Design Guidelines

Ceramic Substrates

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.

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.

Coating

A 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 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; leaving your parts residue free.

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).

CO2 Laser Guidelines

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).

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.

Laser Marking

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.

Diamond Sawing

Diamond sawing is used in the microelectronics industry for cutting very hard, brittle materials as well as softer materials into smaller components with a superior edge quality and tight tolerances.

Saw Streets

The saw street refers to the width of the blade allowance between parts or circuits.  For the best cut quality up to .040” thickness, the typical saw street LPT recommends for 91%, 96% & 99% alumina, aluminum nitride, beryllium oxide and other ceramics, is .010” wide.  Thicker materials >.040” to .100” thickness, a .012” saw street minimum is required.  Materials that are >.100” thickness, the width of the saw street is dependent upon the type of material being sawn.  Please contact LPT to consult with our saw experts about the proper saw street width allowance for your application.

Border Allowance

When designing for the maximum number of parts or circuits achievable on substrate materials, it’s important to remember border allowance.  LPT recommends using a minimum of .100” border on all edges of the parts.  When smaller borders are needed, feel free to contact us about your specific requirements.

Saw Edge and Metalization

From the edge of the part to the edge of the metalized circuit, a minimum allowance of .002” is recommended.  LPT has the capability of cutting through thin layers of metalization up to .005” thick (such as gold, copper, nickel, etc.) on ceramics and other materials with minimal burring.

Window Frames

Different aspect ratios of wall thickness to material thickness are required when designing window frames.  Larger parts that are 2.0” to 3.0” in size require an aspect ratio of 1:1 of wall thickness to material thickness.  For part sizes 1.0” to 2.0”, an aspect ratio of 0.5:1 is recommended.  With parts < 1.0” in size, please contact our saw experts to assist with your specific design.

Composite Materials

When composite materials are used in circuit design, there are no set diamond sawing guidelines.  These materials need to be evaluated by our experts on a case by case basis.  LPT can provide technical assistance with recommending saw street sizes, blades and process types used for a successful result.

Plates to Strips to Chips

If your design requires wrap around circuity (on ceramics or ferrites for example) and you need to diamond saw multiple times starting from plates to strips to chips, LPT has developed specialty alignment and handling processes for  diamond sawing these complex circuit designs.  When designing for the maximum number of strips on a substrate, we recommend using an aspect ratio of 0.5:1, i.e.  strip width minimum .020” on .040” thick substrates.  Once the circuity is printed on the edge of the strip, LPT is capable of aligning and diamond sawing the strips to chips.  Tight tolerances and complex alignments are our forte.  Please contact our diamond sawing experts when you need technical assistance with your custom design for this type of product.

Rods

LPT can diamond saw rods to your preferred segment lengths.  The largest diameter rod LPT can saw at this time is .200” maximum.  Contact our diamond sawing experts for your particular product design requirements.

Plunge Cuts

When making plunge cuts in substrates, the blade does not cut the substrate from edge to edge, but enters it at some point away from the edge and dices a predetermined distance before being raised out of the substrate. The cut may be partial or cut through the full thickness of the substrate. The plunge-cut may be used, for example, to cut a rectangular section out of a wafer.  When plunge cuts are required for your custom design, please contact LPT for technical assistance.

Silicone or Sapphire

An important aspect when diamond sawing these materials is their crystalline structure.  The single crystalline formation of these materials is the best for diamond sawing.  To successfully diamond saw these materials, the orientation of the saw blade to the crystal orientation is critical to ensure high quality edges and performance of the finished product.   Please contact LPT for technical assistance with saw streets and pull back allowances.

Cleaning Processes After Sawing

After LPT has diamond sawn your materials, we clean the substrates to remove debris or residue.  It’s important for you to know if your materials and circuits can withstand solvents or mild detergents, so LPT doesn’t affect the metalization or patterning that was applied so delamination doesn’t occur due to our cleaning methods.  If your parts are sensitive to solvents or mild detergents, please contact LPT to discuss other options for cleaning of your materials or circuitry.

Tape Mount

Our preferred method of mounting substrates up to .025” thickness for the diamond sawing process is to use non-residue tape.  When materials are thicker and edge quality is critical, LPT has other mounting techniques for these materials that we can recommend for your application.

Blade Engineers

LPT has blade engineers available to provide technical assistance with your product design.  They can also assist with developing the proper blades for diamond sawing any type of material.  Please contact LPT when you have custom material and blade requirements for your product.