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Reliability Testing and Qualification – Systematic Demonstration of Durability
A heavy-duty HMI must not only function on the day it is delivered but also operate reliably for ten years or more under extreme conditions. The certainty that a product meets this requirement demands a systematic, scientifically sound approach to qualification based on standardized endurance testing. These tests simulate years of use in a compressed timeframe and reveal weaknesses before the product leaves the factory. For manufacturers, they are the key to minimizing risk; for customers, they provide proof that their investment is protected.
The Philosophy of Accelerated Aging
The idea behind reliability testing is simple: by applying stress factors that exceed normal levels, aging processes that would take years in the field can be simulated in weeks or months in the laboratory. This approach is based on the Arrhenius equation and other physical models that describe how reaction rates scale with temperature, humidity, or mechanical stress. A product that survives 1,000 hours at 85°C and 85% relative humidity has a very high statistical probability of functioning for ten years under more moderate conditions.
However, it is crucial that the tests are not arbitrary. They follow internationally recognized standards developed by industry consortia, standards organizations, and military institutions. These standards define precise test conditions, evaluation criteria, and acceptance limits. For display systems, the IEC 61747 series (for LCD and touch displays), SAE J2412 (for UV aging in automotive applications), AEC-Q100 (for automotive electronics), and MIL-STD-810 (for military and extremely rugged applications) are particularly important.
Thermal Testing: Temperature Cycling and Extreme Temperatures
Thermal stress is the most common cause of failure in heavy-duty HMIs. The combination of extreme temperatures and cyclic changes places the entire system under stress.
The thermal cycling test is the cornerstone of thermal qualification. A typical cycle takes the HMI from -40°C to +85°C and back, with dwell times of 15 to 30 minutes at each extreme and transition rates of 5 to 15°C per minute. For particularly demanding automotive applications, extended ranges from -55°C to +125°C are used. A complete qualification test comprises 200 to 1,000 such cycles, which corresponds to several years of real-world use. This test reveals weaknesses caused by CTE mismatch (Coefficient of Thermal Expansion Mismatch) between different materials. During each cycle, shear stresses build up and then dissipate between the layers, particularly in the adhesive layer. An unsuitable adhesive or a flawed design will fail the test, leading to delamination, cracking, or optical artifacts such as Newton’s rings.
In addition, High Temperature Storage is performed, during which the HMI is maintained at a constant temperature of +85°C to +125°C for 500 to 2000 hours. This test simulates long-term exposure to high temperatures, such as those that can occur in a parked vehicle in the desert. It identifies issues such as yellowing of the adhesive, outgassing of materials, or thermal degradation of the materials. Low-temperature storage at -40°C to -55°C, on the other hand, tests whether materials become brittle in cold conditions or lose their functionality.
Humidity Testing: The Insidious Threat
Moisture is a silent killer. It slowly penetrates materials, corrodes interfaces, and leads to failures that often don’t become apparent until months or years later. Humidity testing is therefore essential for evaluating long-term stability.
The Temperature-Humidity-Bias Test (THB), often referred to as the 85/85 test, is the industry standard. The HMI is kept at 85°C and 85% relative humidity for 500 to 2000 hours. Optionally, an electrical voltage (bias) is applied to accelerate electrochemical migration processes. These conditions are extreme: they correspond to a tropical environment under continuous operation. The test reveals weaknesses in the edge sealing, shows whether the adhesive delaminates due to hydrolysis (water-induced breakdown of the adhesive bonds), and uncovers corrosion issues on metallic contacts or conductive layers. A bonded HMI that passes this test has proven that its seal is effective and that the adhesive remains stable even under the influence of moisture.
The Cycled Temperature-Humidity-Bias Test (CTHB) goes one step further by introducing cyclic changes between different temperature and humidity levels. A typical cycle ranges from 25°C/50% RH to 65°C/90% RH and back. This cyclic stress is even more extreme and thus more demanding than constant conditions, as it places additional strain on the materials through expansion and contraction.
UV and Light Aging: The Challenge of Sunlight
UV resistance is critical for outdoor applications. Sunlight contains high-energy UV radiation that attacks polymers, leading to yellowing, clouding, and mechanical degradation.
The SAE J2412 standard is the gold standard for accelerated UV aging of automotive components. In this test, the HMI is exposed in a xenon arc lamp chamber to a controlled irradiance of 0.55 W/m²/nm at 340 nm, at a black standard temperature of 89°C. A test duration of 500 to 2000 hours corresponds to approximately three to five years of real-world solar exposure. After the test, the Yellowness Index (a quantitative measure of yellowing), transmittance, and haze are measured. An increase in the Yellowness Index of more than 3 to 5 units or an increase in haze of more than 1% is typically considered a failure. Silicone-based adhesives demonstrate their superiority here: they remain virtually unchanged even after extreme UV exposure, while acrylate adhesives without UV stabilizers can yellow significantly.
Mechanical Tests: Vibration, Shock, and Drop
Heavy-duty HMIs are exposed not only to thermal and chemical stresses but also to significant mechanical stresses. Vibrations from engines, shocks from rough terrain, and occasional impacts are part of everyday use.
Vibration tests simulate the continuous vibrations to which an HMI is exposed in a vehicle or machine. In accordance with MIL-STD-810 or similar standards, the HMI is tested over a frequency range of 10 to 2000 Hz with accelerations of 1 to 20 g (depending on the application) for 8 to 24 hours per axis (X, Y, Z). Modern tests use Random Vibration, which replicates a more realistic spectrum than simple sinusoidal vibrations. This test detects mechanical fatigue, solder joint failures, and delamination caused by cyclic loading.
Shock Testing according to MIL-STD-810 Method 516.8 simulates sudden, violent impacts, such as those that occur when driving over an obstacle or during a collision. Typical pulse shapes include half-sine, trapezoidal, or sawtooth with amplitudes ranging from 50 to 500 g and durations from 6 to 11 ms. The HMI must withstand three shocks per axis without glass breakage, delamination, or functional failure. Optical bonding offers a clear advantage here: Full-surface bonding distributes the shock energy across the entire surface and reduces the peak load on the glass, which significantly increases its fracture resistance.
Drop tests simulate the device being dropped. The HMI is dropped onto concrete or steel from heights of 0.5 to 2 meters. The number of drops varies between 5 and 10. This test is particularly relevant for portable or semi-portable devices and evaluates the mechanical integrity of the entire system.
HALT and HASS: Extreme Methods for Extreme Reliability
FFor applications where failure is not an option, HALT (Highly Accelerated Life Testing) and HASS (Highly Accelerated Stress Screening) are used.
HALT is not a lifespan prediction, but a method for identifying design weaknesses. Prototypes are exposed to progressively increasing stress levels—temperature and vibration are continuously increased until the product fails. The failure modes identified in this process are analyzed, and the design is improved accordingly. This iterative process results in extremely robust products that far exceed standard requirements.
HASS is the application of these findings in mass production. Based on the HALT results, defined stress levels are established and applied to 100% of the production run or to random samples. The goal is to weed out manufacturing defects and early failures (infant mortality) before the product reaches the customer. HASS dramatically improves field quality and reduces warranty costs for the manufacturer.
Evaluation and Acceptance Criteria: What Does “Pass” Mean?
A test is only as good as its evaluation criteria. After each test, the HMI is thoroughly inspected and measured. The optical evaluation includes transmission measurements, haze measurements, determination of the Yellowness Index, and a visual inspection for bubbles, delamination, or discoloration. The mechanical evaluation includes peel tests to measure adhesive strength, verification of dimensional stability (warping, distortion), and assessment of surface quality. The functional evaluation ensures that the display and, if applicable, the touch panel continue to function flawlessly.
he acceptance criteria are strict and are agreed upon with the customer in advance. Typical limits are: no visible delamination, Yellowness Index increase < 3 units, haze increase < 1%, adhesive strength reduction < 20%, no pixel defects, and unchanged touch accuracy. A product that meets all these criteria has proven its reliability.
Note on the technical information in this chapter
The technical values, test profiles, and performance data stated in this chapter are provided for technical classification purposes. Actual results depend on the specific display configuration, material system, bonding process, application profile, and scope of qualification. Product-specific values are validated on a project basis using data sheets, measurements, test reports, and customer specifications.