Sensor package assembly having an unconstrained sense die

A pressure detection mechanism having a pressure sense die which may be attached directly to a surface of an alumina-based substrate with an adhesive having an optimum thickness. The adhesive may be stress compliant and may be one or more of silicone, silicone-epoxy, epoxy or any other suitable adhesive material. A compensation and interface application specific integrated circuit may be attached to the surface of the package substrate. The pressure sense die may be electrically connected to the integrated circuit with bond wires. The integrated circuit may be electrically connected to trace conductors on the package substrate with bond wires, and trace conductors may be connected to stress compliant metal conductors or leads for external connection to a mounting surface such as a printed circuit board. Hard plastic, or like material, symmetric covers, with one or more pressure ports or vents, may be attached to both sides of the substrate.

BACKGROUND

The application pertains to sensors, and more particularly to sensor packaging.

SUMMARY

The application pertains to sensors, and more particularly to sensor packaging. In some instances, a sensor package is provided that has a package substrate, with a pressure sense die directly mounted to a surface of the package substrate, sometimes with a stress compliant adhesive to result in an effectively unconstrained sense die. In some cases, the package substrate may include one or more traces and/or bond pads that are electrically connected to one or more package leads. The package leads may be suitable for surface mounting the sensor package to a mounting surface such as a printed circuit board or the like. The package leads may be configured to help provide a level of compliance between the substrate and the mounting surface.

DESCRIPTION

There is a long-standing market need for low-cost high-accuracy pressure sensors. Historically, high accuracy sensors have been complex, expensive-to-produce devices.

In particular, for piezo-resistive silicon pressure sense die based sensors, manufacturers have used many different approaches to isolate the sense element from mechanical and thermal stresses. Large packages, constant-temperature chambers, and most particularly isolation layers between the sense die element and the base substrate or package of the larger sensor have been used. These isolation layers are typically one or more layers of silicon or glass bonded to the underside of the silicon sense die element itself. The bond to the sense die element is usually glass frit or an anodic bond. Specialty glasses are often used to closely match the thermal expansion of the silicon sense die.

It has been found that, in some instances, a sense die may be secured directly to a package substrate, without any intervening isolation layer, while still performing acceptably under normal stress conditions. This may provide a smaller package design for the required components, at a reduced cost. Conventional wisdom would dictate that such a configuration would provide a “low accuracy” sensor. In fact, by careful attention to the details of the entire package, it has been found that directly securing the sense die to the package substrate may result in a stable, high accuracy sensor which may be produced at a lower cost.

It has been found that, in some instances, a thicker alumina based ceramic package substrate may be used, and a pressure sense die may be directly attached or glued to the substrate using an RTV, silicone, epoxy, or other suitable adhesive. In some instances, no intervening isolation layers or substrates are provided between the sense die and the package substrate. Thermal and mechanical stresses may be minimized by careful design of the entire package. An ASIC circuit used for compensation may, in some cases, be secured to the package substrate directly beside the pressure sense die, and direct die-to-die wire bonds may be used to minimize package size and the associated mechanical stresses from a larger package. The ceramic substrate itself may be thick relative to its surface area to improve stability. Covers made from plastic, polyamide, ceramic, or another suitable material, may be attached to the substrate on both sides. These covers may be of virtually identical or similar size and shape, not considering ports and vents, and attach to the substrate with the same “footprint” on each side. Electrical connections to the package may be done with compliant metal leads to minimize mounting stress between the package and a mounting surface, such as a printed circuit board.

In some cases, the sensor may have a piezoresistive silicon pressure sense die which is calibrated and temperature compensated for sensor offset, sensitivity, temperature effects and non-linearity using an on-board application specific integrated circuit (ASIC). The sensor may be configured to measure absolute, differential and/or gauge pressures, as desired. An absolute version may have an internal vacuum (or other pressure) reference, and provide an output value proportional to absolute pressure. A differential version may permit application of pressure to both sides of the sensing diaphragm of the sense die. Gauge and compound versions may be referenced to atmospheric pressure and provide outputs proportional to pressure variations relative to the atmosphere.

The sensor package may, in some cases, have width and length dimensions of about 10 mm, and 10 mm or 12.5 mm, respectively. The materials may include high temperature hard plastic, ceramic, polyamide or other suitable material for the covers of the package, alumina ceramic or other suitable material for the package substrate, and silicone, soft or hard epoxy, silicone epoxy, RTV or other suitable material for the adhesive. In some cases, the package substrate may be 96 percent alumina, 99 percent alumina, or any other suitable percent alumina. It is contemplated that the package substrate may be or include other suitable materials, as desired. The electronic components may be composed of ceramic, silicon, glass, gold, solder and other appropriate materials, as desired.

In some cases, the sensor assembly may have various port configurations.FIG. 1ashows a sensor10with covers11and12without a pressure port but may have a pressure vent13on cover11. The sensor may or may not have a pressure vent13on cover12.FIG. 1bshows sensor10with an axial pressure port14on cover11.FIG. 1cshows sensor10with a radial pressure port15on cover11.FIG. 1dshows sensor with an axial pressure port14on cover11and an axial pressure port14on cover12.FIG. 1eshows sensor10with a radial pressure port15on cover11and a radial pressure port15on cover12. These radial ports15are on the same side.FIG. 1fshows sensor10with a radial pressure port15on cover11and a radial pressure port15on cover12but these radial ports may be on opposite sides. A combination of various ports may be incorporated in sensor10. Sensor10ofFIGS. 1a,1band1emay be situated in a dual inline package (DIP). Sensor10ofFIGS. 1c,1dand1fmay be situated in a single inline package (SIP). Either the DIP or SIP may be used for the port configurations inFIGS. 1athrough1f, or any other port configurations. Sensor10may be situated in another type of package.

FIG. 2shows a side cut-away view of an illustrative sensor10revealing a pressure sense die21, a package substrate22and an ASIC23.FIG. 3is a diagram showing a perspective view of an illustrative sensor package substrate, pressure sense die, application specific integrated circuit, bond wires, trace conductors and leads. Pressure sense die21may be attached to a side or surface24of package substrate22with an adhesive25such as a silicone, RTV, a silicone-epoxy, a soft epoxy, or a regular or hard epoxy, at an optimum thickness resulting in an effectively stress unconstrained die21relative to substrate22. The latter two adhesives may be more applicable to a high pressure sensor10(e.g., over 200 psi or 14 bar). As to an optimum thickness, the thickness of adhesive25may be thick enough for adequate adherence of die21to substrate22, but not so thick as to interfere with the bonding or diaphragm of die21.

In the illustrative example, there is no isolation layer or substrate such as a glass substrate between the pressure sense die21and surface24of package substrate22. The adhesive25may be relatively thin compared to the sense die21and substrate22. The temperature expansion coefficients of sense die21and substrate22may be about the same because of the material of the sense die21being silicon and the material of package substrate22being alumina ceramic. No special effort is necessarily made to select materials for sense die21and substrate22having temperature expansion coefficients very close to each other. The sense die21and substrate22may be of materials other than those stated herein.

It may be noted that package substrate22may be thicker than typical or conventional package substrates. Package substrate22may have, for example, a thickness of one mm and a surface area of 10 mm×10 mm. This would result in an area in square units to thickness in units for a ratio of 100 or a thickness in units to area in square units for a ratio of 0.010. An area-to-thickness ratio for the substrate may be regarded to be equal to or less than 100 square units per linear unit.

The sense die21may have piezoresistive components formed on its outer surface for detection of deflection of the sense die diaphragm to indicate pressure differentiation across the diaphragm of die21. The piezoresistive components may be connected with other detection circuitry in a Wheatstone bridge fashion.

The surface of sense die21facing substrate22may be sealed by surface24of the package substrate, or it may face a hole26through the package substrate22as shown. The hole may deliver a pressure to sense through the package substrate22and to the diaphragm of the sense die21, when desired.

In some instances, an ASIC23may be attached to package substrate22, sometimes with an adhesive27. In some cases, adhesive27may have the same contents as adhesive25, but this is not required. ASIC23may be an electrical interface and compensation circuit between sense die21and connectors or terminals32for connections outside of package substrate22. Sense die21may be bond wire28connected to ASIC23. ASIC23may be bond-wired29connected to trace conductors31on the package substrate surface. Trace conductors31may be connected to the connectors, leads or terminals32. The bond wires28and29and trace conductors31may reduce transfer of thermal and mechanical stresses among sense die21, ASIC23and package substrate22. The outside connectors, leads or terminals32of package substrate22may be malleable so as to absorb thermal and mechanical stress between substrate22and, for example, a printed board (not shown) which sensor10might be mounted to.

FIG. 4is a diagram showing a side cut-away view of the sensor substrate22having a pressure sense die21, application specific integrated circuit23and a cover11with a pressure port14on the same side of the substrate where the sense die21and integrated circuit23are situated.

FIG. 5is a diagram like that ofFIG. 4except for showing the sense die21, integrated circuit23and a cover12having a vent13, on a side of the substrate22opposite of the cover11with port14.

In the present specification, some of the matter may be of a hypothetical or prophetic in nature although stated in another manner or tense.

Although the disclosed mechanism or approach has been described with respect to at least one illustrative example, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.