Method for the dimensionally controlled bonding of surfaces

Abstract

This invention introduces an improved method for bonding two surfaces using silk-screen printing technology. The method according to the invention reduces possible occlusion of the screen, thus improving the print quality of the deposited cement film. This is accomplished by adding particles to the print medium. In addition, by selecting the size of the particles it is possible for these, serving as spacers during the bonding process, to define the thickness of the cement layer. An additional procedural step describes the controlled mutual approach of the bonding surfaces.

Description

BRIEF EXPLANATION OF THE FIGURES

FIG. 1: Silk-screen printing system

FIG. 2: Silk-screen printing system, with the doctor blade halfway across

FIG. 3: Exploded view of equipment for the approach and fixation of elements to be bonded

DETAILS OF THE INVENTION

The four key components required for silk-screen printing are: The print medium, the screen with emulsion areas (defining the pattern to be printed), the surface of a substrate to be imprinted, and a doctor blade that squeezes the print medium through the screen.

FIG. 1 is a schematic illustration of the components of a silk-screen printing system 1. The screen 3 encompasses emulsion areas 5 which determine on the screen those regions that are impermeable to the print medium. This ultimately defines the pattern that will be printed on the substrate surface. In the example shown that pattern is a square 60×60 cm frame, but it is entirely possible to use larger or smaller frames. The cleanly imprintable net surfaces will ultimately be about two thirds of the frame size. The screen is clamped on a frame made for instance of aluminum. Suitable screen materials include woven polyester or other textile-fiber material, or steel mesh, preferably of stainless steel. For the purpose of this description the term screen includes any and all forms of screen material including steel-wire mesh and other netting. The term “filament” refers in a very general sense to the constituents of the screen. The mesh size of the screen is specifically selected for the process at hand, with typical spacings of 60 μm to 300 μm, depending on the application. For the example shown a polyester fabric with a 100×100 μm² mesh was selected, with a filament diameter of about 40 μm. Other filament diameters between 30 μm and 200 μm can be useful as well, with the filament diameter obviously having to be smaller than the mesh size of the fabric. The filament diameter largely determines the density of the print-medium material that can be transferred to the element surface.

Masking of the screen 3 is accomplished by applying a photosensitive emulsion over a large area of the screen and then exposing it through a photomask. The emulsion may be a positive or negative photoresist. In the case of a positive photoresist the developing process will leave intact those areas that were not exposed, whereas those regions will be ablated that were exposed through the photomask. In the case of the negative photoresist the exact opposite applies. In either case the result is a fabric that contains emulsion-occluded regions through which no print medium can be squeezed, whereas the print medium can penetrate through the areas that are devoid of any emulsion. That emulsion as well affects the thickness of the medium applied on the surface of the object substrate. The emulsion causes that thickness to increase by up to 50%. This process permits the implementation of patterns whose smallest components may be about three times the mesh size of the screen employed. For smaller patterns the mesh would interfere with the printed image at least in some particular applications.

As a suitable adhesive print medium 9 according to the invention, epoxy resin is mixed with spacers. Alteratively, the adhesive component may be a UV-hardening, thermally hardening or multi-component chemically curing cement, or one that hardens through the evaporation of solvents. The example shown employs glass-bead spacers 5 μm in diameter. Other spacer sizes up to 80% of the mesh size are reasonably employable. Preferably, however, the maximum dimension of the spacers will not exceed 30% of the smallest dimension of the gaps defined by the mesh. The answer to the question of how high a concentration of spacers should be added is that it must be remembered that too high a spacer concentration will lead to a lumping of the spacers and thus to an occlusion of the screen. Desirable concentrations are between 0.5% and 80%. The preferred amount in the case of spherical spacers is 5%.

For the silk-screening process the screen 3, clamped onto the frame 7, is positioned about 5-10 cm above the target surface of the substrate 13. The screen 3 is aligned with the substrate 13 with the aid of a camera (not shown) that is moved between the substrate 13 and the screen 3, making it possible for instance by means of a beam-splitter prism to control and adjust the position of the screen 3 relative to the substrate 13. Once the alignment has been made, the camera is removed, and the screen is brought up to within a distance of between 0.5 mm and 5 mm, and preferably 2 mm.

Next, the print medium 9 in the form of the spacer-containing cement, preferably epoxy, is placed on the screen 3. Exerting pressure, a doctor blade 11 is then moved across the screen, squeezing the cement with its intermixed spacers through the mesh. Enough pressure must be applied to cause the part of the screen on which the doctor blade is bearing down to make contact with the object surface underneath that is to be imprinted, as shown in FIG. 2. Typical pressure levels are in the 0.2 N/cm range. Reference in this case is made to pressure per centimeter since the doctor blade is usually a kind of spatula. FIG. 2 shows the areas of the substrate 13 on which a structured pattern of cement 15 has been printed after the doctor blade has passed over it.

There are different ways in which the doctor blade can be moved across the screen. A one-time pass of the doctor blade across the screen is usually sufficient. However, there are also many reciprocating dual doctor blade systems in use.

After a structured film of cement has been deposited on the surface of an element, the two surfaces to be bonded must be brought together. When a surface is to be cemented along a specific pattern, the technician usually faces the requirement of creating precisely defined adhesive layer segments, meaning that the width and the thickness of the cement layer must be defined. Moreover, especially in the case of optical elements, bubble inclusions must be avoided. Bubble inclusions are usually caused by the silk-screen printing process itself, as well as by the conditions under which the two elements are joined. The inventors have found that bubble inclusions cannot be avoided merely by heating the applied cement layer to between 30° C. and 80° C., preferably 60° C. There is another condition to be met, whereby the ratio between the width of the applied cement film and its thickness must not exceed 20:1 at least along one dimension. This means that it is possible to deposit very long strips, for as long as the width of the strip does not exceed 20 times the thickness of the strip. Given the surface tension, heating the cement will then cause a degassing of the bubbles. Moreover, the above-described geometry will lead to a convexity in one dimensional direction so that, when the second element that is to be attached is brought up, there will essentially be no formation of bubble inclusions. Now when the two elements are brought together in a precisely defined manner and are ultimately pressed together, they will be joined up to a distance beyond which the spacers will not allow them to come in contact. In the example shown that is the 5 μm mentioned above.

To be sure, when the elements are brought together, the pressure must be applied as evenly as possible. When precision-optical elements with an optical surface are to be cemented together, it will not be possible in many cases to simply press the object elements together with a tool. Another aspect of this invention is therefore dedicated to a method whereby the two elements can be joined in a desirable manner. A method of that nature is feasible when only one of the two elements is of a design whereby a relatively homogeneous channel distribution permits access to the surface of the other element. FIG. 3 is a schematic illustration of that configuration. According to the invention the structured element 105, provided with a cement film 103, is placed in flush contact on an adhesive support 107. The adhesive support contains channels which, by way of a valve, can be selectively connected to a pressure pump or to a vacuum pump. First, the pressure pump is used to generate a gas flow. Next, the second element 110 to be bonded is brought up to the cement film. The gas flow generates a gas cushion capable of suspending the second element without contact. This is usually where the so-called Bernoulli effect comes into play. When next the gas flow is gradually reduced to zero, the second element will be lowered onto the first element in controlled fashion. The cement film and the second element 110 now seal the channels from the environment. This is followed by connecting the channels to the vacuum pump. Opening the valve of the vacuum pump will displace the air from the channels, creating negative pressure. Since the channels are all interconnected, the result will be a well-balanced negative pressure. The pressure of the ambient air will press the second element 110 very evenly against the structured element 105 without the need for applying pressure on the second element by means of an additional tool. In a preferred embodiment the outer rim between the structured element 105 and the second element 110 is provided with an 0-ring gasket 113 that prevents the ambient air pressure from directly bearing on the cement films at the perimeter of the substrate that might otherwise push it inward.

This pressure system, of course, can be modified. For example, the base unit 107 can serve as the bottom section of a pressure chamber in which the structured element 103, the second element 110 and perhaps the gasket 113 can be pressurized, while the channel 115 on the base unit opens up to the ambient atmosphere, thus providing air pressure in the channels.

Claims

1. Method for applying a structured cement film on the surface of an element, encompassing the following steps: preparing the surface of an element on which the structured cement layer is to be applied;preparing a silk-screen printing system with screen and doctor blade, said screen encompassing sealed and unsealed regions;applying a print medium on one side of the screen;positioning said surface in non-contacting fashion near the screen in a manner whereby the screen is situated between the print medium and said surface;squeezing the print medium through the screen using the doctor blade by brushing the doctor blade across the top of the screen, causing it to locally press the screen onto said surface and the print medium in the unsealed regions to be deposited on said surface;characterized in that the print medium contains cement components as well as spacers.

2. Method as in claim 1, characterized in that a print medium is employed whose volume includes a spacer component at a concentration of between 0.5% and 80% but, preferably, approximately 5%.

3. Method for bonding at least 2 elements, characterized in that a first element is provided with a structured cement film according to the method described in claim 1 or 2, and that the surface of a second element is brought up to that of the first element to within a distance defined by said spacers, following which the cement film is allowed to cure.

4. Method as in claim 3, characterized in that the first element is provided with perforations leading to the surface to be bonded, that during the approach a gas flow is first generated in the direction of said surface which gas flow is reduced to zero during the approach and that, upon contact of the structured cement film with the surface of the second element, a negative pressure is generated in the said perforations in a manner whereby the ambient atmospheric pressure presses the second element against the first element.