2025-11-05
Testing for TFT-LCD display modules encompasses numerous performance metrics, stability, lifespan, and related areas. Specific testing domains can be broadly categorized as follows: optoelectronic performance testing, mechanical performance testing, environmental adaptation testing, ESD testing, and others.
Within environmental adaptation testing, numerous specific tests exist. Common examples include: low-temperature storage, low-temperature operation, high-temperature storage, high-temperature operation, thermal shock, high-temperature/high-humidity operation, dual 85°C/85%RH operation, and so on.
Naturally, most engineers are familiar with these standard environmental adaptation tests. However, TFT-LCD display modules also undergo certain “less emphasized” environmental adaptation tests, collectively termed “reliability test items.” After undergoing these “reliability test items,” TFT-LCD display modules may exhibit a series of issues whose complexity is no less significant than that of the environmental adaptation tests mentioned earlier.
Today, we will delve into the “overlooked” reliability test items in LCD display modules. This will cover the test objectives, conditions, procedures, and result evaluation criteria for these specific items.
Pressure Cooker Test(PCT)
PCT testing primarily targets individual LCD panels in small-panel form, not finished LCD display modules. Some customers also conduct this verification to validate the sealing performance of OLED products.
Additionally, many customers have not yet incorporated this test into their standard product acceptance testing procedures. It is currently conducted on a “trial basis” and serves only as a “reference item.”
① Purpose of PCT Testing:
Primarily evaluates the frame adhesive sealing performance and heat resistance of small-sized LCD panel units.
② PCT Test Conditions and Process:
Test conditions are set at 121°C ± 2°C, 100% humidity, and 2 standard atmospheres of pressure (0.2 MPa). Test samples consist of small-sized LCD sub-panels. The test duration is typically 16 hours, with products required to be removed and inspected every 4 hours during the test period.
③ PCT Test Result Evaluation:
After testing, small-sized LCD sub-panels must not exhibit defects such as: leakage caused by frame adhesive damage, frame adhesive cracking, or LCD bubbles.After PCT testing, “LCD liquid leakage” is a relatively common defect. When such issues occur, prioritize inspecting the LCD cutting edges for abnormalities and check whether the LCD bezel adhesive has been punctured or otherwise compromised, affecting its sealing integrity. After completing the inspection, simultaneously submit the defective units to the panel manufacturer for collaborative analysis.
Red Ink Vacuum Test
In addition to the PCT test mentioned above, the red ink vacuum test is also used for evaluating the sealing performance of small-sized LCD panels or other products requiring sealing assessment, such as the hermetic integrity of backlight adhesive-to-metal bonding.
① Purpose of Red Ink Vacuum Test:
Primarily evaluates the frame adhesive sealing performance of small-sized LCD panels or other products/components requiring sealing assessment.
If an LCD panel exhibits liquid leakage defects (in the center of the display area) but the specific location of frame adhesive damage cannot be pinpointed, the red ink vacuum test can also be used for identification.
② Red Ink Vacuum Test Conditions and Procedure:
a. LCD Panel Red Ink Vacuum Test: Completely immerse the small-sized LCD panel in red ink for 72 hours (ink brand: DYKEM 80696, USA). The vacuum pressure in the test equipment should be -0.095 to -0.098 MPa. After completing the red ink vacuum test, clean the sample with acetone before observation.
b. Backlight adhesive iron red ink test: Due to sealing issues, moisture may infiltrate the backlight during reliability testing (primarily through the adhesive holes in the iron adhesive). This increases the risk of waving wrinkles forming when the film material absorbs water.
Therefore, it is generally recommended to also control the sealing integrity of adhesive-backed metal plates. The standard practice involves placing the plate in a container filled with red ink, allowing it to stand at both room temperature and elevated temperatures for a specified duration, then observing any penetration of red ink into the plate.
③ Evaluation of Red Ink Vacuum Test Results:
After completing the red ink vacuum test, visually inspect the tested sample. No adverse phenomena such as red ink penetration should be present.
FOG Binding Water Boiling Test
After water boiling, FOG-state products must not exhibit ACF detachment; particle lightening is permissible, but the number of visible effective particles must still meet the standards of COG ≥5 and FOG ≥10.
It should be noted that slight fading of bonding particles after boiling is a normal phenomenon, provided the number of effective particles remains sufficient to meet requirements.
If concern exists regarding particle fading after boiling, consider evaluating ACF options with “epoxy resin colloid” or verify the compatibility between ACF thickness and bonding finger thickness.
HAST Testing
The three tests mentioned earlier primarily evaluate the physical condition of individual LCD panels, whereas HAST differs. HAST testing, also known as the “Highly Accelerated Stress Test,” is a specialized procedure.
Within the industry, HAST testing is commonly referred to as the “rapid initiation” condition for the dual 85 test.
When conducting reliability risk assessments for finished LCD display modules, if reliability testing time is limited, HAST testing can be considered to quickly assess the product’s risks.
① Purpose of HAST Testing:
The objective of HAST testing is straightforward: to rapidly induce various bonding-related defects such as COG, FOG, FOF, and FOB.
By applying severe stresses far exceeding normal operating conditions, it rapidly exposes potential product flaws and risks within a short timeframe. This enables evaluation of the product’s lifespan and reliability under long-term normal use.
② HAST Test Conditions and Process:
Standard HAST conditions are: 110°C temperature, 85% RH humidity, 0.122 MPa pressure. LCD display modules (32 test samples) are stored under these conditions for 8 hours. After the test chamber cools to 25°C, samples are removed and left at room temperature for 2 hours before powering on for observation.
Some clients impose stricter requirements: after passing the initial 8-hour HAST test, they extend testing to 16 hours to record the failure rate of defective units. Acceptance is determined by the severity of this failure rate. Alternatively, testing may be capped at 32 hours, with the failure rate recorded between 16 and 32 hours.
③ HAST Test Result Evaluation:
After completing the HAST test, allow the tested product to rest at room temperature for 2 hours before inspection. Functional failures caused by bonding are not permitted, such as: touch panel unresponsive, black screen, screen distortion, split screen, line defects, etc. Issues related to polarizers, backlight films, OCA optical adhesives, etc., are not subject to evaluation.
Salt Spray Test
Salt spray testing is likely familiar to many readers, as it is a mandatory requirement for numerous products exported overseas.
Products undergoing salt spray testing are primarily categorized into several states: individual components (such as TP, CG, FPCA, PCBA, etc.), finished LCD display modules, and others.
① Purpose of Salt Spray Testing:
For individual components, the primary objective is to evaluate corrosion resistance and adaptability in mildly corrosive environments. For LCD display modules, the focus is on assessing physical damage and functional degradation.
② Salt Spray Test Conditions and Process:
Place the individual components or finished LCD display modules into a salt spray test chamber containing a 5% NaCl solution with a pH of 6.5-7.2. Expose them to continuous spraying at 35°C for 24 hours, then remove and air-dry for 16 hours.
③ Salt Spray Test Result Evaluation:
After the test, individual components must show no paint peeling, bubbling, delamination, discoloration, or corrosion. For finished LCD display modules, no functional issues such as screen distortion, black screen, screen splitting, or touch panel failure (TP NG) are permitted.
An additional note: Beyond requirements for individual components undergoing salt spray testing, the UV adhesive (UV surface adhesive/UV one-step adhesive) used in finished LCD display modules must also meet stringent salt spray resistance standards.
Inappropriate selection of UV adhesive compromises the protective integrity of the bonding area, frequently resulting in bonding pad contamination, saltwater infiltration causing corrosion, and bonding defects.