I have 15 years of all types of SMT and thou hole soldering/rework.

I recency had an automotive part fail, idle control value in a BMW. straight forward circuit. It was sealed in some rubber type resin i manage to remove.

I tried to remove a small sot-23 diode and could not, I then tried to solder some pins, nothing touched it.

I then used hot air to just experiment and remove a basic 1206 cap, nothing touched it. My thoughts was lead free solder higher temp, so wacked up fuill hot air.

So I basically put a serious amount of heat close up, the cap even slightly started to dis color. Nothing touched it or any of the 20 components on the small board.

Has anyone ever come across an un solder able PCB? the components are standard IC;s and transistors. I manage to get some solder to stick to a leg of a mosfet and the side if the 1206 capacitor, but for 30 minutes of trying to solder or hot air remove a component I failed.

dose anyone have any idea what on earth is going on? i though glue, but there not glued down. its just really strange. plenty of flux, heat etc nothing.

Jason

 

Are you sure the potting compound was not between the SMT device and the board?
Max.

 

dose anyone have any idea what on earth is going on?

yep. It wasn't solder. It was conductive Epoxy. See http://www.mouser.com/pdfdocs/FAQOtherConductiveEpoxy.PDF

Quote:

Conductive epoxies are mainly used in
applications where the risks of mechanical and
thermal cracking are very high, as well as when
there are concerns about damaging heat
sensitive components on the PCB during
soldering

 

Thanks very much - can you tell me if it's pitiable to rework small SO8 ic and Mosfets with the eproxy paste - do I need to smash them off? Do you have any tips for reworking please

 

No. See https://www.tedpella.com/technote_html/Reworking_Cured_Epoxy.pdf

Both MEK and Toluene are very nasty chemicals to work with.

Besides grinding down the component or effectively milling the surface, so all you have are leads to debond, I can offer no other suggestions.