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.

CO₂ Lasers

CO₂ 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).

Drawing Specifications

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

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

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 side.

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.

Laser Drilling

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.

Coating

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.

Annealing

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.

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