IR vs. Hot Air Conveyer Reflow Ovens - Page 1
Forum section: electronics / general

author: NF6X
author_rank: Unknown
timestamp: December 27, 2017, 04:59:03 pm
content: I'm researching low-end conveyer reflow ovens, i.e. the sorts one can find on AliExpress. Most of the cheapest ones use IR heating, and there are a few hot air models. And of course, many listings showing the usual T-962 or some similar drawer oven, calling it some misleading term like "hot air IR". My application would be intermittent in-house production of small batches of niche products in a one-person shop; not contract assembly, and not high volume. Right in that valley of despair between soldering up a few one-offs vs. sending it out to a contract assembler.I gather that hot air is preferable over IR, because I'd think that it would be less prone to hot and cold spots based on differing IR absorption of components on the board being soldered. Can anybody here with real experience in this area explain in detail how I should expect results to vary between a cheaper, smaller, lower-power-draw IR oven vs. a more expensive, larger, higher-power-draw hot air oven?Does an IR conveyer oven even make sense with respect to temperature profile accuracy? I think that one of the rationales of a conveyer oven is to allow heating and cooling rates that are faster than a hot-air system can slew if it has enough thermal mass to regulate worth a damn, without requiring huge peak power draws to slew the temperature quickly. But doesn't IR heating allow rapid enough changes in heat input to the board that moving a board through different zones of constant irradiation wouldn't work any better than leaving the board stationary and varying the IR tube power? And with respect to production rate, does the pipelining inherent in a conveyer oven offer a significant advantage over a few cheaper drawer-type ovens sitting side by side, given that the preceding stages would be a sub-$10k imported PnP machine and a manual framed stencil printer, all operated by one guy who is questioning his life choices?Many of the ovens sold via AliExpress are listed as being specially designed for LED mounting. Does that actually have any technical meaning in reflow ovens, or is it just some labeling that they throw on there to appeal to a market that happens to be making a lot of LED lighting products right now? I think I've seen some PnP place machines that really are specialized for LED lighting manufacturing, in that they claim a high CPH rate and deliberately have only a few feeders.

author: ebclr
author_rank: Super Contributor
timestamp: December 27, 2017, 06:45:57 pm
content: Even with this is possible to solder a boardhttps://www.amazon.com/gp/product/B00006IUWL/ref=oh_aui_detailpage_o04_s02?ie=UTF8&psc=1

author: rhb
author_rank: Super Contributor
timestamp: December 27, 2017, 06:59:30 pm
content: I spent a good deal of time on considering the physics some time back. Not done anything yet though, so take with a cellar of salt.The conclusion I came to was that the best setup was forced hot air accompanied by IR radiation from a source with small thermal mass. In my case, I was considering only single shot rather than a conveyor, hence the small thermal mass constraint.However, forced hot air as the preheat will produce a more uniform temperature profile in the board. The IR then just melts the solder as it passes along the conveyor in your use case.The parts I was interested in were vulnerable to residual stresses.

author: NF6X
author_rank: Unknown
timestamp: December 27, 2017, 07:08:56 pm
content: Quote from: rhb on December 27, 2017, 06:59:30 pmThat makes sense to me. I could imagine doing that in something like a drawer or chamber oven. I'm imagining that I'd standardize my designs around the equipment I'd be using for assembly, and standardizing on, say, a 5"x7" panel size to go along with 8"x10" framed stencils seems like it would make sense for the small line I have in mind. I'd anticipate sporadically making batches of 10-100 panels in a weekend, so maybe that could be practical with a non-conveyer oven, particularly if I could fit two or three panels inside at a time?

author: jmelson
author_rank: Super Contributor
timestamp: December 27, 2017, 11:42:07 pm
content: I've done well over 1000 boards with a GE toaster oven and a thermocouple controller. After a little tinkering, I found that poking the thermocouple into a plated through hole in the board was the best way to get controlled temperature. I did get a roll of very fine thermocouple wire. The controller is a ramp and soak model, so you can program several temperature ramps (ramp to 180 C over about 4 minutes, hold at 180 for 1 minute, ramp to 240 C in 2 minutes, hold at 240 for 1 minute, then ramp down at 5 C / minute.) These occasionally come up on eBay.Jon

author: rhb
author_rank: Super Contributor
timestamp: December 27, 2017, 11:45:33 pm
content: My thoughts exactly.I had in mind using one or two cheap Harbor Freight heat guns for the hot air and an electrically heated slab of aluminum or copper the size of the chamber with a proper black finish for the IR. Then using the heat gun blowers to cool the IR source and the chamber. Decorate with thermistors and PIR sensors to suit.At that point, I think the conflicting temperature profile recommendations for the parts will be the biggest issue.Rotating the boards would compensate for uneven IR emission, so an old microwave with suitable modifications might be a convenient platform.

author: NF6X
author_rank: Unknown
timestamp: December 27, 2017, 11:46:38 pm
content: I still haven't built any actual boards in it yet, but I've hacked my Qinsi QS-5100 to bypass the thermocouple dangling between the IR tubes and instead have a thermocouple in contact with the top of the PCB being reflowed. That probably isn't as good as poking a thermocouple into a plated hole, but in my bare-board tests it still made a significant improvement in the accuracy of the profile. There's still a minor need to fudge the setpoints to get the desired actual temperatures on the board, but it turned soak temperatures from steep and jagged slopes into smooth and level plateaus.

author: NF6X
author_rank: Unknown
timestamp: December 27, 2017, 11:56:16 pm
content: More details of my QS-5100 modifications:Before:After:Pictures of the new thermocouple probe: