Process capability index: A better way to assess equipment capability
Process capability index:
A better way to assess
equipment capability
by Rita Mohanty , Speedline Technologies Inc., Franklin, MA, USA, and Daryl Santos,
Binghamton University, Binghamton, NY, USA
introduction
practitioner may know the formula to use in
Determining the true capability
The electronic material deposition process
order to calculate a process capability index,
of process equipment can be a
is a process by which material, e.g., solder
like Cp or Cpk, what that same practitioner
daunting task. While customers
paste, adhesive, underfill, etc., is deposited
may not realize is the fact that performing
have certain expectations regard-
onto the PCB for assembling and provid-
a process capability calculation on a single
ing the capability of supplied
ing mechanical/environmental support to
set of data may actually yield, depending
equipment, there is no specific
various components. In the early days of
on how the data are stratified, different
existing industry standard requir-
electronics assembly processes, demand on
results. For example, in a recent publica-
ing equipment manufacturers to
accuracy and repeatability was not as high as
tion1 a stencil printing study was performed
report their equipment/process
it is today, due to the larger form factors of
on solder paste deposition for a QFP208
capability. Hence it has become
that time. As the technology in component
component. Thirty prints (that were referred
a game of ‘statistical gamesman-
packaging continues to advance with compo-
to as ‘boards’) were studied, and the data
ship.’
nents and their spacing rapidly decreasing in
could be analyzed from the following points
What is a process engineer to
size, material deposition is becoming highly
of view:
do? We suggest a sound statis-
critical to the electronic assembly process.
tical approach using process
Some of today’s current examples are the
• Total number of deposits: 6,240;
capability indices (C , C ,
following: 0.3mm CSPs, micro BGAs,
• Total number of perpendicular
p pk
and others) before purchasing
miniature passives such as 01005 compo-
(w.r.t. squeegee direction) deposits:
said equipment. Many process
nents, various displays and MEMS devices.
3,120;
engineers are becoming aware,
The primary methods of depositing material
• Total number of parallel deposits:
particularly through 6 σ types of
for component assembly are printing and
3,120;
programs, of the ability of these
dispensing. For the sake of simplicity, this
• Each of the 208 pads had 30 depos-
statistical tools in determining
discussion will be limited to the printing
its; and
process capabilities and subse-
method for solder paste deposition.
• Each of the 30 boards had 208
quently improving upon them.
Typically, printing equipment manu-
deposits.
There is an urgent need
facturers express printing capability as + X
All the data were from the same process;
for building a bridge between
where X is a dimension that is chosen by the
however, depending upon how the data were
product specification reported
machine supplier (example: + 20 microns)
viewed, different PCI values were obtained.
by an equipment manufacturer
without taking the specification (require-
Detail of this study can be found elsewhere
1
.
and the end user in respect to
ments) into consideration. In reality, the
PCIs compare the output of an ‘in-
the process capability. This
true machine capability should be expressed
control’ and normal process to the specifica-
paper presents an overview of
as a comparison of output of an ‘in-control’
tion limits of the product to be produced.
basic concepts and formulas
process to the specification limits.
They basically determine the ratio between
to estimate equipment/process the spread of the product’s specifications
capability and introduces a
process capability indices
to the spread of the process values as typi-
practical, simplified method of
Many process engineers are becoming aware,
cally measured by ±3 σ where σ represents
using statistical concepts and
particularly through 6 σ types of programs,
the standard deviation of the process. The
formulas.
of the need for a sound statistical approach
fundamental PCI is known as Cp and it is
to understand the capabilities of their
often referred to as a measure of ‘inherent’
processes and are learning many of the tools
capability. It is calculated as follows:
that will enable them to determine those ca-
Keywords: Process Capability
As most readers may know, the absolute
pabilities and to subsequently improve upon
Index, PCI, C , C
p pk
them. One such tool for determining the
ability of a process to meet specifications on
This paper was originally presented at
the product is to calculate what is known as
SMTAI 2008.
a process capability index (PCI). While the
22 – Global SMT & Packaging – September 2008 www.globalsmt.net
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