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November 2004

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From:
"Dehoyos, Ramon" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Dehoyos, Ramon
Date:
Tue, 16 Nov 2004 13:06:47 -0500
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        Hi Brian:
                    To obtain good results during SIR testing the voltage should not exceed 100V/mm. Is that a true statement? This is for uncoated boards. For coated boards it should be higher, Shouldn't it?
        Regards,
        Ramon

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Brian Ellis
Sent: Tuesday, November 16, 2004 3:26 AM
To: [log in to unmask]
Subject: Re: [TN] Solder Mask expectations / pushing the envelope?


I'm sorry, as I've said hundreds of times and few seem to think it
important, it is not the test voltage that matters; it is the voltage
gradient. When will this be understood? If a test pattern has spacings
of 0.1 mm and you apply 5 V, the 50 V/mm is moderate. If you apply 400
V, you have to have an 8 mm spacing to achieve the same moderate
conditions. Believe it or not, gradients as low as 5 V/mm still cause a
significant and measurable ionic migration within FR-4 and other
epoxies, even under room conditions, resulting in polarisation and
electrophoresis. At 150 V/mm, the conditions are still just acceptable.
Anything higher and the ionic forces within the epoxy will start to
cause a slow breakdown, which can be accelerated by increased moisture
absorption causing the ions to be more mobile. Note that this migration
is not a conduction, at least until breakdown starts.

You may remember in a recent post, I explained the prepolymerisation of
epoxy resins between epichlorohydrin and bisphenol-A (or TBBPA) in the
presence of sodium hydroxide. I mentioned that the NaOH was used to
capture the chlorine atom of the epichlorohydrin. The resultant sodium
chloride is partially removed by ion exchange in electrical grade resins
and it is the residual quantities of this molecule that cause the
migration I mention (and cause the hygroscopicity, high dielectric
constant and poor power factor of the cross-linked resin). It is also
possible that the sodium silicate in the glass may also play a minor role.

I brought up all these points in various papers in the late 80s, early
90s, particularly with regard to SIR testing, where I suggested that
true SIR can be measured only at very low voltage gradients without
bias, because the very slow movement of ions within the resin falsified
the results, even without conduction.

Brian

Robert Coburn wrote:
> Hi T'Netters;
>
> One of my customers stated they were experiencing what sounded like a
> shorting problem due to ionic contamination.
> Can't tell if the problem originates beneath the mask or not.
> The customer was testing assemblies at 85o C, 85% RH, 400 volts for 168
> hours.
>
> The question is: is the Customer expecting too much from a Solder Mask?
>
> TIA,
> Rob
>
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