of that nickel coating is essentially the density of
pure nickel. But when you are doing electroless
nickel plating, you’re plating a nickel-phosphorus
alloy. The density of phosphorus is 1.8 gm, vs. 8.9
for nickel. The nickel phos coating will have a vastly
different density. You can calculate the density of
that alloy, and for every 1% phos, it’s about a 3%
difference in density, translating to a 3% difference
in actual plated thickness.
That’s why knowing the real number is important.
Adam Hammond is Technical Director at Metal
Surfaces, Inc. (Bell Gardens, CA) with responsibility
for quality operations. He offers this insight: “In
electroless nickel, phosphorus is the key determinant
for magnetism and corrosion resistance. % phos
also impacts a variety of other properties such as
deposit stress, and electrical resistivity.
“Electroless nickel deposits with significant phos
content are nonmagnetic, even though nickel itself
is magnetic. So for military and other programs
where magnetism is undesirable, high
phos EN is an answer – but you must
have good process control to
maintain the correct % phos, and be
able to validate your results.
“The industry overall is becoming
more prescriptive, with more
oversight and tighter flow-down
requirements – oftentimes tighter than
the primary specs. So if you have a
spec that calls-out mid-phos at 5-9%,
it is common to see a purchase order
or blueprint that supersedes it. It’ll
flow down 5-7% or 6-8% – a much
narrower range. In these instances,
the spec defines the majority of what
the engineer or buyer needs; focus
can then be placed on a few
parameters that need to be controlled
more closely.
“If there are forming operations after
plating, like crimping or bending
tabs, an engineer or buyer who uses
electroless nickel and understands its
properties and how phos content can
impact them has great deal flexibility
in part design and can tailor
flow-down requirements accordingly.”
So why not plate electroless nickel with high phos as
the default? One reason relates to processing
constraints, another to deposit properties. High
phos deposit rates are typically the slowest at
roughly 300-500 microinches per hour. With
mid-phos, you can expect as high as 1000
microinches per hour, so absent a spec to the
contrary, the plater may choose the lower phos
option to accelerate throughput.
Regarding the plated deposit, despite the
advantages of high and mid phos EN, lower phos is
harder, both as-plated and following specific heat
treat operations.
About the author:
Paul vanden Branden is Director of SciMed Ltd for
Bowman XRF, a leading US manufacturer of
benchtop XRF plating thickness measurement
instruments.
For more information email paul.
[email protected]
Bowman XRF analyzers with silicon drift detectors are
ideal for measuring electroless nickel and other coatings.
They combine low noise, optimized charge collection, and high
uniformity from detector to detector. The detectors offer best
in class peak-to-background performance and accommodate
incoming count rates up to 1,000,000 counts per second.
Bowman’s L Series XRF coating thickness
measurement instrument accommodates
samples up to 22” x 24” x 13” (LxWxH).
It is manufactured with an Si PIN or
Silicon Drift Detector (SDD
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