Pressure sensor package and method of making the same

Abstract: A method of securing a pressure sensor diaphragm to a pressure sensor base, including the steps of positioning the pressure sensor diaphragm over the pressure sensor base, placing a pressure sensor cap over the pressure sensor diaphragm, applying a tensile force to at least a portion of the cap, and deforming a portion of the cap around a portion of the base. The applying step can occur before the deforming step by engaging a portion of the cap and advancing the base toward the cap to create tension in a portion of the cap. This creates a tensile preload in the cap. The applying step can instead be performed during the deforming step by engaging a first portion of the cap and advancing a second portion of the cap toward the first portion of the cap and around a portion of the base. This can create tension in a portion of the cap when the deformation operation is being performed.

Description: The present invention generally relates to the field of pressure sensor packages, and more specifically to pressure sensor packages having a pressure chip sealed within a cavity by a diaphragm. Pressure sensors are used in a variety of areas, such as automotive and industrial applications, to provide an electrical signal corresponding with a measured fluid pressure. For example, pressure sensors can be used to measure automotive oil pressure and hydraulic fluid pressure. One type of pressure sensor includes a solid state pressure sensing bridge, or chip, for generating an electrical signal corresponding with fluid pressure applied to the chip. Typically, the chip is mounted within a cavity of a base, and a fluid medium (e.g., silicone gel) is placed within the cavity to cover the chip. A diaphragm is secured to the base over the cavity to secure the gel within the cavity, and to create a sealed pressure sensor package. Alternatively, the gel can be injected into the cavity through a separate port after the diaphragm is secured to the base. In operation, pressure applied to the diaphragm deflects the diaphragm, is transmitted by the medium, thereby resulting in pressurization of the chip. The diaphragm is commonly secured to the base by welding. Welding provides an adequate seal, but the operation can only be performed if both the base and the diaphragm are made of compatible metallic materials. In addition, the welding operation can be expensive, and can cause heat distortion of the components. Also, the welding typically must be done without the fluid medium in the cavity, thereby requiring the extra operations of injecting the fluid medium into the cavity and thereafter sealing the cavity without causing an unacceptable pressure offset pressure in the cavity. German Patent Application DE-A-195 07 143 discloses a method of applying a cap on a base by deforming sealing portions of the cap around a neck of the base, to seal an unwelded diaphragm. The present invention as defined in claims 1 to 22 provides a method of securing a pressure sensor diaphragm to a pressure sensor base, including the steps of positioning the pressure sensor diaphragm over the pressure sensor base, placing a pressure sensor cap over the pressure sensor diaphragm, applying a tensile force to at least a portion of the cap, and deforming a portion of the cap around a portion of the base. The applying step can occur before the deforming step by engaging a portion of the cap and advancing the base toward the cap to create tension in a portion of the cap. This creates a tensile preload in the cap. The applying step can instead be performed during the deforming step by engaging a first portion of the cap and advancing a second portion of the cap toward the first portion of the cap and around a portion of the base. This can create tension in a portion of the cap when the deformation operation is being performed. By creating a tension in at least a portion of the cap, a better seal is obtained. The engaging step preferably includes the step of engaging a portion of the cap that is spaced from the diaphragm. For example, the engaging step can include the step of engaging a portion of the cap that is closer to the deformed portion of the cap than to the diaphragm. The method can further include the steps of moving the pressure sensor cap toward the pressure sensor base, and deforming a portion of the pressure sensor diaphragm between the pressure sensor cap and the pressure sensor base. For example, the step of deforming can include the step of deforming the pressure sensor diaphragm along a line of deformation. The pressure sensor diaphragm preferably is formed separately from the cap, but could also be formed integral therewith. In this regard, a portion of the pressure sensor diaphragm can be deformed between the pressure sensor cap and the pressure sensor base, such as along a line of deformation. Fig. 1 is a partial section view of a first embodiment of the present invention. Fig. 2 is a perspective view of a second embodiment of the present invention. Fig. 3 is a section view of a third embodiment of the present invention before the cap is deformed around the base. Fig. 4 is a section view of the package of Fig. 3 after the cap is deformed around the base. A pressure sensor package embodying the present invention includes a pressure sensor base, a pressure sensor diaphragm, and a pressure sensor cap positioned over the diaphragm and secured to the base. The base further includes a cylindrical sidewall defining a cavity 22 and an upper, annular surface. The package also includes a pressure sensing chip 24 which is positioned within the cavity 22. A plurality of wires 26 operatively inter-connect with the chip 24 and extend out the bottom of the base. The base is preferably made from a metallic material, such as stainless steel or cold rolled steel. The pressure sensing chip 24 can be any appropriate chip for sensing fluid pressure, and the selection of which will depend in part on the pressures to be sensed. For example; the pressure sensing chip 24 can be any of a variety of pressure transducers, such as those described in commonly assigned U.S. Patent N° 5,507,171, U.S. Patent No. 4,744,863, U.S. Patent No. 4,853,669 and U.S. Patent No. 4,996,082. The wires 26 are sealed within the base by an appropriate sealing material, such as glass, in a conventional manner. The diaphragm overlies the cavity 22 to provide a barrier to the entry or contaminants into the cavity 22. The diaphragm is not welded to the base as with prior art designs, but rather is held in place by the cap, as described below in more detail. Because the diaphragm is not welded to the base, the diaphragm can be made from non-metallic materials, such as elastomers, rubbers and plastics, and may also be made of metallic materials, such as stainless steel or plated cold rolled steel. In the illustrated embodiments, the diaphragm is made of stainless steel. The package also includes a quantity of pressure transfer media 30 which substantially fills the cavity 22 and covers the chip 24. The pressure transfer media 30 occupies the cavity 22 between the diaphragm and the chip 24 so that little or no air is within the cavity 22. The pressure transfer media 30 transfers pressures caused by deflection of the diaphragm (caused, for example, by a change in pressure applied to the diaphragm) to the chip 24. In this regard, it is typically desirable to choose the pressure transfer media 30 to be a virtually incompressible material, such as is the case with many liquids and gels. In the illustrated embodiment, the pressure transfer media 30 is silicone gel. Fig. 1 illustrates an assembly 49 wherein a cap 50 extends beyond the bottom 52 of a base 54. In this embodiment, the bottom 56 of the cap 50 is plastically deformed around the bottom 52 of the base 54, or an appropriate ledge or groove (not shown) formed in the base 54. In such an arrangement, there is no need to provide threads on the base or internal threads on the cap. Rather, the cap 50 is secured to the base 54 by a simple plastic deformation, or coining, of the cap 50 around the base 54. Assembly of the above-described pressure sensor packages can be performed by filling the cavity 22 with fluid, positioning the diaphragm over the base, placing the cap over the diaphragm, and moving the cap toward the base until the diaphragm is deformed therebetween. The diaphragm of the embodiment shown in Figs. 1 is deformed along lines of deformation that form a closed path adjacent a periphery of the pressure sensor diaphragm. Such closed path forms the desired barrier to the entry of contaminants into the cavity 22. For the package 49 illustrated in Fig. 1, the method further includes the step of deforming a portion of the cap 50 around a portion of the base 54 to thereby fix the cap 50 in position relative to the base 54. Fig. 2 illustrates an example useful for understanding the invention, wherein a pressure sensor package 60 eliminates a leak path between the cap and the base. More specifically, the above-described and illustrated package 49(Fig. 1) generally has potential leak paths between the respective cap and the diaphragm and between the cap and the base. In contrast, the package 60 illustrated in Fig. 2 has only a leak path between the cap 62 and the base 64. This is accomplished by forming the cap 62 from a single piece of material to provide a central diaphragm portion 66 and an integral, peripheral cap portion 68 which is thicker than the diaphragm portion 66. For example, the cap portion 68 and the diaphragm portion 66 could be molded as a unitary structure. The side of the diaphragm portion 66 facing the cavity 70 is optionally arcuately convex such that the diaphragm portion 66 extends below the upper surface 72 of the base 64. Such a design facilitates movement of any air pockets out of the cavity 70 as the cap 62 is being secured to the base 64. That is, when the cap 62 is moved toward the base 64, the center of the diaphragm portion 66 will contact the fluid in the cavity 70 before any other part of the diaphragm portion 66. Further movement of the cap 62 toward the base 64 will result in the fluid contact point gradually moving radially outwardly on the diaphragm portion 66, thereby forcing air pockets to the outer perimeter of the cavity 70 and eventually out through the threads 74. The amount of fluid placed in the cavity 70 before assembly is only slightly more than the amount required for the final assembled package 60. The excess fluid will be allowed to leak out into the threads 74. Another embodiment of the present invention is illustrated in Figs. 3 and 4. The illustrated package is very similar to the package illustrated-in Fig. 1 in that it includes a cap 80 that extends beyond a bottom 82 of a base 84. A diaphragm 85 is compressed between the cap 80 and the base 84. In addition, just like the Fig. 1 embodiment, a bottom 86 of the cap 80 is plastically deformed around the bottom 82 of the base 84 or, alternatively, an appropriate ledge or groove (not shown) formed in the base 84. In this arrangement, there is no need to provide threads on the base or internal threads on the cap 80. Rather, the cap 80 is secured to the base 84 by a simple plastic deformation, or coining, of the cap 80 around the base 84. When the Fig. 1 embodiment is assembled, a punch deforms the cap and the sidewalls of the cap are put under compression. After the cap is deformed around the base, the punch is withdrawn, and the sidewall will typically spring back due to elastic deformation. Such springback can result in a loose fit between the cap and the base. In order to accommodate the springback, an over-pressure stroke is commonly used. However, there is a maximum cap length for which an over-pressure stroke will compensate for the springback. To alleviate the need for an over-pressure stroke, the Fig. 3 embodiment includes an engaging portion along the outer surface of the cap 80 to allow the cap 80 to be supported during the deforming operation. In the illustrated embodiment, the engaging portion comprises an annular ledge 88. The annular ledge 88 is engaged by an upper punch 90 and the base 84 is engaged by an inner support 92 during the deforming operation. The inner support 92 is biased upwardly toward the upper punch 90 by a compliant biasing force F to create a tensile elastic preload in an upper portion 94 of the cap 80, while no preload is formed in a lower portion 96 of the cap 80. The upper portion 94 of the cap 80 is between the annular support 88 and a diaphragm 85, and the lower portion 96 is between the annular support 88 and an annular deformed portion 97 of the cap 80. The pressure sensor package of Figs. 3 and 4 is assembled by positioning the pressure sensor diaphragm over the pressure sensor base, and placing the pressure sensor cap over the pressure sensor diaphragm. A tensile force is then applied to at least a portion of the cap, and a portion of the cap is deformed around a portion of the base. In the illustrated embodiment, the deformation operation is performed by advancing the upper punch 90 toward a lower punch 98. Such movement of the package is accommodated by the compliant nature of the biasing force F that supports the inner support 92. As the upper punch 90 continues its travel downward, the lower punch 98 will eventually engage the bottom 86 of the cap 80, and subsequently deform the bottom 86 of the cap 80 around the bottom 82 of the base 84 to create the deformed portion 97 (Fig. 4). The deformed portion 97 secures the base 84 within the upper portion 94. During such deformation of bottom 86, additional tension is created in the upper portion 94 of the cap 80, and compression in created in the base 84. Once the deformation is complete and the upper punch 90 is retracted, the base 84 will try to elastically expand slightly due to the removed compressive forces, and the upper portion 94 of the cap 80 will elastically contract slightly due to the removed tensile forces. This expansion of the base 84 and contraction of the cap 80 work against each other, resulting in very little springback of the cap 80 and a corresponding secure engagement between the cap 80 and the base 84. It is noted that the lower portion 96 of the cap 80 is under compression during the deformation process. However, because the upper portion 94 is longer than the lower portion 96 and because the upper portion 94 is preloaded, any expanding springback of the lower portion 96 is taken up by contracting of the upper portion 94. In this regard, the relative amount of springback between the upper portion 94 and the lower portion 96 can be adjusted by adjusting the location of the annular ledge 88. More specifically, the closer the annular ledge 88 is to the bottom 86 of the cap 80, the more the upper portion 94 will contract after the deformation operation and, in general, the more tightly the base 84 will be held within the cap 80. If, on the other hand, the annular ledge 88 is located away from the bottom 86 of the cap 80 so that the lower portion 96 is longer than the upper portion 94, the compression of the lower portion 96 may be too much for the upper portion 94 to handle. Depending on the amount of preload applied to the upper portion 94, the result may be a poor engagement between the cap 80 and the base 84. In general, it is believed that it is advisable to located the annular ledge 88 below the middle of the cap 80 so that the upper portion 94 is longer than the lower portion 96. The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and the skill or knowledge of the relevant art, are within the scope of the present invention. For example, the materials used for the base, the diaphragm and the cap could vary from those described, depending on design constraints such as temperature, pressure and environmental issues. In addition, instead of a ledge, the engaging portion could be any appropriate configuration, such as a recess, thread or spline. The embodiments described herein are further intended to explain best modes known for practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with various modifications required by the particular applications or uses of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.

Claims: A method of securing a pressure sensor diaphragm (24) to a pressure sensor base (84) comprising the steps of : Position the pressure sensor diaphragm (24) over the pressure sensor base (84) ; placing a pressure sensor cap (80) over the pressure sensor diaphragm (24) ; applying a force to at least a portion of the cap (80) ; and deforming a portion of the cap (80) around a portion of the base (84) characterizing in that said applying step involves applying a tensile force to at least a portion of the cap (80) and occurs before said deforming step. A method as claimed in claim 1, wherein said applying step comprises the steps of : engaging a portion of the cap (80) ; and advancing the base (84) toward the cap (80) to create tension in a portion of the cap (80). A method as claimed in claim 2, wherein said engaging step includes the step of engaging a portion of the cap (80) that is spaced from the diaphragm (24). A method as claimed in claim 3, wherein the cap (80) is deformed at a deformed portion of the cap (80), and wherein said engaging step includes the step of engaging a portion of the cap (80) that is closer to the deformed portion than to the diaphragm (24). A method as claimed in claim 1, wherein said applying and deforming steps include the steps of : engaging a first portion of the cap (80) ; and advancing a second portion of the cap (80) toward the first portion of the cap (80) and around a portion of the base (84). A method as claimed in claim 5, wherein said engaging step includes the step of engaging a portion of the cap (80) that is spaced from the diaphragm (24). A method as claimed in claim 6, wherein the cap (80) is deformed at a deformed portion of the cap (80), and wherein said engaging step includes the step of engaging a portion of the cap (80) that is closer to the deformed portion than to the diaphragm (24). A method as claimed in claim 1, further comprising the steps of : moving the pressure sensor cap (80) toward the pressure sensor base (84) ; and deforming a portion of the pressure sensor diaphragm (24) between the pressure sensor cap (80) and the pressure sensor base (84). A method as claimed in claim 8, wherein said step of deforming comprises the step of deforming the pressure sensor diaphragm (24) along a line of deformation; A method as claimed in claim 1, further comprising the step of applying a compression force to the pressure sensor base (84) before said step of deforming a portion of the cap (80) around a portion of the base (84). A method of securing a pressure sensor diaphragm (24) to a pressure sensor base (80), the method comprising : positioning the pressure sensor diaphragm (24) over the pressure sensor base (84) ; placing a pressure sensor cap (80) over the pressure sensor diaphragm (24) ; applying a force to at least a portion of the cap (80) ; deforming a portion of the cap (80) around a portion of the base (84) characterized by said force being a tensile force and by removing said tensile force to allow elastic contraction of a portion of said cap (80). A method as claimed in claim 11, wherein said applying step occurs before said deforming step. A method as claimed in claim 12, wherein said applying step comprises the steps of : engaging a portion of the cap (80) ; and advancing the base toward the cap (80) to create tension in a portion of the cap (80). A method as claimed in claim 13, wherein said engaging step includes the step of engaging a portion of the cap (80) that is spaced from the diaphragm (24). A method as claimed in claim 14, wherein the cap (80) is deformed at a deformed portion of the cap (80), and wherein said engaging step includes the step of engaging a portion of the cap (80) that is closer to the deformed portion than to the diaphragm (24). A method as claimed in claim 11, wherein said applying step occurs during said deforming step. A method as claimed in claim 16, wherein said applying deforming steps include the steps of : engaging a first portion of the cap (80) ; and advancing a second portion of the cap (80) toward the first portion of the cap (80) and around a portion of the base (84). A method as claimed in claim 17, wherein said engaging step includes the step of engaging a portion of the cap (80) that is spaced from the diaphragm (24). A method as claimed in claim 18, wherein the cap is deformed at a deformed portion of the cap (80), and wherein said engaging step includes the step of engaging a portion of the cap (80) that is closer to the deformed portion than to the diaphragm (24). A method as claimed in claim 11, further comprising the steps of : moving the pressure sensor cap (80) toward the pressure sensor base (84) ; and compressing a portion of the pressure sensor diaphragm (24) between the pressure sensor cap (80) and the pressure sensor base (84) A method as claimed in claim 10, wherein said step of compressing comprises the step of compressing the pressure sensor diaphragm (24) along a line of compression. A method as claimed in claim 11, further comprising the step of applying a compression force to the pressure sensor base (84) before said step of deforming a portion of the cap (80) around a portion of the base (84).

Inventors: SOKN ERICK LEE, SOKN, ERICK LEE
Applicants: SSI TECHNOLOGIES INC, SSI TECHNOLOGIES, INC.
Application year: 1997
Main classification: G01L7
IPC codes: G01L9, H01L29, G01L7