidation and stuff from the chip and make it reflow the solder better. But i was a bit scared to heat it up too much.

        You just heated it up with a hotair at low speed? and flux off course?

      3. I put like a drop of flux down on the board put the chip in it and then removed the excess flux with a cotton swap.

        Then reflowing at low speed so the stickyness of the flux will hold the chip in place.

        Also in the extra Assembly Guidelines of the TMP006 it says how best to solder it down because the measurement area is quit sensitive to chemicals.

    1. There are also surface mounts with legs around it that can be soldered normally, but bga’s are mean..

    1. The regular tip (the one that came with it). you actually drag solder the side as the flux, surface tension do the rest. no need for some super extra fine tip imho.

      1. The tip that came with mine is fairly fine. I can do drag soldering on fine pitch stuff with my other iron that has a much wider tip. The tip that came with my Aoyue seems like it has a tough time transferring the heat (due to the fine tip), which makes it harder to drag.

        Anyway, I asked, because I wondered if you were using something with the dimple style tip to hold some solder. What heat setting do you use for the iron for that QFN? Maybe you said it in the vid, but that was like hours ago that I watched it. :)

      2. I specially looked at the settings for you but somewhere between 350 and 400 degree celsius That is a setting that works for me for all my soldering work (set it once and never touched it again)

      1. In the US, has a pretty good selection of tips for the Aoyue (and a pretty comprehensive selection of Aoyue equipment, in general).

  2. I was surprised to see you set the hot air temp so high. I’ve soldered lots of QFN devices in a similar way, except at about 250c and for a longer duration, and with a wider air nosel.

    1. the airflow is set near max (80-90%), so I guess the temperature is transferred faster to the areas but will be lower then 380. But that is just a guess, these settings do work for me..

      Did I tell you I can be impatient :)

      1. I find that preheating the board helps *a lot*, I first preheat the board with hot air (same temp only I blow from 10-20cm) for 2-5 minutes and only then start to solder qfn/bga and it works way better then adding a qfn/bga on a cold board and pushing a hot air onto cold board and cold chip – that a recipe for killing the chip, when you spend 2-3 more monutes to preheat them to 150-170C and then solder them it assures better flow of solder and better connections.

        That kester flux you use, it’s piece of … throw it away, it evaporate too fast so when you preheat the board the flux evaporates before you start soldering, use some flux paste (those that come in syringes) its 10000x better for smd, especially qfn and bga

      2. I agree about the flux pen, I have the same one and it’s not that great, I bought a syringe from farnell (the same stuff reviewed here a while ago) and it’s loads better. I’ve tried dispensing solder paste from a syringe but the one I had was impossible to force through the needled – has anyone had any luck with using them? I tried manually applying the paste but for chips like this it is really easy to end up with too much, so I just pre-tin the board now.

      3. @Tom – I have the same problem with trying to spooge solder paste through my (too small for this paste apparently) needle-tips. Great for carpal tunnel though.

        I’m also having trouble getting small dabs of paste onto pads with other tools (like toothpicks and tweezers) as a kludge. I do find that if/when I do get it on, it reflows beautifully. Next time I’m going try experimenting with a slurry of paste and extra liquid flux and just spread it across the pads. I’ve seen other do this with just paste, but controlling the distribution across the pads is still a problem. I’m hoping that taking “flux-is-your-friend” to the next level may do the trick.

        @Arhi – I also wasn’t comfortable about the lack of thermal foreplay before “mounting”. I’d want to spend more time warming-up and cooling-down with a slightly lower (single) temp for the whole process.

      4. To a small extent, the apparent lack of preheating is due to editing. I did have some bad shots (heads, hands in the way the whole time) and cut some things for time.

        I agree though, when I soldered LGA accel chips I also spent a lot of time heating the board, until the board visibly reflows, before making the ‘sandwich’. Neither of us really knew what to do with the BGA chip though, and we were far too timid. Next time, we’ll be sure to preheat more.

      5. Any recommendation for readily available flux pastes? The only one that I’ve seen around is the one that comes with ChipQuik, and the one at Digikey for ~$70 (!!!)/30g.

      6. Yeah, no Edsyn here. It looks like Mouser has a few more items to choose from, with more reasonably priced ways to play.

      7. @Ian it was prophetic that you pointed me to the Soldering supplies page which happens to mention that the hot-air on your 968 died. This happened to my 968 went a few months ago and I was going to check on the forum if anyone else had had the same thing happen to them. I’ll have to add my experience there, also. In my case neither the hot-air nor the smoke absorber work, but the heat in the nozzle is fine. And it still makes that great noise when it tries to work.

      8. Sounds like the pump is shot, obstructed, etc. My problem appears to be a short in the temp probe in the handle. I also discussed this a little in a video a few weeks ago.

  3. wrt “flux don’t go trough needle” – why don’t you just remove needle and use the flux directly from suringe? If you want to use needle – use dremel and SHORTEN the needle a lot. You do not want to push flux trough 5cm of needle, you want to push it trough few mm only :)

    1. I’d get way to much paste out that way — it would be impossible to target to a small pad.
      I did have to do that to get it out of the syringe so that I could try to pick some up with a toothpick to put on individual pads (which didn’t work very well, as it really likes to stick to the pick).

      Not sure if it has to do with the age of my solder paste or not. It’s 2-3 years old but has been refrigerated the whole time, except for the 4-5 times I’ve let it come up to room temperature to use it. I’ve decided that if I ever try to squeegee it through a stencil, I’ll just add some extra liquid flux if it needs it. But trying to dispense it one-pad-at-a-time isn’t working with the needle size (and paste in this condition) that I’ve got.

      1. you can never put too much flux on the board so no worries if you put a lot of it and not only on the pad. It’s just that good flux ain’t free but …

        anyhow, as I said, shorten the needle, go to drug store and get “biggest needle they have” (locally they are like 1c or less) and you cut it to 1cm in length, the flux will flow trough it without problems

      2. Hee hee, in my country you can’t just walk into a drug store and buy needles.

        It’s actually not the needle length that’s a problem with my solder paste syringe, because they’re already about 1cm long. I’d be willing to try to trim them down a little bit more though — we’ll see if I can do it without smashing it and making it even worse. I don’t remember if I had this problem or not the first six months I had this paste. It has been a while, but I think that it originally worked, So it may be that it has thickened over time.

        Thanks for the advice.

  4. Any guidance regarding what’s a good drill size for the backside-solder-through method of getting to the thermal pad of a standard 3x3mm QFN? Something in the 0.6mm range, maybe?

    I would have never guessed that the (super-)tiny holes in the Schmartboard demo would work. With typical ball pitches of 0.5-0.8mm, those can’t be any bigger than 0.3mm, can they?

    1. 0.3mm via’s work like a charm to allow both soldering of the pad as well as to take the heat of the chip to the other side of the board. I use that technique a lot with alegro stepper drivers where you add a bunch of vias under the chip and big copper area under it so that you can better dissipate the heat.

      The only important issue here is that you need metalized via’s, just “holes in fr4” won’t do you any good, you need those copper channels transferring the heat :)

      1. With the pcbservice of seeed i notice they drill first the holes, then metalize it. Every hole was metalized regardless it was a via, pth or a regular hole.

      2. Actually that is not the case. The badge I designed for Make Meeting has holes which are not copper plated. I guess if you use the via tool (mounting holes in libraries are like this) that will create a plated hole whereas if you use the hole tool that will create a normal hole.

      3. @Tayken — Wow, you’re right. The webpage now says no such thing. I would have bet money that it did — and lost. I was certain that I had to think about plated-through drill holes for one design and ensuring that it wasn’t a problem at one point.

        I have a fairly recent board in my hands now, and the mounting holes are not plated. However, one of my other Seeed boards from 2009 does, so plating drill holes used to be the case, and I checked the design and both (the new and old boards) were done with holes, not vias.

        I was wrong about current boards having all holes plated… thanks for pointing that out!

    2. Thanks. Yeah, I knew they had to be plated-through. It just seemed like a 5:1 length:width ratio for the tunnel might be stretching it though. It just occurred to me to check to see what Cree advises also. I remember reading one of their white papers about this (and filled vias) a year or so ago.


      1. I think Cree recommends around 9mil holes for their MX3 series. The PCB house I use only goes down to 15 mils “standard spec” and even these seem to work fairly well at not letting too much solder flow through.

      2. I think that I was confusing apples and oranges. The Cree guidance that I was thinking of was specifically about creating thermal vias between top and bottom layers, not specifically for getting at thermal pad on the part itself. The vias wouldn’t necessarily get solder filled, but just use the electroplated metal for carrying the heat top-to-bottom. Getting the board maker to do filled vias was found to be substantially better, but the unfilled ones would be acceptable if done properly. I think the white paper was published in one of DigiKey’s TechZone “journals” but is probably available directly from Cree.

      3. OK, if you run across the app note, send it along – you’ve got me curious. Generally, you want thermal vias to be directly under whatever is generating the heat – which, most of the time, is a device with a thermal pad anyway. That app note does have some data on unfilled vias as well – but it doesn’t sound exactly like what you’re describing.

      4. “For heat transfer, larger, filled vias are recommended – but if you need to use open vias – they suggest .25 – .3mm diameter to minimize solder wicking.”

        I agree from the standpoint of SMT manufacturing. The original problem that we were trying to solve here was _wanting_ to get solder to wick through for the purposes of adhering the QFN pad bottom (for either thermal or electrical purposes) when handworking, rather than avoiding it. Duane Benson at Screaming Circuits has written about the issues you describe in his blogs and/or articles often. He’s always warning people about vias in pads.

      5. “Ironically enough, this is nearly identical to the PDF I linked to :) or am I missing something?”

        Only that we were responding at the same time!

        LOL – we probably should have taken this entire comments section to the Forums a long time ago. Oh well….

  5. I claim it was Ians fault. Everyone who ever tried know that it requires at least two beer to solder BGAs… no beer for Sjaak over the entire video -> no success

  6. I often solder QFN chips using hot air and solder paste, with an almost 99% success rate.
    Solder paste is much easier, the only drawback is that you have to keep it in the fridge or it will last not more than a couple of months.
    To place it on the board, I use the tip of a cutter, as a good dispenser is expensive and syringes are generally too small for the solder paste, i’ve never been able to squeeze something out of them. Anyway the cutter si ok, it just takes a little longer to place the paste. And off course i don’t place it on every single pad, but spread it over a line of pad, and the flux will do the rest.
    One thing about hot air soldering: dry your chips! If you heat a package from room temperature to 320 degrees in 2 seconds, the risk of pop-corn effect is really high, so I usually heat everything to 150 degrees for 5 minutes, and then I switch to 320 for soldering. Also a small electric oven is useful to dry lot of chips faster.

    1. What do you mean by “cutter”? is that a knife? I normally dip a bit of wire into the solder paste then drag a fine line along pins, but this often ends up uneven so i have to move the chip around a bit before it settles.

      1. Yes, i use a little sharp knife with a 45 degrees tip. For resistors and capacitors I generally dip the cutter a little in the solder paste and then touch the pads with the tip, releasing a small amount of paste. For ICs, I put some paste on the pcb and then make it a straight uniform line through all the pads using the 45 degrees part of the cutter. Not fast, but works well!

  7. The syringe tip which comes with solder paste might be a bit small for hand dispensing. They are probably more suitable for pneumatic dispensers which don’t get carpal tunnel ;). However, it isn’t hard to buy additional tips in various sizes. Most soldering equipment suppliers carry them. Or you can order them quite inexpensively from Buy a selection of sizes and see what works for you. Also, as the paste ages, it gets thicker and so you might have a harder time pushing it through the tip and need to go to a larger bore.
    Some of the comments above seem to be confusing solder paste (a mixture of solder balls and flux) with solder flux. It is true that “you can never have enough flux” but you can certainly have too much solder paste! In fact, when you are putting it on by hand, if you think you have put enough on, you have probably put way more than you should have. Think about how thin of a coat of solder paste is applied by using say a 3 mil thick solder stencil and you will realize that it takes very little solder paste to do a good job. Any more than that and you risk getting a lot of solder bridges which you will have to rework.
    If you are doing QFN, LGA or BGA type parts without exposed leads, solder paste with some sort of reflow system (hot air, hot plate, toaster oven, etc) really is the best way to go. Particularly if you have some sort of temperature control. You have a chance then of approximating the correct temperature profile and less chance of damaging the parts by spending too much time at a high temperature. The damage you do to the parts by incorrect soldering might not be evident at first but could show up in premature failures.
    Pro setups for hot air reflow often use a preheater which raises the temperature of the board to the point where the flux activates and then the hand tool is just used for a short time to actually reflow the solder paste. The latest issue of Circuit Cellar magazine has an article on a DIY preheater made from a heat gun.

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