Lizotte: DOEs and HOEs for microwelding
- From: The Lone Weasel <loneweasel@xxxxxxxxx>
- Date: Sun, 21 Oct 2007 18:37:42 -0500
DOEs and HOEs for microwelding
Todd Lizotte
Many of today's laser microwelding processes involve use of
the multiplexed or splitter-based beam sharing optics. Once
split, each laser beam is delivered to the targeted weld
areas by a fiber beam delivery system with an attached
monolithic refractive optical focusing head.
Fiber beam delivery is employed in coarse to medium
precision applications where the ultimate alignment of the
two elements being welded is not absolutely critical.
However, in certain automotive welding applications, such as
High Intensity Discharge (HID) lamp alignment and welding, a
lack of precision can lead to non-conformance yield loss and
premature failure of the lamp once it is installed. The
balancing of laser energy through coated splitters with the
subtle time delays caused by transferring laser energy
through differing lengths of fibers creates weld variations
and directly affects the performance of the HID lamp.
Figure 1 shows a typical spot-welded tab; there are three
such tabs on the bulb assembly near the base. The position
of the discharge illumination centroid with respect to
automotive head light housings is absolutely critical, as
are both the simultaneous welding of the tabs and the
quality of those welds. Because these bulbs create a
discharge by using high voltage to initiate the arc, a poor
weld will cause arcing at the tab and eventual failure.
In other critical applications, such as welding lids on
electronic packaging to form a hermetic seal or spot welding
optical components into position, these subtle laser energy
variations create specific adverse effects, such as part
deformation and misalignments. These are common traits of
the existing fiber beam delivery processes. Engineers have
developed methods for overcoming or compensating these
obstacles by adding further vision analysis, process time
and spot welding sequences that allow corrections to take
place as parts are being welded. The drawback of these added
process steps is a serious reduction in throughput, and in
high-volume manufacturing. In the worst-case scenario,
process engineers add mechanical deformation motion
techniques to the pick-and-place robotics to mechanically
tweak the parts after the welds are made.
Applications employing fiber delivery often present uneven
beam intensities that create two major problems. First,
overlapping and uneven weld zones create undesired weld
variations. Second, there is an inability to control the
balance between the fiberoptics with the precision necessary
to ensure sub-micron alignment repeatability in a single
process step. Typically, each fiber will have different
optical losses, and precise balancing is only achieved by
adjusting attenuators and/or the focusing optics to maintain
specific on-target laser intensities. Either way, the issue
is one of achieving balanced welding characteristics so that
all the resulting welds have similar properties. Also, no
weld should effect the mechanical positioning of the
precision aligned assemblies that are being welded; however,
it is clear that this is a difficult task under any
circumstances. Figure 2 is an example of a typical fiber
beam delivery-based welding system for the telecom market.
Understanding the dilemma faced by industry, a new
technological solution has been developed. A processing head
can be configured to fit onto the end of a single fiber-
optical delivery system using diffractive and holographic
optics (see Figure 3). This new optical beam shaping,
splitting and focusing head provides a greater degree of
precision welding for applications in telecom devices,
lighting fixtures and hermetically sealed electronic
packages. This new technique takes into account the need for
illuminating a variety of points or surface areas with
balanced laser energy by simplifying the optical system
through elimination of the use of splitting and launching
the split beams into fibers. Diffractive and holographic
optics provide further advantages for applications such as
optimizing laser illumination beam shape, laser micro-level
spot welding, seam welding, plastic welding/bonding and area
surface exposure of photo or thermal cured materials. By
allowing precision to be designed into the optic, the
process precision is increased substantially.
The optics used for microwelding and bonding applications
can be simply classified as either diffractive or
refractive. Refractive designs consist of large-scale
surface relief profiles designed using the laws of geometric
optics, i.e. large curvatures. They treat light by the
refraction and reflection of geometrical rays at optical
interfaces. With geometric optics, a wider variety of
wavelengths can be used with refractive designs as long as
the material can transmit the specific wavelength being
utilized.
Diffractive optics consist of planar elements with zones
that retard the incident wave of light by modulating the
refractive index, or in the case of surface structures, by
modulating the surface profile. The light emitted from the
different zones interferes and forms the desired wavefront
profile, which can consist of a specific shape or an array
of spots. The key limitation to diffractive optics is their
dependence upon interference, making them wavelength-
specific optical elements; one design cannot be used
efficiently over several wavelengths. They are, therefore,
limited to monochromatic applications.
Diffractive and holographic optics are designed to provide
unique and various beam pattern shapes for multi-spot,
stitch-and-seam micro-weld, or bonding applications. Other
laser beam shapes include circular and rectangular zones
that cover an area or circular and rectangular arrays of
individual beam focal points or geometric spots.
Generated by computer software, diffractive and holographic
optics are, by default, versatile when it comes to the
design of elements with specific attributes. A diffractive
optic can assume a specific beam profile, such as those
shown in Figure 4. Each profile offers certain microweld
characteristics, such as large spot welds, small tack welds
or even deep penetrating welds. In the case of bonding,
photo-curing adhesives or photo-initiated bonding of polymer
pre-forms, optimized shaped illumination of diode laser
beams offers a higher degree of control of heat flux on
targeted areas, instead of flood approaches that are
traditionally used.
When multiple and simultaneous microwelds are required, a
diffractive optic splitter coupled with a diffractive
imaging element can provide superior control of the spot-to-
spot beam fluctuations that commonly occur while using
fiber-based splitting beam delivery. Diffractive and
holographic optics can provide a smaller beam delivery
footprint in comparison to multiple fiber-based delivery
systems. When aligning multiple micro-sized optical
components, the compactness of the optical system can
provide added room for further vision or more robust
material handling.
Another key feature is the ability to simplify stitch-and-
seam welding of lids or other polymer or adhesive sealing of
hermetic enclosures or silicon MEMs packaging. Figure 5
shows a scanning system that provides seam welds or bonds
using cost-effective motion control instead of expensive
galvanometer scanning routines. Configuring the optical
imaging head with indexing optical elements, a system can
quickly change spot patterns or beam energy densities on the
target. These added capabilities make changes possible on
the fly, providing further flexibility to an overall
precision assembly process.
Diffractive and holographic optics present many advantages,
although initial investment can be significantly higher than
traditional refractive optics. That said, once produced,
they can be replicated at fractions of the cost of their
refractive counterparts and ultimately their return on
investment (ROI) far exceeds those initial costs due to
increases in process yield and throughput. This should not
discourage those who need to solve problems such as laser
beam delivery footprint issues, spot size limitations,
working distances, tooling configuration, heat flux process
requirements and laser intensity fluctuations. Diffractive
and holographic optics provide real solutions, and those
looking to implement them should take the next step, even if
that next step is the beginning of an incremental journey.
Bibliography & References
Aleksoff, Carl C., Ellis, Kenneth K. Neagle, Bradley D.,
"Holographic conversion of a Gaussian beam to a near-field
uniform beam," Optical Engineering, Vol. 30 No. 5, p.
537?543, May 1991.
Cederquist, J., et al., "Computer-Generated Holograms for
Geometric Transformations," Applied Optics, Vol. 23, No. 18,
p. 3099?.3104, September 1984.
Lizotte, Todd, PC Fab Magazine, "Beam shaping for microvia
drilling," p. 28?33, February 2003.
Sharp, Greg, Kathman, Alan, "Laser beam shapes for the
future," Industrial Laser Review, Dec. 1994.
Veldkamp, W.B., "Laser beam profile shaping with interlaced
binary diffraction gratings," Applied Optics, Vol. 21, No.
17, p. 3209?3212, September 1, 1982. (5)
Todd Lizotte can be contacted at NanoVia, LP, by e-mail at
telizotte@xxxxxxxxxxxx
http://www.industrial-
lasers.com/articles/print.html?id=187799&bPool=ILS.pennnet.c
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--
Yours truly,
The Lone Weasel
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