Firstly, are the comb patterns tested from snap-off coupons from the 
production PCBs? If not, you may have qualified the flux, but not the 
combination of flux, components and production quality boards. Secondly, 
do you think a 196 hour test would be representative of real-life 
conditions over a few thousand hours? Thirdly, if I understand you 
correctly, you have a flux residue porridge in contact with humidity 
sensors, without knowing the hygroscopicity of said porridge. Fourthly, 
I would be very cautious about conformally coating a) over humidity 
sensors b) over potentially hygroscopic flux residue porridges and c) 
over flux residues generally unless you have carefully qualified the 
process (and you would need more than 168 hours testing time). Never 
forget that conformal coating may be a moisture barrier, but it is never 
a long term humidity barrier.

I'm sorry, but only you can answer your questions in detail, by long and 
careful systematic qualification testing.

Brian

John Goulet wrote:
> Is there any information on products with No-Clean flux being effected by 
> moisture over a period longer than that tested in SIR tests with the SIR comb 
> test coupons? Some customers make products for vehicles and toys that are 
> subject to more humid conditions. The No-Clean fluxes used always pass SIR 
> which have high resistance values even after 196 hours. However  if the 
> humidity levels rise and fall but never to the point of drying out the moisture. I 
> thinking that the humid spring and summer months could be a problem if not 
> sprayed with a conformal coat. The other situation I'm testing is when a 
> moisture sensitive part is used. Tests on three of the boards showed that the 
> moisture sensitive part was dragging the surface insulation down. When I 
> localized the heating to this part with a small Leister heat gun, the resistance 
> across the battery terminals with the battery removed went back to infinity. 
> Usually the moisture sensitivity is a factor relative to the SMT reflow process 
> and not an issue in operation. Any info on this? PS this batch was the first 
> ones using Pb-Free solder paste and that means I'll have to check if the BOM 
> changed to a different RoHs version of this special moisture sensitive part.
> 
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