Who would of thought how complex a crystal device really is!

The following reliability study is a 'Must Read' that explains why our crystals are the best in the industry. You will not find details like the followig from any supplier in the industry!

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Therefore, appropriate tests methods, as recognized by the United States Defense Logistics Agency, were selected in an effort to subject enough environmental stress to provide assurance of product reliability.

A key concept in selecting the test profiles is the assumption that environmental stress loads applied to the R/C quartz crystals simulate time. The standard reliability model in figure 5 is a popular way of conveying this concept whereby product introduced into the field is characterized by three regions (early life, useful life, and wearout).

Figure 5 – Standard Reliability Model

Environmental life tests are performed to accelerate the aging rate of a product by elevating and/or cycling the product temperature. This process may be amplified by introducing other environmental factors such as vibration and/or acceleration etc. Regardless of the technique employed, the purpose of this type of reliability testing is two fold.

The first purpose is to weed out the early life failures (as shown in figure 5). The intent is to eliminate this area of the curve before the product is ever shipped to the customer. By doing this, the customer should experience a low failure rate characterized by the useful product life. The second purpose of a life test is to provide a feedback mechanism, whereby the early life failures are analyzed and appropriate design and process changes are implemented prior to design release and continuous manufacturing. This will assure a robust product as depicted (under the blue bell-curve) in figure 3 on the previous page.

Figure 6 – Test Plan Philosophy

Environmental tests and stress levels were chosen so that they increase the probability and accelerate the occurrence of any failure modes. However, they are not intended to introduce failure modes that would never occur under use conditions (i.e., in the light switch example, pounding a switch with a hammer would not be a normal use condition.) Normally, these stress levels will fall outside the product specification limits but inside the design limits. The philosophy depicted in figure 6 was used to construct the test plan in table 1.

Table 1 – Test Plan

TEST SAMPLES

A total of thirty (30) R/C quartz crystals were constructed in three (3) separate groupings (see figure 6). Samples were selected in an effort to capture reliability information across commonly used R/C products and applications (i.e., road, off-road, air, and water.) Each grouping contained ten (10) crystals of the following frequencies; 26.995 MHz, 72.510 MHz, and 75.970 MHz.

Figure 6 – R/C Test Sample Groupings

THERMAL FATIGUE STIMULI

All thirty (30) R/C quartz crystal samples were electrically tested and verified for compliance prior to being subjected to tests outlined in the test plan. The first group of environmental tests applied consisted of four separate "thermal fatigue" type profiles. Figure 7 outlines the various types of thermal environments an R/C crystal may experience throughout its useful life. Although it is unlikely a hobby enthusiast would be operating a R/C device in -65OC or +125OC temperatures, it is the "harsh" user and the ability for the product to withstand various environmental stresses.

Figure 7 – Thermal Fatigue

Thermal fatigue tests were applied as follows:

Stabilization Bake

The purpose of stabilization bake is to determine the effect on the device storage at elevated temperatures without electrical stress applied. Samples were placed into an environmental chamber and subjected to twenty-four (24) hours of dwell time at +150OC per, MIL-STD-883, method 1008, Condition C. Upon completion of the test profile, samples were removed from the environmental chamber and electrically tested in search of device failures. No failures were found.

Burn-In

The purpose of burn-in is to screen for and eliminate marginal devices, those with inherent defects or defects resulting from manufacturing weaknesses, which cause time and stress dependent failures. Ultimately, the selected burn-in profile was chosen to verify the capability of the product to perform and to eliminate infant mortality and/or latent failures (as depicted previously in figure

5).

Samples were placed into a burn-in chamber and subjected to +125.0OC and maintained in this environment for one-hundred sixty (160) Hours, per MIL-STD 883, Method 1015, Condition F. Temperatures were tracked throughout the duration of the required dwell time. Upon completion of the test profile, samples were removed from the burn-in chamber and electrically tested in search of device failures. No failures were found.

Temperature Cycling

The purpose of temperature cycling is to determine the resistance of a part to extreme high and low temperatures, and to the effect of alternate exposures to such extremes.

Samples were placed onto a temperature cycling chamber and tested to MIL-STD 883, method 1010, condition C. This test method exposes the samples to two-hundred (200) cycles of temperature shock for a total of one-hundred (100) consecutive hours. The low temperature extreme was –65OC and the high temperature extreme was +150OC. Each temperature extreme was held for fifteen (15) minutes, with a transfer time between extremes of less than five (5) minutes.

Upon completion of the test profile, samples were removed from the temperature cycling chamber and electrically tested in search of device failures. No failures were found. In addition, samples were inspected mechanically and there was no evidence of physical damage or degradation to the samples.

Thermal Shock

The purpose of thermal shock is to determine the resistance of a device to exposures at high and low temperature extremes, and to the shock of alternate exposures to these extremes, as would be experienced when equipment or parts are transferred to and from heated shelters in arctic areas. Effects of thermal shock may include cracking and delamination of finishes, internal component cracking, opening of thermal or hermetic seals and/or seams, and changes in electrical characteristics due to mechanical displacement or rupture of internal parts, etc.

Samples were placed into a thermal shock chamber and subjected to a two (2) hour and forty-five (45) minute profile running at five (5), fifteen (15) minute cycles (with a transfer time of less than sixty (60) seconds between temperature extremes) @ +125OC hot and -65OC cold per, MIL-STD-202, method 107, condition B. Upon completion of the test profile, samples were removed from the thermal shock chamber and electrically tested in search of device failures. No failures were found.

VIBRATION FATIGUE STIMULI

The second group of environmental tests applied consisted of two

separate "vibration fatigue" type profiles. Figure 8 outlines a theoretical description of how various types of vibration can impact a particular operating environment. In this case we are looking at an automobile and some common types of vibration stimuli.

Figure 8 – Vibration Fatigue

The frequency of vibration is depicted in Hertz (Hz) and the magnitude of random events causes the frequency vibration wave to change over time. Getting into an automobile, slamming the car door and starting the engine alone has introduced various vibration frequency levels. One will note that driving over a pothole in the road causes a sharp jump in vibration frequency. In a harsh user environment, this could be a potential cause for product failure. In an effort to capture the wide range of potential vibration frequencies, both random and swept-sine vibration test profiles were applied.

It must be noted that random vibration applies a continuous application of forcing random frequencies (as in figure 8). However, random vibration does not force specific vibration frequencies across a pattern, which seeks out resonance. Resonance can be defined as; a condition of maximum response (frequency) that allows all weaknesses to have an equal chance for visibility and breakage.

Therefore, some weaknesses will be exposed only when the resonant frequency of that weakness is applied. A visual example would be a female opera singer’s voice sweeping across an array of very high notes and suddenly a wine glass breaks. In essence, the opera singer "swept" her voice across a frequency range and touched the resonant weakness frequency for the wine glass.

The two selected vibration test profiles were applied as follows:

Random Vibration

The purpose of random vibration testing is to determine the ability of the device to withstand the dynamic stress exerted by random vibration applied between upper and lower frequency limits (20 – 2000 hertz (Hz) using the power spectral density) to simulate the vibration experienced in various field environments. Random vibration is characteristic of modern field environments produced by missiles, high-thrust jets and rocket engines.

Samples were properly mounted to a fixture, placed on a vibration shaker table, and tested in accordance with MIL-STD-202, method 214, condition D. Upon completion of the test profile, samples were removed from the vibration shaker table and electrically tested in search of device failures. No failures were found.

Swept – Sine Vibration

The purpose of the sweep sine vibration test is to determine the effect of high-frequency vibration on component parts in the frequency range of 10 – 2000 Hz, as may be encountered in aircraft, missiles, and tanks. This test does not strike random frequency vibrations, but sweeps across the frequency spectrum as prescribed.

Samples were properly mounted to a fixture, placed on a vibration shaker table, and tested in accordance with MIL-STD-202, method 204, condition D. Upon completion of the test profile, samples were removed from the vibration shaker table and electrically tested in search of device failures. In addition, a visual examination was conducted in search of mechanical damage. No failures were found.

FORCE FATIGUE STIMULI

The third category of environmental testing applied consisted of one test profile for "force fatigue." Figure 9 outlines a theoretical depiction of how force fatigue can play a major role in stressing a product throughout its useful life. An automobile, for example, can make numerous 360O turns (the same could be said for an R/C controlled airplane or watercraft), thus creating various g-forces along with the vibration frequencies occurring from normal operation (i.e., driving over potholes).

Figure 9 – Force Fatigue

Constant Acceleration

The purpose of constant acceleration testing is to determine the effects on the types of structural and mechanical weaknesses not necessarily detected in vibration tests. It may be used as a high stress test to determine the mechanical limits of the package, internal metalization, and lead system, die or substrate attachment, and other elements of the device.

Samples were placed into a centrifuge and subjected to constant acceleration per, MIL-STD 883, method 2001, Y1 axis only, for one (1) minute at five-thousand (5000) g’s. Figure 10 provides a schematic outlining the positioning of each R/C quartz crystal in the Y1 axis.

Figure 10 – Schematic for Y1 Axis

Due to success in crystal electrical performance and physical characteristics, a decision was made to step-stress the samples in excess of the five thousand (5000) g-force requirement specified in the MIL-STD 883 test document. Figure 11 provides a visual model of the step-stresses applied up to the maximum eight-thousand (8000) g-force capability of the centrifuge.

Figure 11 – Constant Acceleration Step-Stress Model

Upon completion of each step, samples were removed from the centrifuge and electrically tested in search of device failures. As the test progressed up to the maximum capability of the centrifuge, a total acceleration of eight-thousand (8000) g’s for five (5) minutes was achieved with no failures.

VERIFICATION TESTING

The remaining group of tests were selected as a means of post-stress verification to specified performance requirements.

Seal Testing

The purpose of seal testing is to determine the effectiveness (hermeticity) of the R/C quartz crystal hermetic seal and search for any leaks resulting from thermal, vibration and/or force fatigue testing. All samples were subjected to a fine leak test followed immediately by a gross leak test, per MIL-STD-202, method 112, conditions C & D respectively.

The fine leak test was performed using a sealed chamber and pressurized at sixty (60) psi with grade five (5) helium tracer gas for a one (1) hour duration. Upon completion of a one (1) hour duration under pressure, the chamber pressure was then relieved. Each sample unit was then placed into a sealed vacuum chamber, which was connected to the mass-spectrometer leak detector. The detector was then set to 10-8 cc/sec. The sample was then tested for leakage at a reject parameter of 2.0 X 10-8 cc/sec.

Immediately following the fine leak test, the gross leak test was performed. The samples were immersed in a liquid fluorocarbon at 125° ± 5° C and visually examined under 3 X magnification. All samples met both the fine and gross leak test requirements.

Mechanical Inspection

Each sample was subjected to mechanical inspection in accordance with dimensional requirements specified in the PDI design specification. All samples were measured accordingly with no observed failures.

Electrical Test

Each sample was subjected to electrical testing in accordance with electrical parameters specified in the PDI design specification. Nominal supply voltage was used with specified load. Frequency and resistance readings were recorded. No electrical failures were observed.

STATISTICAL ANALYSIS OF PERFORMANCE

Upon completion of electrically testing all thirty (30) R/C quartz crystal samples, respective data was tabulated for statistical review and analysis. Table 2 provides a summary of frequency (MHz) performance data for each of the three product groupings. Initial electrical data are those readings collected prior to environmental testing and post-test data were collected upon completion of all environmental stress testing.

Frequency data expressed in parts-per-million (PPM).

Table 2 – Descriptive Statistics of Performance Data

In the science of statistics and mathematics, skepticism must always be taken when reviewing such data. Although the means (averages) and standard deviations (dispersion) appear to be non-significant, it is possible that stressing the parts has caused the data to shift in a manner that has significantly changed part performance. This does not mean parts have shifted out of performance specification, but can mean that such stress has caused a shift away from nominal that could trend toward an upper or lower specification limit. It is for this reason that hypothesis testing was performed.

Hypothesis testing is a statistical tool where sample information is used to draw a conclusion about a population of data. In this case we are concerned with the differences observed between initial test and post-test data. From a purely numerical standpoint, the two data distributions are slightly different. The engineering question is; "Are these observed differences due to random variation or due to an assignable cause?" If hypothesis testing yields no statistical significance, then the observed means and standard deviations (in table 2) are in close proximity to one another and thus the differences are due to randomness. Likewise, if there is statistical significance, we conclude that applying environmental stress testing has caused R/C crystal performance to change.

For the purpose of this analysis, the 75.970 MHz data was selected for review. These parameters were selected due to the fact that the greatest magnitude of frequency (MHz) difference was observed between initial electrical data and post-test data sets.

An F-test was used to compare the variances and a t-test was used to compare the means of the data distributions. The following hypothesis tests utilized these two formulas respectively:

In review of table 2 the assumption is that the mean (average) of initial test frequency (MHz) data is greater than the post-test data. Likewise, the assumption is that the sample populations follow a normal distribution and that the variances (standard deviations) are equal.

Therefore, the null and alternate hypotheses for the F-test were:

The null and alternate hypotheses for the t-test were:

At the 95% confidence level, the table value for F.05 (df = na - 1, nb - 1) = 3.18. Since Fcalc(1.22) < Fcrit(3.18), we accept the null hypothesis and conclude that there is not statistical difference in variation.

At the 95% confidence level, the table value for t.025 (df = n - 1) = 2.262. Since tcalc(0.977) < tcrit (2.262), we accept the null hypothesis and conclude that there is not a statistical difference between the two means.

CONCLUSION

A product will fail when applied load exceeds design strength. Selected military test profiles applied to the R/C quartz crystal sample groupings did not exhibit any unusual failure modes, showed no unusual degradation and proved to be as reliable as those required for "High Reliability" (Hi-REL - used in the military products industry).

Figure 12 – Bell Curve Comparison of Test Data

As a confirmation of electrical performance characteristics, hypothesis testing was conducted. Statistical evidence does not suggest a difference in average frequency (MHz) or a difference in distribution variance as a result of the various environmental stresses applied. The initial test and post-test data distributions (see figure 12) are statistically the same. Therefore, it is with high probability that one may conclude Performance Devices R/C quartz crystals are capable of sustaining a wide variety of harsh operating environments.

REFERENCES

1. DeVor, R. E., Chang, T., Sutherland, J. W. (1992). Statistical Quality Design and Control. New York, NY: Maxwell Macmillian International.

2. Edison, L. G., (2004). Demonstrating Reliability Requirements with Accelerated Life Testing. Detroit, MI: General Motors Corporation.

3. Hobbs, G. K. (2000). Accelerated Reliability Engineering, HALT and HASS. New York, N.Y., John Wiley & Sons, Inc.

4. Ireson, G. W., Coombs, C. F. Jr., Moss, R. Y. (1996). Handbook of Reliability Engineering and Management (2nd Ed.). New York, N.Y., McGraw-Hill.

5. MIL-STD-202 (2002). Test Method Standard - Electronic and Electrical Component Parts (Rev. G). FSC: 59GP United States Department of Defense, USA.

6. MIL-STD-883 (1996). Test Method Standard - Microcircuits (Rev. E). FSC: 5962 United States Department of Defense, USA.

7. McLean, Harry W. (2000). HALT, HASS & HASA Explained: Accelerated Reliability Techniques. Milwaukee, WI: ASQ Quality Press.

 

 

 

 

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