Printing tool and printing device for applying a printing medium pattern on a substrate and use of a printing device for forming metallic conductor tracks

Abstract

The invention relates to a printing tool for applying a printing medium pattern on a substrate. The printing tool has a frame and a support structure arranged on the frame and a printing stencil. The printing stencil has at least one recess for forming a printing medium pattern on a substrate. The printing tool also has a stencil holder and a support structure that forms at least one recess of the printing tool in the printing stencil. The printing stencil is attached to the stencil holder and the stencil holder is attached to the frame by means of the support structure, creating a tensile stress. The support structure is designed to leave out at least the recess of the printing template. The invention also relates to a printing device for applying a printing medium pattern on a substrate and the use of a printing device for forming metallic conductor tracks.

Description

[0001] Printing tool and pressing device for applying a printing medium pattern to a substrate and

[0002] Use of a pressing device to form metallic conductor tracks

[0003] Description

[0004] The invention relates to a printing tool according to claim 1, a printing device for applying a printing medium pattern to a substrate according to claim 14, and the use of a printing device according to claim 15.

[0005] Screen printing and stencil printing are known methods for applying printing media to a substrate. These printing processes allow the application of a printing medium using a squeegee or squeegee system.

[0006] Typically, screen printing tools are used that feature a screen printing mesh with a multitude of mesh elements. The screen printing mesh is designed to allow the printing medium to pass through. A negative of the printed image is arranged on the screen printing mesh. This negative of the printed image has openings so that the printing medium only passes through the openings, allowing a structure defined by the openings of the printing stencil to be applied to the substrate in the form of the printed image made of the printing medium. This makes it easy to structure the printing medium on the substrate.

[0007] Typically, stencil printing tools made of plastics and / or metals are used. A stencil of the stencil printing tool has openings corresponding to the negative of the printed image, so that the printing medium only passes through the area of ​​the openings, allowing a structure defined by the openings of the printing stencil to be applied to the substrate in the form of the printed image made of printing medium. This makes it possible to easily structure the printing medium on the substrate.

[0008] Particularly in the field of semiconductor technology, especially for photovoltaic solar cells, it is desirable to structure the printing medium as delicately as possible. There is a requirement to avoid fluctuations in the cross-sectional area of ​​the structures during material application, as a decreasing cross-sectional area results in efficiency losses due to increased conduction resistance. Furthermore, there are requirements for the precise positioning of the printing medium layer to be applied, as the tolerance for mispositioning is becoming increasingly smaller with the advancing miniaturization of components and circuit boards.

[0009] To meet these requirements, printing tools with thin stencils are used. In particular, glass stencils allow for the precise definition of fine line structures in the printing medium.

[0010] A problem when using printing tools with thin stencils, especially glass stencils, is the handling of the stencil within the printing tool. This is due to the point loading of the printing stencil, which occurs due to the forces applied in conventional printing tools. This force is caused by squeegee pressure and squeegee friction. Further forces arise from thermal influences, as well as from bending and twisting of the frame of a printing tool. High forces acting on the stencil lead to the deformation of the stencil and can even cause it to break. This deformation of the stencil can lead to incorrect application of the printing medium to the substrate to be printed.

[0011] Due to the limitation of the tensile force in view of the maximum permissible tensile stress that can act on the stencil, state-of-the-art systems fail to achieve the required tension, which leads to a lack of positional stability during the squeegee movement. An increase in tension leads to an increased risk of breakage and thus a reduction in the service life of the printing tool.

[0012] The present invention is therefore based on the object of forming structures from printing medium onto a substrate via a printing stencil, in particular via printing stencils made of brittle and / or fragile material, and, on the other hand, of providing sufficient tensile force on the stencil without deforming or breaking the printing stencil. Furthermore, the object is to enable high throughput in production while still ensuring a long durability and thus a long service life of the printing tool.

[0013] This object is achieved by a printing tool according to claim 1 and by a printing device according to claim 14, as well as the use of a printing device according to claim 15. Preferred embodiments of the printing tool according to the invention can be found in claims 2 to 13 and 16. The wording of all claims is hereby explicitly incorporated into the description by reference.

[0014] The printing tool according to the invention comprises a frame and a support structure arranged on the frame, as well as a printing stencil. The printing stencil has at least one recess for forming a printing medium pattern on a substrate.

[0015] It is essential that the printing tool has a stencil holder. It is also essential that the printing stencil is arranged on the stencil holder, and that the stencil holder is arranged on the frame by means of the support structure, preferably with the formation of a tensile stress. Furthermore, it is essential that the support structure is designed to at least omit the recesses of the printing stencil.

[0016] This offers the advantage that, due to the decoupling of the support structure from the stencil via the stencil holder, there is no direct force transmission between the support structure and the stencil. This prevents the occurrence of localized tensile loads on the stencil. Such localized tensile loads lead to force peaks being induced in the stencil material, which can cause microcracks and, in the worst case, the stencil to break.

[0017] It is advantageous that the stencil holder is arranged on the frame by means of the support structure, thereby forming a tensile stress. In particular, it is advantageous that the tensile stress is formed by means of the support structure. In particular, it is advantageous that the support structure forms the tensile stress. This can be achieved by arranging a prestressed support structure on the frame and on the stencil holder. It is therefore advantageous that the support structure is tensioned between the frame and the stencil holder to form a tensile stress.

[0018] A further advantage of the printing tool according to the invention is that the support structure is designed to at least leave a recess in the printing stencil, so that no support structure is present in the region of the recesses. The recessed part of the printing stencil is thus free of support material. Another advantage is that the stencil holder, when clamped, absorbs the stresses of the printing stencil as well as the shear forces occurring during the squeegee process. The stencil holder absorbs the forces that occur or at least mitigates them to such an extent that damage to the printing stencil or distortion of the structure to be applied is advantageously prevented. The forces acting on the stencil holder, which occur during the squeegee process, are absorbed by the frame via the support structure when clamped. This advantageously achieves consistent positioning of the printing stencil.

[0019] In a preferred embodiment, the stencil holder of the printing tool has at least one completely enclosed recess. Furthermore, the recess of the printing stencil is arranged within the recess of the stencil holder.

[0020] An advantage of this embodiment is that the arrangement of the printing stencil within the recess of the stencil holder creates an area that is freely accessible for the printing medium to be applied, allowing it to be applied to the substrate through the recess without hindrance. This free accessibility also enables a sufficient supply of printing medium during the squeegee process.

[0021] A further advantage is that the circumferentially enclosed recess allows for a completely enclosed arrangement of the stencil holder, thus providing exceptional stability. This completely enclosed arrangement allows the printing stencil to be attached to the stencil holder with a consistent tensile force. This prevents the printing stencil from breaking due to localized tensile load peaks.

[0022] In a preferred embodiment, the stencil holder is designed as a stencil frame and overlaps the edge of the printing stencil all around.

[0023] The advantage here is that the design of the stencil holder as a stencil frame and the circumferential overlap of the printing stencil create an overlap area, resulting in a larger connection area. This connection area allows for forces to be transferred over a large area. This is also advantageous because the overlap allows for simple joining of the elements during the production of the printing tool.

[0024] In a further preferred embodiment, the printing stencil is connected circumferentially in an overlapping area with the stencil holder in a materially bonded manner.

[0025] The advantage here is that the design of the stencil holder as a stencil frame and the circumferential overlap of the printing stencil create an overlap area, which leads to a better connection. This ensures even transmission of forces, especially the tensile and shear forces that occur.

[0026] In a further preferred embodiment of the printing tool, the support structure overlaps with the frame of the printing tool, in particular the support structure overlaps with the frame of the printing tool all the way around.

[0027] An advantage of this embodiment of the printing tool is that the overlap of the support structure with the frame of the printing tool creates an overlap area, which leads to a better connection for transmitting forces. Furthermore, this embodiment is advantageous in that, particularly in the circumferential case of overlap, an even distribution of the applied tensile forces is achieved. In a further preferred advantageous embodiment of the printing tool, the support structure is designed to leave space for the printing stencil.

[0028] In conventional printing tools, the printing stencil is arranged on a continuous carrier material, which thus also covers the recesses of the printing stencil. This carrier material is typically net-like and has a mesh size. The disadvantage here, particularly when using so-called screen mesh as the carrier material, is that for very small structures that are to be applied to a substrate, a carrier material must be used that has wires with a diameter that is essentially smaller than the structures to be printed. These carrier materials lead to reduced stability and poor dimensional accuracy of the printing stencil. The disadvantages described are prevented by leaving out the carrier structure in the area of ​​the printing stencil.Another advantage is that this enables the use of non-permeable carrier materials, in particular carrier materials made of dimensionally stable but elastically deformable plastics.

[0029] In a preferred embodiment of the printing tool, no direct, material-locking connection is formed between the printing stencil and the support structure.

[0030] The advantage here is that the only indirect connection of the printing stencil, especially via the stencil carrier, prevents the printing stencil from being subjected to localized tensile stress by a connection to the carrier material. This localized stress leads to local stress cracks, which increase the risk of stencil breakage.

[0031] In a further advantageous embodiment of the printing tool, the printing stencil and the support structure are arranged on opposite sides of the stencil holder.

[0032] The advantage of this embodiment is that it improves the print image. This improvement in the print image is particularly achieved when the thickness of the support structure is greater than that of the printing stencil. In another preferred embodiment of the printing tool, the printing stencil and the support structure are arranged on the same side of the stencil holder, with the printing stencil and support structure preferably not overlapping.

[0033] The advantage of this embodiment is that, due to the overlapping areas being on the same side, a one-sided joining process can be carried out in a simple manner for both overlapping areas during production.

[0034] In a further preferred embodiment, the support structure is integrally connected to the stencil holder, and the stencil holder is integrally connected to the printing stencil, in particular by one of the following methods: gluing, soldering, welding, or vulcanization. Thus, the support structure is preferably integrally connected to the stencil holder, and the stencil holder is integrally connected to the printing stencil, by one or more of the following connection types: adhesive connection, soldering connection, welding connection, or vulcanization connection.

[0035] A bonded connection, particularly one of the following processes: gluing, soldering, welding, or vulcanizing, achieves a high maximum tensile strength. The transfer capacity of forces and moments is improved through the bonded connection. Another advantage of the bonded connection is that it does not require additional installation space, as is the case with designs using force-locking connections such as clamps.

[0036] In a preferred embodiment of the printing tool, the support structure is formed from a flexible material, in particular as a fabric, particularly preferably as a fabric made of plastic threads.

[0037] An advantage of this design is that the flexible nature of the support structure allows tensile forces to be transmitted. Furthermore, the construction of the support structure from flexible materials achieves the desired elasticity, which allows the printing stencil to be positioned on slightly uneven surfaces. This is especially true if the surface already contains conductor tracks or other components.

[0038] In a further preferred embodiment of the printing tool, the printing stencil is made of a dimensionally stable material, in particular glass, stainless steel, aluminum, or plastic. It is particularly advantageous to make the printing stencil from glass. In particular, the printing stencil is preferably dimensionally stable against bending.

[0039] The advantages of using the materials mentioned, especially glass, compared to conventional materials, are that glass has increased scratch resistance. Furthermore, glass has a high modulus of elasticity and low thermal expansion. Furthermore, the materials offer high abrasion and chemical resistance. Another advantage of dimensionally stable materials is that they enable a very precise print image and, due to their dimensional stability, are subject to only minor process-related warping.

[0040] In a further preferred embodiment, the stencil holder is not designed to be flexible, in particular the stencil holder is preferably made of a dimensionally stable material, preferably of stainless steel, aluminum or plastic.

[0041] The dimensionally stable design is advantageous because it allows the tensile stress of the support structure to be absorbed without transferring this tensile stress to the printing stencil. The stencil holder is therefore preferably designed to be dimensionally stable against tensile stresses, particularly against tensile stresses emanating from the support structure arranged on the stencil holder.

[0042] The advantages of using these materials are their increased scratch resistance. Furthermore, they offer high abrasion and chemical resistance. Another advantage of dimensionally stable materials is that, due to their dimensional stability, they are subject to only minor process-related warping.

[0043] In a preferred embodiment of the printing tool, the printing stencil and the stencil holder are made of different materials, in particular the printing stencil is preferably made of glass and the stencil holder is not made of glass.

[0044] The advantage of using different materials lies in the ability to leverage material combinations to integrate complementary functions into the printing tool. Thus, the printing stencil can be reliably mounted via the stencil holder, for example, if the stencil holder is made of stainless steel and the printing stencil is made of glass. This leads to a reduction in assembly work and additional rework, and lowers quality assurance costs.

[0045] In a preferred embodiment of the printing tool, the printing stencil and the stencil holder have different thicknesses. It is advantageous for the printing stencil to have a smaller thickness than the stencil holder, in particular for the printing stencil to have a thickness that is at least 50 μm, preferably at least 250 μm, and in particular at least 500 μm smaller than the thickness of the stencil holder.

[0046] A thin stencil allows for precise definition of print patterns with fine lines. A thicker stencil holder, on the other hand, provides better stability against the tensile stress of the support structure.

[0047] Advantageously, the printing stencil has a thickness of less than 1000 μm, in particular a thickness of less than 500 μm, preferably a thickness of less than 200 μm. Advantageously, the stencil holder has a thickness of greater than 1000 μm, in particular a thickness of greater than 2 mm.

[0048] In a preferred embodiment of the printing tool, the printing stencil is formed in one piece. The advantage of an embodiment of the printing tool with a one-piece printing stencil is that it eliminates localized tensile loads on the printing stencil, thus preventing microcracks and stencil breakage. A further advantage is the increased stability of the printing stencil.

[0049] The above-described object is further achieved by a printing device for applying a printing medium pattern to a substrate.

[0050] The printing device according to the invention is formed using the printing tool according to the invention described above or preferably a preferred embodiment of the printing tool according to the invention.

[0051] The advantage here is that the design of the printing tool combines level-compensating printing behavior through the flexible support structure with precise positioning of the printing medium through the dimensionally stable printing stencil. The printing device comprises at least one printing tool according to the invention and at least one squeegee. The printing device is designed to apply a printing medium to the substrate through at least one recess in the printing stencil by means of the squeegee.

[0052] It is advantageous that the printing device is structurally designed in a manner known per se. It is advantageous to construct a known movement unit for moving the squeegee and a feed unit for supplying printing medium in a manner known per se.

[0053] The object mentioned at the outset is further achieved by an inventive use of a printing device according to the invention for forming a metallic structure of a semiconductor component, in particular for forming a metallic contact structure of a solar cell.

[0054] It is advantageous that the use of the printing device according to the invention contributes to time- and cost-efficient production. Furthermore, it is advantageous that manufacturing tolerances can be maintained to a greater extent when using the printing device. Furthermore, the use of a printing device according to the invention is advantageous because the longevity of the printing tool results in lower maintenance requirements, particularly by preventing breakage and microcracks in the printing stencil. A further advantage of using the printing device according to the invention is that the lower probability of failure of the printing tool according to the invention leads to fewer failure-related printing tool changes during production.

[0055] It is advantageous to provide one or more recesses with a greater width in order to enable contact fingers for connecting so-called connectors to a busbar by means of a plurality of recesses in the printing stencil. In particular, it is advantageous to form the recesses for known metallic contact structures for solar cells, in particular for structures with several parallel contact fingers and one or more busbars running perpendicular to the contact fingers.

[0056] The printing tool and the printing device according to the invention are basically suitable for applications in the fields of microelectronic metallization, solar cell metallization as well as the printing of LED / OLED, LTCC and on semiconductor wafers.

[0057] It is advantageous to design the printing stencil and the stencil holder in geometrically different basic shapes.

[0058] It is advantageous to arrange the printing stencil in a replaceable manner on the stencil holder and / or to arrange the stencil holder in a replaceable manner on the support structure. This allows for easy retooling of the printing stencil within the printing tool.

[0059] In alternative embodiments, the elements can also be attached differently or through a combination of fastening methods. However, the material-to-material arrangement of the printing stencil to the stencil holder and the material-to-material arrangement of the support structure to the stencil holder are particularly preferred. Further preferred features and embodiments of the printing tool and the printing device according to the invention are explained below with reference to exemplary embodiments and the figures. The exemplary embodiments are merely advantageous embodiments of the invention and are not restrictive. It shows:

[0060] Fig. 1 is a schematic representation of a first embodiment of a printing tool in front view (1a) and in sectional view (1b),

[0061] Fig. 2 is a schematic representation of a second embodiment of a printing tool in front view (2a) and in sectional view (2b) and

[0062] Fig. 3 is a schematic sectional view of an embodiment of a printing device according to the invention.

[0063] The figures are schematic representations, not to scale. Identical reference symbols in the figures indicate identical or equivalent elements.

[0064] The exemplary embodiment of a printing tool according to the invention shown in Figure 1a is shown in a front view. The present printing tool has a printing stencil 4 with six linear recesses 10. In this exemplary embodiment, the printing stencil 4 also has a square base area. In this example, the printing stencil 4 is made from a 20 μm thick, dimensionally stable glass plate. The so-called LIDE (Laser Inducted Deep Etching) process is used to produce the recesses 10. The printing stencil 4 is arranged on a stencil holder 3 designed as a dimensionally stable stencil frame. The stencil holder 3 has a square recess 10 which is completely enclosed by the stencil holder 3. The printing stencil 4 overlaps with the stencil holder in a circumferential overlap region; this overlap region completely encloses the recess 10 of the stencil holder.The printing stencil 4 is joined to the stencil holder 3 in the overlapping area using laser welding. The printing stencil 4 is arranged under tension. The tensile stress is evenly distributed to the printing stencil 4 via the stencil holder 3 in the overlapping area. The stencil holder, which is designed as a stencil frame, is made of a 1.5 mm thick stainless steel sheet in this embodiment.

[0065] The stencil holder 3 is attached to the frame 1 via the support structure 2. The support structure 2 has a square recess. The recess in the support structure extends into the interior of the stencil holder 3, creating an overlapping area that completely encloses the stencil holder 3. In this exemplary embodiment, the support structure is designed as a flexible plastic fabric. This flexible plastic fabric is integrally connected to the stencil holder 3 in the overlapping area using an adhesive joining method. The frame 1 of the printing tool in this case is made from a hollow stainless steel profile and has a dimensionally stable, square frame shape. The support structure 2 overlaps the frame 1 of the printing tool all the way around. In this circumferential overlapping area, the support structure is integrally connected to the frame 1 using adhesive.

[0066] The printing stencil 4 and the support structure are arranged on opposite sides of the stencil holder. In this embodiment, the printing stencil 4 is the first element to rest on the substrate 7 when the printing tool is placed on the substrate 7 during the printing process by the printing device.

[0067] In this exemplary embodiment, the support structure 2 is arranged under tensile stress between the frame 1 and the stencil holder 3. The support structure does not overlap with the printing stencil 4, nor is there a direct, materially bonded connection between the support structure 2 and the printing stencil 4. The tensile force exerted by the support structure on the stencil holder 3 is distributed equally along the edge of the stencil holder. This holds the stencil holder 3 in its position. The stencil holder 3 decouples the stencil from the support structure and thus protects the printing stencil 4 from tensile stress peaks that arise due to the elasticity of the support structure during squeegeeing. The exemplary embodiment shown in Figure 2 is a modified version of the exemplary embodiment according to Figure 1.To avoid repetition, only the difference to the embodiment shown in Figure 1 will be discussed below:

[0068] The embodiment of a printing tool according to the invention shown in Figure 2a is shown in a front view. Figure 2b shows a side sectional view.

[0069] The embodiment shown in Figure 2 differs from the embodiment in Figure 1 in that the printing stencil 4 and the carrier structure are arranged on the same side of the stencil holder. In this embodiment too, the carrier structure does not overlap with the printing stencil 4. In this embodiment, the carrier structure is thicker than the printing stencil 4. The carrier structure assumes a protective function because it comes into contact with the substrate 7 first during the printing process. Thus, the elasticity of the carrier structure can compensate for unevenness without damaging the printing stencil 4. This is the case, for example, if components or conductor tracks are already located on a substrate 7 during the printing process.

[0070] Figure 3 schematically shows an embodiment of a printing device according to the invention. The printing device has a printing tool according to the invention, according to the embodiment shown in Figure 1. The printing stencil 4 is arranged in the stencil holder 3 and arranged in the frame 1 via the support structure. The printing stencil 4 has recesses 10 which are not covered by the support structure. Furthermore, the printing device has a squeegee 9 and a movement unit 8, to which the squeegee 9 is fastened. The movement unit 8 and the frame 1 of the printing tool are arranged on a holding element of the printing device (not shown), which also has a support for a substrate 7. In this embodiment, the substrate 7 represents a preliminary silicon solar cell yet to be printed, onto which metallic contact fingers 6 are to be applied.By means of a printing medium supply unit of the printing device (also not shown), a printing paste 5 containing metal particles is applied to the side of the printing stencil 4 facing away from the substrate. By means of the movement unit 8, the squeegee 9 is moved over the printing stencil 4, thereby pressing the printing paste 5 through the recesses 10 of the printing stencil 4 onto the substrate 7, so that printing paste 5 is applied to the substrate 7 in the form of contact fingers 10.

[0071] Figure 3 shows a schematic representation. In an actual configuration, the support structure 2 typically bends slightly in the area between the frame 1 and the stencil holder 3. The additional tensile stress peaks resulting from this deformation are absorbed by the stencil holder 3 and not passed on to the printing stencil 4. When the squeegee 9 passes over the printing stencil 4, it is bent slightly toward the substrate 7, despite its dimensionally stable properties.

[0072] List of reference symbols

[0073] 1 frame support structure

[0074] 3 Stencil holder Printing stencil 5 Printing paste

[0075] 6 contact fingers

[0076] 7 Substrat

[0077] 8 movement units

[0078] 9 Squeegee 10 Recess

Claims

Claims 1. Printing tool with a frame (1) and a support structure (2) arranged on the frame (1) and a printing stencil (4), wherein the printing stencil (4) has at least one recess (10) for forming a printing medium pattern on a substrate (7), characterized in that the printing tool has a stencil holder (3), that the printing stencil (4) is arranged on the stencil holder (3) and the stencil holder (3) is arranged on the frame (1) by means of the support structure (2), preferably with the formation of a tensile stress, and that the support structure (2) is designed to leave out at least the recess (10) of the printing stencil (4).

2. Printing tool according to claim 1, characterized in that the stencil holder (3) has at least one completely enclosed recess (10) and the recess (10) of the printing stencil (4) is arranged within the recess (10) of the stencil holder (3).

3. Printing tool according to one of the preceding claims, characterized in that the stencil holder (3) is designed as a stencil frame and overlaps the edge of the printing stencil (4) all around.

4. Printing tool according to claim 3, characterized in that the printing stencil (4) is connected circumferentially in an overlapping area with the stencil holder (3) in a materially bonded manner.

5. Printing tool according to one of the preceding claims, characterized in that that the support structure (2) overlaps with the frame (1) of the printing tool, in particular overlaps all around.

6. Printing tool according to one of the preceding claims, characterized in that the support structure (2) is designed to leave space for the printing stencil (4).

7. Printing tool according to one of the preceding claims, characterized in that no direct, material-locking connection is formed between the printing stencil (4) and the support structure (2).

8. Printing tool according to one of the preceding claims, characterized in that the printing stencil (4) and support structure (2) are arranged on opposite sides of the stencil holder (3).

9. Printing tool according to one of the preceding claims 1 to 7, characterized in that the printing stencil (4) and the support structure (2) are arranged on the same side of the stencil holder (3), wherein the printing stencil (4) and the support structure (2) preferably do not overlap.

10. Printing tool according to one of the preceding claims, characterized in that the support structure (2) is materially connected to the stencil holder (3) and the stencil holder (3) is materially connected to the printing stencil (4), in particular materially connected by one of the connection types adhesive connection, soldering connection, welding connection or vulcanization connection. 11 . Printing tool according to one of the preceding claims, characterized in that that the support structure (2) is formed from a flexible material, in particular as a fabric, in particular as a fabric with plastic threads.

12. Printing tool according to one of the preceding claims, characterized in that the printing stencil (4) is made of a dimensionally stable material, in particular of glass, stainless steel, aluminum or plastic, preferably of glass.

13. Printing tool according to one of the preceding claims, characterized in that the printing stencil (4) and stencil holder (3) are made of different materials, in particular that the printing stencil is made of glass and the stencil holder (3) is not made of glass.

14. Printing device for applying a printing medium pattern to a substrate (7) comprising a printing tool according to one of the preceding claims and at least one squeegee (9), wherein the printing device is designed to apply a printing medium to the substrate (7) through at least one recess (10) of the printing stencil (4) by means of the squeegee (9) of the printing device.

15. Use of a printing device according to claim 14 for forming a metallic structure of a semiconductor component, in particular for forming a metallic contact structure of a solar cell.

16. Printing tool according to one of claims 1 to 13, characterized in that the stencil holder is not designed to be flexible, in particular that the stencil holder is preferably made of a dimensionally stable material, preferably of stainless steel, aluminum or plastic.

Images