SPC and usage difficulties in SMT
1. experiences more wear and tear, the allows more cost-effective production of usage of short and long time capability
piston’s diameter will grow. higher quality products. analysis (MCA and PCA) over the working
A side effect is that SPC produces time of a production process. To begin,
The second effect is systematical and needs
documentation of the behavior of the an MCA will be done during the pre-
to be corrected based on quality require-
processes. That documentation becomes acceptance test at the supplier’s site after
ments. With SPC methods, it becomes
more important as a tool to prove the purchasing a new machine. After the ma-
possible to control the systematic effects
ownership of process quality to custom- chine has been delivered, a second MCA
and to forecast the right time to make the
ers. However, understanding SPC just as a will be done. With a successful MCA after
correction.
Show Program for Customers will not take delivery and installation of a new machine
SPC delivers the tools to monitor pro-
advantage of quality improvements. And it is safe to start production. During
duction processes under the requirements
it certainly will not decrease failure and production it is necessary to monitor and
where measurable quality characteristics
repair costs. control the behavior of the machine over
exist as well as appropriate measurement
the working time with a PCA. This makes
methods and tools. The next requirement
Machine and process it possible to detect weak points in the
is that the process must capture feedback
capability analysis whole process before the yield drops.
to control issues found during the monitor-
Major tools to learn more about process
ing in order to establish a quality control
behavior in terms of SPC include perform- surface mount technology is special
loop for the critical processes.
ing machine and process capability analysis All the advantages of SPC are based on
The quality control loop allows a pre-
(MCA and PCA). These were originally two major assumptions:
dictive detection of weak points to avoid
developed by car industries (especially
defects instead of detecting and repairing 1. All critical quality characteristics are
Motorola) to have an easy-to-use tool for
defects. In the end, an established quality measurable.
estimating yield rates of processes, and to
control loop reduces failure costs and 2. It is possible to find the root causes for
compare the quality of different processes
defects and adjust them in the process,
as well as different suppliers. The idea is to
Purchase
analyzing the measured values.
take random samples out of the produced
Equipment
products and measure the critical quality Unfortunately that is not the case for sur-
characteristics. face mount technology (SMT). At the final
The machine capability analysis (MCA) product, the quality characteristics typically
Short Time Analysis analyzes machine production during a are attributive, meaning either the PCB
By Supplier
Improvement
short period of time, typically to produce works or not, the PCB is reliable or not.
50 products. In mechanical engineering, it The IPC 610D standard is used to evaluate
also is called short time capability. Other these characteristics. An important part
Installation To
influences to the process, such as operator, of these characteristics only can be tested
the Customer material and environment, generally are
after the soldering process; however, the
excluded in this analysis. sources of error generally are located in the
In comparison, the process capability process before soldering, e.g., solder paste
Short Time Analysis
analysis (PCA) considers machine produc- printing.
On Customer Floor
Improvement
tion over a longer period of time. It is Additionally, many SMT production
typical to measure a small random sample environments work with high mixes and
(normally three to 13 products) within a low volumes. This means that even if there
Long Time Analysis
defined time cycle. This analysis considers is useful process information from the final
On Customer Floor
Improvement
not only the behavior of the machine itself, test after soldering, it may not be available
but also all other influences to the process, until it is too late. One way to avoid this
including operator, material, method and is to evaluate each piece of equipment
environment.
Figure 2. Short and long time capability analysis
involved in the production process with an
over the working time of a production process.
Figure 2 shows a typical scenario of the MCA.
One example is the evaluation of the
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placement accuracy of a pick-and-place
machine. It is impossible to use standard
products to judge such a machine because
common FR4 boards simply are not ac-
curate enough. The measurement of real
Printer Placement Oven
TEST produced products will be replaced by the
measurement of process parameters, e.g.,
the placement accuracy of an assembler.
Figure 3. Defects discovered during test can come from anywhere earlier in the process. The usage of dummy products has
been established for this measurement.
Dummy products are designed to run
through production like real products but
also allow measurements to be performed
Printer Placement Oven with the required measurement capabil-
ity. All major placement machine OEMs
develop their own methods for qualifying
Figure 4. Use machine capability analyses to verify machine performance on a regular basis, for each machine in their own equipment. Furthermore, stan-
the production process. dards such as IPC9850 and VDI/VDE
18 – Global SMT & Packaging – September 2008 www.globalsmt.net
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