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Date:	Wed, 10 May 2023 16:04:01 +0200
From:	"Gabriel Paubert (paubert AT iram DOT es) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
To:	"Richard Rasker (rasker AT linetec DOT nl) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Subject:	Re: [geda-user] [off-topic] Large resistance value SMD resistors?
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	Hi Richard,

On Wed, May 10, 2023 at 01:10:55PM +0200, Richard Rasker (rasker AT linetec DOT nl) [via geda-user AT delorie DOT com] wrote:
> Hi all,
> 
> My apologies for this off-topic question, but I'm running into a question
> for which I can't find a good answer online -- and I know that some people
> here have special expertise
> 
> I'm asked to evaluate another tech's design, and apart from the inevitable
> small optimization things, he did a fine job. There is however one thing I'm
> not sure about: the circuit features a couple of 22 megohm resistors, and
> the current choice is simply a 0603 size. As the device in question may be
> used in environments without proper climate control, I think that moisture
> (condensation) may on occasion become a problem given the short insulation
> distance. All voltages are 15 volts and lower, so there's no hazard
> involved, but the circuit's specifications are no longer met if the
> resistance drops below ~15 megohms.
> 
> 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.
> 

Isn't it cheaper to put a encapsulate the resistor and all the solder
joint area in expoxy glue after the assembly (and testing)? 


I would consider products like:

https://epotekeurope.com/wp-content/uploads/2022/08/EPO-TEK%C2%AE-Adhesives-Applications-%E2%80%94-PCB-View.pdf

page 11 (potting compounds).

And yes, it will be difficult oimpossible to replace the resistor if it
fails, but the power dissipated by a 22MΩ resitor with less than 15V
across is so small that it should never be a problem in practice.

I'm not in any way affiliated with Epoxy Technology, and there are other
manufacturers. I've just used their products: H20E silver based conductive
epoxy to attach (ground coonection) backside of bare chips and 360T to
hold conical inductors like:

https://piconics.com/wp-content/uploads/2019/07/G5-Series-Low-Current.pdf

For the record, I bought half a pound (sorry for the units) of 360T in
January 2022, delivered in March, for a bit less than €200. With that
you can probably encapsulate thousands of 603.

The stated shelf life (in theory 6 months for 360T), and 1 year for most
epoxies, is from my experience very conservative. I've just hace a look
and my 360T still looks fine, will try to use it in a few weeks if all
goes according to plans.

	HTH,
	Gabriel

> Thanks in advance for any ideas,
> 
> Richard Rasker
> 
 

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