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From:	"Bob Paddock (graceindustries AT gmail DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Date:	Wed, 10 May 2023 15:17:21 -0400
Message-ID:	<CAOuGh89nK=94FKpG+V1QZCpuXNkH4rB+4qzA+NDtRtf1ToBJdw@mail.gmail.com>
Subject:	Re: [geda-user] [off-topic] Large resistance value SMD resistors?
To:	geda-user AT delorie DOT com
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Reply-To:	geda-user AT delorie DOT com

On Wed, May 10, 2023 at 7:45 AM Richard Rasker (rasker AT linetec DOT nl)
[via geda-user AT delorie DOT com] <geda-user AT delorie DOT com> wrote:

> ... 22 Meg ...
> Would it be better to use a larger resistor (e.g. 1206), or forego SMD
> altogether? I can of course also specify a moisture-resistant PCB
> coating, but that is a rather expensive extra production step.

Coatings are not hermetic seals.

They are good at keeping out particle contamination.
However moisture will still seep through over time.

As it is now devoid of contamination it will never be noticeable in a
digital circuit, due to its high impedance;
perfectly pure water is an isolator.

However in high impedances, or RF tuning circuits, the dielectric changes.
Perfectly tuned transmitters are returned in six months for being off frequency.
Been there, done that. :-(  Water ran off the boards in removing the coating.

This is why some high impedance related parts. such as Electrostatic
Grade Op-Amps,  say "expose to air" in their data sheets.

As Frank pointed out, flux is an issue.

We really try hard to avoid QFNs and anything that is hard to clean
under in our low current (uAs) standby products,
Any residual flux will build up contamination over time, which could
be measured in months or years.
A 10 uA poorly cleaned board built and measured today, might measure
100 uA in six months.
Been there, done that too.

If there are no other through-hole parts consider putting two 10 Megs in series.
To avoid the labor costs of the stuffing charges.

Knowing more about the overall circuit may lead to better/different answers.

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