Cool down too fast after bake | cracking and popping heard during cooldown. - Page 1
Forum section: electronics / general

author: funbags
author_rank: Contributor
timestamp: November 11, 2018, 04:15:42 pm
content: Hey All,TLDR; With a panel with low thermal mass, how long should I let it cool before taking it out of the oven?I've been impatient and usually after 4 min, when PCB is about 120c, I grab them to cool on a silicone pad while I bake the next panel. Would switching to a Pb solder help since the temps would be much lower? As it sits on the pad, I hear cracking and popping as the solder cools.Is this bad?Controleo 3 B&D toaster ovenChipquick SAC305 with peak at 225cI'll keep searching the forum, but I though I'd burn this candle at both ends.Thanks for any help.-a noob

author: Mr. Scram
author_rank: Super Contributor
timestamp: November 11, 2018, 04:20:43 pm
content: Have you tried cooling the board slower? Glass manufacturers prevent glass from breaking by having it slowly cool. Something similar may reduce stresses in PCBAs.

author: funbags
author_rank: Contributor
timestamp: November 11, 2018, 04:36:11 pm
content: MR.Scram. Thanks for the reply.I havent tried cooling them more slowly, because it was just a recent thing I noticed in the last batch. I'm just wondering if the noises I'm nearing are normal, or if I'm setting up the boards for failure. I bought some SnPb paste to try out on the next batch. So I can also try cooling them more slowly. It's just that there is some setup time with new paste profiles, and if I can avoid using Pb I'd like to (mostly for health reasons). I can try it out tomorrow, but I need to dig up some throw away LEDs to test the new profile with.I'll try baking 2 panels at once and then doubling the cool time. That shouldnt impact my flow.Thanks!

author: helius
author_rank: Super Contributor
timestamp: November 11, 2018, 04:37:25 pm
content: Temperature profile includes both ramp up (warm up), soak, reflow, and cooling periods and both the duration and °C/sec of each period should be controlled.Putting a hot board on a cool surface is not a controlled process. Additionally, the temperature below which it is safe to handle a board is more like 100°C, not 120.

author: funbags
author_rank: Contributor
timestamp: November 11, 2018, 05:06:45 pm
content: Thanks Helius.I'll definitely stop taking them out so quickly and let them cool slower in the oven in the next batch.Is the noise expected though?The profile from chipquick only shows a reference temp in the cool down phase of 217c 30 seconds after peak. So I was assuming that's the only important number. Which curiously is somewhat faster than the cool down I'm using since I only have an auto door, and not a fan.Do PCBs themselves have a recommended cool down profile? Like, something that is separate from the reflow profile?Thanks!

author: helius
author_rank: Super Contributor
timestamp: November 11, 2018, 08:15:39 pm
content: The noise isn't normal, it may be caused by the component legs shrinkingbetter to use a rack where the board cools passively in ambient air, than to shock it with a cool surface.

author: mrpackethead
author_rank: Super Contributor
timestamp: November 12, 2018, 02:02:14 am
content: i'd suggest you do not want to exceed abotu 4C/sec for the cool down.

author: jmelson
author_rank: Super Contributor
timestamp: November 13, 2018, 10:09:31 pm
content: Quote from: funbags on November 11, 2018, 04:15:42 pmHow LOUD are these noises? I have heard some relatively soft sounds that I am pretty sure is due to no-clean flux cracking, quite possibly as it absorbs room moisture after going to 0.00% humidity in the oven. I've never worried about it. I can also imagine a panel may develop stresses where the individual boards meet that causes some noises, but that should also be harmless.But, if the noises are loud, then something bad is likely going on, and may be component packages cracking or something like that.Jon