PCB melting

[bittumbler]

Hi,

what causes this PCB delaminating/melting/bubbling?

The PCB is a new PCB from Jlcpcb. Hand populated with some parts by me. Then reflowed with an IR reflow oven Puhui 962, with builtin profile 1 (Pb) or 3 (nonPb).

This problem happens only sporadically. Most of the times, i get good results.
I did some experimenting with blank PCBs and different profiles, locations in the oven, etc., but i am not able to reproduce this.
How can i prevent it?

TIA
Matthias

[tooki]

That happened to me once when our IR preheater’s controls failed, causing it to run at full power uncontrolled.

This is the result of massive overheating. Sounds to me like something’s wrong with your reflow oven.

[Fraser]

That appears to be classic localised overheating of the resin component in the PCB material. Causes could be as follows….

1. Intermittent temperature controller failure to maintain the correct oven temperature.
2. Uneven oven temperature within its chamber. Normally due to the location and shape of the heating elements.
3. Defective PCB material with air pockets or other defects that cause localised overheating.
4. Moisture ingress into the PCB material… the water boils and becomes super heated steam if sealed within the material and bursts the PCB layers which causes a void and overheating.

Fraser

[Fraser]

You could test your ovens temperature distribution with a test plate of plain FR4 and check it with a thermal camera for heat distribution once the oven reaches maximum reflow temperature. (Remove the board from the over to view it)

A more risky test would be to simply place a piece of paper in the oven and monitor it as the temperature rises….it will scorch in the hot spots before combusting ! This is not a test I recommend but it is cheap ! Do it in a safe location. You will likely be dealing with a burning piece of paper though so do consider how you will deal with that !

For information, paper will combust at differing temperatures depending upon thickness, moisture content etc. As a rough idea it will potentially start to combust at 218C (424F) and will very likely combust at 246C (475F). BE SAFE !

Fraser

[bittumbler]

Thanks for the input.

So its likely a problem with the oven. After doing some more searching, i found a site with modifications for this oven. https://github.com/UnifiedEngineering/T-962-improvements/wiki
Will try them, especially the cold junction mod. Then see, if the problem happens again.

Thanks

Matthias

[jpanhalt]

There are several threads on EEVBlog about the T-962, including this: https://www.eevblog.com/forum/manufacture/t962-reflow-oven-233972/msg3929969/#msg3929969

There are 3 important mods:
1) Use the Unified Engineering firmware.
2) Add the cold junction reference.  Be sure to get a version spec'd for parasitic power.  Otherwise, the mod is a little more difficult but can still be done.
3) Replace the masking tape with something.  I used Kapton, but one might just remove it too.

[ogden]

That appears to be classic localised overheating of the resin component in the PCB material. Causes could be as follows….

Whole PCB was badly overheated because flux is nearly charred in other areas.

[Fraser]

Ogden,

Agreed, but the physical damage to the PCB is localised which suggests a localised hotter spot within the oven or a failure on the PCB lamination quality (now considered unlikely since reading of the OP's Oven model known issues)

Fraser

[bittumbler]

Hi,

what still confuses me, it works fine most of the time. I used the oven about 30 times, two times the PCB was burnt like in the picture.
However i just put the boards somewhere in the middle of the tray, not exactly in the same spot. So maybe one of the problems is nonuniform heating.

Thanks for your help.

Matthias

[Martin Miranda]

based on the picture it looked like a thermal runaway.
i suggest adding external over temperature alarm.

omron temperature controllers has a algorithm inside that
when the heater/sensor behavior has changed it will
raise an alarm. 

[Martin Miranda]

the color of the flux also shows over temps.

[Munyua44]

this is caused by overheating. Just check on the reflow oven first to ensure it does the right thing