How to improve reflow with close small components (diy reflow oven) - Page 1
Forum section: electronics / general

author: aaronhance
author_rank: Regular Contributor
timestamp: July 09, 2024, 06:29:44 pm
content: I'm trying to reflow some PCBs in my diy toaster/reflow oven. I've installed a temperature control module, see below, but I think the temperature might by higher than indicated, as the silkscreen turned brown and there's a bubble looking thing in the solder mask. Another issue is some of the components have moved to the vias, 0603 and 0805, I assume this could be caused by poor/messy placement? Another issue is probably that these aren't my final components, but I wanted to test first without £25 worth of stuff, so I scraped some off an old pcb I had lying around. Kind of glad I did.I'm using sn67 and I went to a peak of 210 indicated, took 9mins to reach, then cooled down to 170 before opening to cool faster. Thermocouple in poking in the side, horizontal center, slightly above vertical center, maybe 2 centimetres in.Any advice before I try again, 8 fails left with test components :S.Images attached.

author: jmelson
author_rank: Super Contributor
timestamp: July 09, 2024, 07:55:19 pm
content: I have done well over 2000 boards of various types in a GE toaster oven I got from Walmart about 17 years ago. I use an Omega ramp and soak temperature controller I got on eBay. I poke a micro-sized thermocouple into a plated through hole in the board so the controller is controlling the actual temperature of the board. My oven has two horizontal steel heating elements above the rack, and two more below. I mostly use SAC305 solder. but have used leaded solder too. The flux is really important. For lead-free, GC10 ha proven to work extremely well, and I have had a few other brands that don't work well at all. I set up a profile to ramp up to 180C hold for a minute, then ramp up to 236C, hold for one minute, and then ramp down. That peak temp is for the SAC305, you can reduce the peak for leaded solder. For leaded, a peak of about 215C or so might be good, just to be sure EVERY component has reached at least 186 C for a few seconds. Watch closely through the window to see the solder change from grey to silver. Shine a flashlight in at an angle so you don't get a reflection in your eyes, so you can observe the melting take place.Your component placement before reflow is not real great.Jon

author: newto
author_rank: Regular Contributor
timestamp: July 09, 2024, 08:18:04 pm
content: Whatever temperature you're measuring, it definitely isn't the temperature of the PCB, you absolutely fried that board. I've been using a second had toaster oven and a basic amazon K-type thermocouple thermometer kapton taped to a blank pcb on the side.Even that is more than is absolutely necessary considering the size of the components you're using, just watch it for 5 minutes and turn it off and open the door when you see all the solder melt. Adding a soak phase at around 150C is better practice, but you don't have big heavy components that would take a while to bring up to temperature like inductors or mosfets with heatsinks.If it took 9 minutes to get to temp, I'm guessing your gauge is off by like 50-100C, especially to get it that torched.Edit: For the placement, more care is probably needed, but it wouldn't surprise me if the flux was boiling and bubbling and pushing components around. Covering the vias with solder mask is also a good idea, unless you plan to use them all as test points. You should also be able to put the USB port in for the reflow, once you figure out the temperature and timing...

author: bostonman
author_rank: Super Contributor
timestamp: July 09, 2024, 10:47:59 pm
content: QuoteI've been curious about using ovens to reflow boards. Long story short, I recently acquired a new Ninja oven from someone who got it as a gift and didn't want it. My intention is to keep it should the need to reflow a board ever arise, but would I need to tap into the control panel for this temperature controller to work?

author: MR
author_rank: Regular Contributor
timestamp: July 10, 2024, 12:52:26 am
content: this stencilling is a disaster... fix the stencilling / applying solder first.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: July 10, 2024, 01:49:17 am
content: When the thermocouple is hanging in the air, nearing a setpoint when the controller pulls the handbrake, the air temperature drops easily and hence not able to reach the setpoint and experiencing an oscillation. Therefore it will try for a long time to reach the setpoint. Hence, your controller is taking 9 minutes and that too is by luck it finishes. Plug the thermocouple tip into the through hole. Also, your paste looks suspicious.

author: Kasper
author_rank: Frequent Contributor
timestamp: July 10, 2024, 04:57:56 am
content: Quote from: bostonman on July 09, 2024, 10:47:59 pmI started with a used $20 IR oven and haven't used it enough or had enough failures to justify an upgrade. I use a thermocouple probe on my DMMand control the oven manually.Print out the reflow profile of the solder.Tape probe to similar PCB and place it near the PCB(s) being reflowed.On until a bit below soak temp.Off until a bit before soak complete.On until a bit below peak temp.Off for a bit.Open door.If you're doing a bunch and want to fine tune: record the temp as you go and watch out for overshoot.

author: Pinkus
author_rank: Frequent Contributor
timestamp: July 10, 2024, 11:45:52 am
content: Well this looks to me like the use of a cheap/useless or expired paste. Try a fresh paste, which is also known for its good soldering performance in DIY ovens. Avoid the stuff on Ali!

author: jmelson
author_rank: Super Contributor
timestamp: July 10, 2024, 03:31:33 pm
content: Quote from: bostonman on July 09, 2024, 10:47:59 pmYes, I cut a wire and spliced a solid state relay in series with the existing thermostat. If you don't want to do that, you could just make a cord with a plug, SSR and socket, and then plug the oven into that.Jon

author: jmelson
author_rank: Super Contributor
timestamp: July 10, 2024, 03:33:29 pm
content: Quote from: 48X24X48X on July 10, 2024, 01:49:17 amYes, first try with my toaster oven, I put the thermocouple in the air, and the board got severely toasted! The tiny thermocouple doesn't absorb IR like the board does.Jon

author: jmelson
author_rank: Super Contributor
timestamp: July 10, 2024, 03:36:44 pm
content: Quote from: Pinkus on July 10, 2024, 11:45:52 amYes, good paste is essential. Did you use a stencil? Or did you apply blobs of paste by hand? Looks like maybe too much paste in some places, and water absorbed in the paste splattering and causing components to move.Jon

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 10, 2024, 05:37:52 pm
content: What brand of solder do you use?Apparently some of the cheap brands are completely unusable.The rate at which the temperature rises is also an important factor. If it's too quick, you get uneven temperatures and bad soldering. If it's too slow, then the flux may evaporate before the solder melts, and it does not flow properly.

author: Kasper
author_rank: Frequent Contributor
timestamp: July 10, 2024, 05:53:39 pm
content: Quote from: Doctorandus_P on July 10, 2024, 05:37:52 pmCheap stuff works well for me, even a year or so beyond expiry. I'm just building little prototypes and testing them briefly, no vibration testing.Chip Quick: NC191SNL15T5https://www.digikey.ca/en/products/detail/chip-quik-inc/NC191SNL15T5/11480392

author: aaronhance
author_rank: Regular Contributor
timestamp: July 10, 2024, 06:05:03 pm
content: So, it looks like I did everything wrong.I used "MECHANIC XG-50 Liquid Soldering Solder Paste 42g SN63/Pb37".

author: Kasper
author_rank: Frequent Contributor
timestamp: July 10, 2024, 06:08:24 pm
content: Quote from: aaronhance on July 10, 2024, 06:05:03 pmNow that's out of the way, you can do some things right!

author: aaronhance
author_rank: Regular Contributor
timestamp: July 10, 2024, 08:15:54 pm
content: Quote from: Kasper on July 10, 2024, 06:08:24 pmThis time it went soo much better. The resistors were so cheap I picked up 100x of them, so now they've arrived I'm using them, that eliminates an unknown. I also attached the thermo couple to the pcb, and turned it off when I saw shiny balls. But I am still dreading the 7 pin IC.

author: JURP
author_rank: Contributor
timestamp: July 11, 2024, 10:25:14 am
content: 1) Too much paste2) Bad thermocouple housing. How many has a response time constant? 0.5, 1, 2 minutes? Or even more?

author: woody
author_rank: Frequent Contributor
timestamp: July 12, 2024, 10:20:40 am
content: Why not use a 'bare' thermocouple? Then you can place the little bead (where the two metals are fused) right on the PCB or even better, in a hole. I used to put a plated-thru hole in the middle of my designs just for this purpose. It might be that a bare thermocouple reacts faster?Before I changed to vapor phase I used a modified toaster oven for lots of prototypes without any problems. Always using lead-free solder. Biggest problem was that I had to babysit it to open the oven door right after the 10 seconds of reflow; the cooling phase temperatures would otherwise not be near where they should be. Without manual intervention the thing stays way to hot for way too long a time.

author: bob8819
author_rank: Newbie
timestamp: July 16, 2024, 12:06:32 am
content: It is better to use an air fryer, its temperature is more uniform

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: July 16, 2024, 01:59:47 am
content: Just left the stencil thickness to be fixed. Just reduce the thickness, then the excess solder paste will not be there.

author: Kjelt
author_rank: Super Contributor
timestamp: August 12, 2024, 01:03:15 pm
content: Quote from: bob8819 on July 16, 2024, 12:06:32 amInteresting idea, just thinking is the airflow not way too much, blowing parts away on the pcb?