Design Guideline for Ceramic
Ceramics used for
electronic circuitry (usually “thin film” or “thick film”
metallization) are typically alumina (Al2O3,
several types), beryllium oxide (BeO) or aluminum nitride (AlN).
These materials are available in standard sizes, thicknesses and
flatness (“camber”)1. In
designing electronic circuitry for ceramic substrates, it is not
always optimum to use a standard size of ceramic for the
circuit. It is frequently best to optimize the size and shape of
the circuit and then process arrays of these on standard
ceramic. Also frequently, circuitry is placed on both sides of
the ceramic, and there is need to connect one side to the other,
perhaps by putting holes in the ceramic and metallizing the
holes. There are several methods for “singulating”2
the parts, including sawing, laser machining, and scribing &
breaking. Laser machining, scribing and drilling are often the
most economical methods for creating these features. When
designing laser machined ceramic substrates, it is important to
understand the limitations of the material during the laser
machining process. Although ceramic is a material with hardness
just under diamond, it is also brittle and susceptible to
breaking, chipping or cracking due to impact.
Material Flatness and Surface Finish
As-fired ceramic substrates typically have
a camber of .003 inch per inch, as produced by the manufacturer.
If the application requires a tighter material flatness, there
are two other processing options to select from: camber sorting
of as-fired material or mechanically lapping and polishing the
substrates. A flatness (or camber) of .0005 inch per inch can be
achieved by lapping and/or polishing the substrates.
Lapped substrates have a surface finish
range of 20 up to 60 micro inches for thick film applications;
polished substrates typically will have a surface finish that is
<2 micro inches. The polished substrates are primarily used for
thin film applications, as these surfaces require a finer finish
for plating or sputtered metals to form good adhesion.
lasers focus an intense, coherent beam of infrared (10600 nm)
radiation onto the ceramic which rapidly heats and vaporizes the
ceramic. Because a lens can focus only on a single plane, the
exit side of the ceramic is usually the focal plane and thus the
cuts or holes will have a taper of about 2° rather than being
perpendicular to the surfaces as would be a saw cut or
mechanical drill. Laser machining and drilling are done
essentially the way single-axis mechanical milling and drilling
are done on a CNC machine, with an x-y table providing the
motion instead of the tool (laser) moving. Location tolerance
for the x-y table is about ±1 mil (25 µm). Beam diameter is
typically about 3 mils (75 µm) which practically limits hole
size to about 5 mils (125 µm) on the entrance side.
Holes and laser machined features will be
larger on the entrance side than on the exit side due to the
taper mentioned above. Near the cuts or holes, the walls of the
ceramic are melted leaving a glassy surface. Some of this glassy
material (slag) will also deposit on the entrance surface in a
“heat affected zone” which is about one or two mils (25 – 50 µm)
wide. Metallization typically does not adhere as well to this
heat-affected zone. Also, since the laser creates intense
localized heating, stresses or even microcracks may be induced
which can be detrimental to further handling of the parts,
because ceramics are relatively brittle. These phenomena can be
mitigated by annealing the ceramic (see Annealing).
For the circuit layout, it is important
for the designer to depict on the drawing the laser entrance and
exit side because of the taper inherent in the laser process and
proximity of the metallization to laser features. Typically, the
view shown is identified as laser entrance or exit side with a
separate cross section detail showing the direction of the
taper. From this information, the laser house can then provide
the proper set-up for the application.
Laser scribing involves using the laser in
pulsed mode to create lines of holes that are spaced 5 +2/-1 mil
(125 + 50/-25 µm) center to center with a depth of 40% to 50% of
the ceramic thickness. This standard will be used unless
otherwise specified and these parameters adjusted on a
case-by-case basis. The scribing allows the ceramic circuits to
be processed in an array and finally singulated by “snapping”
apart the individual circuits along the scribe lines. Circuitry
should allow at least 10 mil (0.25 mm) “streets” for the scribe
lines. If the scribe lines are post-machined (added after the
circuitry has been processed) at least 4 mils (100 µm) should be
allowed from the metallization edges to the edges of the scribe
to avoid the laser harming the circuitry. The width of the
scribe (i.e. diameter of the scribe holes) will increase
somewhat with ceramic thickness, so it is best to be generous
with this allowance.
Laser machining is done with the laser in
CW (continuous wave) mode, where the beam is left on
continuously to cut all the way through the material and provide
a smooth edge. The standard tolerance is ±2 mil (50 µm); a
tolerance of ±1 mil (25 µm) is achievable based on the exit side
of the ceramic. Hence, it is better to put circuitry on the exit
side of the ceramic, which has the added advantage of avoiding
the heat affected zones. To avoid cracking, the distance from
feature edge to feature edge should be at least equal to or
greater than the thickness of the ceramic; two times the
material thickness is recommended. Circuitry for the entrance
side must allow for the 2° taper and heat affected zone. If one
side is a ground plane, it’s best to put this on the entrance
When parts are post-machined after
metallization, it is best to allow at least 4 mils (100 µm)
between the laser machined edge and the edge of the
metallization to avoid harming the circuitry. This should be
about 6 mils (150 µm) for circuitry on the entrance side. Also,
if singulating by laser machining, streets between the circuits
in an array should be a minimum of 0.100 in (2.5 mm) to avoid
cracking, since the ceramic is subjected to much more heating
with the CW beam, and for a longer period, than in scribing.
Generally, the guidelines for laser
machining above apply to laser drilling as well, since holes are
simply a special case of laser machining. The smallest practical
hole diameter is about 5 mils (125 µm) on the entrance side and
3 mils (75 µm) on the exit side. Again, the minimum distance
between the edges of the holes should be at least equal to the
thickness of the ceramic; a distance of two times the material
thickness is recommended. The guidelines for post-machining
above also apply for post laser drilling.
Our CO2 lasers can mark and engrave many different
types of materials including metals, ceramics, glass,
plastics, fabrics, phenolics, and polymers. Precision graphical designs,
alpha-numeric characters, bar codes, and other identification markers can
easily be added linearly or circumferentially to the material
over a 17” workspace with a positioning tolerance of 5 mils (125 µm) and a
minimum feature size of 3 mils (75 µm). As a non-contact marking method that
uses no inks or solvents, laser engraving is a permanent and biocompatible
process suitable for use on surgical tools, medical implants, and other
components that require traceability.
A polyvinyl alcohol (PVA) coating is
typically applied to the ceramic substrates prior to laser
processing to prevent the slag from adhering to surfaces which
will later be metallized. The PVA coating also protects
metallization during post laser machining operations (after
circuitry is printed). This coating is water soluble and easily
removed after laser processing is completed.
Since the slag can act as a barrier
between the ceramic and metallization, (causing poor adhesion
with some metallization systems), a process called annealing can
be used to ‘resurface’ the ceramic substrate. Annealing is a
heat treatment process that is typically done in excess of 1275
degrees C. Annealing can also relieve the stresses produced from
the ceramic manufacturing as well as those introduced during the
laser machining process. During the annealing process, the
material is softened to a point where the stresses are relieved.
Annealing, however, can adversely affect tight dimensional
tolerances by up to 1.5 mils (38 µm).
Cost Saving Tip
Whenever possible, the designer should
use standard as-fired materials and laser tolerances for the
most cost effective approach to circuit design.
1 Typical “as fired” camber is
0.003 in/in (30 µm/cm); lapped &/or polished ceramic can be as
flat as 0.0005 in/in (5 µm/cm).
2 “Singulation” is a term coined by the industry to mean
disassembling the array into single parts.
3 These phenomena can be induced with other means of machining
and drilling also.