I think this maybe choice of words. What you describe as moist could be wet,
and dry ones could be solid. I think that's what I mean or am I just
generalising from tantalum capacitors to the whole world?
Regards
Mike
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: Wednesday, October 13, 2004 8:25 AM
To: [log in to unmask]
Subject: Re: [TN] Aluminum electrolytic Cap's
Hey, wait a minute, Joyce. I haven't seen a WET elco for at least 50
years. All elcos today, to the best of my knowledge, are dry and neither
need nor have venting. A wet elco literally has liquid in them and, if
you shake them, you can hear/feel it slopping around. Wet elcos had the
advantage they were self-healing. I think the last manufacturer was
possibly Philips in the 1940s.
Dry elcos are not really dry, but have a moist absorbent material
between the electrodes, but they are not self-healing, therefore do not
generate gas. The dielectric is a pre-anodised layer on the anode,
before the capacitor is wound, so there is no electrolysis within the
case and no gas generation. The moist layer is actually there just as a
contact from the cathode to the thin dielectric.
And if what you say were true, there would be two problems:
a) you could not have hermetic components
b) if the component were vented, the small amount of moisture in the
absorbent layer would be lost by evaporation and the elco would become
pF, instead of uF!
Brian
Joyce Koo wrote:
> Disagree on the the coating... although the coating may be softer
initially,
> the acrylic coating can be hardened during the life of the product, may be
> interfere the venting mechanism. Agree with your customer, it should not
be
> coated (see attached).
>
> my 1.6 cents.
> jk
>
> "In the operation of a wet electrolytic capacitor a certain amount of gas
is
> generated from the electrolysis of the electrolyte and unless means is
> provided for the purpose quite high pressures may be built up inside the
> container. For this reason it has been found necessary to provide the
covers
> with some form of vent. In practically all instances this vent takes the
> form of a thin sheet of gum rubber which is perforated by a very small
hole.
> The thickness of the rubber and the size of the perforation determine the
> pressures at which venting action will occur. In practice it is desirable
to
> obtain venting action at the lowest pressure which will prevent actual
> leakage or escape of electrolyte through the perforation. "
>
>
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Brian Ellis
> Sent: Monday, October 11, 2004 2:32 AM
> To: [log in to unmask]
> Subject: Re: [TN] Aluminum electrolytic Cap's
>
>
> I see no reason why hermetically sealed elcos cannot be conformally
> coated. If an elco generates pressure, it is because it is already in a
> fault condition and should therefore be changed, anyway.
>
> Brian
>
> Lum Wee Mei wrote:
>
>>Hello,
>>
>>Since we are on electrolytic cap, how about conformal coating (acrylic) on
>
> them?
>
>>We have been coating them since day one as no one has instructed that
>>electroltic cap cannot be coated.
>>
>>Recently, there have been opinion that we cannot coat them as there will
>
> be no
>
>>gap for the pressure to escape - similar to the below issue?
>>
>>What are your opinions?
>>
>>Regards,
>>Wee Mei
>>
>>
>>Quoting Brian Ellis <[log in to unmask]>:
>>
>>
>>
>>>It depends on what cleaning medium you are using. If you are using a
>>>halogenated solvent and just a microgram penetrates inside the casing,
>>>you will see customers complaining of failures after 6 months to 2
>>>years. This is well-known and was publicised by Sprague in the 1960s.
>>>This company recommended using only hermetically sealed elcos if
>>>cleaning in halogenated solvents. They also tested aqueous cleaning and
>>>determined zero effect by aqueous cleaning, including with saponifiers
>>>which could cause superficial corrosion of the outer case. However,
>>>IMHO, it always makes sense with aqueous cleaning to have each stage
>>>slightly warmer (say 5 deg C) than the previous one, so that you always
>>>have positive pressure inside the cans while they are wet, to prevent
>>>the ingress of contaminated water.
>>>
>>>Brian
>>>
>>>Scott Lefebvre wrote:
>>>
>>>
>>>
>>>>We have a problem with the plastic disk coming off after being process
>>>
>>>through our In-Line PCB washer. What concerns if any should I have.
>>>
>>>
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