New simple, low-cost reflow hot plate - Page 1
Forum section: electronics / projects

author: Peabody
author_rank: Super Contributor
timestamp: March 21, 2019, 10:30:59 pm
content: This reflow device consists of the $10 hot plate from Walmart, a $5 circular saw blade from Home Depot, and three screws and washers from Ace Hardware, for a total cost of under US$20. No thermocouple or controller is needed. Links to the related video and Github repo are at the bottom. Links to the parts are in the repo.The basic idea is that if you develop a power cycle sequence that produces a temperature profile pretty close to the datasheet, you can produce the same temperature profile every time simply by repeating that power sequence, starting at room temperature of course. This eliminates the need for a feedback thermocouple or controller, and the scorching which can result from improper placement of the thermocouple in a feedback system. All you need is a stopwatch or timer app. Through trial and error, I've developed the power sequence, so in effect the tuning has been done in advance. But of course the user can adjust the power sequence as he wants.Originally I thought I would need a controller, so I built one which allowed micro-managing the power duty cycle over every 2-second period, and got a good profile from that. But then I discovered I could get essentially the same profile just by turning the power fully ON for specific periods at only two points during the process. So while the controller is included in the repo, it's not really needed.This system could be assembled by pretty much anyone in the US, but something similar could be developed for other locations. You would just need a thermocouple to help develop the pattern. And the known melting points of 63/37 and 60/40 leaded solder are helpful in checking thermocouple accuracy. The main issue with hot plates, and probably toaster ovens, is that there is a very significant lag in the response of the system to power, so you have to turn the power ON or OFF well in advance of your target event. Otherwise you will get overshoots and scorching.This system has resided at my local OSH group's maker space for a month, and at last night's meeting I was told it works fine, with no burning or scorching. So it seems to me it's a really simple, safe, easy, cheap solution for occasional reflow needs.https://github.com/gbhug5a/Reflow-Hot-Plate-and-Controller

author: cv007
author_rank: Frequent Contributor
timestamp: March 22, 2019, 04:39:11 pm
content: Getting burned, and cut, at the same time, looks like funAnything that works is good, but why not skip the saw blade and get the hot plate with a solid surface ? I'm not sure what material is the 'solid', but they all seem to look the same- a surface with small grooves in a circular pattern. You could even keep the saw blade if the risk factor is too low for you.I have a cheap solid surface hot plate that works pretty well. I can't keep solder paste around long enough to remain good, so I just pre-solder all pads- liberal use of amtech-nc-something flux, get all pads hit with a little solder using soldering iron, drag solder ic type pads with as much solder as pad will hold (flux does all the work, and you can't fail). Leave flux alone, place components with flux being the sticky glue to hold in place. Put board on hot plate, set hot plate to warm (in my case), watch for reflow of all parts, turn off hot plate, slide board(s) off to something else, let cool, clean board. Add any through hole parts. Done.

author: Peabody
author_rank: Super Contributor
timestamp: March 22, 2019, 05:17:53 pm
content: I looked for a cheap hot plate which included a solid surface, but the ones I found like that had a single circular heating element right in the middle. Target sells one like that. But I just didn't think that would work as well as the Walmart hot plate. The combination of the three rings of heating element and the air gap between those and the saw blade seems to provide a broad reflow surface with reasonably consistent temperature all over.It sounds like you have a good system. And I think hot plates in general are remarkably forgiving. More than one guy I talked to just puts the board on the hot plate until it reflows, then quickly removes it. I just wanted something a bit more controlled than that.But I think the main point is that you don't have to spend the day rebuilding a toaster oven, and buy or build a fancy controller, not to mention getting a thermocouple and an SSR, and spend a lot of money, to be able to reflow an occasional board. As long as all the parts are on one side. :-)

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: March 22, 2019, 05:29:57 pm
content: Search eBay for "lcd screen separator". These are cheap and work very well for reflow purposes.I'm slightly concerned mine might contain asbestos but probably I'm just paranoid.

author: TimCambridge
author_rank: Regular Contributor
timestamp: March 22, 2019, 06:24:53 pm
content: Quote from: 0xdeadbeef on March 22, 2019, 05:29:57 pmWhat temperature do they manage. 120C for LCD glue softening would not work too well for soldering!

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: March 22, 2019, 07:10:57 pm
content: Mine (cube design, blue metal case) goes at least to 350° so temperature is not a concern.

author: cv007
author_rank: Frequent Contributor
timestamp: March 22, 2019, 07:16:01 pm
content: QuoteIt took me a while to figure out that the temp control only needed to go to 'warm' (on mine at least) to get to reflow without having a fine line between reflow and burned boards. Now I can take my time to some degree when watching for reflow and I don't have to panic while getting the boards off. I'm not quite sure why I initially thought I needed to turn up the temp so much.What I do like about a hot plate, is you have direct access to the board, to both watch closely and can move a part if needed or tap a part if in doubt about its situation.You are right that it doesn't take much hardware, money, or effort to get into smd, and is easier than it appears to be. So grab your hot plate, saw blade, frying pan, whatever, and have at it.