PCB Laser Technology for Rigid and Flex HDI – Via …

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PCB Laser Technology for Rigid and Flex HDI – Via Formation,
Structuring, Routing

Dr. Dieter J. Meier, Stephan H. Schmidt*
LPKF Laser & Electronics AG
Garbsen, Germany and Wilsonville, OR*

Abstract

A new versatile laser technology is available that is capable of working with both ridged and flexible boards
using only one laser source. This system is based on a THG-UV laser (355 nm) and vector data software. This
system can be used for drilling, cutting and structuring. Small and medium board manufacturers will be able to
enter the HDI market with a minimum investment and a guaranty of high yields for each technology step.
Various materials and combinations including glass fiber reinforced substrates can be drilled, cut and structured
with the same equipment. This paper will introduce special applications in the area of micro via formation
(minimum diameter of 30µm at 250 holes per second), laser direct structuring (minimum line widths of 0.8mil at
13.8 inches per second) and routing (compounds of various materials) and will discuss the technological
benefits.

Introduction

The trend to further miniaturization continues. For
rigid and flex circuits the industry predicts
dimensions1 that can’t be produced economically
with the current technology. Low yield and
technical requirements that can’t be matched
require a technology change. High density
interconnect (HDI) circuits require microvias with
diameters of less that 40µm and tracks with less
than 50µm width.

The well-established laser technology will
definitely play a leading role in this technology
change2. When choosing a laser system it is
necessary to answer the following questions:

• Does the laser system’s performance meet

the demand for current and future
generations of both flexible and rigid
circuit board?

Is the chosen laser technology capable to
handle the selection of materials (metal
layers, glass fiber, different substrates,
adhesive, solder mask, galvano- and photo
resists, etc.)?

• Which wavelength is best, considering the
absorption ratio and ablation performance
of the desired material?

•

•

Is the performance of the laser including
power, regulation and consistency wide
enough to cover the range of desired
material properly?

• Can the mechanical design of the system

deliver the required accuracy and
repeatability in the micrometer range?

An ideal concept would be an inexpensive system
with only a single laser source, yet flexible enough

to cover a broad range of applications on flexible
and rigid boards. This would enable small and
medium sized businesses to enter laser technology.
For better return on investment, an ideal system
would be versatile enough to drill holes but also
structure circuitry as well as scribe and route
boards.

Laser and System Concept

The laser source has been designed particularly for
this application field. It is based on a Q-switched
Nd:YAG Laser with a wavelength of 355 nm (UV).
At this wavelength most of the metals (Cu, Ni, Au,
Ag) that are to be ablated in printed circuit
applications show absorption rates of more than
50%. Organic materials can also be accurately
ablated. The high photon energy of UV lasers at
3.5-7eV8 cracks the chemical bonding as the
ablation process in the UV spectrum is partly
photo-chemical and partly photo-thermal. These
capabilities make a UV laser system the first choice
for applications in the printed circuit board
industry7.

The system should only use a single laser source for
budgetary reasons, but still have to provide an
energy density (fluence) of more than 4J/cm² that is
needed for opening the Cu surface when drilling
microvoia4 holes or for structuring Sn layers5. The
ablation process of organic material such as epoxy
resins and polyimide requires only an energy
density6of around 100mJ/cm2. To address this wide
spectrum the laser would need a very precise and
sophisticated energy control as the drilling of
microvias requires a 2-step process. The first step
opens the Cu with a high fluence and the second
step removes the dielectric with low fluence.
Another aspect in using one single laser source is
the spot size. CO2 lasers with their usual spot size
of 70µm9 can’t drill state of the art microvias with

less than 50µm in diameter directly. Instead it is
necessary to use a pre-etched conformal mask on
the board to limit the laser beam to the desired
diameter10. UV lasers on the other hand typically
have a spot size of approx. 20µm at a wavelength of
355nm. The frequency of the laser pulses is
between 10 and 50kHz at a pulse length of less than
140ns. Detailed investigations of the resulting blind
vias indicate that 140ns pulse length causes no
Heat-Affected-Zone (HAZ) in the material. Fig. 1
shows blind vias in Cu with a diameter of 70µm.
The substrate material is FR4.

Fig. 1: Heat-Affected-Zone (HAZ) test pattern

Therefore both flex and rigid materials can be
properly processed with this pulse length.

Fig. 2 shows the basic principal of such a
system11,15. The laser beam is positioned with a
computer-controlled scanner / mirror system and
focused through a telecentic lens that allows the
beam to maintain a right angle to the drilled
material. This scanning process allows the software
to generate a vector pattern and it compensates for
both material and layout deviation. The scanning
area measures 55 x 55 mm (2.2” square).

Fig. 2: Scanner concept
This system is compatible with a CAM software12
that imports all common data formats such as
GerberTM RS-274S and RS 274X, ExcellonTM I and
II, Sieb & MeierTM, DXFTM, BarcoTM DPF, HP-GL
and ODB++TM among others.

The mechanical design is based on a rigid granite
construction precisely polished to a surface

accuracy of less than 3µm. The table rests on air
bearings and driven by linear motors. The
positioning accuracy is controlled with glass scales
that guaranty a repeatability of ±1µm. An optical
sensor integrated in the table itself compensates for
the optical distortion and long-term drift based on
an accurate alignment of the laser position at
various mirror locations. The software creates an
array of correction data based on the alignment that
is overlaid on the entire scanning area. The
calibration for the drift compensation takes about 1
minute and can be done while a work process is
executed. Any variation in the substrate itself, such
as inaccuracies in the positioning caused by
deviation of the fiducials, is detected by a high-
resolution CCD camera and compensated for by the
control software.

Inconsistent planarity is compensated by surface
sensing with a resolution of 1µm that controls the
topography of the board, which allows the control
software to adjust the laser focus.

The substrate fixture is based on a vacuum unit
with a honeycomb design. Eventually emitted gases
will be extracted and filtered through an active-
charcoal system. The laser system is covered and
compliant with laser safety class I.

Laser Drilling

A wide variety of material can be laser drilled but
the speed and therefore the throughput of the
drilling process depends on the material properties.
In general there are two main ways to maximize the
throughput: 1) Reduction of the Cu layer thickness
to 5µm (1/8 oz.) to allow the removal of the copper
(step 1 of the drilling process) with high fluence
and 2) to develop substrate material that has good
laser ablation characteristics (step 2 of the drilling
process). DuPont, for instance, developed a
material called THERMOUNT14 that contains non-
woven aramid fibers. The manufacturer claims this
material is easier to ablate with laser technology,
than material with woven glass reinforcement.
Other new developments of dielectric substrates
that can be more easily ablated by laser utilize so
called “Hotmelts”17.

For HDI application some special build-up and
connectivity techniques have been established that
use RCC (resin coated copper). Those are thin Cu
foils (min. 5µm), which are covered with an epoxy
resin system and are usually laminated on both
sides on rigid substrates. Fig. 3 shows blind vias,
which were formed into this material. The diameter
of these holes is 30µm. With this material a speed
of up to 250 holes per second could be achieved.

depending on the material, can reach up to 300mm
per second on a work area of 640 x 560mm² (25.2”
x22”) with a total thickness of up to 50mm (2”).

Fig. 5: Direct circuitry structuring using Laser
and Sn resist technology.

As previously mentioned, polymere materials and
layers can be ablated, without damaging the
substrate material. This is accomplished with the
accurate energy control of the laser. This offers
further application possibilities with the decrease in
the Packaging pitch (Wafer Level Packages, CSP,
µBGA®). The photolithographic structuring of
solder masks presently can handle apertures as
small as 150-200µm. Also here laser ablation
allows further integration and smaller apertures.
Fig. 6 shows the opening of a solder mask
(manufacturer: Lackwerke Peters / Germany) with
the developed laser system. With a resist thickness
of 25µm an ablation speed of 60mm per second
could be achieved. The opening width here was
100µm, however elements can eventually be as
small as 50µm. The Cu underneath could be plated
after a usual cleaning process with Ni / Au without
problems. This technology can also be used for
HDI applications to open PI or PET based cover
coats or cover layers.

Fig. 6: Solder mask structured with laser

Investigations of conductive material (Ag based)
embedded into polymer substrates to create
conductive structures, found that the UV laser
system can successfully be used here. In Fig. 7 such
structures are shown. A special polymer system on
epoxy resin base with a layer thickness of 15µm
could be structured at a speed of up to 300mm per
second. The minimum structure width was obtained
with 25µm.

Fig. 3: Cross section of blind vias in RCC

The resin layer thickness is 50µm. Interesting to
mention is, that the residue (smear) by the ablation
process on the hole’s wall is so insignificant that
the plating process could be performed using only
the common cleaning chemicals15. This also
supports the use of a UV laser versus a CO2 laser
for this application. If the laser parameters are
optimized, it is to expect the smallest HAZ and
little debris and recast. The Cu surface in the hole’s
bottom is slightly textured by the laser, producing
good adhesion capabilities for the following plating
process, as shown to Fig. 4.

Fig. 4: Cross section of metalized blind-vias in
glass reinforced FR4

After opening the Cu layer with high fluence, both
the resin and the glass fibers will be ablated and
vias can be plated, as shown in Fig. 4. It shows
blind vias in an FR4 inner layer with 17,5µm (1/2
oz.) Cu. The drilling speed was determined here
with 100 holes per second.

Laser Structuring

The uses of lasers for the structuring of metallic or
polymer surfaces intersperses itself more and more.
In the beginning only Sn layers 18 over Cu foils
were structured and turned into conductor tracks
after etching and stripping. As a result of the
decrease in the spot size (down to 20µm) another
possibility arises, that allows ablation of lacquers
and photo resists directly (Direct Write UV Laser
Photolithography)19 as an alternative to the
traditional photo lithography.

Fig. 5 shows a structured printed circuit board
developed with UV laser. The minimum track
clearance is 30µm. The maximum structuring speed

Summary

The presented laser technology was developed for a
broad application field including, drilling,
structuring, and cutting. It enables, in particular,
small and medium-size companies to enter the HDI
technology. Since only a single UV laser source is
used, this system is very economical. The laser
source was developed exclusively for this system,
in order to achieve maximum performance at low
maintenance cost and superior uptime. An arc lamp
pumped laser is used, which allows the users to
change lamps independently. The lamp life is
specified with 300 hours working time and the
calibration of the system requires a minimum
expenditure of time.

Such a system is well suited for prototyping, since
it both drills and structures. It also represents an
alternative to photolithography. Since it operates in
the UV spectrum at 355nm, the possible material
range extends from flex to rigid PCBs including
polymer materials such as solder masks, cover
coats, galvano resists to name a few.

References

1.

IPC, The National Technology Roadmap
for Electronic Interconnections 2000/2001

2. Cutting Edge Manufacturing- Applying
Lasers in Electronics Production,
PRISMARK Partners LLC, Dec. 2001

3. “When is the Right Time to Buy a Laser
Tool“, Schaeffer, R.D., CircuiTree, Feb.
2002, 56

4. “Improvements in High Speed Laser

Microvia Formation, Using Solid State
Nd:YAG UV Lasers“, Cable, A., IPC
Printed Circuit Expo 1997 , Technical
Paper p. 17-7, San Jose, 1997

5. “Current Alternatives to Direct Imaging/
Patterning (Part II)“, Vaucher, Ch.,
CircuiTree, 46, July 2001

6. UV-Lasers, Effects & Applications in

Material Science, Duley, W.W.,Cambridge
University Press 1996

7. “UV Laser Drilling of Multilayer Blind

Vias“, Raman, S., Schreiner A.F. IPC
Printed Circuit Expo 1998 , Technical
Paper p. 17-1-1, Long Beach, 1998

8. Excimer Laser Ablation and Etching, by J.
Brannon, American Vacuum Society
Monograph, 1993

9. “Research for innovative PCB technology
and experience with laser tool“, Krabe, D.
Scheel, W., Glaw, V. , Printed Circuit
Europe, Jan.-Febr. 1998, p. 9-13

Fig. 7: Laser scribed tracks for Ag-Paste

Laser Routing

The freely programmable and flexible mode of
operation make UV lasers particularly suitable for
precision cutting of HDI applications11,21
(depanelization, singulation). With the developed
laser the most different material combinations could
successfully be processed. Fig. 8 shows an HDI
multilayer with 6 layers of different materials (FR4
/ polyimide / epoxy resin and acrylate resin). The
cutting velocity was 10mm per seconds.
Delamination did not occur.

Fig. 8: Multilayer build-up, laser routed

In addition, glass-fiber reinforced FR4 could be
processed with UV laser, as represented in Fig.9.
The edges are clean and don’t need any post
processing, as usually would be required with
mechanical routing or punching or when cutting
with CO2 laser.

Fig. 9: FR4, laser cut

The cutting velocities, which can be reached, are
material dependent and are typically within a range
of 50mm to 500mm per second.

10. ”CO2-Lasers for Microvia Drilling and
other PCB and Flex Applications“,
Schaeffer, R., CircuiTree, September
2001, p. 90

11. ”Laser Processing of Flex“, Venkat, S., PC

FAB 2001, p. 28-34

12. CircuitCAM- LPKF Laser & Electronics

AG, Software

13. “Copper Foil for HDI Applications“,
Cheskis, H.,et al., IPC Printed Circuit
Expo 2001, Technical Paper p. 13-2-1,
Anaheim 2001

14. “Empfehlungen zur Herstellung von
Löchern mittels Laserablation in
Leiterplatten und Multilayern, die mit
THERMOUNT nichtgewebten Aramid
verstärkt sind“ Powell, D., Weinhold, M.,
Dec. 1993

15. “Ultraviolet laser system and method for

forming vias in multi- layered targets“ US-
Patent 5.593606, 1997-01-14

16. “Thermal Reliabily of Laser Ablated

Microvias and Standard Through- Hole
Technologies“, Young, T., Polakovic, F.;
IPC Printed Circuit Expo 2001, Technical
Paper, Long Beach 1999

17. “A Novel SBU Dielectric and Coating

System“, Wall, C., PCFAB 2000, p. 36-44

18. “Direkte Laserstrukturierung-50µm-
Strukturen mit mindestens 80%
Ausbeute“, Krause, J., PLUS-Produktion
von Leiterplatten und Systemen 3/2000,
395-398

19. “A High-Density, Resin-Coated-Foil

(RCF) Substrate Utilizing Mask and Direct
Write UV Laser Photography“, Corbett, S.
et al., International Conference on High-
Density Interconnect and System
Packaging 2001, Copenhagen

20. “Microvias Using a Conductive Paste to

Replace Electroless Plating“, Gandhi, P. et
al., IPC Printed Circuit Expo 2001,
Technical Paper, p. 13-3-1, Anaheim 2001

21. “Solid State Lasers-Provide Manufacturing
Solutions for Flex Circuits“, Venkat, S.,
CircuiTree November 2001, p. 46-53