LASER DRILLING & MACHINING OF CERAMIC SUBSTRATES

© Accu-Tech 1994

APPLICABILITY

This document provides general guidelines and considerations for the laser drilling and machining of fired ceramic substrates typically used in the manufacture of microelectronic circuits and multichip modules. The specifications and tolerances given here will generally produce the MOST COST EFFECTIVE design approach. Tighter tolerances may be achieved at some increased cost and leadtime.

PURPOSE OF LASER DRILLING AND MACHINING

The CO2 laser has become an important tool in the precision fabrication of technical ceramics. The reasons for this lie in the technological changes that have occurred within the electronic industry to miniaturize parts and produce them using batch fabrication methods. A brief history:

  • In the early days of ceramic substrate fabrication, individual substrates where small in overall size, relatively thick, and substrate features were generally large. These small parts were typically metallized one-at-a-time using automated feeders and loaders. The state of the art in substrate tolerances was +/- 1%.
  • Early fabrication methods in fired ceramic involved machining substrate features with carbide, diamond or ultrasonic tools. Although these techniques were not really cost effective and were limited in the type and size of features they could create, there were all that was available at the time when precision locations of features were required.
  • A subsequent method was developed that utilized precision tooling to punch the required features in "green" ceramics before firing. This method improved the cost situation when the quantities produced justified the cost of prototyping and tooling. Tolerances improved but were limited by variations in the firing process. Green punching technology can be quite effective for volume production runs where substrate features are relatively large and the lot-to-lot and part-to-part tolerances are +/- 1% or larger. Feature size is generally limited to holes or shapes greater than 0.010 in. across the smallest dimension.
  • In recent years the high circuit densities and cost reduction efforts demanded by the electronic industry have required that batch fabrication methods be used to cost effectively manufacture ever shrinking miniature substrates. As a result, new hardware, tooling, and techniques have been developed to fabricate multiple parts with high precision on large substrate sheets.
  • Under these pressures, the CO2 laser has developed into the most precise and versatile method of fabricating ceramic substrates. Under software control the laser can create features of virtually any planar shape and can maintain tolerances to within +/-0.001 in. The laser is extremely flexible and permits close location of features with considerable layout flexibility. Hard tooling is not required, turnaround is quick, and the cost is low.
  • The combination of green punching and laser machining can be cost effective for manufacturing substrates with large, non-critical holes and small, high density features requiring precision.

TYPES OF CERAMIC MATERIALS

Materials covered include thin, flat substrates of Alumina, Beryllia, and Aluminum Nitride. Call the Accu-Tech factory directly for technical information on other special materials that may be laser machined.

GENERAL MATERIAL CHARACTERISTICS

Alumina, 96%

  • Excellent overall substrate material for cost effective manufacturing and laser processing. Typically represents over 90% of the microelectronics volume.

Alumina, 99+%

  • Similar to 96%. Provides a superior surface for fabricating Thin Film circuitry.

Beryllia, BeO

  • Typically used for its superior heat conductivity. Beryllia requires special handling and safety precautions due to the potential hazard of the dust produced during cutting, grinding, drilling, or breaking operations.

Aluminum Nitride

  • Roughly equivalent in heat conductivity to Beryllia but the safety issue is avoided. Call Accu-Tech for additional information on this material.

SUBSTRATE SIZE AND LIMITS

 Typical Substrate Size: 4.5 in. X 4.5 in. - Sizes up to 16 in. Square can be processed.
 Typical Substrate Thickness: 0.010 in. TO 0.060 in. - Thicknesses greater than 0.100 can be processed.

COST EFFECTIVE DRILLING OF HOLES

In order to achieve cost effective manufacturing, multiple parts are typically created on a single large substrate. The parts are then processed in batch form and later singulated into individual substrates by breaking along scribe lines. The processing of either individual substrates or arrays requires accurate registration at each operation.

SUBSTRATE ALIGNMENT OPTIONS

Several options for substrate alignment are listed below in order of increasing laser processing cost.

1. SCRIBED EDGES - After scribing the substrate, the borders are broken off to produce accurate outside reference edges for the subsequent processing operations.

2. AS-FIRED EDGES - The pattern to be cut by the laser is located on the substrate in relation to the original edges. Two adjacent edges on the substrate are used to form a reference corner. The entire substrate can be utilized with this method but the alignment accuracy may be poor due to irregularities in the fired edges.

3. ALIGNMENT FLATS - The alignment repeatability can be improved for subsequent processing by the addition of precision, laser machined flats along the outside reference edges of the substrate. These flats provide a smooth surface to make accurate contact with the tooling pins. Using this method avoids the expense of laser machining the entire substrate edge.

4. POST ALIGNMENT - With this method, cut features can be optically aligned to the substrate metallization or other surface features such as holes, edges or other existing scribe lines. Accuracy is excellent.

Figure 1 illustrates a substrate designed with breakaway borders into which three alignment flats have been machined. The substrate is shown being registered against three alignment pins. For illustration purposes there are sixteen individual parts (defined by scribe lines) shown on the substrate. Each part contains six holes and a cutout that requires laser machining.

 

 FIG. 1 - Substrate Registration using Alignment Flats

METHODS OF LASER DRILLING HOLES

There are two basic methods of creating a hole:

1. Pulsed or percussion method -

This method is suitable for drilling small round holes up to 0.005 in. by rapidly vaporizing the substrate material at high laser power. Hole creation is very fast and clean. The illustration in Figure 2 shows the crossection of a drilled hole. The hole shape is dependent on energy distribution within each laser pulse.


 

 Fig. 2 - Crossection - Laser Cut Hole

2. Contoured or trepanned method -

This method is used to produce a hole of any size or shape. The method consists of selecting a punch- thru point inside the periphery of the feature, cutting to the feature edge, and then following the edge outline to complete the cut that defines the feature.

 

 Fig. 3 - Photo of Laser Contoured Hole

THE ANATOMY OF A LASER CUT

The method of hole creations (either pulse or contour) does not materially affect the cross-sectional shape of the cut edge. The sketch in Figure 2 shows the cross-section of a typical laser cut edge. The laser beam has entered at the top and exited at the bottom. As the beam vaporizes the material, the entrance edge becomes slightly rounded. The cut also develops a slight taper. See the table on at end of this document for the taper vs thickness.

DESIGNING MACHINED FEATURES

Virtually any planar shape can be cut in ceramic substrates. These shapes include circles, curves, rectangles, polygons, rounded objects, thin slots, etc., and any combination of the above. Since ceramics are strong but brittle materials, the designer should consider a radius as large as practical on inside corners. All inside corners will have a minimum 0.002 in. radius due to the laser beam diameter. Rounding outside corners can also reduce chipping.

An important consideration when designing a machined feature is the location of the start and stop points for the cut. Some general guidelines:

  • Provide for the location of the start and stop points in a benign area away from corners. For example when cutting a rectangle, a start point near the center of the long edge is ideal.
  • Provide clearance around the area where the laser will punch through so that a potential slag bump or chip will not affect the adjacent features.

LAYOUT CONSIDERATIONS

In order to ensure a high yield of finished parts, the designer should attempt to maintain at least the minimum recommended distance between the edges of machined features. See the table at end of document for recommended values.

  • Avoid aligning scribe lines and machined features in such a way that the break along the scribe line might deviate to include the machined feature.
  • When parts in an array are separated by waste strips, the recommended width of the waste strip should be 0.100 in. or greater.

DEALING WITH SURFACE MATERIALS

Other materials may appear on the surface of a ceramic substrate or within the substrate and affect the cutting of the substrate. Examples include:

  • Metallization- Such as thick film or deposited thin film metals.
    1. Metallization on the surface tends to reflect the laser energy. The result can be a slight change in the kerf line and an undercutting of the ceramic beneath the metallization. Cutting through metal on the laser entrance side can cause discoloration of the cut edge.
    2. Metallization on the backside in contact with the laser beam tends to melt and bead.
    3. Refractory metallization within the substrate can protrude slightly from the cut edge.
  • Dielectrics-Such as glasses or polymers.
    1. Glass dielectrics may tend to chip when cut, especially on the exit side of the laser beam.
    2. Polymers may be affected when the laser beam approaches within 0.002 to 0.005 in.
  • Components such as chip capacitors, chip resistors, and semiconductors may cause clearance problems or require special fixturing.

The effect of these materials may be minimized with proper layout and good laser machining techniques.

HANDLING AND CLEANING

Substrates are generally coated with a water soluble material to protect them during scribing, breaking, machining or shipping. The coating may be removed by water wash. Normally, the coating is removed by Accu-Tech unless otherwise specified by the customer.

REMOVAL OF SLAG

Slag occurs when the substrate material is melted by the laser. Slag buildup is primarily found on the beam exit side of the substrate and is removed after laser processing.

TOLERANCES FOR DRILLING AND MACHINING

The SPECIFICATIONS and TOLERANCES provided in the table below will generally produce the MOST COST EFFECTIVE laser processing. Tighter tolerances may be achieved at an increased cost and leadtime. All dimensions and tolerances are given in decimal inch units. Metric units are also available.

 SUBSTRATE
THICKNESS
TYPICAL EDGE TAPER EXCLUDING ENTRANCE ROUNDING RECOMMENDED DIAMETER TOLERANCE (NOTE 1) MAXIMUM CHIPOUT MINIMUM FEATURE EDGE TO EDGE MINIMUM FEATURE EDGE TO SCRIBE C/L MINIMUM FEATURE EDGE TO METAL EDGE
.010 .001 +/-.002 .005 .010 .010 .005
.015 .001 +/-.002 .008 .015 .015 .005
.020 .001 +/-.002 .008 .020 .020 .005
.025 .001 +/-.002 .010 .025 .025 .005
.030 .001 +/-.002 .010 .030 .030 .005
.035 .001 +/-.002 .010 .035 .035 .005
.040 .002 +/-.003 .010 .040 .040 .005
.050 .002 +/-.003 .012 .050 .050 .005
.060 .002 +/-.003 .015 .060 .060 .005

Note 1. Optical measuring techniques are generally used to verify these dimensions.

For tighter diameter control, pin gauges are recommended.

As a general rule it is helpful to coordinate and/or specify specific measuring methods when attempting to measure dimensions and tolerances of the magnitude shown here.

Feature location tolerance............ +/-.002, centerline to centerline.

Kerf width.................................... .004 +/-.001 measured at the beam exit side.

Slag.............................................. .001, Max. residual after removal


DRL1194

© Accu-Tech 1994

Accu-Tech©
LASER PROCESSING INC.

 

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