SCIMED LTD
What they often don’t realize
is that phosphorus can be
quickly and precisely
measured with XRF, a proven
and familiar quality tool – if
the instrument has the right
detector.
XRF systems use one of
several types of detectors.
Among the most popular for
the measurement of coatings
are Silicon PIN diode
detectors, which deliver
spectral resolution superior
to the “prop” (proportional)
counters that preceded them,
so thinner deposits and lower
concentrations can be
measured, along with multi-layered samples and
samples involving “mystery” materials. Silicon PINs
combine low noise, excellent resolution, low
detection limits and high stability. They are
“standard” for most XRF equipment lines,
but they are unable to measure phosphorus.
Silicon drift detectors (“SDDs”) are widely preferred
for the measurement of coatings. Their count rates
and spectral resolution are highest by far; they are
best at detecting very low kV photons. SDDs offer
substantial peak-to-background ratio improvement
over the alternatives, particularly when they are
paired with an x-ray tube that uses optimal target
material. They can also measure phosphorus, and
everything else on the periodic table between
sodium and uranium. The only downside is their
higher cost.
For a plater that wants – or whose customers
demand – verifiable phos numbers, the SDD is a
slam-dunk. For shops that envision more – or more
tightly specified - EN in the future, there’s a
conundrum: make the added investment now, or
wait.
Fortunately, there’s a new “middle ground.”
Companies can opt for a lower cost XRF with an Si
PIN detector – a system that will measure every
major industrial element except phosphorus - and, if
and when it becomes important to measure phos
precisely, or inline, (or at least in-house) swap-out
the Si PIN detector for an SDD.
Modularity is not revolutionary but it is new to this
Components used throughout automotive, aerospace and heavy
construction - and the printed circuit boards that run virtually
everything - depend on electroless nickel to function, and to last.
application. Upgrading to an advanced new SDD
has become more attractive as costs have declined
over time.
But let’s consider an alternative view. Some platers
(you know who you are) say “we pretty much know
where we are on phos, and it doesn’t vary that
much. So it’s fine to measure once or twice a
month.” (This is an actual quote.) Others
acknowledge that routine measurement would be
advantageous, either to prevent rejects, or to allow
the quoting of a broader range of more lucrative,
albeit tighter spec, programs.
In electroless nickel, phosphorus is in solution with
the nickel; the elements plate simultaneously.
Notably, if you have an XRF that can’t measure %
phos, you have to then “tell” the XRF unit what you
think the %phos is for it to function. So, you have to
know it, or assume it – and either way, commit to
that number.
So, you’re entering a number without knowing it’s
valid? In a competitive environment where precision
is increasing, this seems unwise.
And here’s another common scenario: companies
send samples for testing, and they come back with a
reading of, say, 9.5% phosphorus. But there’s a
place-holder for 8 in their calibration and often, no
one bothers to change it. (“It’s close enough!”) The
difficulty here: a 1% difference in phos translates to
a 3-4% difference in EN thickness – and that’s often
sufficient to produce rejects.
If you’re doing electrolytic nickel plating, the density
168 2020 - 2021 read online: www.surfaceworld.com