For long term reliability my choice would be to use an OA flux and wash the boards. This leaves behind no "porridge" (great term Brian) to do mysterious things over time. We have seen strange instabilities in circuits and spent countless hours trying to design them out. Problem solved by simply washing the boards.
My 2¢.
Phil
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: Sunday, November 09, 2008 11:00 AM
To: [log in to unmask]
Subject: Re: [TN] No-Clean flux & Moisture sensitive sensors
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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