Full beam - materials processing applications using lasers

Materials World magazine
,
1 Aug 2011
Laser cut SS and polymeric stencils. Image courtesy of LPKF

Dr Ronald D. Schaeffer, Chief Executive Officer at PhotoMachining Inc, Pelham, USA, looks at the
processing benefits micromachining provides over existing technology.

Lasers have been used for at least 40 years
in industrial environments, but early lasers
really were not suited for precision plastics
machining. However, the invention of highly focusable
CO2 lasers, UV lasers and ultra short pulse lasers has
opened up new fields, allowing plastics to be
processed with high speeds and very clean edge
quality. Lasers can also be used to weld and mark
plastics and other materials, but here the focus is on
material removal such as cutting and drilling.

Some of the important factors in materials
processing applications using lasers are edge quality,
aspect ratio, taper, minimum achievable spot size,
processing speed and cost.

Around the edges

Are the edges smooth and straight? Is there a heat
affected zone? If so, how far does it go into the material? Smooth edges can be affected by several
things, including the shape of the beam and pulse
overlap. If the beam is Gaussian, heat effects may not
be avoidable on the wings and, in this case, a flat top
beam homogeniser could be needed. If an imaged
beam is used, different shapes, including lines
and circles, can get imaged on to the target. As a
general statement, the further into the UV the laser
wavelength or the shorter the laser pulse, the better
the edge quality. An example of where edge quality is
important is in making flow orifices for microfluidics
and drug delivery.

Rough edges and surfaces with lips can be
problematic for flow dynamics. The images below show polyimide cut with three different lasers:

The first (Figure 1a), was cut using a CO2 laser, the
second (Figure 1b) a 355nm, 50ns laser and
finally (Figure 1c) a 355nm, 12ps laser. The kerf width
is about 75 microns and it is obvious that the cut
quality improves with shorter wavelength and
pulse length.

Aspect ratio is the difference in material thickness
with respect to hole size. For example, a one
millimetre exit hole in five millimetres of material gives a 5:1 aspect ratio. One of the benefits of laser
processing over, for instance, chemical etching is the
ability to produce aspect ratios greater than 1:1.
Aspect ratios of 10:1 are comfortable under almost
any conditions and ratios of up to 100:1 have been
produced in some cases – with the caveat that
the exit diameter is described, while the entrance
diameter is larger than the exit a considerable amount
because of taper (see below).

The image below shows some high aspect ratio
holes drilled in alumina ceramic:

The entrance hole is
about 0.0042 inches while the exit is 0.0038 inches
going through 1.5mm of material. The resulting taper
is very low, which is desired for this application as it
involves pushing small wire pins through the material
for high density probe cards. Similar effects can be
seen in some plastics.

With almost any laser process, there will be a taper
associated with features so that the laser entrance
side is bigger than the bottom or exit side. The taper
angle is a function of the material, spot size, focal
length of the lens, divergence of the beam and laser
fluence on target. Minimal taper is achieved with
higher fluences on target. Sometimes after going into
the material at a certain depth, we see waveguiding.
This is when the hole drilled takes on more of
a trumpet shape than a conical one. Unless
sophisticated optics are used, taper will be present in
laser-made features. The image below shows a
blind hole in polyimide, 200 microns in diameter. Even
in this low aspect ratio example, both the taper and
the edge roughness can be seen.

Coming into focus

Minimum achievable spot size is determined by
a constant multiplied by the laser wavelength,
focusability of the laser (called M2), focal length of the
focusing lens, all divided by the clear aperture of the
lens. Therefore, infrared lasers have a much larger
minimum achievable spot size than UV lasers. As a
general rule, the best way to achieve low taper
and clean processing is to have high peak power intensity. Peak power is the energy per pulse divided
by the pulse length and peak power intensity is peak
power divided by the spot size on target. Therefore,
in order to get high values, one has to have either
high energy per pulse, short pulse length or small
spot sizes. Since small spot sizes are a function
of wavelength, UV lasers can produce high peak
powers – especially as most UV lasers also have a
short pulse length relative to infrared lasers. High
photon density on target appears to be the key to
getting low taper and high quality features.

Small spot sizes are nice when needed, but they
mean processing speed can be slower, because the
pulse-to-pulse overlap on a small spot is such that it
takes many more pulses to define a given line
segment than using a bigger spot – assuming kerf
width is not an issue. Ultra violet photons are
absorbed within a small interaction volume with
respect to infrared photons, so control is possible.

Beam delivery is also important. Galvanometre
(galvos) beam delivery systems are much faster
than using table motion alone, even when using highspeed
linear tables or air bearing tables. In general,
higher power and higher repetition rate lasers are also
faster to a point, but we usually find that it is difficult
in most cases to use more than a few hundred kHz
on target because the beam cannot be moved fast
enough. One must also be careful when using galvos
because they are inherently less accurate than table
motion, although new all-digital devices are on the
market with built-in encoders that can be both highspeed
and accurate. Smaller input apertures can be
faster than larger ones because of the inertia of large
mirrors, but this also limits spot size. Higher speeds
in any case result in less accuracy.

Speed is related to cost, but if the lasers and optics
are a lot more expensive to achieve faster processing
times, the results may be a wash. As a general rule, infrared lasers cost less than UV lasers to buy and
also to operate. For example, a 20W CO2 laser will
cost about US$250/W to purchase, a 20W fibre laser
will cost about US$600/W, a 20W, 355nm DPSS
laser will cost about US$8,000/W and a 20W, 10ps
pulse length fundamental wavelength laser will cost
about US$15,000/W. Therefore, UV and short pulse
lasers are best used where edge quality and precision
considerations outweigh cost considerations.
Frequently, compromises in design must be made so
that the best combination of speed, accuracy, spot
size and cost can be achieved.

Suitable materials

Here are some examples of plastics that have been
processed with high quality, high resolution features.
This picture shows a bioabsorbable stent being manufactured with an 800fs Raydiance
laser:

This laser operates at the infrared fundamental
wavelength, but because of its extremely short pulse
length, even heat sensitive materials can be laser
processed with no apparent degradation of material
on the edges. Post-processing involves only a
light cleaning. Technology for producing stents uses
stainless steel, but embedding metals in the body can
be problematic if complications arise. Bioabsorbable
materials dissolve within a few weeks and avoid
complications inherent with metal stents.

This image shows both a polyimide
and stainless steel solder mask:

Again, technology
involves using infrared lasers to process thin stainless
steel into masks, but these masks cannot meet the
demands resulting from decreasing size of features.
Polymeric stencil masks, processed with UV lasers,
allow the generation of much finer features than can
be achieved using infrared lasers and stainless steel.

This image shows microvia laser-drilled
holes in resin coating over a copper conductor layer:

The vias are 30, 40 and 50μm in D. The taper
increases as the hole D decreases and the resulting
aspect ratios get bigger. Sometimes a bit of taper
is actually beneficial, as in drug delivery orifices and
microvias such as these that are subsequently plated
for conductive paths. There are hundreds of lasers
around the world drilling billions of vias in dielectrics
for satellites, cell phones, medical devices and
automobile components, to name but a few.

Lasers can provide a cost effective and
technology-enabling solution to manufacturing
problems involving putting high quality, high
resolution features in a host of different plastic
materials. New laser sources, such as high-power
UV lasers and ultra short pulse lasers are being
commercialised and are allowing work to be done
that was not previously possible.

Further information

Dr Ronald D. Schaeffer, Chief Executive Officer, Photomachining Inc, 4 Industrial Drive, Unit 40, Pelham,
New Hampshire, 03076, USA. Tel: +1 (603) 882-9944. Email: rschaeffer@photomachining.com
Website: www.photomachining.com