RE: Quick hot-air soldering video

baldengineer — Sun, 05 Mar 2023 17:09:04 GMT

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).

RE: Quick hot-air soldering video

Andrew J — Sun, 05 Mar 2023 15:38:52 GMT

It might be worth getting new paste. I can paste up a multi-part board (say 50 parts) and then place the parts without the solder drying out. Also, I’ve heard that solder paste can be “revived” by mixing some flux in with it - never tried it though. What I have found the most easiest way in a million miles is to use a stencil to lay on the paste - it is so worth the £5 to get made with the PCB that it’s a no-brainier. It makes laying on the paste way quicker so even less time to dry out. I’ve also found the second most easiest thing to do is pre-heat the board - again, it makes the job so much easier than hot air alone. I should add, I’m no expert either!

Your board looks fine to me - I would probably have more solder on the inductor terminals (less now I use templates) but I don’t know if that is necessary if it is mechanically sound. Besides, those things are a nightmare to get off if needed to be replaced and more solder doesn’t help that.