Reflow soldering improved with carbon dioxide

co2_reflow-oven

This is exactly what it looks like. [Oleg] calls it soldering in inert atmosphere, but it’s just a toaster oven reflow hack dropped into a container full of carbon dioxide.

Why go to this trouble? It’s all about solder wetting. This is the ability of the molten solder paste to flow into all of the tinned areas of a board. [Oleg] talks about the shelf life of hot air leveled PCB tinning, which is about six months. After this the tin has oxidized. It will certainly not be as bad as bare copper would have, but it can lead to bad solder joints if your PCBs are more than about six months off the production line. This is one of the reasons to use solder flux. The acid eats away at the oxidized layer, exposing tin that will have better wetting.

But there is another way. Soldering in the absence of oxygen will also help the wetting process. CO2 is heavier than air, so placing the reflow oven in a plastic container will allow you to purge air from the space. CO2 canisters are cheap and easy to acquire. If you keg your own homebrew beer you already own one!

If you’ve got everything but the reflow oven just look around for a few examples of how to build your own.

30 thoughts on “Reflow soldering improved with carbon dioxide

  1. Hmm… I’m a little surprised the plastic didn’t melt or deform. (especially with the lid on) You’d think the heat would build up fairly quickly in there. Good idea though…

      1. Hmm, I’m not sure all toaster ovens are so well insulated. Mine gets quite hot outside, and fast. But I do have a CO2 cylinder on hand for planted aquariums, so I’ll definitely keep this tip in mind for the future.

        1. protip: “hot” to a human and “hot” to a plastic are two very different things. In my experience 50C or higher are uncomfortably hot (can’t touch for more than a few seconds) whereas the threshold for most plastics, including polypropylene, for deformation, is generally above 100C.

          1. This is correct. However, my toaster oven actually has deformed a piece of plastic, that I’d assumed was safe because it was sitting on an elevated wire rack on top of the oven. I did a quick test by setting it to 200°C and turning it on, after 5 minutes the hottest spot on the top was 74°C, and still climbing. It might still be ok for the described reflow configuration, but I’d do a dry run while monitoring temps first.

  2. good idea. I’ve read about some manufacturers using nitrogen for the same reason. Gold plated PCBs help too :)

    1. Nitrogen works well. However, the cylinders are not cheap so if you already have CO2 cylinder you can use it.

  3. I like the idea. as well. It will also help to stop fires, AND maintain a cooler outside temperature with the right amount of CO2 in the contained Atmosphere.

  4. Aww ok nitrogen is inert co2 isn’t. Also if you want mixed gases welding supply stores have a great selection. I would suggest argon or argon/co2 mixes. Argon is very good at displacing o2.

      1. CO2 is quite non-inert in many, many situations. If you had the choice, you should certainly choose N2 wherever available (I suspect it is actually cheaper, as well). The simplest situation to consider, and one common enough that many applications avoid it, is simply that CO2 will readily dissolve in water to form carbonic acid. You may remember demonstrations where an unbuffered solution of water will readily drop in pH (measured with a sensitive meter) when you breathe on it due to the dissolution of CO2. So as you can imagine, in many applications (if not most, as there will be some humidity in almost all cases) CO2 atmospheres are very much NOT inert and have the potential to cause problems. In fact atmospheric concentrations of CO2 participates in a good number of corrosion and other reactions.

        Point is, I have issue with calling CO2 “inert.” When there is water involved, it’ll inevitably have some influence, especially in a 100% atmosphere, and many reactions produce CO2, which means that in many cases a high concentration of CO2 will slow down the reaction (think Le Chatelier’s principle). Having considered all that, ask yourself how often you see a reaction consume or produce N2. N2 is unquestionably much more inert.

        With that said, in the conditions of a reflow soldering oven, I doubt this would be a substantial problem. I don’t know for sure, however, as weird things do tend to happen on the surface of metals at high temp, and for all I know there may be some reaction (possibly in conjunction with small amounts of water vapor or adsorbed water vapor–which, by the way, doesn’t just magically “boil off” beyond 100C). I certainly wouldn’t want to store things in an “inert atmosphere” of CO2 for long periods of time, blissfully assuming that this will prevent anything bad from happening to what I put into it.

    1. Evocube : priced Argon lately? Your money must be burning a hole in your pocket. And mazurov is correct – for this purpose CO2 is inert – meaning non-reactive. If you were more informed you would know that CO2 is commonly used for MIG welding. It’s not inert in that setting but it does the job of keeping oxygen away.

      1. For this purpose, CO2 is inert. At welding temperatures small amounts of CO2 split and form CO and O2. This is required in some processes for the small amount of oxidation it provides.

      2. Yes I know co2 is used in welding hence the welding supply reference. Co2 is used in welding not because its inert it is used for heat transfer ability. That is why when you weld stainless its limited by less than 3 percent by most codes as it causes precipitation which causes rusting. I see the point here just sharing my opinion and if you want to compare brain pans on welding let me know.

        1. Yep. MAG is normally used in Europe. All codes refer to Mig/Mag as GMAW (gas metal arc welding) which takes out the confusion with the gas references. However Mig is much easier to say in conversation :)

  5. “This is one of the reasons to use solder flux. The acid eats away at the oxidized layer”

    Not with anything electrical it doesn’t! Don’t use acid flux when soldering anything which will have electricity running through it.

    1. To be fair, all fluxes are acidic. It’s just that the “acid core” fluxes are considerably more corrosive, and require extreme amounts of cleaning to safely remove (basically, you need to use abrasives strong enough to strip solder mask, which makes it useless for electronics). Rosin fluxes are usually designed to turn acidic at high temperatures.

  6. Nice minimalist “good enough” applied hack. BUT!

    There’s few enough people that consider such hacks for us to lightly risk LOSING any …eh?

    CO2 tends to be a bit less subtle=you might feel a choking/desire to flee – or not.

    N2 however, kills more than a few folks by simpler accidents. Like a tank capable of displacing oxygen from a really hard to grasp area and often quicker than you can survive.

    Darwin bats last. Be careful- this is a hack for outside or a fully open garage door etc and even then- BE AWARE of the low spots hazard persistence. That pet sleeping on the floor… a kid in a carrier half the house away. The basement filled with death. Or- a gas burner extinguished.. partially? by either oxy displacer. There’s a VERY plausible flashback scenario lurking too.

    Propane water heater- N2/CO2 smothers enough flame for a gas puddle to form, thence flashback…

    Damn hard to do for proving- scary easy by accident.

    1. Yeah, CO2 in small quantities is really dangerous! It’s not like our bodies produce it naturally in similar amounts used in this technique over a short period of time, or anything! As for N2, I like to live vicariously, so I moved to a planet where the atmosphere is over 3/4 N2. Every day is an adventure!

      Fan+window, problem solved forever. Spray paint is more dangerous than this.

      Heck, my breakfast was more dangerous than this.

      Improper storage of *large* gas cylinders is frightening though! Anyway, keep up the safety trolling, it genuinely brightens my day. It’s a nice break from fast motorbikes and faster code. Carpe diem!

      1. AFAIK it’s not really CO2 that kills people, it’s CO (Carbon mono-oxide). Have a well ventilated environment and heed the signs. Get drowzy or ill, go to a “safe” area. When using a CO2 cannister CO shouldn’t be a problem yeah?

        1. Eirinn,

          if you believe that inhale a little of the white fog from dry ice and tell me you still think CO2 wont kill you. (Dont really try this but seriously anything that displaces oxygen in your lungs will kill you).

          1. In real-life situation, it’s quite difficult to suffocate on CO2 because of hyperapnic alarm response -> http://en.wikipedia.org/wiki/Hypercapnia (the one one would experience trying to breathe CO2); you would really want to die to stand it long enough to actually die. On the other hand, nitrogen suffocation is painless and sudden, see Suicide Bag -> http://en.wikipedia.org/wiki/Suicide_bag . If you prefer working alone, CO2 is much safer since you have a chance to detect it and run away.

            BTW, I converted my oven to N2 the other day. Results are the same but soldering is easier since I don’t need a bath.

  7. I made a different attempt to reflow my laptop gpu card like this. I cut an aluminium can in half put some alcohol in it and burned it in the oven to create an inert atmosphere. it worked well.

  8. It seems that if you’ve got a container and some CO2 around, you could also purge the O2 from the container and store your etched boards in it. With a lid, it’d probably keep the O2 out for a very long time, and then your boards wouldn’t oxidize nearly as much…

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