Flip chip pressure sensor assembly

A flip chip pressure sensor assembly. The flip chip pressure sensor assembly comprises a substrate; a pressure sensor die comprising a sensing diaphragm, the die having a top side and a bottom side that is reverse to the top side, where the top side of the die is electrically connected to the substrate by flip chip mounting technology; a cover defining an aperture disposed over the pressure sensor die, where the aperture defined by the cover aligns with the sensing diaphragm to provide a path for pressure to be transmitted through the aperture to the bottom side of the sensing diaphragm; and a gel disk disposed within the aperture in intimate contact with a bottom side of the sensing diaphragm, where the gel disk is domed above an outer shoulder of a rim defined by the cover.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Technology advances continue to enable reducing the size of sensors. As sensor features and/or components are reduced in size, new manufacturing problems are encountered that are not experienced when manufacturing sensors having larger feature sizes. It is a challenge to mass manufacture sensors while maintaining performance standards and production yields as sensor size is reduced. At the same time, a variety of markets for end products using sensors are aggressively pushing continued size reductions.

SUMMARY

In an embodiment, a flip chip pressure sensor assembly is disclosed. The flip chip pressure sensor assembly comprises a substrate; a pressure sensor die comprising a pressure sensing diaphragm, the die having a top side and a bottom side that is reverse to the top side, where the top side of the die is electrically connected to the substrate by flip chip mounting technology; a cover defining an aperture disposed over the pressure sensor die, where the aperture defined by the cover aligns with the pressure sensing diaphragm to provide a path for pressure to be transmitted through the aperture to the bottom side of the pressure sensing diaphragm; and a seal between the cover and the pressure sensor die.

In another embodiment, a pressure sensor assembly is disclosed. The pressure sensor assembly comprises a substrate; a pressure sensor die comprising a sensing diaphragm, the die electrically connected to the substrate; a cover defining an aperture disposed over the pressure sensor die, where the aperture defined by the cover aligns with the sensing diaphragm to provide a path for pressure to be transmitted through the aperture to the sensing diaphragm; and a gel disk disposed within the aperture in intimate contact with the sensing diaphragm, where the gel disk is domed above an outer shoulder of a rim defined by the cover.

In yet another embodiment, a pressure sensor assembly is disclosed. The pressure sensor assembly comprises a substrate; a pressure sensor die comprising a sensing diaphragm, the die electrically connected to the substrate; a cover defining an inner shoulder at a first end of the cover, defining an aperture disposed over the pressure sensor die, and defining one of a deformable wall or a collapsible wall at a second end of the cover opposite to the first end, where the aperture defined by the cover aligns with the sensing diaphragm to provide a path for pressure to be transmitted through the aperture to the sensing diaphragm and where the second end of the cover is in intimate contact with the substrate; and a seal between the inner shoulder of the cover and an outer perimeter of the bottom side of the pressure sensor die.

DETAILED DESCRIPTION

Force sensors and/or pressure sensors are used in a wide variety of applications including, for example and without limitation, industrial robots and surgical robots (e.g., in joints of robot arms to sense force), in infusion pumps used to deliver medications and/or IV solutions into human beings, in aspirators to measure lung capacity, in other medical devices, and in measuring fluid pressures in airplane applications and/or HVAC applications. It is understood that a sensor that responds to force inputs may be employed to measure pressure, for example by dividing the measured force by an area to which the force is applied. Said in another way, a force sensor may, in some circumstances (e.g., pressure applied to a constant surface area of a mechanical plunger coupled to the force sensor), output a signal that is proportional to a pressure (e.g., pressure×area=force, hence pressure is proportional to force) and may be said to sense pressure. The present disclosure refers to flip chip pressure sensors, but it is understood that the teachings may also be applied to flip chip force sensors.

There is an ongoing push from the industry to reduce the size of pressure sensors. For example, by reducing the size and expense of medical devices sufficiently, in part by reducing pressure sensor size, it may be possible to provide medical patients with personal and/or disposable devices that obviate the need of hospitals to own and manage these devices. The deployment of medical devices owned by patients may increase the amount of out-patient care versus in-office care or in-hospital care, thereby reducing healthcare expenditures. Further in the future, drug delivery mechanisms may move away from the pill form for delivering drugs to patients to quasi-continuous direct infusion of the active drug with dosage controlled, at least in part, based on feedback from small pressure sensors.

A micro mechanical electrical system (MEMS) pressure sensor die may be the component that senses pressure in a pressure sensor assembly (e.g., transduces mechanical force to an electrical property such as current). As pressure sensor dies get smaller, the area consumed by wire bonds on the MEMS pressure sensor die becomes the limiting factor on further size reduction. The space consumed by wire bonds may be negligible when the size of the MEMS pressure sensor is about 25 millimeters (mm) on a side (e.g., 25 mm×25 mm). The space consumed by wire bonds becomes a consideration when the size of the MEMS pressure sensor die approaches about 5 millimeters (mm) on a side (e.g., 5 mm×5 mm). Wire bonds in a MEMS pressure sensor die about 2 mm on a side (e.g., 2 mm×2 mm) may consume about 30% of the space of the die. Wire bonds in a MEMS pressure sensor die about 1 mm on a side (e.g., 1 mm×1 mm) may consume about 50% of the space of the die. The present disclosure contemplates dispensing with wire bonds for connecting the pressure sensor die to the pressure sensor assembly, for example electrically connecting the die to a substrate and/or printed circuit board, and relying instead on what is commonly referred to as flip chip mounting technology. The use of flip chip mounting technology can largely overcome the limitations on reduction of size in pressure sensor dies posed by wire bonds.

MEMS devices are typically manufactured using processing methods employed in the semiconductor industry. Many MEMS devices are fabricated on a single semiconductor wafer, and the wafer is later sawed up to separate the MEMS devices. The structures of the MEMS devices are built up using successive layers, from bottom to top where the “bottom” is the semiconductor wafer and the “top” is the last layer deposited (i.e., the layer deposited last in time). These definitions of “bottom” and “top” are used throughout this specification when referring to the pressure sensor die. During fabrication of MEMS devices intended for use in flip chip mounted applications, metallized pads are deposited on the top layer of the MEMS devices that are electrically connected to electrical conducting paths (e.g., “wires” or “electrical traces”) within the MEMS devices, and solder bumps are deposited on these metalized pads. Later, after sawing up the wafer to separate the MEMS devices, a MEMS device is flipped over (i.e., its “bottom” side is turned to face up while its “top” side is turned to face down), its solder bumps are aligned with matching pads on an external circuit (i.e., a substrate or printed circuit board) positioned below the MEMS device, and the solder in the solder bumps is re-melted (i.e., reflow solder processing) to provide an electrical connection to the external circuit. Other methods of providing electrical connection of the flip chip to the external circuit are, for example, thermosonic bonding and thermocompression bonding.

Turning now toFIG. 1AandFIG. 1B, a pressure sensor die10is described.FIG. 1Ashows a sectional view of the die10, whileFIG. 1Bshows a plane view of the die10. In an embodiment, the pressure sensor die10may sense or measure pressure incident on the die10. In an embodiment, the pressure sensor die10may sense or measure force incident on the die10. In an embodiment, the die10is a MEMS device manufactured with MEMS fabrication processes. The die10is provided with solder bumps or solder balls for use in flip chip mounting to a substrate and/or printed circuit board. In an embodiment, the die10comprises a pressure sensing diaphragm12, an outside edge14, an edge-to-diaphragm joint16, and a plurality of solder bumps18. While two solder bumps18are illustrated inFIG. 1A, it is understood that the die10may comprise any number of solder bumps18. In some contexts, the outside edge14may be referred to as the outside perimeter of the die10. In an embodiment, the die10may be generally square in planar shape. In another embodiment, the die10may be rectangular in planar shape. In another embodiment, the die10may be polygonal in planar shape, for example triangular, pentagonal, hexagonal, etc. In yet another embodiment, the die10may be generally circular in planar shape or oval in planar shape. In an embodiment, the die10may be less than about 5 mm×5 mm in planar size (e.g., less than about 25 square millimeters (mm2) in planar area), less than about 2 mm×2 mm in planar size, less than about 1 mm×1 mm in planar size, less than about ½ mm×½ mm in planar size, or smaller yet in planar size. Over time it is contemplated that the planar size of the pressure sensor dies10will get smaller as MEMS fabrication processing improves and as other associated manufacturing processing improves. In an embodiment, the diaphragm12may be about 50 micrometers (μm) or microns thick.

The outside edge14of the die10may be referred to in some contexts as the “picture frame” of the die10. The outside edge14or picture frame narrows from a greater thickness to the thinner thickness of the diaphragm12over a sloped surface15. In an embodiment, the sloped surface15makes an about 125° angle with the “bottom” side of the diaphragm12. Alternatively, the sloped surface15makes an about 125.3° angle with the “bottom” side of the diaphragm12. In an embodiment, one or more piezoresistive element is located within the die10proximate to the edge-to-diaphragm joint16, close to the surface on the “top” side of the die10, for example about 10 μm below the surface of the “top” side of the die10, about 5 μm below the surface of the “top” side of the die10, about 2.5 μm below the surface of the “top” side of the die10. While the above-described shapes, dimensions, thicknesses, and angles are generally representative of the physical geometry of the pressure sensor die10, it will be appreciated that other pressure sensor dies10that differ in one or more aspects of their physical geometry from those described above are also contemplated by the present disclosure and may also benefit from the teachings of this disclosure. For example, in an alternative embodiment the diaphragm12may be as thick as the outside edge14and hence the die10would not have the sloped surface15in that alternative embodiment.

As pressure and/or force is applied to the diaphragm12, the diaphragm12experiences mechanical stress, and the resistance of the one or more piezoresistive elements changes. When stimulated by an externally applied voltage (e.g., via the solder bumps18connected to an external circuit such as an external printed circuit board) an electrical current through the piezoresistive elements changes as the diaphragm12experiences mechanical stress from applied pressure and/or force, and this difference in electrical current can be mapped to an indication of pressure and/or force. In some contexts the pressure sensing diaphragm12may be referred to as a sensing diaphragm, bearing in mind that it may transduce either pressure or force.

Turning now toFIG. 2A,FIG. 2B, andFIG. 2C, a first pressure sensor assembly50is described. In an embodiment, the first pressure sensor assembly50comprises the pressure sensor die10, a cover52, a first seal54between the cover52and the outside edge14(or “the picture frame”) of the die10, and a second seal56between the cover52and a substrate58. The die10is electrically connected to the substrate58by a plurality of solder joints62using flip chip mounting technology. In an embodiment, the seals54,56may be provided by a glue or by an adhesive. In an embodiment, one of the seals54,56may be a compliant media (e.g., O-ring or seal gasket) and the other one of the seals54,56may be an adhesive or glue. In an embodiment, one of the seals54,56may be a compliant media (e.g., O-ring or seal gasket) and the other one of the seals54,56may be a snap or locking feature to hold the cover52in place on the substrate58. In an embodiment, both seals54,56may be a compliant media (i.e., O-ring or seal gasket).

The substrate58may be a ceramic substrate and/or a printed circuit board. It is noted that the substrate58is different from and not to be confused with a semiconductor substrate on which a MEMS component may be fabricated. The substrate58provides some mechanical stress isolation (e.g., rigidity to the die10and to the first pressure sensor assembly50generally. The substrate58comprises electrical wires, electrical conductors, and/or electrical traces (not shown) that connect to the die10via the solder joints62and that promote connecting the first pressure sensor assembly50to external circuitry (not shown), for example to a plug or jack terminating a wire leading to an embedded computer, for example a computer embedded in a medical instrument.

The flip chip mounting of the die10to the substrate58by the solder joints62electrically connects the die10to the substrate58. The solder joints62additionally mechanically attach or couple the die10to the substrate58. In traditional flip chip mounting of MEMS devices to substrates, an underfill comprising an electrically insulating adhesive is commonly flowed under the “top” side of the MEMS device, filling the space between the “top” side of the MEMS device and the substrate. This underfill may provide a stronger mechanical coupling of the MEMS device to the substrate and may promote better heat flow away from the MEMS device. The improved mechanical coupling provided by the underfill can reduce mechanical loads on the solder joints, thereby reducing the risk of one or more of the solder joints failing which would result in the function of the MEMS device failing or becoming unreliable and/or inconsistent.

In the pressure sensor assembly50, however, underfill can induce mechanical stress on the diaphragm12which in turn can offset the output of the first pressure sensor assembly50. While this offset output could theoretically be compensated for by external circuitry and/or processing, underfill suffers from the further disadvantage that the mechanical stress that it induces changes over time and hence even a compensated output would drift over time, rendering the pressure sensor output inaccurate. Consequently, in at least some embodiments of the pressure sensor assembly50, underfill is not employed in mounting the die10to the substrate58. It is understood, however, that in other embodiments, underfill may be employed in mounting the die10to the substrate58, for example using an underfill material that does not induce mechanical stress or that induces a mechanical stress that does not change over time or, for example in an application where even the time varying mechanical stress of underfill material can be compensated for during processing. The seals54,56and the case52provide some improved mechanical stabilization of the pressure sensor die10, reducing the mechanical stress on the solder joints62.

The cover52defines an aperture60or opening. When the cover52is sealed to the die10and to the substrate58by seals54,56, the aperture60is aligned with the diaphragm12so as to provide a path for pressure and/or force to be transmitted through the aperture60to the “bottom” side of the diaphragm12. In an embodiment, the cover52defines a shoulder66on an inner edge of the cover52, and the first seal54is disposed between the shoulder66and the “picture frame” (e.g., the outside edge14) of the die10. In some contexts, the shoulder66may be referred to as an inner shoulder. The cover52may be a molded plastic component, a stamped metal component, or a component formed of a different material or in a different way.

In an embodiment, the cover52defines at least one vent hole64. The vent hole64may be round, oval, square, rectangular, polygonal, or another shape. The vent hole64provides fluid communication between an exterior ambient pressure and an interior chamber of the first pressure sensor assembly50, where the interior chamber is defined as the space between the first seal54, the second seal56, the cover52, the substrate58, and the “top” side of the pressure sensor die10. Thus, the pressure sensor die10of the first pressure sensor assembly50provides an electrical indication of a pressure differential between ambient pressure (or any other reference pressure that may be provided via the vent hole64) and a fluid pressure applied to the “bottom” side of the diaphragm12, for example an air pressure, a gas pressure, a liquid pressure, or other fluid pressure. In another embodiment, one or more vent hole may be located in the substrate58(e.g., a drilled hole) and no vent hole64may be provided in the cover52. If the substrate58is a printed circuit board, a vent hole may be provided in the substrate58by routing out the vent hole in the substrate58. If the substrate58is ceramic, a vent hole may be provided in the substrate58by laser drilling. In some embodiments, however, it may be less expensive to provide the vent hole64in the cover than to provide a vent hole through the substrate58in a precise location that aligns with the small size of the die10. Alternatively, in an embodiment, the second seal56(e.g., a seal adhesive) may not be continuous and may provide communication over those discontinuous portions of the second seal56.

In addition to providing a reference pressure (e.g., ambient pressure) to the “top” side of the diaphragm12, the vent hole64promotes curing and/or drying of the seals54,56. Without providing the vent hole64, the first seal54and/or the second seal56may not cure and/or dry properly.

In an end application, the aperture defined by the cover52aligns with and is mechanically coupled to a tube, channel, or manifold containing a fluid whose pressure is desired to be sensed (e.g., an edge of an exterior wall of the cover52mechanically couples to the tube or channel). The tube, channel, or manifold may be sealed to the cover52in such a way that the pressure in the tube, channel, or manifold does not propagate to the vent hole64and/or does not propagate to the ambient. For example, the pressure sensor assembly50and the cover52may be secured to the tube, channel, or manifold with an O-ring providing a seal between cover52and the tube, channel, or manifold.

Turning now toFIG. 3, a second pressure sensor assembly100is described. The second pressure sensor assembly100may be referred to as a gel coupled pressure sensor in some contexts. In an embodiment, the second pressure sensor assembly100comprises the pressure sensor die10, a gel disk102, a cover104, a first seal111, a second seal110, and a substrate112. The cover104defines an aperture in its middle and an outer shoulder106having a flat surface that stops at an outer edge defined by a sharp drop-off of the surface of the cover104. The angle made by the flat surface of the outer shoulder106with the drop-off surface of the outer edge of the cover104may be between about 220° to about 270°. When the cover104is sealed to the die10by first seal111and sealed to the substrate112by second seal110, the aperture defined by the cover104is aligned with the diaphragm12so as to provide a path for pressure and/or force applied to the surface of the gel disk102to be coupled by the gel disk102through the aperture of the cover104to the “bottom” side of the diaphragm12.

In an embodiment, the seals110,111may be formed of glue or adhesives. In an embodiment, one of the seals110,111may be a compliant media (e.g., O-ring or seal gasket) and the other one of the seals110,111may be an adhesive or glue. In an embodiment, one of the seals110,111may be a compliant media (e.g., O-ring or seal gasket) and the other one of the seals110,111may be a snap or locking feature to hold the cover104in place on the substrate112. In an embodiment, both seals110,111may be a compliant media (i.e., O-ring or seal gasket).

The cover104further defines a sloping interior ran108that is generally funnel-like in shape, albeit possibly not conical in cross-section, because the die10may be square or rectangular rather than circular in planar shape. The cover104may be a molded plastic component, a stamped metal component, or a component formed of a different material and/or in a different way. The second pressure sensor assembly100further comprises a plurality of solder joints116that electrically connect the die10to the substrate112using flip chip mounting technology. In an embodiment, no underfill is disposed between the die10and the substrate112. In another embodiment, however, underfill is disposed between the die10and the substrate112. See the description above with reference toFIG. 2A,FIG. 2B, andFIG. 2Cfor further discussion of the use of underfill iii flip chip pressure sensor assemblies.

The second pressure sensor assembly100further comprises at least one vent hole114which may be located in the substrate112(e.g., a drilled or a routed through hole) or in a side wall of the cover104. The same remarks about the manufacturing costs of providing a vent hole in the cover104versus providing a vent hole in the substrate112apply here, butFIG. 3illustrates the vent hole114located in the substrate112to illustrate this alternative location of vent holes versus the location illustrated inFIG. 2A. The vent hole114provides fluid communication between an exterior ambient pressure and an interior chamber of the second pressure sensor assembly100, where the interior chamber is defined as the space between the gel disk102, the cover104, the seal110, the substrate112, and the “top” side of the pressure sensor die10. Thus, the pressure sensor die10in the second pressure sensor assembly100provides an electrical indication of a pressure differential between ambient pressure (or any other reference pressure that may be provided via the vent hole114) and a fluid pressure applied to the “bottom” side of the diaphragm12by the gel disk102, where the gel disk102couples a fluid pressure incident on its surface, for example an air pressure, a gas pressure, a liquid pressure, or other fluid pressure, to the “bottom” side of the diaphragm. In an embodiment, the second seal110may not be continuous and may provide fluid communication between the exterior ambient pressure and the interior chamber of the second pressure assembly100and no separate vent hole114may be provided.

When liquid gel is poured into the aperture defined by the cover104, the liquid gel flows onto the “bottom” surface of the die10, fills up onto the sloping interior wall108of the cover104, up to the edge of the outer shoulder106, up onto the flat surface of the outer shoulder106, and flows toward the sharp drop-off surface of the outer edge of the cover104. As the liquid gel flows out to the sharp drop-off surface, the liquid gel stops and beads-up, forming a generally domed or convex shape across the aperture defined by the cover104. Without wishing to be bound by theory, it is thought that fluid dynamic properties of the liquid gel, specifically with reference to surface tension effects and adhesion effects, account for this behavior: while the relative contact angle between the liquid gel and the surface does not change, the absolute contact angle with respect to the horizontal of the flat surface of outer shoulder106increases dramatically at the outside edge of the outer shoulder106, due to the drop-off of the surface of the cover104. In an embodiment, the liquid gel is a dielectric gel.

It has been found that narrowing the outer shoulder106to such an extent that there is no flat surface for the gel to flow out onto does not promote formation of the desired gel dome, even with a sharp drop-off surface outside of the excessively narrow outer shoulder: in this case the liquid gel spills over the outer edge without any significant doming. Additionally, it has been found that if the outer edge of the outer shoulder106does not drop off sharply, even with an ample flat surface in the outer shoulder106, the liquid gel spills over the outer edge without any significant doming. The combination, however, of the sufficiently wide flat outer shoulder106and a sufficiently sharp drop-off of the surface outside of the outer shoulder106does promote the forming of the dome.

In an embodiment, a sufficiently sharp drop-off may be achieved by keeping the radius of the transition from the flat surface of the outer shoulder106to the drop-off surface to less than about 0.1 mm. In other embodiments, the transition from the flat surface of the outer shoulder106to the drop-off surface may be maintained to less than a different radius, for example less than about 0.5 mm or less than about 0.05 mm. It will be appreciated that maintaining a radius in a manufactured part such as the cover104to less than 0.1 mm (or less than 0.5 mm or less than about 0.05 mm) poses some challenges. For example, a specialized tool for producing the cover104may be designed to employ replaceable inserts that are removed and replaced relatively frequently during manufacturing of the cover104as the inserts wear down. Replacing the inserts may be desirable to maintain the radius of the transition from the flat surface of the outer shoulder106to the drop-off surface below the desired maximum radius. In an embodiment, a sufficiently wide flat outer shoulder106is at least 0.5 mm wide, at least 0.1 mm wide, or at least 0.05 mm wide.

After filling the liquid gel into the cover104to form gel disk102, the gel is cured. In an embodiment, a vacuum is applied to the second pressure sensor assembly100during the gel curing phase of manufacture and heat is applied to the gel. After curing, the gel disk102retains the domed shape. In an embodiment, the gel disk102comprises a soft dielectric gel.

In an embodiment, after curing, the dome of the gel disk102may project about 100 μm (about 4/1000thinch) above the flat surface of the outer shoulder106. While this may seem an insignificant feature, it has been found that this small dome promotes advantages of more reliable and more consistent linear pressure sensing response in gel coupled flip chip pressure sensor assemblies. Some techniques of forming the gel disk102in larger pressure sensor assemblies are thought to not work well on the smaller physical scale of flip chip pressure sensor assemblies. For example, pouring a gel disk that is larger than the outer edge of the cover104and then cutting off and removing the excess gel would be expected to produce a gel disk that introduces inaccuracies related to gel compression aberration due to inconsistent gel skin performance (the cut gel skin doesn't perform like uncut gel skin).

When force is applied to the gel disk102, for example by a finger press or by a mechanical plunger pressed by a finger or other mechanical force, the force is transmitted through the gel disk102to the diaphragm12. The domed shape or convex shape of the gel disk102contributes to linearity of response and/or pressure indication output of the second pressure sensor assembly100. Additionally, the sloping interior wall108of the cover104also promotes linearity by minimizing any distortion effects that may occur if pressure is applied locally closer to the outside edge of the gel disk102.

Turning now toFIG. 4A,FIG. 4B, andFIG. 4C, a plurality of alternative covers are described. In one or more embodiments, the covers described with reference toFIG. 4A,FIG. 4B, andFIG. 4Cmay be used in manufacturing the first pressure sensor assembly50described with reference toFIG. 2A,FIG. 2B, andFIG. 2Cor in manufacturing the second pressure sensor assembly100described with reference toFIG. 3, with the modification that in the first pressure sensor assembly50the second seal56would be omitted, and in the second pressure sensor assembly100the seal110would be omitted. Each of the covers150,170, and190share a common property that the lower end of the covers (the end that would be in contact with the substrate58,112) is collapsible or deformable by design. As used herein above, the term “collapse” does not mean total structural failure of a cover but a controlled, progressive, and/or modulated deformation of the cover. While not illustrated inFIG. 4A,FIG. 4B, orFIG. 4C, it is understood that each of the covers150,170,190defines an aperture that aligns with the pressure sensing diaphragm12of the pressure sensor die10so as to provide a path for pressure and/or force to be transmitted through the aperture to the “bottom” side of the diaphragm12(via the gel disk102in the case of the second pressure sensor assembly100) when used in manufacturing the pressure sensor assembly50,100.

As the size of the pressure sensor die10is reduced, the size of the cover employed to make the pressure sensor assembly50,100is likewise reduced in size. The manufacturing processes for fabricating the cover52,104and of making the seals54,56,110,111may not be as precise as the MEMS manufacturing processes used for fabricating the die10. Manufacturing process variations in making the cover52,104may create challenges in establishing the seals54,56,110,111. Using extra adhesive or glue to make up seals54,56,110,111—in order to compensate for manufacturing variation in cover dimensions—may produce undesirable inconsistencies in the performance of the pressure sensor assemblies50,100. By using covers with partially-collapsing or controllably-collapsing walls, the cover may be pressed down to make a seal with the die10, and dimensional variation in the walls may be accommodated by the collapse of the lower portion of the wall—more collapse for a taller cover, less collapse for a shorter cover. The partially- or controlled-collapsed wall would still provide some mechanical support for the cover, carrying some of the load of the cover that might otherwise be borne by the die10alone, thereby stressing the solder joints between the die10and the substrate58,112.

The cover150illustrated inFIG. 4Ahas walls154that are thinner at a lower end152that would be in contact with the substrate58when the cover150is used in fabricating the first pressure sensor assembly50and in contact with the substrate112when used in fabricating the second pressure sensor assembly100. In an embodiment, the lower end152is less than 80% as thick as the upper end of the wall154before it meets the top of the cover150(e.g., the portion forming the shoulder of the cover). In an embodiment, the lower end152is less than 50% as thick as the upper end of the wall154. In an embodiment, the lower end152is less than 30% as thick as the upper end of the wall154. In an embodiment, the lower end152is less than 15% as thick as the upper end of the wall154. The wall thickness may progressively thin from the upper end of the wall154to the lower end152. The thinness of the lower end152of the wall154may be modulated during fabrication to make the collapse of the lower portion of the wall154during fabrication a controlled, progressive, and/or modulated collapse. This may also be referred to as a partial collapse or a graceful collapse.

This collapse may take the form of the lower portion of the wall154of the cover150deforming by splaying outwards away from the die10. In an embodiment, the wall154of the cover150or a lower portion of the wall154of the cover150may angle outwards, away from the die10, to promote the splaying deformation. The collapse may take the form of the lower portion of the wall154of the cover150deforming in an accordion-like or bellows-like manner. In an embodiment, the wall154of the cover150or a lower portion of the wall154of the cover150may be pre-bent or pre-stressed to promote such an accordion-like collapse mode. While no vent hole in the cover150is illustrated inFIG. 4A, in an embodiment, the cover150defines a vent hole in the wall154.

The cover170illustrated inFIG. 4Bhas feet172at a lower end of a wall of the cover170. In some contexts, the feet172may be referred to as narrow fingers. The narrow part of the feet172is a concentrated point of structural weakness of the wall of the cover170and will be the place where controlled, progressive, and/or modulated deformation of the cover170may occur. The collapsibility of the cover170may be controlled by making the feet172more or less thin, making the number of feet172more or less, and/or making the cut-out area between the feet172deeper or less deep. The lower part of the outside wall of the cover170may slope outwards, away from the die10. In an embodiment, the feet172may be pre-stressed or pre-bent to promote an accordion-like or bellows-like collapse mode. In an embodiment, the cover170may define a vent hole or may have a vent hole formed in a wall of the cover170. Alternatively, the space between the feet172provides communication with ambient and may obviate the formation of another vent hole.

The cover190illustrated inFIG. 4Chas collapsible ribs or spring-like structures192. The collapsible ribs may be formed in manufacturing by cutting a hole leaving the thin collapsible rib192in place. During manufacturing, the cover190may be pressed down onto the substrate58,112, causing the collapsible rib192to collapse partially. In an embodiment, the cover190may define a vent hole or may have a vent hole formed in a wall of the cover190. Alternatively, the space between the ribs192and the solid wall of the cover190may provide communication with the ambient and may obviate the formation of another vent hole.