Topic: Effects of battery leakage on calculator hardware (p)
The four pictures below are to show the effects of corrosion on a calculator PCB ater a battery remained too long inside the device.
The calculator is a "portable" (as they said in the 70s') Commodore 796M, in other words an old pocket calculator.
The battery compartment is placed at the top of the calculator, hence it leaked over the display PCB, and the leaking product flowed within the tiny space between the small display PCB and the magnifying plastic block, in such a way that is was hardly visible at first glance.
Only the removal of the magnifying plastic block allowed to discover the disaster :
A : 7-segment block of LEDs, at the right end of the calculator display.
B : copper tracks finished with a gold flash that feed the LEDs.
C : bonded gold wires that link the blocks of LEDs to the tracks.
D : effect of corrosion on the tracks. The green colour of the corrosion tends to show that it is corroded copper, while gold, being a noble metal, remains intact. Actually, the leaking liquid attacked the copper from the edge of the tracks, and it developed underneath the thin layer of gold plating.
E : pad with tin-lead solder joint that receives the signals from one of the display drivers.
F : we can see that the corrosion has lifted the edge of the gold layer, it is very tiny, but clearly visible.
G : battery liquid that remained between the PCB and the edge of the magnifying plastic block, it finally hardened with time.
So, all of this corrosion had to be cleaned away, and I have selected a chemical approach for this purpose.
Half-way cleaning :
Cleaning completed :
Now, we have a clear view of the situation : the gold flash has disappeared in the cleaning, as the missing copper substrate was not supporting it anymore.
H : examples of where the copper track was totally corroded and disappeared.
I : copper was partially corroded only, but the gold flash does not adhere anymore to the copper, and the end of the gold layer is lifted in the air.
J : bonded wire on the copper layer. The bonds are still OK, but very fragile, as they are on the lifted gold layers.
The purpose was then to repair the corroded copper track.
I used a 100-110 micron copper wire from a common electrical cable of any household equipement. I just chose the thinner wire I could find in my collection of surplus cables.
Then, with a soldering iron, I soldered the wire and gave it the shape of the original track, well, I tried to, as it is very small, and it was all hand made.
K : added tin-lead solder to reinforce the weakened track.
L : areas were the remaining copper track allowed to firmly solder the wire.
M : ends of the gold layer on which the gold wires for the block of LEDs are bonded.
And that's it, the calculator was assembled again, and the display works perfectly.
Just one bad point : a row of keys from the keyboard doesn't work anymore, but this time it is due to an internal failure inside the integrated circuit, and I cannot do anything for this ! :-)