[volt-nuts] Oil and molded components

[Ed Palmer]

Dr. Frank Stellmach wrote:
> Hi,
>
> I would be careful on the selection of the oil for the molded Z201.
> The oil must be highly isolating, very pure, absolutely humidity free, 
> and must not contain acids or other substances which might affect the 
> resistor element.
>
> From the vishay site, one learns, that the metal can type is 
> hermetically tight, and if its oil filled, neither oxygen, nor water 
> vapor can degrade the resistor element. These two properties only lead 
> to the ultra stable 2ppm/6years drift.
>
> On the other hand, the molded type has a drift of 20ppm/yr., which 
> means the resitive element is vulnerable to both, despite the mold 
> compound.
> It's well known, that molded ICs also need a coating on the chip, and 
> that water vapor diffuses into the mold, if the device is exposed to 
> free air. (cracking during reflow soldering will occur)
>
Earlier in this thread there was mention of resistors that were coated 
with shellac or lacquer.  Would it be possible to coat a molded resistor 
to seal it from air, moisture, or oil?  Perhaps one of the high-voltage 
varnishes like this:  
http://www.action-electronics.com/msds/GC10-9002%20Spec%20Sheet.pdf

Ed
> So, if the Z201 is put into an oil bath, the mold copound will soak up 
> the oil like a sponge, and  the oil (and other substances)might 
> diffuse to the resistor element.
>
> On the other hand, if the appropriate, pure oil has been selected, 
> such an oil bath would reduce temperature drift AND degradation drift 
> in one instance.
>
> Btw.: I have seen a designation of such an oil recently, perhaps it 
> was on the Vishay technical site, describing the VHP types.
>
> Best regards - Frank
>