A new angle on printing
A new angle on printing
by George Babka and Scott Zerkle, Assembléon Americas, Alpharetta, GA; Frank Andres, Rahul Raut and
Westin Bent, Cookson Electronics Assembly Materials, South Plainfield, NJ; and Dave Connell, Research in
Motion, Waterloo, Ontario
introduction
In part, this is because printers cannot
Although blade contact angle The challenge of successful 01005 feature
program a variable contact angle. The ability
is a critical stencil printing
solder printing has been well documented,
to change blade contact angle as a process
parameter, screen printers
driven by the continuing trend to miniatur-
parameter would greatly widen the process
have so far been unable to
ize electronic assemblies. Successful imple-
window for printing fine features.
vary it ‘on-the-fly’, in soft-
mentation of this technology will demand
ware. The recently released
critical improvements to the process, push-
Theory
Assembléon/Yamaha YGP
ing beyond the limits of current technology.
Good quality in stencil printing ultimately
One solution, which involves a large
printer has changed this, and
means delivering the right amount of solder
capital expenditure, is to use two printers
has made new application
paste to the right place on the substrate. The
and two stencils: one printer with a thin
final paste layer should be flat, with even
research possible to study a
stencil to print the miniature components,
thickness across the deposit and the correct
crucial 01005 process vari-
and a second in-line printer with a thicker
shape (pattern resolution).
able for feature printing that stencil to cover the larger solder paste
SMT stencil printers have two blade
previous researchers have
volume requirements. A second, and more
angle parameters: a static contact angle and
ignored.
economical, solution would require process
a dynamic attack angle (Figure 2). The con-
We have designed the first
optimization and a very tight process
tact angle is a function of the blade holder
robust solder paste printing
window. This means identifying exact-
and is formed between the stencil and blade,
process for 01005 compo-
ing process parameters, including circuit
achieving contact with the stencil with
nents that uses only a single
board fabrication (pad designs, etc.), stencil
no force between them. The attack angle
(fabrication method, thickness and aperture
printer and stencil. We have
is a function of the contact angle, blade
size), solder paste (particle size, rheology and
studied transfer efficiencies
compliancy, print speed and paste rheol-
activator robustness), printing parameters
ogy; it is the sum of the static and dynamic
across all the major param-
(speed, separation, pressure, etc.), placement
forces acting on the blade during the print
eters, with important results
(pressure, speed and accuracy etc.) and re-
stroke. Although the dynamic attack angle
for reliable high-density flow optimization (atmosphere and thermal
is complicated to determine, it is primarily
equipment assembly. Our
profile).
established by the static contact angle.
findings show that a vari-
Improvements have been made over the
Earlier theoretical and applied research
able blade contact angle can
years to most of the above process inputs.
has demonstrated that solder paste rolling
print fine features with wider
Various studies have evaluated fabrication
generates the downward force vector that
process window, and reduce
methods, investigating, for example, elec-
fills the stencil aperture with paste. A funda-
overall process variation be-
troform versus laser-cut and comparing the
mental requirement of solder paste printing
effect of stencil thickness. Stencil printing
tween boards produced from
is the generation of the correct amount of
studies have regarded the effects of print
a line.
downward force in the paste roll to properly
speed, print pressure, and separation speed
fill stencil apertures. Too little force and
to optimize solder paste transfer efficiency
the aperture will not fill properly; too much
(TE). However, one crucial area that has not
force and the result is a premature break-
Keywords: Screen Printing,
been examined is the blade contact angle.
down of the solder paste. Our work shows
Stencil, Blades, Blade Angle,
Transfer Efficiency
!
!
Figure 1. Comparison of the YGP 3S (Swing Single
Squeegee) to the standard double trailing edge blade
configuration. Figure 2. Contact angle vs. attack angle.
36 – Global SMT & Packaging – February 2009
www.globalsmt.net
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