ISQFD 2002 >
QFD - A Method for Describing a Process?
QFD - A Method for Describing a Process?
The manufacture of modern, multilayer printed circuit boards is a highly
complicated process. The overall process involves a sequence of
individual, very complex sub-processes, some of which are carried out many times.
The result is that the manufacture of a multilayer printed circuit board
can consist of a sequence of 40, 60 or even more individual processes.
This involves a wide range of different processes and processes of widely
differing complexity. For example, mechanical tasks (drilling, milling,
scoring, brushing, pressing), chemical/electrochemical steps (cleaning,
galvanising, coating, etching etc.), photo-technology (board patterns,
solder resist masks, etc.), printing, optics, etc.. Modern printed circuit
boards are highly sensitive and have precision structures; track widths of
100 mm and smaller are standard today; 4, 8, 10 right up to 14 and sometimes
even more layers are pressed onto each other in order to manufacture a
printed circuit board that provides a high packing density. The position
of the individual layers with respect to each other is very important here
as metallic feed-throughs have to be manufactured between the layers. Quite
often position tolerances of less than 100 mm have to be maintained.
I think it is patently clear that at the end of the whole process chain
there can only be a satisfactory number of good printed circuit boards
if each individual process can be run with an extremely high yield. For
example, error propagation shows that in order to obtain a final yield
of 90% with a sequence of 50 individual processes, on average each process
must be operated with a yield of about 99.8%. This corresponds fairly
closely to a ±3 sigma process.
This requirement, which is often taken as standard today, can only be
achieved by having an excellent knowledge and control of each individual
process. The magnitude of the challenge becomes apparent when one realises
that we are concerned here not just with processes which are influenced by
a few parameters (up to 10) but also some processes with 100 and more
In order to demonstrate this complexity, a QFD-approach is used to describe
and classify the process parameters. Here, a team works together,
comprising the following participants: process owner and specialists in
the process to be described, process owner and specialists in the
subsequent process (internal client), quality experts, and possibly also
specialist personnel of the manufacturer, developers, etc. of the process.
For this, the central matrix of a HoQ (House of Quality) is used in the
The team defines the essential output (process results) of the relevant
process (that is why the internal client is also very important). This
output, which consists of the individual requirements (the output parameters),
is put into the rows of the matrix and can, if necessary and desired, also
be weighted (e.g. pair-wise comparison).
All the parameters which control or influence the process, namely the
process parameters, are then put into the columns of the matrix. When doing
this, it is beneficial to work through the process from the start to the
end, from module to module.... Attention to detail is an advantage. Indeed
it is recommended to pay very close attention to completeness and to
include all possible parameters whose importance will be determined later.
As in a classical QFD, the relationships between the individual process
parameters and the output parameters are defined following standard
conventions: the relevant process parameter has either a large (9),
medium (3), small (1) or no (0) effect on the relevant output parameter.
Here, the standard QFD symbols are used. After making this link between
the output parameters and the process parameters, the sequence of
importance of the process parameters is calculated, once again following
standard QFD conventions. What one finally obtains is "an order", a
sequence of the process parameters according to their importance, i.e.
their effect on the process and hence on the output parameters.
This represents a key step in understanding a process: the definition of
the process results (output parameters), their rank and the process
parameters which control the process, and affect the results - ordered
according to their importance for the process.
In the following two steps the matrix is extended to the right and
downwards. The reference value and tolerances of each output parameter,
and how they are measured, are recorded to the right, in the rows of the
The matrix is extended downwards with the reference values and the upper
and lower limits of the process parameters and how they are measured,
controlled or monitored.
One now has a matrix which provides a virtually complete representation
and description of the process. From this, key properties of the process
can be gleaned, including whether and how all output parameters and
process parameters are defined, quantitatively recorded and measurable
or whether there are still gaps, even perhaps for the key parameters.
This matrix cannot only be used to identify the key process parameters
but can also be used to promote thought about how they can be measured
and controlled. It can also be used for training purposes, to draw up
maintenance plans, etc..
In addition, a variable comparison can be carried out, at least for processes
having process parameters which are relatively east to vary. To do this, one
runs the 5 - 20 main process parameters at level 1, which roughly corresponds
to the standard process, and afterwards at level 2, at which poorer process
results are expected. If one has recorded the key process parameters in their
sequence when drawing up the matrix, the process results should significantly
change when this relatively simple experiment is undertaken. If this is not
the case, the most important process parameter(s) has/have still not been
Dr. Klaus Bischoff
Studied Physics and Mathematics at the University of Karlsruhe
Worked 5 years as Vendor Engineer for Digital Equipment, at that
time (1985 - 1990) one of the biggest computer manufacturer of the world.
From 1990 to 2000 quality manager at Endress + Hauser Maulburg: Endress + Hauser
is a manifacturer of equipment for measuring and automation with about 5.500
employees world wide.
Since 2000 quality manager of the photo print electronics a company of the
Endress + Hauser group that produces printed circiut boards.
© 2002 QFD Institut Deutschland e. V. All rights reserved.