I have exactly that issue on my desk these days. Battery packs that consist
of a small PWB with a hard impregnated paper on each side, a number of Lion
cells and these together are wrapped in a black plastic for protection. They
live in a hostile environment for years. The PWB is a regulator that
controls that the cells are accurately charged and recharged. Now, a certain
percentage are returned regularly because of failures. My analysis showed
that the chinese manufacturer indeed made nice PWB, but left lots of flux
residues. Exposed to humidity, copper dendrites grew between high impedance
conductors and caused the biasing going mad. In the worst cases the
recharging was stopped. My recommendation to the manufacturer was simply to
clean the boards and conformal coat them.
Inge
----- Original Message -----
From: "Robert Kondner" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Sunday, November 09, 2008 4:57 PM
Subject: Re: [TN] No-Clean flux & Moisture sensitive sensors
> Hi,
>
> Just another data point from a similar problem I once found:
>
> Current drain from a small Li battery can be increased by unit software
> failure. If initialization code or hardware is not properly initialized a
> board can arrive at a customers with a bad battery. In the process of
> testing the board the initialization completes and operating current drops
> to a normal value.
>
> Bob Kondner
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of John Goulet
> Sent: Sunday, November 09, 2008 11:32 AM
> To: [log in to unmask]
> Subject: Re: [TN] No-Clean flux & Moisture sensitive sensors
>
> I'm manufacturing the boards for the customer. We use his parts, his board
> design and the specified flux. The SIR board I referenced is that used by
> all flux manufacturers to qualify their solder paste and liquid fluxes and
> is not a coupon on the assembly. The IPC specifies a conformal coat
> thickness of .003 to .1xx". I'll have to research the different conformal
> coating specs in light of what you said. We meet with the engineers from
> Humiseal last week relative to product selection; acrylic, UV cure and
> water
> based products. They stated that they are designed to protect electronic
> circuits from harsh enviroments, out side products and that is why
> Humiseal
> is specified on all military products.
> This is why I know that if we build and test the board here and coat them
> the resistance won't change and discharge their lithium battery. Currently
> we build, ship and they test.
> Then they ship them out and return those that don't have an infinite
> reading
> across the battery or 3.3 to 3.1 volts on their 3 VDC battery. Baking the
> board on the returns at 60C (battery spec not to exceed 85C) has fixed
> many
> that didn't have bad batteries of a bad clock chip. Form now on we are
> testing the batteries before we put them in since I found 5 bad ones in
> the
> shipping container. The discharged batteries measure a resistance so it
> wasn't our boards in those cases.
> Now I need more information on "Bloc" Moisture sensitive parts. My
> recommendation Friday was to have us perform the testing and then
> conformal
> coat the board within 24 hours. Based on what I've seen and not knowing
> what
> happens to the board when it leaves us, I feel this will solve the
> problem.
> It will certainly remove the unknowns if we control all the handling and
> process steps.
> Looking forward to your reply.
>
>
> -------------- Original message --------------
> From: Brian Ellis <[log in to unmask]>
>
>> 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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