There is no specification, nor can there be. There are far too many
variables. Ionic contamination testing cannot correlate the reliability
of assemblies with flux residues thereon. However, your ionic mix sounds
very suspicious, although you do not mention the quantities of each
anion. I suggest that your problem MAY be due, at least partially, to
your bare boards coming from the PCB fab guys complete with a hefty dose
of contamination. Most RMA fluxes contain small quantities of EITHER
bromides OR chlorides, but I don't know of any with BOTH. This would
suggest, e.g., the board is HASLed with a Br- flux (common) and soldered
with a Cl- one (also common) and insufficiently washed in between. Of
course, the bromine could also come from the board itself (flame
retardant), if your measurement process would allow it to be leached
out. As for nitrates, and more especially, nitrites, I cannot imagine
where they came from. Neither are anions commonly used in electronics,
to the best of my knowledge. Is there any possibility that the boards
could become contaminated after they were put in service.
Nitrates/nitrites are commonly used in agriculture and meat processing,
as well as various chemical processes, and are also common in the gases
released after the combustion of explosives and rocket fuels.
It could also be helpful to know what cations were found on the boards,
as well.
OK: the practicalities. If you seek a reasonable reliability with
"no-clean" soldering, firstly, you must routinely check incoming bare
boards for ionic contamination. Here, it is possible to set up an
in-house standard, and I would suggest that 0.5 µg/cm2 eq. NaCl would
not be exceptionally severe. Then, make sure there is no process within
your own factory which would introduce further ionic contamination,
other than the flux itself. Then the flux itself should come under
suspicion. RMA fluxes are perhaps not the best for "no-clean" processes:
they were not designed for this and some of them do leave residues which
contain fairly high halide levels. Personally, I would plump for a
low-residue non-rosin flux. In all cases, I would qualify the
post-solder flux residues with ECMR testing and, for critical circuits,
SIR testing, before adopting ANY flux.
Of course, if the contaminants reach the circuits in service, then you
need additional protection and, possibly, even cleaning and conformal
coating.
Now for your basic question. Post-solder (with "no-clean" chemistry)
ionic contamination testing CAN be useful, but only as a means of
process control. The MO is simple. When your process is working at its
optimum, with clean incoming boards and components, measure, say, 50
identical boards and calculate the mean and standard deviation of the
results (the SD should be quite narrow, say, < 10% of the mean). These
will be your baseline. Repeat for all your different circuits. In
subsequent production, take hourly samples, measure them, and check that
they fall well within the statistical probability of your baseline
measurements. If they consistently fall outside, IN EITHER DIRECTION,
for that type of circuit, then your fluxing/soldering process has gone
off kilter and corrective action may be required. Note that the absolute
values of the means are meaningless (no pun intended) and bear no
correlation to the reliability.
Hope this helps.
Brian
YH Koh wrote:
>
> To TechNet,
>
> Wondering if there is a specifcation to indicate the limit of ionic
> contamination for an after reflow boards. Understand that the amount of the
> flux activator remain on the boards is depending on the amount of solder
> joints on the boards. So, is there a criteria to address the amount of
> ionic versus the pads area or other related criterias?
>
> We are currently having corrosion on the board (after 1 year of application
> in the field). The flux we are using are no clean RMA flux. After
> extracted the flux remnant from the boards and subjected for Capillary
> Electrophoresis (CE-MS) test, we found high amount of bromide, chloride,
> nitrite and nitrate. I am not sure if thease are the elements that caused
> the corrosion.
>
> >From here, I am not sure if it is practical to specify the ionic limit on
> the after reflow boards? Or should continue to work on the driven factor of
> the corrosion like water, voltage, electromigration etc?
>
> Appreciate any inputs.
>
> Best Regards,
> YH Koh
>
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