Toaster oven reflow power - Page 1
Forum section: electronics / general

author: davegravy
author_rank: Regular Contributor
timestamp: January 14, 2022, 04:58:28 am
content: I picked up an old Black and Decker toaster oven and like I've seen done a bunch on YouTube, am trying to use it for reflow.I'm trying to achieve the profile of my solder paste which is here:http://www.chipquik.com/datasheets/SMD291AX.pdfI'm practicing with just a thermocouple in the oven for now. The profile expects the temperature to rise from 300F to 455F in 1min30s however the fastest this oven can seemingly do it is 2min. Is this a big problem?It also takes well over 30s to get from room temperature to 200F, I'm guessing that's less important however.

author: geggi1
author_rank: Frequent Contributor
timestamp: January 14, 2022, 09:55:57 am
content: I would do at least one of two things.1. Check that the heating element is working correctly with the correct wattage by checking the resistance and supply voltage.2. if the unit got a glass door I would insulate the door.

author: Peabody
author_rank: Super Contributor
timestamp: January 14, 2022, 03:59:52 pm
content: If the heating elements are wire, that may be the best you can do. Response will be slow in both directions. I think the preferred oven would have ceramic elements, which turn fully on and off very quickly.

author: jmelson
author_rank: Super Contributor
timestamp: January 14, 2022, 05:12:27 pm
content: Put the thermocouple on a board, in air it doesn't register the same at all. The thermocouple will read very low while the boards LITERALLY catch fire! Don't ask how I know this.I use a ramp and soak temp controller and an SSR that controls the heating element, so I can program the temp profile, and poke a micro-size thermocouple through a plated through hole in the board. I have done over 2000 board with this setup.Jon

author: davegravy
author_rank: Regular Contributor
timestamp: January 14, 2022, 05:30:41 pm
content: Quote from: Peabody on January 14, 2022, 03:59:52 pmThey are ceramic it looks like. White tubular elements.

author: davegravy
author_rank: Regular Contributor
timestamp: January 14, 2022, 05:34:16 pm
content: Quote from: jmelson on January 14, 2022, 05:12:27 pmThanks, I will try it with a board inside. I'm hoping to avoid needing a controller since I only need to make a few boards and a fair number of people seem to say they manage without

author: DLE
author_rank: Contributor
timestamp: January 14, 2022, 08:59:22 pm
content: Testing with a real board and components (no paste) on it is the best way to verify the profile as the board mass will affect your profile. However, you don't want to exceed 2 reflow cycles without starting to risk component life. If you don't plan to use the board from your testing it's no big deal.If your just doing a few boards at a time I wouldn't worry about a controller either. Turn the toaster at full power and watch the reflow through the window (if it has one) and keep an eye on your thermocouple inside. The thermocouple should be on the board, not in the air. Once you hit peak temp, open the door and you can add fan(s) to aid in cooling the board faster if necessary. If you are doing typical hobbyist boards, you should be fine. You may find, that you need a slight pause in the temp climb to allow the board to come up to temperature for higher mass boards/components.

author: sleemanj
author_rank: Super Contributor
timestamp: January 14, 2022, 09:36:58 pm
content: I'm pretty space constrained, so I made my "reflow" oven from a literal popup toaster. It even still actually looks like a popup toaster, plastic case, dial and all.Popup mechanism and timer circuit was ripped out, an aliexpress triac dimmer/speed controller module was put in place of the original timer, and a rocker switch added. The steel "chassis" was wrapped in kapton and silicone wires used.Both "slots" were left functional (it would have been a pain with the design of the toaster to lose one since they were in series), the "power setting" required is almost as low as it can possibly go (the wires certainly do not glow like you're making breakfast), the only controller is me watching a thermocouple reading from on PCB scrap, turn on, wait to hit about 200, turn off, wait until it's cooled down below solidus and remove.For doing a couple boards every now and then, there's no point in going into the rabbit hole too far.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 14, 2022, 11:31:15 pm
content: If you just want to see how it works use a skillet. Sparkfun and Adafruit both have videos of the process somewhere on their sites.If you want faster heating cover the door with aluminum foil. Also, if you progress past the manual control you may want to use a PID controller with soak func. The hole in the top right corner of the toaster on the picture is a place where such a controller used to be.For reference: my oven goes as fast as 2C/sec after soak. IIRC the 1C/sec rate from the paste datasheet is a maximum, not minimum. You don't want to heat too slowly though. Run an unpopulated board or two through your profile and see if pads are still shiny.

author: davegravy
author_rank: Regular Contributor
timestamp: January 14, 2022, 11:33:58 pm
content: So I played around with my thermocouple and IR thermometer gun. As the temperature rises on my thermocouple, if I kill the heater and take a quick IR reading of the solder stop, the IR readings are 100F - 125F higher than the thermocouple. After resting 15-20 seconds the thermocouple and IR readings equalize (the thermocouple reading increases slightly, mostly just the IR readings drop).I suppose the IR gun reads the very surface "skin" temperature whereas the thermocouple has a slower response that's more representative of the internal board temperature? So I could see why relying on the thermocouple wouldn't be great since surface temps could get extremely high and cause problems.Challenge is that while I have a couple spare PCBs, a couple critical components I could only get 1 of (supply chain).So I have to get this right on the first try (not my preferred way of operating). I'm going to throw some random unneeded parts on that I have sitting around to approximate the added thermal mass and do a few more trial runs.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 14, 2022, 11:34:26 pm
content: You also may want to get a special "crayon" which will melt at specified temperature. The one on the pic is for SAC305, for SnPb you need 218-ish C.

author: davegravy
author_rank: Regular Contributor
timestamp: January 14, 2022, 11:37:51 pm
content: Quote from: mazurov on January 14, 2022, 11:31:15 pmI assume you mean unpopulated board with solder paste applied? Or do you mean check that the bare copper pads are still shiny (they're quite matte straight from OSHPark).Sorry, new at this if it's not obvious

author: mazurov
author_rank: Frequent Contributor
timestamp: January 14, 2022, 11:42:21 pm
content: Quote from: davegravy on January 14, 2022, 11:33:58 pmNot only that but the inertia of the oven is also non-trivial. If you turn the heater off at the specified temperature you'll overshoot 10-20C. A simple remedy is to set final temp. accordingly.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 14, 2022, 11:47:10 pm
content: Quote from: davegravy on January 14, 2022, 11:37:51 pmI meant the board with HASL (tin) finish. OSH Park uses ENIG (gold), they will stay shiny long after the board starts smoking.You want to stay at the peak for as little time as possible, but not less.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 15, 2022, 12:17:59 am
content: Another option. It reflows at ~150C.

author: Kasper
author_rank: Frequent Contributor
timestamp: January 15, 2022, 07:48:03 am
content: Quote from: davegravy on January 14, 2022, 11:33:58 pmIR gun might be reflecting off the board and measuring the back of your oven.That is less likely to happen if the gun is perpendicular to the board.