The importance of Cpk
The importance of C
pk
by Darren Brown, DEK, Weymouth, UK
For an industry characterised by acronyms, bullseye, you have a high C
pk
.” A general
Long used by the semiconductor
here’s another one: C
pk
. Three letters— rule of thumb states that a C
pk
value of
and circuit board assembly
short, simple, and to the point. Expressed less than 1.33 indicates a process that is
sectors as a tool for maximising
as a number that is equally short, simple incapable of producing a repeatable part,
end of line yields, reliability and
and to the point, C
pk
, or the ‘process capa- whereas a C
pk
of 2 indicates a process that
bottom-line performance, Cpk is
bility index,’ is in fact a sophisticated and is accurate, repeatable and absolutely under
soon to become a critical metric
powerful metric that enables us to see at control.
a glance how accurate and repeatable our So that, in a nutshell, is C
for the PV industry too.
pk
. But why it
processes are and what guarantee they give is due to become so important for the solar
us for future performance in this respect. industry?
Long used by the semiconductor and Before we answer this question, let’s
circuit board assembly sectors as a tool for first consider that, until now, conventional
Keywords: Cpk, Print-on-Print
maximising end of line yields, reliability silicon solar modules have typically been
Technology, Seletive Emitter
and bottom-line performance, C
pk
is soon laid up manually, by workers armed with
Cells, Machine Stability
to become a critical metric for the PV solder irons in factories in low labour cost
industry too. As new, more demanding areas. They have worked to placement
solar technologies come on stream, C
pk
will tolerances in the order of +/- 200 µm
be essential to the evaluation of processes that have been driven principally by
and their fitness for the future. It will aesthetic requirements—the features on
also provide invaluable information that neighbouring cells had to look reasonably
will allow PV manufacturers to compare well aligned.
equipment and make the right capital This is all about to change, as now
investment decisions. there is a functional reason for needing
It is therefore important to understand absolutely identical cells. Firstly and
what a C
pk
value really means. To this end most simply, the lay-up process is being
it is helpful to start from the elements that automated, so the grids and bus bars must
comprise C
pk
. be positioned accurately and repeatably,
First, capability. The results of most and justified according to module
numerically measurable processes, like, manufacturers’ requirements.
for example, print alignment accuracy, are But there are many other reasons why
distributed around a central value. If these C
pk
is fast becoming so important. One
are tightly grouped, the distribution curve such is print-on-print technology, which is
will be steep and narrow, indicating high being developed to increase cell efficiencies
process capability, or repeatability. by decreasing the surface area occupied,
Process capability (C
p
) takes this a step and therefore shadowed, by the collector
further, expressing the distribution curve’s grids. It does this by allowing grid fingers
breadth as a ratio relative to specification to be printed as fine as 50 µm without
width, indicating how comfortably the compromising their capacity to carry
distribution curve fits within tolerances. In current. This is achieved by printing the
other words, degree of process repeatability grid twice over, doubling its height and,
relative to specifications. therefore, its current carrying capacity. As
It is possible to achieve a brilliant we at DEK know from our experience with
C
p
but with a curve so off centre as to this procedure for the semiconductor and
be out of specification, and this is where biomedical sectors, repeatable accuracy
C
pk
comes in. This ratio is similar to C
p
is key here, primarily because, without it,
but also takes into account the distance this degree of fine line work would simply
between the central value and the closest be impossible. Consider too that a high
tolerance limit, indicating how well the definition screen print relies on an effective
process distribution curve is centred within gasket between the substrate and the
specification limits. underside of the print screen, so alignment
To borrow a simple analogy from a must be perfect. This is particularly true
Six Sigma forum, “If, when shooting at a of the second pass, where the landing
target, your gunshots form a tight group, area—formed by the first print—is so limited
you have a high C
p
. If you adjust your that a misalignment of even 5 µm can
sights so that this tight group falls on the result in printing paste flooding out and
12 – Global Solar Technology – July/August 2009
www.globalsolartechnology.com
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