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Crystal Blanks

A crystal blank is the quartz wafer exclusive of any mounting structure or hermetic package. Blanks can be any of the typical crystal cuts, AT, BT, SC, FC, IT and GT. Crystal blanks are commonly supplied to OEMs to be processed in to various frequency control products: hybrid oscillators, hybrid VCXOs, sealed crystal units and sensors.

There are two basic types of banks: un-plated blanks and plated blanks . Un-plated blanks have no electrodes deposited on each side of the quartz wafer. Common to both un-plated blanks are surface finish and blank geometry.

Surface finish is the texture and quality of the crystal blank's surface. It is specified by the particle size, in microns, of the final abrasive used in the processing of the blank: 5 micron, 3 micron, and 1 micron. Surface finish better than 1 micron is a "polished" finish used in the manufacture high-precision resonators.

Blank geometry refers to the, diameter, thickness and cross section of the crystal blank. For AT, BT and doubly rotated blanks, frequency is a function of thickness. Diameter has a direct relationship to thickness but also is constrained by the mechanical limitations of the package. A good rule of thumb is the higher the fundamental frequency, the smaller the blank and vice versa. The cross section of the blank may have to be manipulated/shaped to provide optimum performance of the resonator. The cross section designs include but are not limited to plano-plano, beveled (edges), plano-convex, convex, and ring-supported (inverted-mesa).

Common Blank Cross Sections:

Crystal blank cross sections.

Inverted-mesa/ring Supported Crystal Blank:

Inverted-mesa/ring Supported Crystal Blank

Since diameter and frequency are directly related, a number of blank diameters (in inches) are available to provide products over the broad range of frequencies from 1-250 MHz:

.150 .270 .390
.200 .295 .420
.220 .323 .550
.240 .350 .600
.250 .370 .650

2.5 mm X 5.0 mm (rectangular strip)

UN-PLATED BLANKS:


Un-plated blanks are typically supplied to oscillator OEMs who will finish the plating process by adding their own electrodes and inserting them into crystal holder, hybrid oscillators or sensors. Since a raw blank is in such an early stage of the manufacturing process and further processing steps will have a direct effect on the electrical performance of the blank, only very rough approximation of frequency, stability are part of the specification. Frequency is typically specified as the nominal plus or minus approximately +/- 0.5% or +/-500 ppm. Stability, verified only by x-ray diffraction would typically +/- 1 minute of arc.

PLATED BLANKS:

Plated blanks are similar to un-plated blanks except that they include the electrodes for electrical interface to the circuit. Again, the OEM's intent is to use them as a building block for the final product, hybrid oscillator, crystal unit or sensor. Due to a plated blank's being further along in the resonators finishing process, the specification can be more stringent that that of an un-plated blank. Calibration tolerance is typically specified at +/- 0.2% or +/- 200 ppm. Stability for the blank, excluding any exogenous effects can be verified by placing a sample group into holders and testing over temperature to verify performance.

Several items are part of any electrode specification for plated blanks. They include type of metal(s), electrode size, and configuration.

Electrodes provide the electrical interface to the circuit requiring they be conductive. Various conductive metals are used in the manufacture of plated blanks, not all of which adhere well to quartz. Often chromium is first evaporated onto the blank to provide a good interface with the quartz. After the chrome is placed on the plank, an overlay of nickel, gold or silver is added. Aluminum is another metal that is used without the chrome underlay since it bonds very well with the quartz. Aluminum oxidizes quickly establishing a non-conductive layer which must be removed before further processing. Tuning to frequency is accomplished by either adding or removing metal to the electrode.

Electrode size is a design criteria and is selected to achieve specific resonator performance such as pullability/deviation, spurious suppression, spurious free band-pass, Q, phase-noise, motional resistance, capacitance, inductance etc. within the confines of frequency and package size. Since all are inter-related, it is nearly impossible to change one with out effecting other. Electrode diameters range from 0.010 through 0.500 inches and are spaced in such a way as to provide over-lap in crystal motional parameters.

Electrode configuration refers to the angle formed by the electrodes (on opposing sides of the blank) to each other. The three standard configurations are:

90 degrees

120 degrees

180 degrees

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