PRECISION LASER CUTTING OF PTFE SUBSTRATES FOR HIGH
© Accu-Tech 1997
This document provides general guidelines and
considerations for laser
cutting PTFE laminates used in the manufacture of high frequency and
circuits. Precision PTFE laminates are used as substrates in the
of amplifiers, filters, mixers, couplers, phase shifters, transmitters,
antennas, etc. PTFE laminate sheet material is sold under the Duroid,
Cuclad, Diclad and other trade names.
TYPES OF LAMINATE MATERIALS
The laminates typically used in microstrip, stripline
circuit fabrication consist of PTFE (Polytetraflouroethylene) that has
reinforced with either glass fibers, woven glass fabric, or proprietary
ceramic materials. Certain types of filler materials are used to
the substrate dielectric constant and its temperature coefficient.
PHYSICAL AND MECHANICAL CHARACTERISTICS
Laminates are normally supplied with copper or brass
cladding that may
be etched and then die cut or precisely machined into the final circuit
configuration. The completed laminates are typically between 0.005 and
inches thick and are clad with 0.125 to 0.5 oz. per ft2 copper. The
may then be assembled into an electronic module that serves as a ground
plane, a mounting base, and a heat sink. In other cases, the laminates
be bonded to a thick metal plate that serves as the mounting base and
CUTTING LAMINATES REQUIRES PRECISION
Considerable dimensional precision and stability are
required of the
fabricated circuits to obtain acceptable performance at high frequency.
This is particularly true for microstrip and stripline circuits. It is
uncommon to require that line width and spacing dimensions be held to
± 0.001 inch and that dimensional stability over temperature be
than 25 PPM per C. Workmanship during fabrication must be absolutely
to avoid scratches and micro-dents to edges and surfaces.
The non-rigid nature of unbonded laminates together with
dimensional tolerances place a substantial demand on the quality and
of fixtures, tooling, and workmanship.
CIRCUIT LAYOUTS ARE CHALLENGING
Layout patterns for high frequency circuits are often
complexity is created by layout nesting techniques that are employed to
fully utilize costly laminate materials. Finally, the fabrication
provided may be somewhat inadequate to ensure a precision fit in a
Dimensional "fine tuning" may be required to optimize performance
of the circuit.
CIRCUIT FABRICATION METHODS USED
Fabrication methods currently used in the industry
consist of six basic
- Masking and etching
- Routing with a router
- Cutting with a CO2 Laser
- Through-hole plating
- Drilling with a bit
- Punching with a die set
COMPARISON OF CIRCUIT FABRICATION METHODS
Precision photo masking and etching are generally
adequate to produce
metal patterns of the desired accuracy on the substrate. The table
attempts to qualitatively show three common methods used to shape a
substrate. The "best" method will depend on the laminate materials,
the geometry of the specific circuit configuration, the tooling
and, as in most precision operations, the skills, tools, and ingenuity
the operator. The selection of a skilled, experienced, and committed
is the best recommendation for achieving a high quality, cost effective
| DESIRABLE ATTRIBUTE OR CIRCUIT APPLICATION
ROUTER AND DRILL
| ACCURACY OF
|ABILITY TO MAKE CHANGES
|LOW VOLUME APPLICATIONS
|SUBSTRATE WITH CERAMIC
|SUBSTRATE WITH GROUND
|ABILITY TO CUT THROUGH
|PROGRAMMED PATH DESIGN
|SCALING TO MATCH
|OVERALL PART QUALITY
HOW THE LASER CUTS PTFE LAMINATES
The laser cutting process uses infrared light focused to
a small spot
(approx. 0.005 in.) to cut through the substrate material. PTFE, glass,
and filler materials readily absorb this light and are easily cut. The
is very reflective to the laser beam so it resists cutting. This
nature of the CO2 energy absorption enables it to cut along the edge of
metallization without damaging it. In some cases it is possible to
the substrate material down to a metal layer to form blind vias and
- The laser is a single tool that can perform most of
the required material removal processes without the need for additional
handling and multiple tool changes.
- The laser is effective for "Programmed Path",
"Programmed Scan" and "Raster Scan" methods of cutting and material
removal. This flexibility permits optimization of the layout based on
the types of features being created and the requirement to save costly
laminate real estate.
- Holes can be virtually any size or shape due to the
small beam size and programmable motion.
- Beam focus provides a modest degree of control over
the shape of the substrate edge.
- Either backside or frontside processing can be
performed by the laser.
LASER CUT QUALITY
Compared to other cutting methods, the laser cut is
quite precise and
does not distort adjacent material.
- The laser cut leaves an extremely clean, sharp edge
and produces square corners.
- Laser cutting of tight, intricate patterns is
generally fast and accurate.
PROGRAMMED PATH METHOD
The shape of the cut is determined entirely by the
programmed path in
the motion control system. This technique usually utilizes the laser
registration system to precisely locate and control the cut with
to alignment targets or to a specific feature on the laminate such as
edge of the etched metal foil. In this case, a set-back is required
the edge of the laminate to the edge of the etched foil. If the cut
be right up to the edge of the metallization, the programmed scan
can be used (see below).
PROGRAMMED SCAN METHOD
Using this method, the etched metal foil on the
ground-plane side of
the laminate is being used as a mask to create a cut flush with the
of the foil. The laser beam is travelling along the edge of the
on a path defined by the motion control program to overlap the edge of
metal slightly. The part of the beam that hits the metal is reflected
the part that hits the substrate cuts through. The result is a cut that
is flush with the metal even if the metal edge varies from the
location. If the variation is large it may become necessary to make
parallel passes along the edge of the metal.
CUSTOM SCALING OF THE CIRCUIT
Due to the soft nature of PTFE materials, they are
subject to dimensional
distortion during fabrication. This typically occurs during metal
and mechanical processing. When tight tolerances are required, this can
cause rejection of the parts. Yields can sometimes be improved by
scaling both X and Y cut dimensions during laser processing to
for the distortion.
- The laser control system uses high resolution optical
registration that permits the precise location of machined features in
relation to etched metallization patterns.
- There is no substantial relaxation of laser drilled
holes after they have been cut.
- There is no tool wear when laser cutting PTFE
substrates. Conventional tool life (drill bits and router bits) can be
reduced to minutes when cutting ceramic loaded substrates.
- Due to the small kerf of the laser beam, tighter
nesting of parts and features is achievable in order to save expensive
substrate real estate and reduce finished part costs.
The laser is an excellent tool to create prototype
circuits and to fine
tune them for performance. Setup is quicker and less costly than
tooling and changes are much easier to make. Turnaround time for
parts can generally be accomplished within a few days.
DESIGN INFORMATION FORMATS
Layout and design information can be provided as a line
drawing or in
electronic format. Electronic files may be transferred via modem using
electronic bulletin board or may be sent on disk. The following is a
list of popular electronic drawing formats supported by Accu-Tech:
.DXF .CDL ASCII IGES .DWG .PRT PRN HPGL GERBER
Other electronic drawing formats are supported.
Accu-Tech is willing
to work with special formats.
© Accu-Tech 1997
LASER PROCESSING INC.
1175 Linda Vista Drive
San Marcos CA 92078