Mail Archives: geda-user/2023/05/10/11:01:26
You haven't told us enough about the circuit, its assembly, nor use lifetime to properly evaluate. However one more thing to consider is the solder/flux chemistry. Some of these leave fairly low sheet resistivity that could result in fairly mediocre performance. With good chemistries (low residual ionic content) some condensation could be tolerable with the fairly easy 30% tolerance band. 1206 doesn't gain much if anything (assuming uniform contamination) as the potential conduction broadens to the same degree as it lengthens.
Hope that helps,
-F
On 5/10/23 04:10, 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.
>
> Thanks in advance for any ideas,
>
> Richard Rasker
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