MATERIAL AND BONDED DISPLAY THEREOF

D E S C R I P T I O N

Technical field

[0001] This application relates to wet optical bonding of full displays, and more particularly to a method of using adhesives to assemble components.

Background Art

[0002] The optical bonding technology is based on the application of an optically clear adhesive between the cover layer and the display in order to improve the optical performance of the display in bright light environmental conditions. The adhesive eliminates the air layer between the cover substrate and the display, thereby reducing the total light reflection, improving the contrast. The adhesive is required to be optically transparent, to have a suitable refractive index (i.e., a refractive index coincident with the both contact interfaces) and to keep its properties over time. Furthermore, the adhesive should be applied under appropriate controlled conditions such that there are no significant differences in the optical properties at the interface and no defects on the adhesive layer. Furthermore, the adhesive must provide a strong adhesion between the display and the cover substrate. Improved processes are still required for optical bonding of displays with higher throughput, at reduced costs and with higher product quality.

[0003] Several solutions regarding optical bounding have been presented.

[0004] Rockwell Collins developed processes and tools to optically bond any two components with a pre-cut sheet of optical coupling adhesive (OCA). This fast, low- labor technique requires no dispensing or curing of the adhesive, reduced surface preparation and no clean-up of excess material compared to liquid bonding. Direct Dry FilmTM bonding scales from very small surfaces to very large surface areas. The main technical disadvantages in this dry optical bonding process, consist in: the needs of autoclave that requires also, vacuum, batch production, big machine size and it is a slow process. Furthermore, dry optical bonding is not suitable for filling up nonuniform regions (e.g. warped regions) and for obtaining a constant thickness of bonding material.

[0005] LG process makes the bonding process in an early stage, so that it just bonds the polarizer before assembly. In the case of scrap they only loose the low cost of the polarizer. LG process is constituted by: cover substrate as substrate layer; filling the cover substrate with liquid resin on the jig region; spread the resin uniformly on the jig; pre-cure by heating through the glass; remove the jig; assembly of the cover substrate plus resin over the cover substrate; bubble removal in an autoclave and final cure/aging. However, this process is not applicable for complete displays, it demands autoclave process and is based on a jig reference system. Heating through the glass and attachments on its surface could cause non uniform cure of bonding material.

[0006] US 8657992 document refers to a method and apparatus for the bonding of one or more material layers to a display panel, which yields a resulting bonded assembly that is transparent and substantially free of optical defects, such as entrapped dirt and air bubbles. During the bonding process, a moving dispensing probe, or needle, dispenses a liquid bonding material into an air gap between the surfaces to be bonded in a manner which continuously touches (wets) each surface simultaneously. By simultaneously wetting each surface, and by dispensing the liquid bonding material in a predetermined pattern between the surfaces, the introduction of entrapped dirt and the formation of air bubbles is prevented as the two matching surfaces are subsequently pressed together and the adhesive is cured. Also the application of a uniform compressive force holds the material layer and the display panel together, entrapping the liquid bonding material in an uniform layer, before being cured. This technology presents some disadvantages, namely: it requires UV light for curing the bonding material; there is no containing system (e.g., dam layer that acts as a border for the bonding material), being necessary extra UV edge cure for entrapping the bonding material; there is a high risk of waterlog for low viscosity materials.

[0007] US 2014/0246148 refers that structures in an electronic device, such as substrates associated with a display, may be bonded together using a liquid adhesive. No dam layer is used. In this method fiber optic-based equipment may be used to apply an ultraviolet light to the peripheral edges of an adhesive layer during bonding (no containing system). Three-dimensional adhesive shapes may be produced using nozzles with adjustable openings, computer-controlled positioners and other adhesive dispensing equipment. Ultraviolet light is applied to cure the liquid adhesive through a mask with an opacity gradient, and an adjustable shutter controls the adhesive exposure to the UV light. The exposure to UV light can be used to create an adhesive dam that helps create a well-defined adhesive border (containing system). Multiple layers of adhesive may be applied between a pair of substrates. The main disadvantages of this invention consist in: requiring UV light for curing the bonding material; no containing system; restrictions/difficulties in using multiple bundles of optical-fibers in inaccessible or difficult access areas; technology too complex and expensive technology.

[0008] US 2012/0325402 refers to a method of making a display assembly that includes the steps of attaching a first and a second layers with an optically clear heat activated adhesive to form a laminate. Each of the first and second layers has opposing major surfaces. At least one of the first and second layers has a three dimensional surface topography covering at least a portion of one of its major surfaces or it is distortion sensitive (e.g., warped). At the heat activation temperature, the adhesive is pressure sensitive. The method also includes heating the laminate to the heat activation temperature of the adhesive causing it to flow. The adhesive is selected from the group consisting of thermally reversible cross-linkable adhesives and post cross-linkable adhesives by means of irradiation, thermal or moisture curing. The main disadvantage consist in: there is a high risk of bubbles formation due to the flow and heating of the adhesive; only pressure sensitive adhesive can be used; the use of UV light for bonding material cure is only possible for complete transparent covers.

[0009] Generally, the main limitations of the existing methods are related to the use of UV light, requiring transparent layers; the use of complete displays restrains the use of autoclave processes, where pressure can damage the displays; the absence of a containing system (no dam material) requires an extra edge cure solution or leads to a bad-defined adhesive border. In all the cases, no accurate positioning system is described for relative placement of both layers with tight tolerances.

[0010] These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.

General Description

[0011] This new process applies to the wet optical bonding of full display assemblies using a liquid optically-clear adhesive that fill the space between layers to bond, for reduction of light reflections at the interfaces, increasing display readability. It consists in a sequential number of steps: surfaces cleaning, surfaces treatment (e.g., by a plasma treatment), dam dispensing, followed by filling material dispensing over the display, cover and display assembly and joining, and oven cure. The process has the following features and advantages:

It is used for optical bonding directly the cover on the surface of the full display ensemble (including, LCD, PCB and frame);

It is highly flexible, fully automated and suitable for any kind of flat displays shapes and sizes;

It is adjustable for different designs and types of layer covers: transparent, partially or fully painted;

It can be used with a wide variety of liquid bonding materials, including the ones with low surface energy; It can be used with a wide variety of liquid bonding materials, with thermoplastic of thermoset nature;

It can be used with a wide variety of liquid bonding materials that can be heat or mixture activated and then solidify (polymerize or passing through a phase transformation);

It can be used with a wide variety of liquid bonding materials that can be post- cross-linked by irradiation (e.g., UV light, laser), by thermal curing, by moisture curing or other type of curing process;

It dispenses a dam material as a border for entrapping the bonding material, with adequately positioned of air exhausts (during bonding material filling), and ensuring a correct distance between layers (e.g., display, cover);

It allows using a dispensing pattern of the adhesive material that allows a smooth filling, without air entrapments, fully covering the region to bond;

It combines the cover assembly over the display with the smooth filling of the bonding material, without air bubbles generation or air entrapments;

It allows an accurate optical positioning system of layers, being suitable for their tight tolerance positioning;

It allows an uniform cure/solidification process, even for covered regions, in just one step;

It can incorporate in-line inspection steps (e.g., automated optical inspections,

AOI) along the process to assess the product quality (e.g., cleanness of the substrates, bubbles on the bonding material/interfaces);

Despite using a liquid bonding material, it is controllably clean;

The process steps can be performed simultaneously and sequentially, so to reduce cycle time;

It improves the quality of the optical bonding products, by eliminating air entrapments and bubbles on the adhesive and interfaces;

It avoids the need of autoclave or vacuum application in order to eliminate air entrapments and bubbles; It uses cheap bonding material and ensures no MURA effects during the lifetime of the display with only one thin bonding layer.

[0012] Despite of being preferable implemented for full displays, it can be used for optical bonding of any type of two or more flat layers.

[0013] The present disclosure relates to the bonding of layers (e.g., display and cover) with a liquid optically clear adhesive and more particularly to the process of bonding the same to produce optical assemblies that include such optical clear material.

[0014] The procedure consists of two parallel processes that converge at the assembly station, where both layers are joined together (Figure 1). The first of these two parallel series of steps is the preparation process of the cover layer, comprising: a cleaning system (e.g., washing and drying device, followed by a surface activation technique (e.g., atmospheric plasma). Simultaneously, the other layer (e.g., a display) initiates a series of steps, starting with the cleaning station (e.g., ionized air jet). Then, this layer is subjected to an AOI step. If acceptable, the display is moved to the next station. In the dam dispensing station, the dam made of high viscosity resin, is dispensed on the display with a given pattern. In the following step, the filling material, which in this case is a liquid optically clear adhesive, is dispensed in such a way that the later guarantees the spreading throughout the entire bonding without trapping air to avoid bubble formation.

[0015] After preparation/cleaning of both layers and dispensing of materials in one of them (finishing of the parallel steps), they are assembled in one station (assembly station), which acts as the converging point of these two parallel processes. The assembly is done via a robotic arm, which position is guided via a novel alignment system, ensuring this way the precise alignment between the two layers. After the assembly is done, the device is transported to the curing station.

[0016] The present invention consists in a new method for bonding two layers with a liquid adhesive, in a quick process, guaranteeing that air entrapments, air bubbles or voids are not formed (improved bonding quality). The method compromises the dispensing of a dam material with a special pattern on one layer (typically, a display), and on the same layer also dispensing, with a special pattern, the bonding material. After those steps, the other layer is positioned over the previous one at a tilt angle and brought vertically down to contact the bonding material 3D pattern. As the layer is tilted over, decreasing the angle between the two layers, the bonding material is squeezed, spread over in a control way and covers the area to be bond. This ends when the two layers are parallel and fully touching the dam material (gap height).

[0017] In this invention the special patterns in which the dam and bonding materials are dispensed allow controlling the flow of the bonding material. The dam material pattern have in an embodiment only two vents in order to allow the air to escape and to control the bonding material overflow. The chamfers at the opposite corners of the vents facilitate the smooth flow of the bonding material without air entrapments and air bubbles formation.

[0018] The bonding material has a 3D pattern. The spatial triangular-like shape of an embodiment with a thick band at the larger side allows to feed the chaffered dam side during its flow and to converge the flow into the air vents position. This allows the air to be pushed away through the vent exhausts, producing air/bubbles-free bonding of the two layers.

[0019] The patterns of the dam and bonding materials and the way the layers are joined together (e.g., tilt angle and rate) are correlated. They are designed to ensure that when the bonding material starts to spread over the bonding area, the flows push all the air to the flow front, forcing it to escape through the vents on the dam. This ensures that no air is trapped in the bonding area and in consequence this area is bubble free.

[0020] It is disclosed a method for producing a display assembly by bonding a cover layer and a display layer, said method comprising:

cleaning and preparing the cover layer for surface activation and activating the surface of said cover layer; and in parallel to the previous step, cleaning the display layer and dispensing a liquid bonding material over it, using a previously placed dam to confine said liquid bonding material;

positioning the cover layer over the display layer and lowering the former over the latter layer such that both are joined in parallel by the bonding material into a display assembly;

curing the bonding material of the display assembly.

[0021] Separately applying the activation the surface of said cover layer, e.g. by plasma activation, i.e., only to the cover layer and not the display, avoids damages to the display by the surface activation process. Some prior art processes apply the surface activation plasma treatments to the cover layer together with the display, which is not yet joined causing damage to the display.

[0022] In an embodiment, the curing the bonding material of the display assem bly is by a heat treatment.

[0023] In an embodiment, the surface activation is by a plasma treatment.

[0024] An embodiment of the method comprises for the dispensing of the liquid bonding material over the display layer:

placing on the display layer a peripheral spacer dam for confining the liquid bonding material, wherein said dam has a substantially rectangular shape having air vents;

dispensing the liquid bonding material in a predetermined position and with a predetermined shape within said dam;

lowering the cover layer over the display layer, until the former touches, at a tilted angle, a first side of the dam rectangle;

gradually reducing the tilt angle of the cover layer until it is in contact with the side of the dam rectangle which is opposite to the first side, and the two layers are parallel and joined at a distance of the dam height, wherein the liquid bonding material predetermined position and shape are such to feed the bonding material along the dam sides and to converge the bonding material flow into the direction of the air vents.

[0025] In an embodiment, the dam is arranged extending along substantially the whole periphery of the display layer.

[0026] In an embodiment, the dam is a liquid bonding material having a higher viscosity than the liquid bonding material.

[0027] In an embodiment, the two parallel method steps concerning the cover layer and the display layer are carried out simultaneously.

[0028] In an embodiment, the two parallel method steps concerning the cover layer and the display layer are carried out in sequence.

[0029] In an embodiment, both layers have the same refractive index.

[0030] In an embodiment, the liquid bonding material is a liquid optically clear adhesive comprising a refractive index coincident with both layers.

[0031] In an embodiment, the cover layer is a protective overlay glass or a polymer layer.

[0032] It is also described a display assembly obtainable by any of the described methods.

[0033] It is also described a television, a computer, a plasma TV, an LCD TV, a mobile phone, a watch, a tablet, a display or a an automotive dashboard comprising any of the described display assemblies.

Brief Description of the Drawings

[0034] The following figures provide preferred embodiments for illustrating the description and should not be seen as limiting the scope of invention. [0035] Figure 1: Schematic representation of an embodiment of the process flow for optical bonding of two layers.

[0036] Figure 2: Schematic representation of an embodiment of the joining of the two layers, wherein:

100 represents the display layer;

101 represents the dam material or periphery spacer material;

102 represents the dispensed bonding material;

106 represents the cover layer.

Detailed Description

[0037] The present disclosure relates to the bonding of layers (e.g., display and cover layers) with a liquid optically clear adhesive and more particularly to the process of bonding the same to produce optical assemblies that include such optically clear material.

[0038] The procedure consists of two parallel sequential processes that converge at the assembly station (Figure 1). This has the advantage of reducing the total cycle time. The first branch of these two parallel series of steps is the layers surface preparation process that starts with a cleaning step, for example, using a washing and drying equipment. This is then followed by a surface activation technique, such as atmospheric plasma treatment in order to raise surface energy and reactivity, thus improving wettability and adhesion.

[0039] At the same time, the other series of steps initiates with preparation of the display. This starts with the display cleaning station, e.g., based on an ionized air jet. This is then followed by an automatic optical inspection, AOI, step that checks the display's surface for any particles. This allows to dramatically diminish the exposure to airborne particles. If the display passes this rigorous verification, it proceeds into the dam and seal dispensing station. In this station, the dispensing of the dam and/or seal is done. The seal material is used to close the gap between the bezel and the display, in order to avoid the infiltration of the filling material in this zone. The seal material is only used when this gap is exposed to the bonding material. The dam defines the layer ^' s gap and bonding area. It will guide the spreading of the filling material, and it will avoid any overflow and bubble formation due to air entrapment or incorrect airflow. This material also has the ability to better fixate the layers, which will be later assembled into the display. The dispensed dam material has a high viscosity that allows fixing its shape, without need of an immediate cure step. This allows the posterior and simultaneous cure of all dispensed materials in a single cure process. The process then continues directly into the filling material dispensing system. In this device the filling material is dispensed in a specific pattern. Both the dam and the filling materials can be dispensed in such a way that the subsequent spreading of the filling material over the entire visible area of the display is smooth and without air entrapments and bubbles. After these dispensing steps, the display goes directly into the assembly device.

[0040] The assembly station acts as the converging point of the two parallel processes above described, holding two different input systems: one for the display covered with the seal, dam and filling materials, and the other for the cover layer. Firstly in this station, the cover layer is manipulated by a 6 DOF robotic arm that precisely position it over the display at a tilt angle. The tilted layer touches the filling material and the dam, where a hinge point is defined. Further tilting this layer forces the filling material to spread over the display. The flow of filling material stops when the two surfaces are parallel, finishing the assembly process. In this station, this assembly process can be replicated with incorporation of another robotic arm, operating out of phase with the other one. This allows reducing substantially the cycle time. The assembled device is then conveyed to the curing station, where all dispensed materials are cured.

[0041] Between all these stations the devices are transported by an automated transportation system, e.g., based on a conveyor system, making a fully automated process. To avoid contaminations all equipment are located in a space with controlled environment (e.g., pressurized, clean room). [0042] The use of a thorough cleaning process of the displays followed by an AOI station allows reducing the dependency on the supplier's quality parameters, as well as highly affecting the rejection rate by strongly reducing the number of contaminated surfaces.

[0043] The AOI of the displays early in the assembly line has the key advantage of creating a buffer that separates cleaned surfaces for the optical bonding from non- cleaned ones, sending these latter back to be cleaning station again, before any more steps are taken.

[0044] The adoption of an integrated process comprising display preparation, material dispensing, assembly and curing process within closed stations allow reducing the number of contaminations after the inspection process, as well as reducing the amount of possible human errors to nearly zero.

[0045] The adoption of a fully automated process allows including self-maintenance procedures in the machines that helps decreasing the number of rejections. A few examples of these procedures are: the programmed automated purging of critical dispensing points, and the automatic degassing of key materials.

[0046] The adoption of two parallel processes allows increasing line versatility/flexibility, reducing machine size and individual complexity to the minimum possible. This reduces the possibility of any problems, defects and cycle times. It also allows keeping the process simple and fully automated.

[0047] In sum, this process is almost free of human errors because it is fully automated; it is a highly versatile/flexible process due to the organization in several stations; it has a reduced cycle time as several steps are run in parallel; it has an extremely low rejection rate due to the process optimization and intermediate inspection activities. Furthermore, the process shows all the required capabilities to produce bubble free displays, with no-overflow for the correct dispensing patterns and assembling.

[0048] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0049] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof.

[0050] The embodiments described above can be combined with each other. The following claims further define the preferred embodiments of the disclosure.