LASER SCRIBING OF CERAMIC SUBSTRATES

© Accu-Tech 1994

APPLICABILITY

This document provides the General Guidelines and Considerations for the laser scribing of ceramic substrates that are typically used in the manufacture of Microelectronic Circuits and Multichip Modules. The specifications and tolerances given will generally produce the MOST COST EFFECTIVE design approach. Tighter tolerances may be achieved at increased cost and leadtime.

PURPOSE OF SCRIBING

Scribing makes possible the cost effective manufacture of arrays (multiple ceramic parts) that may be then singulated by breaking them apart. Scribing also provides a cost effective means of improving part quality by allowing the removal of the outside borders that contain defects such as rough or non-parallel edges.

TYPES OF MATERIALS SCRIBED

Typical materials scribed are thin, flat substrates of Alumina, Beryllia, and Aluminum Nitride. Please call the Accu-Tech factory directly for technical information on other special materials that may be scribed.

GENERAL MATERIAL CHARACTERISTICS

Alumina 96%

  • Most commonly used in Microelectronics.
  • Represents over 90% of the volume manufactured.
  • Excellent overall substrate material for cost effective manufacturing and laser processing.

Alumina 99+ %

  • Similar to 96% but typically provides a superior surface finish 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 toxicity of the dust produced during cutting, grinding, drilling, or breaking operations.

Aluminum Nitride

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

DIMENSIONAL CONSIDERATIONS

 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.

The processing of arrays involves the registration of the substrate at each operation. Precise alignment at each step is of critical importance. Alignment is typically accomplished by registering the part against three pins where two are located along one edge of the part and one on an orthagonal edge. Since "as fired" or scribed edges may lack the required precision, an additional step to laser machine alignment flats may be required. The figure below (fig. 1) shows substrate alignment flats registered against the alignment pins.

SCRIBE CHARACTERISTICS

A laser scribe line consists of a series of small, closely spaced holes in the substrate that are produced by pulses of laser energy. Viewed under magnification, the scribe holes should appear essentially clean and free of recast. Under backlighting a plugged hole will appear dark. An occasional plugged hole will not affect the scribe. Hole depth is controllable and should generally be 1/3 to 1/2 of the substrate thickness.

SEM PHOTO OF SCRIBED HOLE

The SEM photo above shows a cross-section of the tapered hole that has been created by the penetration of a laser pulse into the substrate. The minor amount of slag shown on the substrate surface has been deposited around the hole as a result of the melt. The slag is benign, and is usually removed by a scraping process if the substrate has not been metallized.

The hole depth can be measured by inking the scribe line with a black marker pen, snapping the substrate at the scribe line, and viewing the cross section with a calibrated microscope.

LAYOUT /LOCATION OF SCRIBE LINES

The distance from a scribe center line to the edge of other features such as holes, cutouts, or metallization should be 0.010 to 0.050 inch minimum, depending upon the part design.

The border width or distance from the last scribe line to the edge of the substrate should be 0.100 inch, minimum. The border areas are shown in the figure below. Alignment flats have been laser machined into the borders to permit precision alignment of the substrate into work fixtures.

It is recommended that the scribe lines should completely cross the border areas to produce a good break and clean, square corners when the parts are singulated.

 

THE FINISHED PART

In the drawing above, the finished part is shown as being separated from the array. A corner of this part is magnified to show the somewhat rough edges that normally result when singulating a part from a scribed substrate. In applications where the edge roughness must be overcome, the scribe lines can be laser machined at an increased cost.

SUBSTRATE COATING

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.

 

SCRIBING AND ALIGNMENT OPTIONS

  1. AS-FIRED EDGES - The laser scribe pattern 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 irregularities in the original edges.
  2. SCRIBED EDGES - After scribing the substrate, the borders are broken off to produce accurate outside reference edges for subsequent operations.
  3. ALIGNMENT FLATS - Alignment repeatability can be improved by the addition of precision, laser machined flats along the outside reference edges of the substrate. These flats provide a smooth, accurate surface to make contact with tooling pins. The expense of having to laser machine the entire substrate edge is avoided.
  4. POST SCRIBING - Using this method, the scribe lines can be optically aligned to substrate metallization or other surface features such as holes, edges or other existing scribes.

SINGULATION OF THE SUBSTRATES

Even with the best scribing, the ability to hold extremely tight tolerances after the break will depend on the substrate material and the skill of the operator. Skilled hand breaking is usually preferred to machine breaking.

Common effects of breaking that cause a variation from a "perfect" straight line are hooks or flares, breakouts, and chips. The majority of these defects occur at the ends of the scribe line and the corners where scribe lines cross each other. The examples below show common variations encountered when breaking substrates. The specifications section below provides cost effective guidelines for acceptance.

   
 HOOK AT CORNER  CHIP AT CORNER

 

INSPECTION OF SCRIBED SUBSTRATES

The following equipment or its equivalent is recommended for the inspection of scribed and/or broken substrates.

  • Ceramic thickness
 - Micrometer
  • Substrate Features, distances, locations
 - Optical Coordinate measurement machine
  • Pulse spacing, Scribe Depth, Hole Taper, Pulse Hole Diameter, HAZ, Slag Height
 - Microscope with calibrated graticule
  • Breakout Tolerances
 - Calipers
  • Camber
 - Parallel Plates

TOLERANCES FOR SCRIBING

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
 DEPTH OF
SCRIBE
DEPTH
TOLERANCE
+/-
PULSE (1)
SPACING
96+% AL2O3
PULSE (1)
SPACING
99+% AL2O3 
 MAX
CHIPOUT
FINISHED
PART
BREAKOUT
.010 .004-.006 .001 .005 .005 .005 +.005/-.002
.015 .006-.008 .001 .006 .005 .008 +.005/-.002
.020 .008-.008 .002 .006 .005 .008 +.005/-.002
.025 .009-.012 .002 .006 .005 .010 +.005/-.002
.030 .012-.014 .002 .006 .006 .010 +.005/-.002
.035 .014-.016 .003 .006 .006 .010 +.010/-.004
.040 .016-.018 .003 .006 .006 .010 +.010/-.004
.050 .019-.023 .004 .006 .006 .012 +.010/-.004
.060 .023-.027 .004 .006 .006 .015 +.010/-.004

NOTE (1) Pulse spacing tolerance is +/- 0.001 in.

Surface Slag height, Max. . . . . . . .001

Scribe Location Tolerances

Note: All measurements made to a scribe line are made to the scribe centerline.

Scribe, line to line. . . . . . . . . . . +/-.001

Scribe, line to laser machined feature. . . . . . . . . . . +/-.002

Lasered feature to scribed exterior edge for ceramic thicknesses of:

.010 to .030 . . . . . . . . . +/-.003

.035 to .060 . . . . . . . . . +/-.005

Scribe line with respect to a fired edge. . . . . . . . . . . +/-.005


CL0694

© Accu-Tech 1997

Accu-Tech©
LASER PROCESSING INC.

 

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