Repair COB (blob) electronics

[prampec]

Hello,
I have some faulty calculators I would like to fix, but there are way too few components in them. Just a COB and 3 passives. First of all, what could go wrong in these circuits (not mentioning the visible problems)? E.g. COB can fail? Is there a method to fix it?
Thanks,
Balazs

[schmitt trigger]

I could be wrong, but those hard epoxy blobs cannot be removed without damaging the silicon chip itself.

But, let’s assume you can remove safely all the epoxy without cracking the chip itself or pulling the bonding wires… What comes next? Do you know the chip’s part number? Do you have the means to detach and re-attach the bonding wires?

[Haenk]

unfixable, sorry
as mentioned, the blob is not removable, also it encloses the die and bonding wires - thermal attack will destroy the pcb, chemical attack will destroy the pcb, physical attack will likely destroy the die and certainly the bonding wires
However COB calculators are usually not very rare, so just buying another one is probably cheaper than working time anyway.
Btw. the first to use COB is likely the Novus 650. Which was really built to budget.

[prampec]

Thanks for the answers.
COB is not fixable, I was expecting that. But does your answer means, that in these cases the COB failed? It is quite hard for me to believe a blob covered silicon chip can go wrong.
E.g. does it contains some EEPROM that can be reprogrammed? (In theory, because nobody has a firmware for that, that's for sure.)

[Haenk]

It's probably not too hard to reverse voltage in a calculator (switching battery direction or wrong polarity/voltage of external PSU), and COB always means "lowest costs possible", i.e. not a lot of protection, if any at all.
Usually, these CPUs are mass-produced with masked ROM, so no EEPROM.

If the CPU is broken, the unit is done.
However there sometimes are a protection diode and a small electrolytic cap for voltage stabilization - these can fail and could be replaced.

[CaptDon]

Could be some of the tiny electrolytic or tantalum capacitors are bad? I recently fixed an electronic 'Master Mind' game that was built very similar to a vacuum flourescent display calculator by replacing EVERY capacitor. Even the mylars were bad!! Probably really poor quality to begin with. Also, some of the soldering was suspect with years of mechanical shock. Some of the Sorenson power supplies with digital panel meters using the black blob technology were not repairable. The meters drifted and had no way to be zeroed. We just used analog meters as replacements.