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By JohnLeung
Iam using a DMM to record the "online" temperature during the course of reflow in a 1500W oven. I've found even though the max temperature never got up to 250C in 7 minutes, the PCB is still over-cooked. Over cooked is so clear from the yellow color of the PCB substrate. It shouldn't be like that, right?

Any preheat in 100C, ramp up to 200C and shut down the oven in 7 minute was clearly followed, but I still have over-cooked PCB. Why?

Any idea / suggestion is greatly appreciated.

By moorejl
I have learned by experience that you just set your oven to 450F and let it come up to temp with boards on top getting preheated on a piece of foil, put your boards in on the foil and let reflow about 45 secs inspecting with flashlight for complete reflow and then open door, turn off oven and pull out tray to let cool. perfect every time and no controller needed.
By JohnLeung
Thanks mooreil

Actually after several times of over cooking, I am now switching over to two ovens. One of them set at 100C, and the other stays at 250C. After preheat the PCB (on a tray with foil) in 100C for 3 minutes, I just take the whole tray to 250C for 15 seconds. All parts got reflow, and I believe even though I need two ovens, the thermal stress on smt parts will be minimal. Agree?
By moorejl
Sounds like a good method. Allows for running multiple batches with one preheating while the other reflows and cools down. My cheapo oven ($30 US) doesn't have any insulation so the top of the oven gets close to 100C and I use that as my preheat "hotplate". Like you, the reflow happens very quickly and doesn't seem to melt the plastic on SMD rated connectors etc...