Establishing a precision stencil printing process for miniaturized electronics assembly
precisely under control. ciency implied in the box plot, but instead are shown in this paper are from data
Since it is expected that there will be a 75-85% with low variation. The VMR collected using customer test vehicles. A
need to throw off many common process line charts will be used to show qualita- common feature of customer test vehicles
control freedoms, as we come to know the tive and quantitative differences. The is to offer a range of pad sizes and spaces.
0.3 mm pitch process we should expect in- actual volume, SPC specification limits, In this manner, the aim for variation in
cremental insight and experience with new- and other information remain important transfer efficiency is to capture the accept-
ly introduced product design. The message knowledge to clearly characterize the 0.5 able process and tooling window from a
of some individual aperture variation may mm pitch process. However, the VMR line range of sizes and shapes. Unacceptable
not necessarily signal the precision process chart technique for precision stencil print variation results of the larger sizes (or small
needs further optimization. Clearly, the process comparison will show its benefits spaces) and the unacceptable results from
concept for comprehensively presenting for characterizing attributes for smaller the smaller sizes (or large spaces) could
visual variation of the stencil printing assembly designs. help determine the acceptable range of
process for 40 aperture size and pad design Test vehicle (board) layout and stencil sizes in the middle, as measured by the
combinations can help characterize the aperture design selection for miniaturized variation in transfer efficiency. At this
challenge. However, activity for comparing assembly can easily be judged to have both writing, none of the customer test vehicle
multiple trials suggests using multiple sets too much opportunity available, or too lit- designs comprehensively captures the range
of 40 box plots and 40 standard devia- tle. The test vehicle can contribute to exces- of design criteria suggested in Figure 16. Fig-
tion charts. The task of collecting all of sive variation in transfer efficiency if it has ure 16 shows conceivable design proposal
this data and making all of these figures alternatives for 0.3 mm pitch.
is daunting in itself, but trying to com- One final comment about the method
pare multiple sets of this many apertures for reporting variation in transfer efficien-
becomes overwhelming.
“As we come to
cy is that the sequence of the apertures in
For short-term decision-making using the box plots will not be arbitrary, random,
many data sets of 40 box plots would be
know the 0.3 mm
or alphanumerically ordered. Clearly, the
prohibitive, but fortunately a short-cut sequential order of the test vehicles should
method can put entire sets together on pitch process we remain in consecutive order. This will
a single page. Consider that the (100%)
target value of the axis setting on the box
should expect
allow for changes of variation in transfer
efficiency to be observed over time. The
plots indicates the transfer efficiency data
could have a 1:1 relationship with the
incremental insight
sequence of apertures will be presented
in descending order by area ratio, but in
variance. This is because the standard
deviation is the square root of the variance.
and experience with
groups. This can offer advantages that
are similar to the visual effect of a Pareto
7
Consequently, good print quality will
newly introduced
chart. The first group is often rectangular
have a transfer efficiency variance value aperture shapes, the second group is typi-
less than 100%. To compress the massive
product design. ”
cally square apertures, and a third group
amount of information from multiple is circular apertures. Within each group,
print trials, the variance-to-mean ratio solder mask defined (SMD) pads are a
(VMR) can be taken from the measure- separate sub group, followed by non-solder
ment data for each combination of mask defined (NSMD) pads. The general
aperture size and pad data design. There also been designed for SIR, pin-in-paste, idea is to begin with the larger volume ap-
could then be a set of VMR values plotted wave soldering, or large-scale components ertures, and descend to the smaller volume
on a line chart. Points on the line would that require a stencil thickness greater aperture. Anyone reading a report of varia-
represent the aperture size and pad design than 4 mils. There is likely to be interac- tion in transfer efficiency gets lead down a
combination. Points that remain below 1.0 tion variation from unnecessary printing benchmark of acceptability until the small-
indicate that the print quality is good. An features during the data collection process. est aperture attributes show otherwise.
entire line below 1.0 will serve to visually The test vehicle dimensions should at least As a customer comes to better know
indicate that the entire combination of be similar to new product introduction solder volume optimization, clear pro-
apertures and pad designs has good print designs with 01005 and 0.3 mm pitch duction limits on transfer efficiency can
quality. Several lines on the VMR line components. The important consideration be specified. Mathematical models have
chart can represent an alternate attribute is to recognize not only attributes available revealed techniques for estimating the for-
in the process, for example, stencil separa- on the test vehicle, but also those that may mation of soldered connections – optimal
tion conditions, room temperature, or be unavailable. For example, determine the solder volume
8
. However, from paste print
under screen wiping. Entire VMR lines location of representative 0.3 mm pitch inspection, representative calculated C
p
,
that remain close will show similar print aperture sizes and pad dimensions on the C
pk
and DPMO tables can be formed from
quality. VMR lines or points on a VMR test vehicle. Recognizing the opportuni- actual transfer efficiency measurement
line that diverge can show precise differ- ties available, as well as, the opportunities data on paste deposits. These tables can
ences in the stencil printing process. unavailable on the test vehicle can help in be done specifying 150% as an UCL, and
At times the transfer efficiency planning the method used to characterize 50% as a LCL, and then later updated as
specification limits may be unknown. An the results. Showing variation data from an production yield determines acceptable
example shown is a 0.5 mm pitch process unrepresentative location of component process tolerance for defects.
using a 5-mil stencil thickness. In a produc- pad features may differ from those that are
tion setting, an adequate volume of paste in a dissimilar location because of inherent stencil printing process data—
is present from 75-85% transfer efficiency tooling and process. results of variation in transfer
on 305 µ (12 mil) circular apertures. The The test vehicles used in case studies efficiency
true target may not be 100% transfer effi- from which transfer efficiency data results The data and results are given here in
12 – Global SMT & Packaging – August 2009
www.globalsmt.net
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