SPC and usage difficulties in SMT
SPC and usage difficulties
in SMT
Mathias Keil, CeTaQ, Radeberg, Germany
history The definition of statistical
SPC considers that each process
The idea of monitoring manufacturing process control
is influenced by random and
processes using statistical methods goes back SPC considers that each process is influ-
systematical events. That makes
to the 1920s when Walter A. Shewhart, an enced by random and systematical events.
it necessary to understand the
American physicist, engineer and statisti- That makes it necessary to understand the
behavior of a process and to
cian, was engaged by the Western Electric behavior of a process and to judge when it
judge when it runs normally
Company in Chicago to find a way to runs normally and when to make adjust-
and when to make adjustments.
produce their products with a certain quality ments. The major goal of implementing
The major goal of implementing
level. SPC in production is to produce higher
He understood that the quality of a final quality at lower costs.
SPC in production is to produce
product depends on the deviation of all As an example, consider turning pistons
higher quality at lower costs.
parameters involved in the production and on a lathe, where the diameter of the piston
that it is necessary to monitor the critical is a critical quality characteristic. SPC always
parameters during production. This made starts by taking a random sample out of
it possible to understand which deviations the produced pistons from a well working
Keywords: Statistical Process
were just random, being in the normal process. The next step is to measure the criti-
Control (SPC), Production
behavior of the process, and which were cal characteristic—in our case, the diameter.
Process Monitoring, Machine
caused by systematical effects like misaligned Doing so over a certain period of time will
Capablity Analysis (MCA), Pro-
machines, failures in material, operator show two things:
failure, etc. Mr. Shewhart published the
cess Capability Analysis (PCA)
principles of his theories in his book Eco-
1. Each measured diameter will have a
nomic Control of Quality of Manufactured
slightly different value. Over a short pe-
Product. (Van Nonstrand, New York, 1931).
riod, that deviation is only influenced by
This became the standard for successfully
the precision of the lathe, the material
improving quality in the manufacturing
used and other random events.
of different types of products by means of
2. Over a longer period of time, we will
statistical process control (SPC).
see a tendency of the diameter to grow.
This is influenced by the wear and tear
breakdown of the lathe tool. As the tool
Figure 1. The quality control loop.
!
16 – Global SMT & Packaging – September 2008 www.globalsmt.net
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