On Monday, February 25, 2019 at 2:21:30 PM UTC, snipped-for-privacy@gmail.com wrote :
gned. These are 1206 22uf 10V ceramic caps, we do not specify a manufacture r. The caps are on the output of a 3.3V 1amp 2MHz dc to dc converter (5 amp inductor 2.2uH), there are two in parallel. They become resistive, around
200 ohms. The ripple current is around 0.4 amps total, so I cant see that i t is excessive ripple current, they are not near mounting holes so I cant s ee mechanical issues, and they initially pass test but fail in the field. H eating the cap does not have much effect on the resistance.'t
the cap of flux residue and tried heating and fault stays. It measures 186 ohms, another cap reads 400 ohms. Only one of the two parallel caps fail ( Both same part) and no other caps fail (Different parts and smaller). I hav e looked for ceramic cracks and cant see any but I am not sure if I would s ee them even if they were there!
, Rises in 10ms to 3.3V as expected
n parallel, both with very little resistance. The equivalent resistance is half the resistance of a single cap. With the failure, no matter how high i s the resistance of that cap, the equivalent resistance is still less than the resistance of the single good cap. Is it not enough? I suppose you use two caps to double the capacitance not lower the resistance. btw, 47uF/10V at 1210 (B size) tantalum is quite common. Is it not enough? Too expensive? In this case I bet you can find poscap capacitors these days with better r esistance (in fact VERY low) and probably less expensive than tantalum. Har d to say though if less expensive than two ceramics of this size.
When the caps fail the load current of the board in sleep mode goes from 20 uA to 5mA, hence batteries go flat. When I remove the two caps board curren t returns to 20uA. If I measure the resistance of the caps one is high impe dance, and one around 186 ohms.