Patent Application
20080222884
Embodiments are generally related to pressure sensors and transducers. Embodiments are also related to Chip-on-Board components and configurations. Embodiments are additionally related to chip-on-board pressure sensor packages.
Many processes and devices have been used in the field of pressure sensing. Pressure sensors are generally used and deployed wherever a need for monitoring and responding to pressure changes is necessary. Pressure sensors are commonly used in a variety of automotive, aerospace, commercial, industrial, and consumer applications.
The operational environments in which pressure sensors are required to operate in these applications with high accuracy and repeatability can be very demanding. For example, extreme thermal conditions including thermal shocks in ranges from 160 C to −55 C, exposure to harsh and/or conductive media, withstand high overpressure (proof pressure) cycling without change in calibration and survive high peak (burst) pressures to protect system from potentially catastrophic leaks.
In the case of a pressure sensor that relies upon the use of a pressure transducer (or sense element) consisting of piezoresistive silicon on an etched silicon diaphragm a most cost effective solution for operating in such environments is to use so called “back-side” sensing. With this arrangement the only parts of sensor which are exposed to the media are the electrically isolated cavity side of the pressure transducer, the adhesive used to bond the transducer die to a substrate and finally the substrate itself.
For such a design to operate reliably under the conditions described above typically requires the use of attach materials which have high strength and chemical resistance. The rigid mounting of stress sensitive die (e.g., pressure transducer or pressure sense element) onto a substrate where there is large mismatch in thermal expansion coefficient between the die and the substrate can introduce high levels of package stress, which can result in output errors, non-repeatability and potentially, mechanical damage.
Pressure sensor apparatus 100 depicted in
The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is, therefore, one aspect of the present invention to provide for an improved pressure sensor.
It is another aspect of the present invention to provide for a pressure sensor configuration that includes a mounting of a stress-sensitive die onto a substrate.
It is a further aspect of the present invention to provide for an improved package for a chip-on-board pressure sensor.
The aforementioned aspects and other objectives and advantages can now be achieved as described herein. A packaging method and apparatus for a chip-on-board pressure sensor is disclosed. A chip-on-board substrate can be provided and a pressure transducer connected to the chip-on-board substrate. A buffer layer is generally located between the pressure transducer and the chip-on-board substrate in order to form a pressure sensor in which thermally induced package stresses are prevented or reduced from being transferred to the pressure transducer.
In general, such a buffer layer requires a coefficient of thermal expansion (CTE) intermediate between the pressure transducer and the chip-on-board substrate. Additionally a high stiffness (i.e., elastic modulus) of the buffer layer prevents or reduces flexural (e.g., bending) stresses from being transferred to the pressure transducer. In a preferred embodiment, buffer layer such a ceramic can be adhesively bonded to pressure transducer which in turn can be adhesively bonded to the main chip-on-board substrate. Importantly the proposed buffer layer also allows greater flexibility in adhesive selection and/or bonding methods when compared with direct chip-on-board mounting methods and devices, wherein several technical trade-offs have to be made to reliably attach the pressure transducer.
The chip-on-board substrate typically carries the electrical circuitry and/or other die components and/or mechanical features for protection of components. In alternative embodiments, additional components and/or circuitry and/or mechanical features may be incorporated directly into the buffer layer.
The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein.
The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof.
Referring now to the drawings and in particular to
The ceramic buffer layer 202 can be bonded to the pressure transducer 102 by an adhesive 204 which in turn is adhesively bonded to a main chip-on-board substrate 106 (such as the substrate disclosed in the configuration of
Advantageously, the CTE of the ceramic layer 202 such as Alumina 96% Al203 is typically 6 ppm/K at 25° C. which is intermediate between that of the transducer and the substrate. Therefore, the level of differential thermal expansion between the die and its point of attachment is greatly reduced by the presence of the buffer layer. Furthermore the elastic modulus of ceramic is typically 300 GPa, which can be considered to provide sufficient stiffness to prevent flexure stresses from the chip-on-board substrate and external packaging from transferring to the pressure transducer.
One possible fabrication process for implementing the method 400 depicted in
Step 1: Create wafer of pressure transducers
Step 2: Optional: Bond Silicon wafer to glass constraint wafer
Step 3: Singulate pressure transducer die from wafer
Step 4: Bond die to buffer layer
Step 5: Optional: Singulate buffer layer assemblies from array/panel
Step 5: Bond buffer layer to chip-on-board substrate
Step 6: Wire bond pressure transducer to chip-on board substrate
Step 7: Complete assembly of chip-on-board substrate (additional components, protective cover etc).
Step 8: Assembly of the chip-on-board platform into external packaging for application.
Note that in an alternative embodiment as described with respect to Step 2 (i.e., also see block 402) above, a pressure transducer can be provided without the additional anodically bonded glass layer. Also as indicated by Step 5 (i.e., also see block 405) above, the manufacturing and handling of multiple buffer layer structures in an array (i.e., batch) form can be achieved. In another alternative embodiment, the use of material other than ceramic such as metal or plastic, but having a CTE intermediate between that of the transducer and the chip-on-board substrate and a higher elastic modulus than the substrate can be used. Additionally, Step 6, although not specifically shown in
In yet another embodiment a buffer layer can be attached to the substrate using a eutectic metal solder. Additionally, this could be used to provide a pressure seal to the substrate or alternatively an electrical connection to the substrate with the addition of an insulating under-fill adhesive to complete the seal.
In another embodiment to enable additional electrical connections to the pressure transducer and/or chip-on-board substrate, electrical circuit traces, vias, and/or bond pads can be provided on the buffer layer. As further alternative embodiment involves implementing the buffer layer in a multilayer structure with a process such as high temperature co-fired ceramic (HTCC) or low temperature co-fired ceramic (LTCC). Such features can also be used to enable additional mechanical features such as cavities for protection of the pressure transducer and/or buried channels for fluid path to back-side, and/or circuit traces to simplify design and assembly
Based on the foregoing, it can be appreciated that the pressure sensor method and apparatus described herein therefore overcomes the aforementioned problems associated with the prior art by creating a compact packaging for a pressure sensor that includes chip-on-board assembly capabilities, which enables efficient and robust fabrication for various aerospace, automotive, industrial, consumer, commercial and other applications.
It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
