By Jocelyn “Justin” Lauzon, Senior Technical Advisor at Reflex Photonics Inc.
Reliability is Critical for Components Operating in Space
In space and avionics applications, failure is not an option. Components must stand extreme heat, cold, radiation, shock, and vibration, yet deliver reliable performance. Devices must be tested beyond what is specified to ensure performance in harsh environments to avoid failure.
What are the criteria of a good reliability test program and how should one be designed? In this article, we examine the reliability requirements of such devices and the steps involved in designing a test program to ensure reliability and performance.
Diagram of the possible connections between the causes of failure. Connections are indicated with solid lines, no connection with a dashed line. Preordained failures have an internal mechanism that produces the same outcome, failing at a set rate, from the moment of creation. (Source: AT&T Tech Journal, 1985)
Electronic devices in space are bombarded by various types of radiation including visible, infrared, ultraviolet, x-rays, gamma rays and others.
The Advantages of Optical Fiber Extends to the Accuracy Demands of Space
In space, high performance components must be able to deliver reliably in the punishing environment. It is optical transceivers that drive transmissions, converting signals to and from a copper-resident format. Fiber optics communications provide high bandwidth and low latency signaling. Signal transmissions through fiber optic cables (FOCs) provide immunity to EM/RFI interference, crosstalk, and voltage level surges. Fiber optics’ accuracy and reliability exceeds traditional cabling. Covering 1,000 feet requires four pounds of FOC versus 39 pounds of copper wiring, and fiber optics also consume less energy than copper. To convert electrical signals from circuitries with copper output to fiber optics, optical fiber transceivers are usually required.
Reflex Photonics’ optical fiber transceivers support multiple electrical interfaces.
Fiber optics adds tremendous value to the entire system, therefore a suitable example within the scope of this article is a short-reach parallel multimode fiber optics transceiver supporting a large bandwidth (up to 28Gbps/channel). Intra-satellite communications require a high bit-rate. In prior years, optical transceivers represented high-performing technology, yet exhibited weakness in harsh environments. However, ruggedized, sealed optical transceivers now survive rocketing into orbit, extreme temperatures, and radiation. Testing optical devices primarily ensures quality and reliability. How does one test optical transceivers for reliable operation in harsh environments?
What Tests Are Needed to Ensure Critical Devices Can Endure the Harsh Environment of Space?
Several tests are necessary to ensure critical devices such as optical transceivers can provide reliable operation in space, including space applications testing (including radiation), mechanical, environmental, Life Tests, Live Tests, and rigorous screening tests for ensuring the reliability of subsequent lots.
How to Perform the Five Critical Tests?
In addition to a controlled set up, each test requires visual inspection for anomalies before and after the evaluation. A significant performance degradation associated with a test is considered a qualification failure. The focus of the discussion relates to mandatory testing of fiber optic transceivers (FOTs) intended for operation in space or a similarly harsh environment. All devices need mechanical and environmental testing to verify long-term integrity. Mandatory Tests also include Life and Live Tests. Such tests can help root out design or process flaws. Finally, even if units pass all tests, every subsequent lot of units must undergo screening tests.
Mandatory Mechanical and Environmental Tests
Three consecutive mechanical tests are executed on the same FOTs. Before and after completion of the mechanical integrity evaluation, units must be visually inspected and characterized over the operating temperature range to confirm no significant performance degradation occurs. Mechanical integrity evaluation tests are executed on non-operating units, in the order below:
- Vibration tests across all three axes (20g between 20Hz and 2000Hz at sixteen minutes per axis)
- Mechanical shock tests of five repetitions each over all six orientations, using 500g shock on a half-sine pulse duration of 0.5ms
- Thermal shock tests consisting of twenty cycles between 0 and 100°C with ten minutes dwell time and transient time less than five seconds
Three environmental stress tests for FOTs include:
- A temperature cycling test to evaluate mechanical fatigue over the component’s lifetime. For example, a mismatch of dissimilar materials subjected to thermal expansion can cause failure.
- A damp heat test to ensure that a sealed FOT can continually provide resistance to a moisture-filled atmosphere.
- A sequence of tests, applicable only to products with a BGA electrical interface compatible with solder reflow, is performed. These test for possible impact to reflow profile and cold temperature storage.
Life Tests validate long-term reliability via extensive, hastened lifetime tests with several units from different lots. Life tests are not intended to test for infant mortality, for which burn-in tests would suit. Life tests forecast performance degradation over a lifetime. To emulate operating for more than 20 years, an FOT operates at a bias current exceeding operating condition by 80 percent for 4000 hours at a case temperature of 100°C.
Live Tests, although more complex, thoroughly evaluate the performance of several electrical and optical interface configurations.
Live testing is performed with high-speed digital signals operating through every channel of the DUT (Device Under Test). As the DUT is stressed, transmission errors are measured to maintain a BER (bit error rate) that is better than 1 error in one trillion bits while operating under harsh conditions. Tested under stress, a significant signal degradation of an FOT will demonstrate a cumulative effect from both transmitter and receiver areas that would impact the error count.
Live test setup configuration.
Space Applications Tests
A major threat in space is radiation. Radiation testing must be split into three different categories for evaluation, covering potential complications related to geostationary or low-earth orbit environments. At least five units should be collected for each test. The tests should be repeated when FOT critical components arrive from new fabrication lots. The radiation tests are:
- A non-operational test with a Total Non-Ionizing Dose (TNID) protons (5e12proton/cm2total dose).
- A live test, Single Event Effects (SEE), at both room temperature and 85°C, including heavy ions (Ho, Cu, Ar, Ne, N, each for a total fluence of 1x107ions/cm2).
- A biased and un-biased test with Total Ionizing Dose (TID) of gamma rays (100 krad cumulative dose).
The number of errors/events caused by live radiation tests must be compared to those occurring at acceptable levels. For non-operational tests, the performance of FOTs before and after the application of radiation dosages are compared and can only differ negligibly to pass. A ruggedized FOT that is sealed with encapsulation material must undergo an outgassing test. Live thermal vacuum tests, maintaining a vacuum of at least 5x10-5hPa for at least twenty thermal cycles extending from -40°C to 85°C, ramp temperature at ~5°C/minute, with 5-minute dwell times must also be successfully completed. Other tests for space applications include decompression tests, which can also be used to qualify parts for avionics applications. Decompression tests demonstrate that performance is not affected at pressure levels emulating a 2,438 m or 8,000 ft altitude, ramping to 15,850 m or 52,000 ft (in less than 15 s) and remaining there for one hour.
Finally, Screening Tests of six thermal cycles ramping at 8°C/minute with five-minute dwell times are completed for continuous assurance of quality COTS devices. Added to thermal cycling is a burn-in with a duration of 168 hours and a case temperature 100°C, operating at normal bias current. After a new FOT design is qualified via the aforementioned testing, units from subsequent production lots are validated with the screening process. Temperature cycling tests tend to reveal assembly defects, whereas burn-in tests exhibit infant mortality rates.
As discussed above, in applications such as space, avionics, and defense, failure is not acceptable. Components and systems must stand extreme heat, cold, radiation, shock, and vibration to deliver reliable performance. Finding failures, defects and marginal components through stringent testing is essential, and leads to products with a long life and high reliability in the field and in space. The five critical tests include Mandatory Mechanical and Environmental Tests, Life Tests, Live Tests, Space Applications Tests, and finally, Screening Tests. Advancing through a significant number of tests designed to demonstrate reliability in harsh environments results in fiber optic transceivers that meet or exceed operational requirements, and typically outweighs cost saving considerations for many critical applications.