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  • Author Author: shabaz
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Quick hot-air soldering video

shabaz

Very bad and shaky video quality, but I've decided to upload this 2-minute video anyway, in case it helps anyone, because I struggled for years with hot air tools, and it only slowly dawned on me the issues I'd had were related to things like no pre-heating and severely underpowered (300W from memory) hot-air tool. 

Any tips will be gratefully appreciated! Nothing in this video is best practice; I'm just a beginner. It's definitely not normal to apply paste on a pre-heated board for instance, but it seems to prevent the paste from drying up rapidly (and I am using several-year-expired paste).

The video shows capacitors being soldered using the hot-air tool, to a thick copper 4-layer PCB. The soldering iron struggled with it, so I decided to use the hot air tool instead. The video doesn't actually show the placement of the capacitors, but that was just done with tweezers, placed on top and lightly tapped to make sure they were sitting in the solder. I needed two hands for that, and I was shooting video handheld.

Whether the capacitors work or not I have no idea! : ) That's yet to be seen. I was trying to heat evenly, because they can very easily crack and go open- or short-circuit. 

Direct video link

Photos of the board:

image

View from a different angle; the board is still a work-in-progress, still things to solder, but the rest can mostly be done with a soldering iron.

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  • First, I don't think they look THAT bad. With those BACs, after they reflow, I will sometimes go back and flood them with flux, then hit them again with the hot air. My working idea is that parts like high-layer count MLCCs are pretty effective heat sinks, so all of the flux in the paste burns up before the alloy melts, which prevents a clean reflow. By hitting them again with a bunch of flux, you're giving the added flux a chance to break down oxides that started to build up from the (relatively) long period of time you soaked them with the heat gun.

    Maybe my explanation is wrong, but I have found flooding with flux and reflowing again really cleans them up.

    One trick I've been trying recently is setting my air to 150C and giving the board a chance to soak up some heat. Then after about 2 minutes, I crack up the air to my intended temperature. So, basically, minimally activate the flux and pre-heat the FR4, copper, and components. I really think a downside to hot air soldering is that the flux burns off faster (and is less effective overall.) Also, I really worry about thermal stresses on MLCCs. My experience at the capacitor company taught me how frustrating thermal cracking can be (and how easy it is to do.)

    I didn't catch which paste you're using, but 300C sounds low to me. When I solder with BiSnAg, I run my air between 225 and 250C, depending on the area, even though it reflows around 140-145C. When using SnPb, I set the air to at least 350C. And when using SAC, I just throw everything away and pour a stiff drink. (j/k, at least 450C).

  • in reply to baldengineer

    Hi James,

    Thanks for the info! Hehe yes the B.A.C.s suck up all the heat, they were problematic with a soldering iron on a scrap board before I decided to hit them with hot air. I too was concerned about cracking. Does it affect smaller capacitors more than BA ones? I've had capacitors go short-circuit when they were 0402-sized, but I have not done enough reflow to know if larger capacitors would be less at risk, or higher risk.

    Also, nice tip to re-flux, the quick drying up of flux made me uncomfortable, so that sounds like a workable approach.

    I'll crank up to 350 deg C in future. I was using this stuff, Qualitek 818, Sn62Pb36Ag2:

    image(Image source: Somerset Solders).

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  • in reply to baldengineer

    Hi James,

    Thanks for the info! Hehe yes the B.A.C.s suck up all the heat, they were problematic with a soldering iron on a scrap board before I decided to hit them with hot air. I too was concerned about cracking. Does it affect smaller capacitors more than BA ones? I've had capacitors go short-circuit when they were 0402-sized, but I have not done enough reflow to know if larger capacitors would be less at risk, or higher risk.

    Also, nice tip to re-flux, the quick drying up of flux made me uncomfortable, so that sounds like a workable approach.

    I'll crank up to 350 deg C in future. I was using this stuff, Qualitek 818, Sn62Pb36Ag2:

    image(Image source: Somerset Solders).

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