Direct-Dry-Film Optical Bonding: Finding New Applications
Originally developed for avionics and military displays, Direct-Dry-Film (DDF) optical bonding is now moving into industrial and consumer applications such as touch screens, TVs, smartphones, tablets, digital signage, and medical imaging.
by Birendra Bahadur, James D. Sampica, Joseph L. Tchon, and Vince P. Marzen
OPTICAL BONDING in display products was first used for CRTs and then for flat-panel LCDs around 1980. The technology was confined mostly to low-volume high-performance avionics and military displays for a long time afterward. During the last 6 years, optical bonding has exploded in many commercial and industrial applications, such as iPhones, touch screens, tablets, digital signage, and medical imaging.1-4 Optical bonding has grown to a multi-billion (~ $2 billion) industry and is still growing at a fast pace.1,2,4 Liquid bonding has been the most popular optical-bonding technology for many years,1,2 but dry-film optical bonding is also gaining in popularity.2,3
Rockwell Collins started the optical bonding of LCDs in the late 1980s, using conventional liquid bonding.1,2 Certain limitations of the technology (described later in this article) were realized during this period, which prompted the company to start developing dry-film bonding in 1993. During early developments, avionics products used a hybrid technology; i.e., subassembly layers were bonded using optically clear PSA (dry film) and then those subassemblies were laminated to the display using liquid adhesives.2 Since 2006, however, all of Rockwell Collins’ avionics and military products, except legacy ones, have used dry-film optical bonding. The company realized the benefits of its Direct-Dry-Film (DDF) optical bonding 2,3 over conventional liquid optical bonding1,2 for emerging commercial applications and decided to license the technology and provide optical-bonding solutions for a variety of consumer and industrial applications.2,3
The Ins and Outs of Optical Bonding
Optical bonding is defined as the bonding of two or more optical components using a clear optical index-matched adhesive. In its simplest form, it fills the air gap between the cover glass and the display with an index-matched material that reduces the specular reflectance and increases the contrast of the display stack.2 An anti-reflective (AR) coating is usually applied to the top surface of the cover glass, along with possibly an anti-smudge (AS, hydrophobic) coating and often an anti-glare (AG) treatment. These materials reduce the specular reflectance further and increase the display contrast and legibility significantly in high ambient lighting.2,5,6 The optical bonding of many components reduces the total reflectance of the stack drastically. It also improves the environmental performance and structural integrity of the display stack simultaneously and provides design flexibility.2,3,6,7
The specular reflectance (RS) of an interface between two non-absorbing media of refractive indices n1 and n2 is given by8
For a glass (n1 = 1.5–1.52) and air (n2 = 1.0) interface, RS is ~ 4.0–4.25%. If two materials have identical or very close refractive indices, the RS from their interface will be close to zero. Filling the space between two identical layers with index-matched adhesive cuts their specular reflectances to almost zero. This index-matching principle is widely used in optical bonding.
The simplest interference AR coating consists of a single quarter-wave layer of transparent material whose refractive index is the square root of n1 × n2. For air and commonly used crown glass (n1 = 1.52), it comes out to be 1.23. The material having the closest refractive index with good physical properties for coating is MgF2 (n = 1.38).8,9 Many layers of graded refractive indices may be used to fill the space between the two layers of significantly different refractive indices. Multilayer thin coatings with destructive interference for reflected light, such as high-efficiency anti-reflective coating, are used to cut the specular reflectance significantly.8,9 Recently, developed “Moth-Eye” type AR films provide even better results.9-11 A diffuse surface, using microstructures, reduces the specular reflec-tance but increases the diffuse reflectance. Sometimes a combination of AG and AR is used to reduce the white-shirt effect or front-surface image reflection by diffusing the remaining specular reflectance.2 A circular polarizer (CP) is used to cut the specular reflectance in cases such as OLED, LED, CRT, and EL displays and touch screens. It cuts the reflectance from metallic and dielectric coatings very effectively, but also reduces the display luminance significantly.
Display Contrast and Legibility
The impact of various layers and optical bonding on the display reflectance and contrast ratio (CR) can be easily understood by examining Fig. 1 and Eqs. (2) and (3).
Figure 1 shows a typical backlit LCD with cover glass. R1, R2, and R3 are the reflected lights from surfaces S1, S2, and S3.2,5 Each surface reflects ~4%. The specular reflectance is additive from every layer, so the total specular reflectance for un-bonded glass is ~12%. It is 8.2% for un-bonded good AR glass (R1= 0.2%, R2 = R3 ~4%). For optically bonded plain glass (R1 = 4%, R2 = R3 ~0%), it is 4% and for optically bonded AR glass, it is reduced to 0.2% (R1 = 0.2%, R2 = R3 ~0%).
Fig. 1: Ambient light is reflected from an LCD with cover glass.
The CR of a commercial backlit LCD, with on-pixel luminance (Lon) of 75 fL and off-segment luminance (Loff) of 0.30 fL, can be calculated using formula 2:
where RS is the specular reflectance, Rd-on and Rd-off are the diffuse reflectances of the on and off segments, and S and D are the specular and diffuse light components of the high ambient lighting. In the dark, S and D are both zero, so the display has a very high contrast ratio (250:1) and is quite legible. To simplify the calculations, neglect the diffuse reflectance and the equation becomes12
Let us calculate the CR of the display in 2000-fC ambient lighting. It comes out to be 1.31 for un-bonded plane glass, 1.45 for un-bonded AR glass, 1.93 for optically bonded plain glass, and 18.37 for bonded AR glass.
The optical bonding of the AR glass increases the CR from an unreadable 1.45 to a very readable 18.37. To obtain the same CR (18.37) with un-bonded AR glass, one has to drastically increase the display luminance to 2969 fL from 75 fL, which is impractical. In practice, the glass-optical adhesive and adhesive-polarizer interfaces have very low specular reflectances (~0.035% and 0.015%, respectively), which generate a CR = 15.09. This is still a highly readable CR.
In real life, there are a few reflecting surfaces inside commercial TFT-LCDs. The color filter and black matrix contribute the most (~0.3–0.4%), while the other layers (polyimide, liquid crystal) add negligibly. The total specular reflectance from the display stack (AR glass, polarizer, LCD cell) comes out to be ~0.65%, which reduces the display CR to 6.62. Further reduction in CR may come from the diffuse surfaces and diffuse light contributions [Eq. ( 2)]. The maximum high ambient lighting is substantially higher for commercial avionics (8000 fC diffuse) and fighter planes (10,000 fC diffuse + 2000 fL specular). To achieve a high ambient CR of ~4–9, these applications require much higher luminance (~ 100–300 fL). Besides drastically increasing the high ambient contrast of the display, optical bonding also increases its luminance moderately by reducing the backward reflections.2 It also eliminates the parallax issue observed in some air gap units.
Environmental Performance
As mentioned earlier, optical bonding also improves environmental performance and structural integrity and provides design flexibility. Some benefits include:
1. Impact and Shock Resistance: Optical bonding of a strong glass on top of the display increases its impact and shock resistance drastically. The impact and shock resistance of the bonded unit is much higher than that of an air-gapped unit,2,7 as the adhesive layer absorbs the shock to some extent and the bonded cover glass and display front substrate together provide much better mechanical strength than they would separately. The impact tolerance increases with increasing cover glass strength and thickness. Bonding increases resistance to damage by vandals in outdoor vending and bank machines and increases drop tolerance in cell phones, etc. Proper glass bonding helps displays tolerate severe shock, vibration, and boot-kick impacts faced in military and avionics uses.2,3 It reduces pressure (pen, finger) induced liquid-crystal deformations in LCDs. Optical bonding also enhances user safety by keeping shattered pieces together when glass breaks due to severe impact.
2. Improved Visibility at Low Temperatures: The air-gapped designs generally trap atmospheric humidity in between the cover glass/touch screen and display, which may affect display legibility at low temperatures due to vapor and ice condensations. Optical bonding prevents this degradation by filling the air gap with adhesive.
3. Improved Environmental Performance: LCD’s iodine-based sheet polarizers deteriorate quickly in high-humidity and high-temperature conditions. The covering of the film’s edges, top, and bottom by the adhesive in optically bonded displays improves the humidity/temperature performance of the polarizer and other optical films significantly. Many un-bonded displays fail the high-humidity/high-temperature and thermal cycling requirements of military and avionics products.13 The adhesive covering also protects the polarizers from short-term accidental water contact.2
4. High-Altitude Performance: Optically bonded displays usually have better performance than un-bonded displays at higher altitude due to mechanical strengthening.
5. Design Flexibility: Optical bonding provides many design flexibilities such as narrower bezel and gaskets. The zero-bezel look of certain smart phones and tablet PCs is only possible by optically bonding cover glass to a pro-cap touch panel. Design capabilities such as these are not possible without optical bonding.
6. Added Functionality and Other Improvements: Bonding the display with appropriate complementary components such as touch screens, NVIS, ITO heaters, privacy films, and EMI and RFI absorbing layers can increase its functionality.1,2,6 A display’s solar and UV performance can be greatly increased by optically bonding it with solar rejection layers (IR filters, hot mirrors) and UV absorbing films.2,6 Bonding of UV and low visible wavelength cut-off filters can drastically increase the life of some types of LCDs.14 One example of this occurred when dichroic LCDs were used in direct sunlight in the deserts around the Persian Gulf. Those displays exhibited significant failures because proper UV filtering was not implemented.14
Optical Bonding and Adhesives
In liquid bonding, the adhesive is used in liquid form to bond the optical elements and is then cured by heat, light (UV or visible), chemical reactions, moisture, or a combination.1,2,15-18 In the case of dry bonding, the optical adhesive is contained in sheet form and may or may not require curing.2,16
Bonding Adhesives
The sheet material used in dry bonding may be composed of either (1) fully cross-linked pressure-sensitive optically clear adhesives (OCAs) that do not require further curing19 or (2) thermoplastics that can be reflowed at higher temperature and pressure and adhere after assembly, or (3) UV-curable sheets16 that can be reflowed to take the shape of the bonding space and cured by UV later on. Acrylic and silicone are the two most commonly used materials in dry-film bonding.
Regardless of chemistry, process, and type of bonding used, the key characteristics of adhesives include the following attributes:
• Low (or ideally zero) birefringence
• Refractive Index = 1.47–1.52
• Low moisture absorption
• Low cost, readily available, non-hazardous ingredients
• Haze-less, optically clear (high transmission) and particle/defect free
• Resistant to thermal soak and cycling
• Good UV, IR, and life stability
• Nonreactive with glass and other optical films
• No out-gassing, bubble formation, or latent formations after bonding
• Flexibility for repairing products later on and removal of parts from partial assembly
• Superior adhesion to both high (glass) and low (plastics) surface energy materials
• Suitable for glass–plastic–glass laminations; various TCEs
• Processing temperature for bonding < 90°C
Liquid-Optical-Bonding Limitations
Liquid optical bonding (LOB) has been described very well by Mozdzyn and Rudolph,1 Bahadur et al.,2 and many commercial firms15. The main weaknesses of liquid-bonding technologies are
1. The material preparation is cumbersome, costly, and defect-prone. In two-part systems, the materials must be mixed thoroughly in ratios as prescribed by the manufacturer. The air and byproducts, generated due to mixing or chemical interactions, must be removed. The raw materials and mixture should be filtered to minimize the foreign materials. Proper dispensing is a must to remove the entrapped air. Depending on the automation and process used, ruggedizers need to develop some level of their own custom expertise. In general, lamination processes using liquid adhesives are labor intensive with long cycle times.
2. Radiation curing of UV-curable liquids can also be limited due to light-blocking masks and uniformity of cure affecting display performance over temperature.
3. A considerable shrinkage may take place during the curing of liquid adhesives, which makes the control of the bond-line quite difficult.
4. Many liquid adhesives, especially silicones, require a primer application to achieve adequate adhesion to many low-surface-energy substrates.
5. Liquid-optical-bonding processes are messy. The cleanup of display and tooling are essential after lamination. This significantly increases the cycle time and cost of equipment, material, and solvent; this can also lead to further yield loss.
Direct-Dry-Film Optical Bonding
Figure 2 shows the flow chart for DDF lamination. The first major step of the process is material preparation. Substrates must be thoroughly cleaned using conventional cleaning methods with automated or manual systems. The bonding process must be done in a clean room to avoid the particles and contaminations. The optically clear adhesive (OCA) is typically a sheet of adhesive with double-sided liners that is die-punched or laser-cut to the desired size. After material preparation, the OCA is roller laminated to the first substrate (aka cover glass) using a semi-automated machine commonly used for polarizer lamination. Once this is completed, the DDF bonding process involves three key steps inside a specially equipped chamber.
Fig. 2: The DDF optical-bonding process is shown from left to right. Ambient light is reflected from an LCD with cover glass.
First, the OCA laminated cover glass and the rigid LCD are loaded into the chamber with a means to maintain a small gap in between them. The OCA faces the LCD front surface. A vacuum is subsequently induced to a desired level for a set time. The substrates are then allowed to fully contact each other while aintaining this vacuum followed by a method for applying external pressure via flexible membranes. To reduce the cycle time and improve the product flow, an autoclave is typically used to remove the vacuum voids quickly. The process and equipment are described in detail in several patents.2,20 Depending on the configuration and chamber size, a takt time of 26 sec can be achieved. Further optimization of tooling and process may reduce it to <10 sec. The process maintains the LCD cell gap very precisely and creates no optical distortion or cell-gap non-uniformity. Materials to bond can include components such as AR glass, protective glass, touch screens, ITO heaters, NVIS filters, EMI shields, and even additional displays. These components can be bonded to the front/rear or both sides of a display or to another optical component.
Proper design, process optimization, and tooling are essential in producing a high-quality DDF optical bonding. Insufficient attention to detail in these areas will fail to achieve good results. The next section lists the advantages of Rockwell Collins’ DDF bonding process in particular, and also clarifies some of the misconceptions about dry-film optical bonding.
Advantages of DDF Optical Bonding Over Liquid Optical Bonding
Some comparisons of dry-film bonding with liquid approaches are listed below:
• Cleaning: The overall process for DDF is much cleaner. The display is laminated without residuals to clean up after bonding and the tooling remains ready for immediate re-use.
• Handling: OCA is easier to handle than liquid optical adhesive because it is in sheet form.
• Adhesion: Dry film and liquid optical bonding both produce good adhesion.
• Material Usage: No wastage of optical adhesive due to spreading, leakage, and overflow. Cleaning solvents are not required. The DDF process is “green.” The material utilization can be increased to > 80–90% by optimizing the use of the OCA sheet area for a few display sizes.
• Thickness Uniformity and Dimensional Superiority: The DDF bonding produces the required thickness and uniformity of the adhesive layer over the entire area because it uses the adhesive in a uniformly thick sheet form. The thickness control and uniformity is tough to achieve in LOB. Sometimes, liquid adhesive may overflow, leaving some parts of the gap unfilled. In DDF bonding, there is no need to account for shrinkage, bond-line control, or varying cure rates that can influence bond line and internal stress.
• Productivity: The overall productivity of the DDF process is much better than liquid optical bonding. It is much faster and simpler with fewer steps and less equipment.
• Automation: It is much easier to automate because the materials are solid; no mixing, de-airing, or pouring.
• Yield and Reparability: The process produces high yield and is repairable, which is not the case with some liquid-bonding technologies. It is difficult to repair the field-returned liquid optically bonded parts.
• Simultaneous Bonding of Multiple Components: Multiple components may be bonded simultaneously, which is not possible with LOB. This also reduces the bonding cost significantly.
• Cost: DDF optical bonding is cheaper than LOB in mass-scale manufacturing.
• Bonding Different Types of Components: DDF is the ultimate bonding technique for flexible (soft) to soft and soft to rigid (hard) surfaces. In hard-to-hard bonding, the properly developed techniques, optimized processes, and improved materials produce equally good or better results than those produced by LOB, which is better for bonding curved and uneven surfaces.
• Size and Large-Area Bonding: The DDF process works very well from small–to–large sizes. It has been scaled up to a 65-in. TVs.3 Larger tooling may be quickly fabricated to support larger sizes when the demand arises.
• Environmental Performance: DDF-bonded displays exhibit better environmental performance (vibration/shock and temperature) than LOB displays. They show no delamination at elevations >100,000 ft.
DDF Optical-Bonding Applications
DDF optical bonding is becoming popular in industrial and consumer applications that require outdoor readability and durability to withstand impact, vibration, extreme temperatures, altitudes, dust, and rough handling. This methodology also integrates touch screens very effectively. DDF optical bonding provides the following attributes to displays:
• Enhanced sunlight readability (~5–10 times, depending on application).
• Increased impact resistance (~3–8 times depending on the bonded cover glass).
• High shock and vibration tolerance.
• Very low reflectance for touch screens in desired applications.
• Greatly enhanced life by protecting the display materials from the humidity.
• High-quality optical bonding of performance-enhancing auxiliary components, such as AR, AG, AR/AG/hard coat/anti-smudge and heater glasses, touch screens, and UV, NVIS, EMI, and sunlight filters.
Some particular optical dry bonding applications are listed below:
Military and Avionics: Military, ground vehicles, and avionics displays operate in notoriously rugged conditions. Challenges include rough terrain, extreme temperature changes, high ambient lighting, high altitudes, electromagnetic interference, shock, and vibration. A Boeing 787 cockpit using many DDF-bonded displays is shown in Fig. 3.
Fig. 3: A variety of dry-film optical bonded displays are used together in this cockpit. Image courtesy Boeing.
Marine Electronics: Displays used in marine electronics, including ships and yachts, are regularly exposed to harsh environments such as high humidity/high temperature, high ambient lighting, salt water, rain, shock, and vibration. DDF bonding helps to satisfy these requirements. It also provides lamination capability for many displays on a single sheet of large glass, which is often desired in yachts (Fig. 4).
Fig. 4: Direct-dry-film bonding works well for combining multiple LCDs on one sheet of large glass, as is often desired in luxury yachts. Image courtesy Kessler–Ellis Products.
Medical and Other Applications: Medical displays require mobility, sharp pictures, touch screens, and reliability features in demanding environments with high ambient lighting, shock, vibration, frequent temperature changes, and sterile conditions. DDF optically bonded displays provide these attributes. Other potential applications for DDF technology include smart mobile devices with enhanced displays for better outdoor readability – a requirement well-suited to DDF optical bonding.
Branching Out
Optical bonding has been widely used in military and avionics for a long time, where it improves the optical as well as environmental and functional performances of a display stack. Liquid and dry-film optical bonding both fulfil the requirements of most of today’s new applications. DDF optical bonding is superior in many ways to the more popular liquid optical bonding in optical and environmental performances as well as cost, material usage, and volume production. It is a highly “green” technology and is growing very fast in numerous applications.
Acknowledgments
Thanks are due to Alyssa Hahn for her help.
References
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2B. Bahadur, J. D. Sampica, J. L. Tchon, and A. Butterfield, “Direct Dry Film Optical Bonding – A Low Cost Robust and Scalable Display Lamination Technology,” J. Soc. Info. Display 19 (11), 732 (2011); ibid., Invited Talk at IDRC/Latin Display Conference held at Sao Paulo, Brazil, Nov.16–19 (2010).
3Rockwell Collins Booth at SID Display Week 2008–2012; http://www.rockwellcollins. com/Products_and_Systems/Optics/Optical_ Bonding.aspx
4D. Doyle and P. Oehler, “Optical Bonding: Critical Technical Challenges for Performance, Manufacturing, and Supply Chain,” SID Symp. Digest Tech. Papers 43, Paper 50.1, 667 (2012).
5G. Walker, Veritas et Visus (August, 2006), p.44.
6I. Bita et al., “Front-of-Screen Display Components and Technologies,” Information Display 28 (9), 12 (2012).
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8J. A. Dobrowolski, “Optical Properties of Films and Coatings,” in Handbook of Optics, Vol. I, 2nd ed. (McGraw-Hill, Inc., New York, USA, 1995).
9http://en.wikipedia.org/wiki/Anti-reflective_coating.
10T. Taguchi et al., “Ultra-Low-Reflective 60” LCD with Uniform Moth-Eye Surface for Digital Signage,” SID Symposium Digest Tech Papers 41, paper 80.3 (2010).
11S. Kajiya et al., “High Transmission Optically Matched Conductive Film with Sub-Wavelength Nano-Structure,” SID Sympoosium Digest Tech. Papers 43, Paper 44.1 (2012).
12B. Bahadur, “Display parameters and requirements,” Liquid Crystals: Applications and Uses, Vol. 2, ed. B. Bahadur (World Scientific Pub. Co., Singapore, London, New Jersey, 1991).
13Radio Technical Commission for Aeronautics “RTCA/DO-160, Environmental Conditions and Test Procedures for Airborne Equipment”; Mil-STD–810, “Department of Defense Test Method Standard for Environmental Engineering Considerations and Laboratory Tests” (Jan. 1, 2000).
14B. Bahadur, “Dichroic LCDs,” Liquid Crystals: Applications and Uses, Vol. 3, ed. B. Bahadur (World Scientific Pub. Co., Singapore, London, New Jersey, 1992); B. Bahadur, “Void Formation in Liquid Crystal Displays,” talk at LCI, Kent State University, August 1994; J. Doyle and K. Wisler, “Unexplained Voids in LCDs,” SID ‘95 Applications Digest, paper A2.2, 24 (1995).
15Literature on Dupont™ Vertak Bonding Technology from DuPont (DuPont Display Enhancement, Inc.); General literature from Optical Bonders and Display ruggedizers such as Planar, Vartech Systems, Inc., General Digital, Dontech, Occular.
16ThreeBond 1630 UV-Curing Adhesive Sheet, Literature from Three- Bond Co., Ltd., Tokyo, Japan. http://www.threebond.co.jp/en/technical/technicalnews/pdf/tech74.pdf
17Data Sheets on optical adhesives from Three Bond, 3M, DuPont, Master Bond, Summers Optical, Norland/Edmond, Epoxy Technology, Henkel Loctite, Dymax Corp., Permabond, Delo, Sekisui Chemical, Ellsworth Adhesives, Polytec PT GmbH
18A. Clements, “Selection of Optical Adhesives,” Clements Tutorial 1, College of Optical Sciences, University of Arizona, USA (Dec. 12, 2006).
19Data Sheet from 3M on Optically Clear Adhesives.
20Many U.S. patents on dry-film lamination technology have been issued to Rockwell Collins. These are 5,592,288 (1997); 5,920,366 (1999); 6,266,114 (2001); 6,284,088 (2001); 6,520056 (2003), 7,435,311 (2008); 7,566,254 (2009); and 7,814,676 (2010). Many more have been filed. •