[R-390] A question about silver-mica capacitors failures.

[Charles Steinmetz]

Jacques wrote:

> It was flagged by many contributors to this site that the number of silver
> mica capacitors failures is rising.
>
> As the failure mechanism seems to be understood (silver migration within the
> capacitor package), it is not clear for me if this occurs only when a
> potential difference is applied to these capacitors, or if it can be
> triggered only by the time, even if the radio containing those capacitors
> was left unpowered for years.

First, note that two common types of silver mica capacitors are found in 
boatanchor radios.  First came molded mica capacitors, mostly seen as 
the "postage-stamp" type.  As their name implies, a plastic body was 
molded over a naked capacitor assembly (generally, with leads arranged 
axially).

Then came the more modern dipped silver mica caps, overlapping in time 
with the molded plastic types.  These used a similar internal capacitor 
assembly (usually with the leads arranged radially rather than axially), 
which was literally dipped into an epoxy encapsulant.

The notable difference between molded and dipped mica caps is that the 
dipped epoxy type has *much* better environmental sealing than the 
molded type.  As we see below, this is a clue to the failure modes we 
experience.

The failure rates of mica capacitors are correlated directly with the 
environmental insults they have suffered during their lives.  Water and 
moisture, in particular (but also atmospheric pollutants like gasses of 
sulfur and peroxides) seep into the innards of mica caps and cause 
silver migration and failure.  So, we see a very high failure rate in 
mica caps that have spent time underwater (same with ceramic caps, BTW), 
and a lower but still considerable rate for mica and ceramic caps that 
have been stored in humid environments (i.e., the way surplus radios and 
capacitors are very often stored).  Here, caps in unused radios and caps 
sitting on the shelf are *more* at risk than caps in radios that are at 
least occasionally used (the heat from operation tends to drive out the 
moisture).

In all cases (water, humidity, and atmospheric pollutants), the failure 
rates for dipped mica caps are quite substantially lower than for molded 
mica caps; but poor storage conditions of unused radios and caps on the 
shelf eventually get to them all, so failure rates of dipped mica caps 
are expected to increase in the coming decades (note that the dipped 
caps have a double advantage -- besides being better sealed, they are 
also generally somewhat newer than molded mica caps).

Best regards,

Charles