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Reflow Soldering Still Doesn’t #*&@ Work

After four semi-failed attempts, I am not destined for success with reflow soldering using an electric hotplate. At least not with chips having a 0.5 mm pin spacing. At this point I’m only motivated to continue on by pure stubbornness, as any hope that this would “save me time” is long gone.

Building on the results of my most recent previous attempt, I tried again with a new aluminum plate and new solder paste. I was pinning my hopes on the new solder paste, after having discovered that my original solder paste syringe had already expired by the time I bought it. I suspected that the poor wetting behavior and large number of solder bridges were due to spoiled paste. Judging by its date code, the new solder paste was only five months old, and before purchase it was stored refrigerated by DigiKey to prevent spoilage. They shipped it in a cold pack, at least, so I assume it was refrigerated before that.

The results were sadly the same as my prior attempts: poor wetting and tons of solder bridges. Apparently it wasn’t my solder paste’s fault, but my technique. The video shows the story in all its gory detail.

After having more-or-less exhausted the other possibilities, I’ve come to the conclusion that using a stencil must be essential for success. Maybe no-stencil application of solder paste direct from the syringe can work for components with nice big pads, but not for chips with 0.5 mm pin spacing, unless you want to spend time afterwards manually fixing solder bridges with an iron. Lacking a stencil, I prepared a second PCB by carefully smoothing out the solder paste with a cotton swab, trying to approximate the result of a stencil. I couldn’t avoid getting solder paste between the pads, of course, but I tried to get a thin and even coating over the entire pad area. Here’s what this second attempt looked like, prior to placing the components:

Looking at this photo now, the solder paste coating is kind of terrible, but at the time it seemed good. It’s hard to appreciate just how small those pads are, and it’s very difficult to spread the solder paste around evenly with a cotton swab.

After reflow, this second board turned out better than the first. There were fewer solder bridges, and the overall distribution of solder was more uniform. It was still far from acceptable, though.

Now it’s time for a decision. Do I order a stencil, and try again? Out of pure stubbornness and a desire to see this finally work, I want to say yes. But the practical part of my brain says no, I’ve already proven that it’s faster to assemble these boards with my current technique of drag-soldering. That makes success or failure with reflow and a stencil irrelevant. Maybe I’ll try it anyway for “educational purposes” – a sure way to justify any questionable idea.

Read 16 comments and join the conversation 

16 Comments so far

  1. Andrew March 31st, 2017 5:11 pm

    I’m going to double my comment from the earlier post here just in case;

    “The solder paste that I used (linked above) is a mix of solder and no-clean flux. Should I be applying more flux beyond that?”

    YES

    GOD YES

    ALL OF THE FLUX

    The flux inside the solder is enough flux to wet the solder balls together into one blob of solder. It’s not enough flux to wet the pins or the pads on the circuit board; if you’re not flooding the area with a flux pen prior to reflowing, shit just isn’t going to work.

    Since I can’t find any mention of using a flux pen / flooding the area with flux in any of these reflow posts… I’d say that’s your issue. Hot air pencil, flux flood immediately prior to applying hot air. I’ve managed to solder up some incredibly fine-pitch and tiny components with this method, including QFN packages.

  2. Steve March 31st, 2017 5:37 pm

    So you put solder paste on your pads, then position the components on the pads, then put more flux on top of everything before doing the reflow? How do you avoid bumping all the 0.5 mm pitch components out of alignment when you apply more flux? I could maybe see it if you’re using liquid flux from a bottle, but with a flux pen or gel flux it’s going to be tricky.

    When you reflow the way you’ve described, do your fine-pitch components come out near 100% good, without having to do manual fixup of bridges?

    One of the other commenters in the earlier post said “Don’t use flux with paste”, which was my assumption also, but maybe I’m very confused.

    Here’s some nice documentation of a DIY reflow including a 0.5 mm pitch, done the same way I did, with no stencil and no extra flux beyond what’s already in the solder paste. They get 25 bridges on a 208 pin chip, that must be fixed with an iron. They also conclude “if we had a stencil, this would be great, but hand soldering all these components may have taken less time than applying paste, placing components and then fixing bridges.” That’s essentially the same conclusion I reached. http://store.curiousinventor.com/guides/Surface_Mount_Soldering/Solder_Paste_and_Toaster_Oven

  3. Joe M. March 31st, 2017 7:00 pm

    Flood with flux first! (A flux pen is ideal for this task.) Cleanup of any excess solder or bridged pins can be usually be done with solder wick / desoldering braid afterward. Then a quick swab of isopropyl alcohol to clean up any leftover flux and residue.

  4. Steve March 31st, 2017 7:45 pm

    Joe do you mean “first” before putting down the solder paste, or before reflowing but after placing solder paste and components? I did try fluxing the pads as a first step in one of my earlier attempts, before putting down the solder paste, but it didn’t seem to make any difference. Maybe I didn’t use enough, or it all evaporated by the time I did the reflow.

    You’re right the solder bridges aren’t too difficult to remove with desoldering braid – it’s just the extra time required that bugs me. If “reflow and touch-up bridges” were the only way to assemble a tricky prototype, that would be fine. But for assembling dozens of identical boards, where the goal is speed, it’s not really a winning solution.

  5. Dillon Nichols April 1st, 2017 4:37 am

    You can get solder stencils for ~$25. I’d say it’s definitely worth it to buy at this point in your process.

  6. kbob April 1st, 2017 7:02 am

    I’ve been happy with polyimide stencils from OSHstencils.com. $15 or so shipped, usually within 24 hours, and they also sell solder paste. A more experienced friend also likes their steel stencils, but I haven’t tried one. I’ve only done two designs so far, and I’ve ruined one board out of 8 total. Have successfully soldered 0.5mm pitch parts. I use a skillet, but also have a hot air gun for rework.

    So it can be done.

  7. Adrian April 1st, 2017 9:14 am

    I think you used too much solder, which has nowhere to go. Try just using a thin trail of solder across the pads. Good quality paste on a clean board will distribute over the full pad during reflow. No additional flux needed.

    You can experiment without components to see how much/little is needed to cover the pads. Small pitch leaded packages are probably the hardest to reflow as excess solder gets sucked up between the leads.

  8. Adrian April 1st, 2017 9:22 am

    PS I’m all for flux when it comes to reflowing pre-tinned pads. E.g. using an iron to put solder on the pads of a QFN, clean with isoprop, apply flux, put chip down and then reflow with hot plate or hot air. Flooding paste with flux won’t do anything unless your paste was dried out.

  9. Benjamin Gillies April 2nd, 2017 1:30 am

    Aside from trying what everyone else has mentioned, I will put a huge plug for stencils.

    Did SMD reflowing with a solder rework station and some generic stencils, perfect results first time.

    I also have to add the above experience was during the soldering phase of my electronics course, so only just learnt to hand solder…

    Although I will say a big +1 to flux… flux makes the worse of us pros, you can never have too much flux…

  10. Leeland Heins April 3rd, 2017 9:20 am

    Since a disposable pie tin or a big block of aluminum didn’t work I’d be tempted to try something in between like an aluminum cookie sheet, maybe even a teflon coated one. A cookie sheet ought to be flat, stiff and not warp like a pie tin but heat through much more quick than a plate.

  11. Roy Wood April 3rd, 2017 9:45 am

    I’d really like to see this work out, just so we can get closure on the whole thing! Keep going!

  12. Steve April 3rd, 2017 2:17 pm

    I’ve ordered a stainless steel stencil from oshstencils.com. Now I guess I need a tiny squeegee, and solder paste in a jar instead of a syringe. Here’s hoping this works…

  13. kbob April 3rd, 2017 3:03 pm

    (a) OSHstencils includes a squeegee that looks like a credit card.

    (b) I use a syringe. Just put a few dabs in front of each cluster of holes.

  14. Steve April 3rd, 2017 3:20 pm

    I didn’t notice any mention of a squeegee when I placed the order – is that a freebee? Cool!

    If you’re using a syringe instead of a jar, how do you reclaim the extra solder paste after you clean up the stencil? Or do you just dispose of it? From watch videos of other people doing this, they put on a LOT of solder paste, but end of scraping most of it back into the jar afterwards.

  15. kbob April 3rd, 2017 3:32 pm

    Freebie. There’s been one in each of my three orders. If you don’t get one, you could use your library card. (-:

    I do not reclaim mine. I don’t expect to use up a syringe before it expires, so I wipe off the excess and throw it out. If you’re doing volume production, you’ll want a jar.

  16. Steve April 4th, 2017 10:33 am

    While I wait for the stencil, I tried another test where I inundated everything in a lake of flux immediately before reflowing, as some people have suggested. Unfortunately it didn’t help, and the results were largely the same as before. It appeared that the liquid flux all boiled away well before the solder paste reached melting temperature.

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