SPC and usage difficulties in SMT
3712 recommend using dummy Figure 6 shows the measured values The next image shows the results after
products to perform MCA studies on only of head #2 separated by the placement calibrating with OEM tools. The result is
SMT placement and dispensing machines. angles. It shows that each placement angle better but the machine still cannot reach
CeTaQ’s measurement methods also are works with a good deviation. Each ellipse the desired specification limits. Now the
based on the usage of highly accurate glass is small enough to fit in the specification reason is systematical mean shifts of each
boards, excluding the influences on the limits easily. The problem is that only the individual head from the nominal values
used material (printed boards and compo- 0˚ placements are nicely centered to the of up to 50 µm. A global mean shift is easy
nents). nominal values. All other placements are to adjust in most common SMT equip-
With this method, it is not possible to systematically shifted up to 140 µm. With ment. In this case, those offsets have been
perform extended evaluations exactly as this knowledge, the next step for a thor- adjusted directly in the machine data. After
discussed above. To solve this problem, a ough SPC is now to find a way to adjust that, a third measurement was performed
double strategy has been established over the systematical shifts in the machine, to verify the adjustments.
the last few years: which requires some experience and some- Figure 8 compares initial and final
times the help of the OEM. In this case, measurements. The machine now performs
1. Using MCAs to verify machine perfor-
the solution was simple. The machine was safely inside the desired specification lim-
mance on a regular basis. Performing
calibrated by OEM calibration tools. its, and the differences between initial and
multiple MCAs as opposed to a PCA
allows the machine’s systematical influ-
ences to the production process to be
controlled.
2. Monitoring influences of materials,
components, solder paste, etc,. by using
AOI/AXI machines.
Only if the systematical influences are
under control will it make sense to worry
about the random influences. This re-
quires performing MCA tests on each type
of machine involved in the production
process on a regular basis. The frequency
of doing such testing depends on different
factors:
• Quality philosophy of the company
• Process requirements vs. results of test
done in the past
• Customer requirements.
The tests can be done anytime from weekly
to once every two years. Generally, it is
strongly recommended that an MCA is !
performed after major events in the pro-
duction machine’s lifecycle:
Figure 5. X-y plot of the initial measurement.
• After moving machines
• After major maintenance tasks on qual-
ity critical machine parts like bearings,
heads, cameras, etc.
• After mishaps with the machine
example of an Mca on a chip
shooter machine
The following example comes from a
medium-sized electronics manufacturer
using chip shooting machines with two
individual heads and a typical component
size of 0603. There were specification
limits of ±100 at 4 sigma that the machine
was supposed to reach.
Figure 5 shows the x-y-plot of the initial
measurement. To reach the required
specification limits, the ellipse must be
inside the red lines of the lower and upper
specification limit (LSL/USL). In the im-
age, this is not the case, which means that
the machine tested does not perform in !
specification, and that is where a thorough
Figure 6. Measured values of head #2 show that all but the 0˚ placements are shifted.
SPC analysis starts.
20 – Global SMT & Packaging – September 2008 www.globalsmt.net
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