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

author: axxe
author_rank: Newbie
timestamp: May 30, 2019, 11:26:23 am
content: Hi experts,I have made a few PCBs and used a relatively cheap Chinese reflow oven to solder the surface mount components. I have done boards up to 100x100mm previously without any problem. But one of my latest designs seems to fail whilst in the oven. After the initial failure I asked the board house whether there was any issues with the batch that I had received, but their replay was that it was 5 months since receiving them and they suspected water ingress into the board causing the failure (The black board in the image - tested bare after a failed populated board). I tweaked the design a little and had the PCB remade using a different board house. The failure mode in the oven is pretty much exactly the same. (Red board in photo).It appears that the board is getting locally too hot in the oven. The area that fails is near the back of the oven, and part way through the heating cycle there is a bad smell and when the oven is opened I have a blistered board and oozed burnt board innards.Given that this has failed on 2 boards from different PCB manufacturers it must be to do with either my design or the oven. The only thing I can think about the design is that I haven't put a ground plane on either side of the board in this design. A ground plane would at least spread the heat more evenly across the board and may stop this localised burning. I have tried running a cycle of the oven with a thermocouple placed in through the draw and it seems to reflect the temperature expected in the profile, but it is possible that there is an exposed element close to the back of the oven.Any ideas on this?ThanksVance

author: Rerouter
author_rank: Super Contributor
timestamp: May 30, 2019, 11:39:01 am
content: Where is your temperature sensor? If it is a Type K thermocouple style, you can wire them in parallel to average out the temp across the enclosure, or just stick it to a problematic part of the PCB, so its regulating based on that part of the PCBDepending on where the sensor is currently, Its likely cooking your board before that reads the correct temperature. yes the air might be at the right temperature, but you care about the copper temp.

author: axxe
author_rank: Newbie
timestamp: May 30, 2019, 01:07:54 pm
content: Thanks for your reply. I may have a look at the oven, but its a hackspace owned item and they may not want me to "modify" it :-)I have been successful with other boards as I said. Do you have any thoughts on what the difference might be here? Smaller boards may not be close to the direct heat source, but could this also be the lack of ground plane on this board? I guess the way to find out is to create a version 3 of the board with a ground plane top and bottom and see how it fares...Vance

author: Yansi
author_rank: Super Contributor
timestamp: May 30, 2019, 01:10:30 pm
content: Why would one even want to bastardize PCBs in a hacked oven? You can more than easily solder those 10 or 20 components by hand.

author: OwO
author_rank: Super Contributor
timestamp: May 30, 2019, 02:01:12 pm
content: From that picture it's pretty obviously a severe overtemperature on that area of the board. IR ovens are ass, for a board like this just use a hot air station.

author: Peabody
author_rank: Super Contributor
timestamp: May 30, 2019, 02:21:45 pm
content: You might look at a hotplate. Here's a setup I made using US parts on 120VAC, but the idea would be the same for you:https://github.com/gbhug5a/Reflow-Hot-Plate-and-Controller

author: jmelson
author_rank: Super Contributor
timestamp: May 30, 2019, 06:44:36 pm
content: Water absorption is a REAL problem, but it will NOT cause burning. it can cause blistering of the substrate or bubbling of large copper areas. A bake out at 50 C for one hour before stuffing and reflow will generally solve that problem.But, you black board really looks like the temperature control failed, and the board got SEVERELY overheated. I have a setup that requires a thermocouple to be poked into a plated-through hole in the board. One time I forgot to place the thermocouple, and the boards caught fire, and looked a bit worse than your black one.It appears both boards had an uneven temperature across the boards. The tops (as in the picture) got much hotter than the bottoms.I use a toaster oven with thermocouple controller, and the only way to get proper temperature control is to have the thermocouple touching the board. With the thermocouple in the air, it can read 100 C or more BELOW the actual board temperature.Jon

author: ogden
author_rank: Super Contributor
timestamp: May 30, 2019, 07:07:39 pm
content: Quote from: axxe on May 30, 2019, 01:07:54 pmYou don't have to "modify it", they have to fix or replace it. Those boards are *burned* in temperatures far exceeding any SMD reflow profile. It is result of inferior/faulty oven.

author: NorthGuy
author_rank: Super Contributor
timestamp: May 31, 2019, 12:03:02 am
content: Since this is one-sided, the heat has not been applied evenly. Therefore, I would guess it's rather a problem with uneven heating rather than with sensing. I've seen similar damage to PCB at as low as 300oC.

author: axxe
author_rank: Newbie
timestamp: May 31, 2019, 08:33:15 am
content: Thanks to all for your comments. I am learning to create PCBs with surface mount components and as you can see its a bit trial and error. I will try hot air reflow and see whether it works better for me as I usually only have a few components, although again we have a cheap hot air reflow gun, so possibly variable results there too, but I can at least monitor and adjust as I go. The hotplate idea is possible, but only really usable at home (Not suitable for the hackspace).I'll also check out the oven/thermocouple.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 31, 2019, 08:48:25 am
content: Use old-school leaded solder instead of lead-free to give you an extra ~30 degC margin for error. All margin you can buy helps when the process isn't properly in control.Note that severely excessive temperatures might look just OK, usually the components (except plastic connectors) survive the extra stress. So I would start tuning the process by first finding a point where the solderdoes notmelt, then go up until you have it, instead of trying to back off from severe overtemp.