PCB goes smoky in reflow oven - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: July 24, 2016, 09:29:45 am
content: I have one of those cheap reflow oven's from China the T962. I have used it to produce a number of boards now mostly all of them to inches by 2 inches and 1.6 mm thick. I also successfully assembled a board that boards about 100 x 90 mm and 1.6 mm thick. I have just had the same board made and I offered it and things got a bit smoky as the FR4 started to delaminate and smoulder. The main difference from the original design is that the board is now 0.8 mm thick but I have also not put any filled planes on it.The overheating of the board is starting just as the parts reflow. After the 1st one went up in smoke I tried another one and observed it and as soon as I saw the PCB start to bubble I pulled the tray open and it was saved and the parts were perfectly re-flowed.So has this been caused by the thinner board which I did in the hope of saving material yes I am an environmentalist but also saving weight for when these things are shipped in volume from China or was this caused by the lack of copper pours all over the board? Does the copper covering do something to protect the a FR4 from the infrared heat?Do I simply need to use a different reflow profile? I'm not sure that these ovens actually allow me to create my own profile. Time perhaps for a new controller for it?

author: janoc
author_rank: Super Contributor
timestamp: July 24, 2016, 09:55:50 am
content: AFAIK, those cheap ovens are notorious for irregular heating - you get cold and hot spots. The thinner board and lack of groundplanes could be enough for the board to burn if placed in such a hot spot. The copper helps to spread the heat out, so if you don't have it, poof.You may want to try to move the board around until you find a colder place. Or borrow a thermal camera and check.Another possibility is that you need a different profile for the thinner material - which is quite normal, those profiles need to be tweaked.You could also simply have a crappy laminate - I have also stock of some which delaminates and burns even under a normal soldering iron at 320C

author: Simon
author_rank: Unknown
timestamp: July 24, 2016, 10:07:28 am
content: I see when in fact the small amount of copper plane I am using as heat sinks and for LEDs does seem pretty much untouched.I do in fact have a thermal imaging camera. I'm not sure how it would help me though. This is a very small oven I have previously tried to do 9 of my 2 in.² boards all at once but this meant that I had 3 boards in between the 2 heating elements and then to lots of 3 boards either side of one heating element and the boards that were on the 2 outer rows did not properly reflow. So the only way of getting proper heat onto the board is to put them down the middle where they are going to get the maximum temperatures anyway.The boards came from PCB cart I've had a few deliveries from them and generally not had a problem. This is however my 1st attempt with 0.8 mm boards. I think I will add some ground planes to help distribute the heat. With the copper actually reflect the heat from the infrared bulbs? Even on the bottom of the board I can see that the bottom has heated up around small copper plane but under the copper plane the board looks fine.I'll have to read chinglish manual and see if I can create my own profiles it looks like these boards need less preheating and a faster rise to reflow temperature as there is less material to preheat.

author: bookaboo
author_rank: Frequent Contributor
timestamp: July 24, 2016, 12:26:26 pm
content: Do you have the temperature probe positioned correctly? Best practice is to secure it to the top of the PCB you are reflowing, or if you are doing several boards attach the probe to a blank PCB of same design and place in the oven near the PCB to be reflowed.The one disaster I had on my oven was when the probe came loose and the oven went beserk trying to hit the target temperature profile, that is not a nice smellI would guess that if the probe and oven did its job on the 1.6mm it should do the same job on the 0.8mm, it should just automatically adjust?

author: Simon
author_rank: Unknown
timestamp: July 24, 2016, 01:23:41 pm
content: that is what I thought. I'll have a look at the sensors, They usually just hang above the board.

author: nctnico
author_rank: Super Contributor
timestamp: July 24, 2016, 01:36:05 pm
content: I'd say you'll need less energy to get a 0.8mm board up to temperature so my guess would be you'll need a profile with a shorter heat-up time. Maybe you can find various profiles for board thicknesses somewhere (not specific to your oven) and extrapolate from there to get a base profile with some chance of succes.

author: ehughes
author_rank: Frequent Contributor
timestamp: July 24, 2016, 04:36:13 pm
content: I have been using this oven successfully.Step 1:https://github.com/UnifiedEngineering/T-962-improvementsImplement the cold junction compensation they recommend and flash the new firmware. It is much better.The new firmware is much better and controls the heat much better. It allows custom profiles.I would recommend getting another thermocouple and verify the temperatures it is reading is what you think they should be

author: Simon
author_rank: Unknown
timestamp: July 24, 2016, 05:38:31 pm
content: Quote from: ehughes on July 24, 2016, 04:36:13 pmlooks like someone needs to set themselves up doing the mod for a fee

author: Niklas
author_rank: Frequent Contributor
timestamp: July 24, 2016, 07:50:55 pm
content: The amount of copper and epoxy could make a difference, but are you sure that both boards are made with the same FR4 material? Laminates and prepregs could have different Tg-ratings (glass transitioning temperature). This could also affect the reliability of the via hole plating as the thermal expansion increases above Tg, typically from 60 (below Tg) to 300 ppm/'C in the Z direction.

author: Simon
author_rank: Unknown
timestamp: July 24, 2016, 07:54:26 pm
content: no they came from different factories. But I have had no problems with the 1.6 mm board from PCB cart. I'm guessing I need to change my oven settings if possible.

author: IanJ
author_rank: Unknown
timestamp: July 25, 2016, 06:22:21 pm
content: Quote from: janoc on July 24, 2016, 09:55:50 amThese guys do an upgrade, which includes a new LCD. When I got my T962A I never even switched it on before I did the upgrade.http://www.estechnical.co.uk/reflow-controllers/t962a-reflow-oven-controller-upgradeT962A Reflow Oven v4 Controller UpgradeAccurate temperature control;Even heating of PCBs;Prevents wastage through damaged or destroyed boards and components;Prevents the need to rework incomplete reflow soldering;30 easily adjustable reflow profiles;Pre-programmed with a profile ideal for lead-free soldering;Firmware v2.7.Ian.

author: Simon
author_rank: Unknown
timestamp: July 26, 2016, 06:57:10 am
content: I might get one when they are back in stock

author: Simon
author_rank: Unknown
timestamp: July 31, 2016, 07:23:01 am
content: I did another board today on profile 2 of the oven, the top temperature is slightly lower. It went fine but this board has a 16x16 electrolytic cap and a large inductor on it as well as being fully clad in 2oz copper.

author: DerekG
author_rank: Frequent Contributor
timestamp: November 05, 2016, 02:22:22 am
content: Quote from: Simon on July 24, 2016, 09:29:45 amInstalling a small low flow fan to push the heat around inside will even up the temperatures. Also "preheating the oven at a lower temperaure with the board in it", then increasing the temperature just at the end of the cycle to fully melt the solder paste often helps.

author: Simon
author_rank: Unknown
timestamp: November 05, 2016, 07:49:30 am
content: , there is the small problem of the oven being at up to 250C where most electronics and plastics get a bit upset at 125C. It would have to be a shaft into the over from outside so that the motor can be external.

author: Simon
author_rank: Unknown
timestamp: November 05, 2016, 05:11:15 pm
content: The lack of air movement is certainly a problem in these oven's. I did a batch of nine 2 in² boards and the upper and lower row of boards did not properly reflow because they were only being shone light from one direction. I don't know how much of the infrared heating process relies on the infrared praise directly hitting the object being heated or if wafting air around the oven would move the heat around enough to give more usable space in the oven and more even heating. As things stand the other is only any good for boards that fit between the bulbs will only overlap slightly.

author: SeanB
author_rank: Super Contributor
timestamp: November 06, 2016, 05:51:10 pm
content: Easy way to move the air is to go get a Defy thermofan oven spare fan motor and blade. The motor sits outside the oven, and has a long shaft that projects into the oven with the fan on it to stir the air. It will work up to 130C motor surface temperature with reduced life, and in the oven use it normally has there is little air circulation and no insulation there in any case, so they run hot. Run it in a smaller oven and blow cooling air over it and it will outlast the oven.

author: wraper
author_rank: Unknown
timestamp: November 06, 2016, 06:14:21 pm
content: Fan will somewhat help but not a whole lot. Speaking from experience with different IR oven which has a fan.

author: salbayeng
author_rank: Frequent Contributor
timestamp: November 21, 2016, 11:12:44 am
content: Quote from: DerekG on November 05, 2016, 02:22:22 amWhile I have access to proper factory with reflow oven, I normally reflow prototypes using a $20 Kmart toaster oven, (and a digital thermocouple readout) using almost the exact method you described. Preheat (with toaster thermostat at 200C) with both elements to ~ 120C, switch to bottom element wait till about 160C (it's fairly slow getting there, but a nice soak at ~160C is good for bigger parts)Then both elements on, watch for reflow from 190C especially on the big parts (usually happens ~ 200C) then at 205C (or 210 if big parts are slow to reflow) power off and pull door open. Wait a minute or so and slide PCB's onto a thick metal sheet (to cool down).The top element on the toaster is basically all IR heating, and it's heating isn't uniform, the secret is to get a good preheat using convection (bottom element).If you try just IR (top element) you will burn the PCB in places before it reflows.