Establishing a precision stencil printing process for miniaturized electronics assembly
tion will concentrate efforts for smaller carefully planned tables. The second step both (a) simulate the experiment applica-
aperture openings, selection of the test in preparation for a paste print trial is tion (ultimately in a production setting),
vehicle and analysis of print data typically listing the key control attributes during the and (b) duplicate replicate figures (graphs
include a broad examination on a range printing activity. Typically these include and analysis) from similar transfer efficien-
of aperture sizes and pad designs. There identification of the stencil, the squeegee, cy data collection. The critical parameter
exists a major challenge to control the board support, print speed, print pressure, used to quantitatively confirm printing
assembly process conditions during print separation conditions, aperture count, process results have been duplicated is the
trials. However, subsequent quantitative squeegee overhang, inspection time, date, variation in transfer efficiency. The preci-
reporting approaches of the findings (in times, paste identification, alloy, powder sion of the stencil printing process will be
addition a significant challenge) will also size, metal %, room temperature, relative judged by the changes in variation of the
be shown. The aperture sizes and pads will humidity, under screen cleaning setup/ transfer efficiency (for any of the combina-
range from 6-40 combinations. These print tions of aperture sizes and pad designs).
trial examinations aim at process develop- Ultimately (in a production setting), any
ment of miniaturized electronics assembly. printing process will not be sufficiently
Data collection, tabulation, analysis and
“Since it is
precise, if variation increases and causes
reporting will include the results of varia- more assembly line defects to occur.
tion in transfer efficiency not only from
expected that there
In this paper, box plots will be used to
the multiple combinations, but also the show variation in transfer efficiency. Box
changes in the variation for various tooling
will be a need to
plots are an effective visual summary of the
and process attributes. All of this emphasis data
5
. Several approaches can be consid-
on further perfecting the stencil printing
throw off many
ered for axis settings on the box plots.
process includes using statistical tools for Learning about transfer efficiency variation
quality planning and analysis. Juran and
common process
for a stencil thickness is important. In
Gyrna offer a well-written book on treat- a 0.5 mm pitch stencil printing process,
ment of statistical and technological activi-
control freedoms, as
the box plots include specification limits
ties related to analysis techniques
3
. on the axis settings. For example, 100%
The first step before any report of print
we come to know
transfer efficiency is shown, suggesting
performance is prepared is to establish that 100% transfer efficiency is a standard
measurement assurance that the varia- the 0.3 mm pitch axis target for each aperture size and pad
tion in transfer efficiency is acceptable combination. When specification limits
for decision making. This step demands a process we should are set at 150% for the UCL and 50% at
thorough evaluation of the measurement the LCL, then the axis setting on each box
system. For the case study examples in this expect incremental plot figure will be conveniently standard-
paper, automated solder paste inspection ized at 0-25-50-75-100-125-150%. In this
equipment is programmed for each test insight and manner, it becomes readily easy to see how
board and each test stencil. The inspection closely transfer efficiency data surrounds
program is then tested by collecting paste experience.” the (100%) target value.
measurement data on the same printed Placing a standard deviation chart
board 32 times. The results for each and below the box plot will help the reader gain
every stencil aperture and board pad are insight about the level of transfer efficiency
individually examined statistically to char- frequency, paste date of manufacture, variation in the box plot. A good print has
acterize precisely how much variation exists paste lot #, and details of controlled pause a standard variation less than 10%
6
. The
in the paste measurement system. This times in the experimental procedure. At axis setting on the standard deviation chart
variation is later summarized for different the conclusion of the print trial, the raw will be standardized at 0-10-20%. Obser-
deposit sizes and shapes, but the repeatabil- measurement data is immediately extracted vations of standard deviation below the
ity of every individual aperture/pad com- from the paste inspection system so that (10%) mid-line will represent good print
bination gets observed. The relationship a data table can be created in a statisti- quality. Consistent observations below the
between the measurement precision and cal software program. Usually tabulation (10%) mid-line will represent consistently
the paste deposit tolerance are expressed as activity involves proper insertion of at- good print quality. Changes in the observa-
a precision/tolerance ratio (P/T ratio)
4
for tribute columns that associate the control tions will signal changes in the consistency
every aperture in the stencil. The measure- attributes to the measurement data so of print quality. These consistencies or
ment system is routinely re-checked to es- that each individual board data result can changes will show how significantly an
tablish its stability over time. Copies of the be distinguished. A standard file format attribute responds with changes in various
measurement assurance study are available for the columns in the data table allows tooling and process attributes, such as aper-
with every case study. It should be noted for comparative analysis of the transfer ture area ratio, pad design, or print speed.
that repeatability results from the measure- efficiency among print trials. Attribute As sufficient insight about the charac-
ment system are excellent for every shape columns for individual stencil aperture ter of print quality is gained for multiple
and size. The exercise is tedious, but the size and shape are inserted, as well as, a tooling and processes, a precision stencil
overwhelming assurance of measurement column for area ratio of each aperture, and printing process may be established. The
acceptability gives credibility to ensuing often a column with some designation to box plots and standard deviation charts
decision-making. describe the pad design. can later be useful for determining that
Among the tools for quality planning One of the subsidiary aims for using the ideal tooling and process has indeed
and analysis, it appears essential to use a visual presentation of the transfer effi- been followed. Variation in the transfer ef-
those which visually present data in figures ciency variation will be to help anyone who ficiency that does match earlier print trials
(graphs). The data must be formatted in sees printing process results to be able to could indicate that the print operation is
www.globalsmt.net Global SMT & Packaging – August 2009 – 11
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