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Alexander Holiat MSE 542 Flexible Electronics Term Paper I
May 11,2006 I I
Laser Drilling for Electrical Interconnections in Flexible Electronics
Recent years have introduced products which continue on a trend toward smaller,
lighter, and more powerful. These include cell phones, PDA's, digital cameras, MP3
players, and medical electronics. These products are requiring an increased number of
chips per product with an even greater number of 110 signals per chip in a smaller
package. To incorporate a greater degree of electronic function into a smaller volume,
circuit traces and the holes used to connect them must have smaller physical dimensions
[I]. The use of flexible circuits and substrates are also becoming more important to
decrease the volume and weight of the product. Industry has been on a continual struggle
to keep pace and to reliably and inexpensively manufacture these products.
Since the invention of printed wiring boards (PWBs) in the 1950's, vias have been
drilled with high-speed mechanical drills [2]. The process has since then become very
sophisticated, fast, and accurate. The vias that are usually made with the mechanical
drills are in the range of 250 - 400 pm. As technology pushes for better performance and
smaller footprints, the sizes of the vias continues to shrink.
I Around 1995, the production of circuit boards with microvias started in Japan and
was referred to as high density interconnect (HDI) [3]. Microvias are holes with a
diameter less than 150 pm. Other advances such as the use of blind vias and the use of
two HDI layers on each side helped continue the technology trends. Though holes of 150
p are achievable with mechanical drills, the cost effectiveness of (his process under 200
p is no longer practical 121. Mechanical drills are also inapt for the formation of blind
vias.
V. .. - 3
Future technologies are also introducing the application of polymeric materials as - 3
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the substrate which would produce flexible substrates. Polyimides are also being used as ::
protective and insulating layers of the interconnect substrates which is advantageous for
the cost and performance of the circuit module [4]. Current technology offers a wide
range of materials for circuit boards, from polymers to ceramics. They are primarily
made with a dielectric between two layers of copper. PCB substrates are either
diglycidyl ether of tetrabromobisphenol (FR4)- or bismaleimide triazine(BT)-based with
glass fiber reinforcement for rigid boards, polyimide for flexible circuits, or alumina for
high-performance devices [5].
75 urn Blind Via 100 pm Through Via Cu (18 pm) I Epoxy (60 pm) I Cu (18 pm) Slasfi-Ulxir-reinforeed muMlayei
Figure 1 -Holes in various PCB materials [I]
The new technology to replace mechanical drilling must be able to drill through a variety
of materials and produce blind vias, not to mention to maintain the usual cost and
throughput variables. Three technologies began to compete to take over microvia
drilling: plasma etching, photo via formation/etching, and lasers. [l] Any of these
processes must also fulfill the following requirements for vias to have reliable
interconnections, they include: clean vias without residue, no delamination of copper and
substrate, tapered sidewalk without undercut, big land diameter for robust
interconnection to the inner layer, and no perforation of the inner layer [6].
Plasma etching requires a very large capital investment. It is also limited to
certain substrates which makes it not very attractive to small businesses and would only
be feasible for very high volume production Photo via forming process is limited to
photo imageable dielectrics and does not have a very high yield. However, these first
two methods are able to form vias in each layer simultaneously while lasers drill each
hole one at a time. Lasers, however, are very versatile and provide the user with many
options. The wide varieties of lasers such as C02 Laser, solid state UV Lasers, and
excimer lasers all have different strengths and weaknesses. Laser drilling is also the
easiest and cheapest option to implement as it provides the user with the ability to
implement it with established techniques and substrates, as well as providing future
benefits to drill other materials [5]. Other laser advantages include: direct large amounts
of energy to specific locations, localized heating, high heating and cooling rates (10"-10~'
Klsec), low thermal budget useful for plasticlflexible substrates, patterning, and ability
for large scale manufacturing (roll to roll). It is no surprise that they have become the
most popular method of microvia formation.
The term used when lasers drill holes is ablation. Ablation is the removal of
material using light. It can be done either by vaporization of the material due to heat, or
[7]. The laser energy per unit area is called the fluence. There are many key parameters
of interest when dealing with ablation in laser drilling. One must take into account
several laser beam and processing parameters. The first is wavelength. Figure 2 shows
the spectrum of light with the various laser types at different wavelengths. The choice of
wavelength is very important depending on the absorption spectra of the copper, epoxy,
polyimide, or other materials. You want the material of choice to absorb highly at the
laser wavelength being used. Having a small absorption depth is also important to ensure
a high energy deposition in a small volume for rapid and complete ablation.
Figure 2 -Laser Type vs. Wavelength [I]
The second parameter is pulse duration and energy per pulse. It is ideal to maximize
peak power and to minimize heat transfer to the surrounding material. The result may
induce thermal degradation of the surrounding material known as the Heat Affected Zone
(HAZ) [S]. The third parameter is the pulse repetition rate. If the rate is too low, ablation
will not be as efficient because the material will cool. If the rate is just right, the material
will ablate more efficiently because more of the previous energy will be available with
the next pulse. The fourth parameter is the beam quality. Beam quality is measured by
the energy, beam spot size, the focusabiity, and the homogeneity. Lenses, beam shapers
and expanders, and masks are used to control the beam [4]. If the beam is not controlled,
the ablation will not be as efficient and the shape of the hole will not be as expected.
There is also the interaction between the laser beam and target material with the beam
focus depth [S]. Each of these variables must be optimized in the processing
environment in order to obtain the correct properties desired for the given application.
The most common types of lasers are CO; and UV. They all have their
advantages and disadvantages which are studied below. The differences between the
beam properties, quality of the laser types, and the various material interactions strongly
determine their applicability fields. [4]
COi Lasers
CO; lasers emit infrared light with a wavelength 10.6 (un and the beam is focused
using lenses. This long wavelength limits the minimum focus diameter which reaches its
physical limit around 75 (un. The short depth of focus associated with this laser also
limits via diameter. However, the wavelength is ideal to ablate organic materials such as
polyimide and epoxies. COz lasers can not effectively ablate metals due to the absorption
of a few percent of the beam and the reflection of the rest [4]. However, if one treats the
surface of copper typically with black oxide and has the optics optimized for the process,
CO; has been able to ablate copper for 100 (un holes [9].
very clean ablation with no side thermal damage. However, they are very big, have a
high cost of ownership, and are inefficient. They lose approximately 95% of the output
power due to masking [12]. They have a very low repetition rate (several hundred pulses
per second), and do not remove metals as well as solid state lasers. [I] The advantage of
excimer lasers over solid state lasers is that they selectively remove material over large
areas uniformly, as opposed to solid state lasers which rely on focusing the beam to a
very small area to achieve sufficient fluences. New advances in optics and new materials
are continually making excimer lasers more attractive, however solid state lasers
dominate over excimer lasers for microvia applications [13].
Solid State Lasers
W solid state lasers are primarily third-harmonic Nd-doped designs (YAG and
Vanadate) at 355 nm, though other harmonics are available 121. The light is emitted in
very short high power pulses that can be focused to very small spots. The size of the
focus spot is directly proportional to the wavelength of the light. To achieve smaller
diameters, smaller wavelengths must be used. Solid state lasers can create microvias that
are less than 25 pn in diameter. Unlike excimer lasers, solid state lasers exhibit
repetition rates ranging from 1 - 100,000 pulses per second without losing beam quality
and are capable of ablating metals [I]. Other advantages include high reliability,
minimal service downtime, lower operating costs, and greater simplicity [12].
There has been a steady progression of in ig power and pulse repetition frequency
come from advancements in beam positio technology since the processing speed is "t
through the years. As this trend continues,
dominant in the manufacturing of circuit
determined by the time spent drilling each and the time required to move between
vim [2].
UV solid state lasers will soon become
boiads. An increase in throughput can also
CO2 vs. UV - Hybria
Using C@ is essentially a thermal process in which the intense beam melts and
vaporizes the material, either by conformal mask drilling or large window drilling [14].
This usually results in thermal degradation of the surrounding material known as the Heat
Affected Zone, HAZ. The HAZ reduces precision and can create problems in circuits by
creating conductive paths between circuits that are supposed to be isolated. In contrast,
UV lasers interact with materials primarily through a photochemical process in which the
high energy photons actually break molecular bonds. The result is a clean cut with
minimal effect on the surrounding material [15]. Figure 4 illustrates the different hole
profiles by drilling at different wavelengths in the same material.
CO,
Figure 4 - Som 2 HAZ using higher wavelengths in polyimide material [7]
Harmonics YAG of YAG
The limit of CO; laser via size is 75 pn while solid state UV at 355 nrn can go to 25 pn.
Another comparison is the greater absorption of many materials with UV, especially
copper which is mostly reflective in the IR. UV lasers also eliminate the need of using
masks or large windows on the surface since the beam defines the shape and size of the
hole. There are also fewer alignment problems than CO; lasers because vias are aligned . . .;.
with respect to internal fiducials [14]. CO; lasers are known for their high average power . ' 2
and high drilling throughput, while UV lasers can drill through metals easily and make + :
the smallest vias.
In PCB manufacturing, hybrid lasers have been developed that integrate the two
lasers together; using each laser's strengths to increase throughput. The copper is drilling
with the W laser and the dielectric with the C@ laser in parallel processing [14]. This
hybrid laser provides manutkdwers with the flexibility of ddling through any variety of
circuit board materials or configurations.
Lasers have already become a standard in manufacturing of PCB. In 2006,0.4
mm chip scale packages on telecom products will be in volume production. This dictates
design parameters of stacked microvias with 2-3 HDI layers on each side, lineslspaces
below 60 urn, and three times the amount of laser vias with the diameter down to 70 pn
[9]. The laser drilling technology could switch completely to UV when the next
generation of 0.3 mm chip scale packages with thinner dielectrics and 50 pn diameter
microvias are used [9]. This will only be possible with the continuing trend of high
power UV solid state lasers that can provide higher throughputs and will provide longer
lifetimes. Also, if a company invests on UV laser systems for a specific product, it will
most likely be able to use the same system on different PCB materials for future products.
r
. .
References
1. Egitto, Frank: Laser drilling for electrical interconnection in advanced flexible electronics applications. (Endicott Interconnect, 2006).
2. Dunsky, Corey: High-speed Microvia Formation with UV Solid-state Lasers. Proceedings of the ZEEE, 90, 1670, (2002).
3. Moser, David: Lasers tool up for via formation. Laser Focus World, 37,213, (2001).
4. P. Gordon and R. Berenyi: Laser processing of flexible substrates. (Department of Electronics Technology, Budapest University of Technology).
5. J. Paulus and M. Petti: Microvia materials: enablers for high-density interconnects. Proc. ZPC Printed Circuits Expo 1997, San Jose, CA, S17-7-1.
6. Lange, B.: PCB laser technology for rigid and flex HDI -via formation, structuring, routing. LPKFLaser & Electronics AG, (2002).
7. Sun, Y.: Micro-via drilling on polymeric with laser pulses. (Electro Scientific Industries, Inc.).
8. Schmidt, H.: Ultraviolet laser ablation of polymers: spot size, pulse duration, and plume attenuation effects explained. .I Appl. Phys., 83,5458, (1998).
9. Stahr, Hannes: The laser drilling horse race. Printed Circuit Design & Manufacture, 23,28 (2006).
10. Savage, Neil: UV lasers make their mark. OE Mag., 1,46, (2001). 1 1. Kincade, Kathy: Excimers take more refined approach to micrornaching. Laser
Focus World, 41,69, (2005). 12. M. Keirstead and P. Somerville: UV lasers are workhorses. Laser Focus World,
37,197, (2001). 13. A. Masters and T. Geiiking: Beam-shaping optics expand excimer laser
applications. Laser Focus World, 41,99 (2005). 14. Justino, Pierre: A comparative review of laser manufacturing processes. PC FAB,
25,44, (2002). 15. R. Schaeffer and T. Hannon: UV solid-state lasers exhibit precision and
dependability. Laser Focus World, 37,115, (2001).
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