Anti scratch, Anti fingerprint, Fingertip abrasion
Display is nowadays widely integrated in many types of products:
Automotive: car, truck, bike, motorcycle
Telecommunication: mobile phone, pad, watch, E-book
White goods: refrigerator, washing machine, dish washer, stove
Medical: button, switch
When there is an issue with the display, the customer will only return the product, not the display!
IPSA provides a comprehensive display testing scope including the mechanical, durability and optical measurements. All these are carried out under the following standard environmental criteria:
• temperature: 25 °C ± 3 °C
• relative humidity: 50% ± 10%
• atmospheric pressure: 86 kPa to 106 kPa
Fingertip motions: tap, zoom, swipe
How does the human fingertip interact with display?
We first shall understand our human fingers and hands.
Firstly, our human fingers are highly flexible due to the combination of multiple joints and degrees of freedom. Additionally, the finger pads are viscoelastic, not like a metal. Finger pads also have rough skin structures, and they are not homogeneous. Most important is that the motion of human finger is nonlinear with high dynamics.
Secondly, when human hand approaches any surface, it is always with a specific 45° angle, instead of 90°, because human being feels hurt with a “90° touch”.
Thirdly, there are dandruff, fat, or dirt on the fingertips and hands. We normally sweat, and sometimes apply lotion or cream on our hands.
All those critical factors mentioned above are related to human physiology and the real daily use scenario. In order to achieve sensible test results, those shall be included in the test criteria, instead of performing any test only because it is a standard!
Here are 3 common motions of the human fingertip on the display: tap, zoom, and swipe.
Fingerprint and Soiling affinity
In our daily life, all kinds of particles, natural and artificial fat as well as sweat and other contaminants could easily adhere to the display surface and stay on it. This will not only bring a negative impression on the appearance of the display, but also creates further damage due to its chemical stresses.
This test is named as “Fingerprint & Soiling Affinity”.
Human beings tend to clean and remove those disturbing contamination imprints, and this cleaning procedure is to determine the “easy-to-clean” or “cleanability” property of the display.
Visual and quantitative evaluation on the cleanability is recommended to be carried out under standard light D65 and a certain image processing software.
Due to the current COVID-19 pandemic situation, more frequent disinfection actions have been carried out in our daily life. Either the display is under constant exposure to the disinfection spray/ cleaning products, or people apply the disinfection gel on their hands and rub around their fingers, then continues their daily use on the display product.
Therefore, an additional chemo-mechanical stress is created due to the residues from the disinfection products.
Display surface haptic study
What is your feeling when you touch a display or touchscreen?
Silky? Sandy? Sticky? Froggy? or Bumpy?
The human perception on the display product is a new topic for discovery. We have been in this field since 2012. We have finalized 4 haptical properties: microfriction, haptical topography, macro roughness and creeping/recovery. Together with our panel study team which includes minimum 30 participants, we could present clear and distinct haptic properties of the display/touchscreen in terms of stiffness, roughness, creeping, stickiness, slipperiness, pleasantness.
The purpose of the general abrasion test is to evaluate the robustness of the display surface from actual use, such as storage with abrasive objects or outdoor installations in abrasive environments.
This test is carried out with a reciprocating motion driven with a linear motor, with the abrasive tip rubbing vertically and repeatedly over the display surface at a constant speed.
The purpose of the general scratch test is to evaluate the anti-scratch behaviour of the display surface against various scratch tips from actual use, such as stylus scratch, key scratch or finger mar scratch.
This test is to determine the display durability from random short contact events, such as the use of a sharp tip or accidental gouging during storage.
The test is especially designed for testing the surface with a film coating or organic compounds.
This test is carried out with either a one-direction motion (with or without delay) or a reciprocating motion driven with a linear motor, with the scratch tip moving vertically and repeatedly over the display surface at a constant speed, either automatically or manually.
Stylus/Pencil hardness scratch
Pencil hardness scratch test is a common evaluation for display durability from sharp contact events such as the use of a sharp tip as a stylus. ISPA provides this testing service according to ISO 15184. The test is performed automatically or manually, the pencil scratch stylus moves at a constant speed of between 5 mm/s and 10 mm/s for a distance of at least 25 mm in accordance with ISO 19252. The applied load of the scratch test is freely programmable.
Scratch could easily appear on surfaces, e.g. metal, polymer, glass, optics, ceramics, with the size in macro, micro and nano scales. In many case, a deep and long scratch occurs during a fast and high dynamic motion, e.g. a key scratch on the car exterior paint. This high dynamic scratch, namely Dyna-scratch, is done at the speed up to 150 cm/s with a scratch depth in centimeter range.
So far, the conventional scratch testers normally run at quite slow speed (max. 4 cm/s) with short distance, and what it generates is not a scratch, but a mar, which characterizes as the ability to resist light damages. However, materials, especially polymers, have certain viscoelasticity and thermal recovery capability. Therefore, those temporarily removed surface materials will be then spread out over the surface, and this viscoelastic recovery only shows as slight and fine scratches.
A special coating technology that protects the display from fingerprints and smudges, brings a nice functionality to the display product.
The customer does not have to constantly rub or clean the display surface as frequently as before.
Display surface damage evaluation
In general, there are two methods: visual inspection and WCA measurement. Visual evaluation takes place under standard light D65. Starting from a frontal viewing point, the viewing angle is varied until a maximum contrast can be recognized and documented. WCA measurement or optional the surface free energy (SFE) with the polar and dispersive parts is a quantitative method to evaluate the display coating abrasion rate.
The homogeneity test is a very important to evaluate the display surface coatings consistency for manufacturing quality control purpose, and it also helps to determine the right durability testing area.
Normally with a standard WCA system, 3 parallel lines with 20 droplets per line are applied for the surface homogeneity test.