Patent Description:
Sensors, for example pressure sensors or inertial sensors (such as accelerometers and gyroscopes) are used in many applications, including inertial navigation, robotics, avionics, and automobiles. In inertial navigation applications, such sensors may be found in self-contained systems known as "inertial measurement units" (IMUs). IMUs typically contain a plurality of accelerometers and/or gyroscopes, and provide an estimate of an object's travel parameters such as angular rate, acceleration, altitude, position, attitude and velocity, based on the outputs of gyroscope(s) and/or accelerometer(s). Each inertial sensor in an IMU is a self-contained package. <CIT> describes a microphone package comprising a MEMS microphone mounted on a substrate and sealed within a cavity using a thermoplastic lid, connected by conductive trace to an electrical component mounted on the thermoplastic lid. <CIT> describes a multi-layered packaged semiconductor device having fluidically sealable and fluidically exposed compartments. <CIT> describes a resin-sealed semiconductor device comprising a semiconductor chip mounted on a chip mounting section of a lead frame. <CIT> describes a dynamic sensor such as an acceleration sensor comprising a sensor chip and a circuit chip mounted within a common package, in which a mounting surface and an adhesive film are used to reduce thermal stress transfer to the circuit chip.

Microelectromechanical systems (MEMS)-based sensors can be used e.g. to measure pressure or temperature, or linear or angular motion without a fixed point of reference. MEMS pressure sensors often work on the principle of mechanical deformation of a MEMS structure due to fluid pressure. MEMS gyroscopes, or strictly speaking MEMS angular rate sensors, can measure angular rate by observing the response of a vibrating MEMS structure to Coriolis force. MEMS accelerometers can measure linear acceleration by observing the response of a proof mass suspended on a spring in a MEMS structure. High performance MEMS inertial sensors are defined by their bias and scale factor stability.

Due to the applications of such sensors, they may on occasion be subject to extremely high forces e.g. accelerations on the order of ><NUM>,<NUM>. Under such conditions, components within the sensor package may begin to fail. Typically, the first failure within the sensor package occurs due to the wire bonds connecting the package to the sensor. The wire bonds may short circuit one another, or experience a joint break at the place where the wire bond is joined to the pad of the sensor or package. This is obviously undesirable as it decreases the functional range of the sensor, and furthermore requires replacement of the unit in either breakage scenario described above.

The present disclosure seeks to address at least some of these shortcomings.

According to a first aspect of this disclosure, there is provided a sensor package according to claim <NUM>.

According to a second aspect of this disclosure, there is provided a method of making a sensor package according to claim <NUM>.

The present disclosure relates to a sensor package and a method of making a sensor package. It will be appreciated that using an interposer, wire bonded to both the sensor and the at least one external wall, is an improved approach to electrically connecting a sensor and a sensor package as compared to the prior art. The interposer allows for short wire bonds from the sensor and the at least one external wall to the interposer, replacing the single, long wire bond from the sensor to the at least one external wall in the prior art. This provides improved resilience of the sensor package under high stress. Furthermore, it allows an existing sensor and package combination to be improved without needing to redesign either component.

Some non-limiting examples of a sensor package and a method of making a sensor package are described in further detail below.

<FIG> shows a cross sectional schematic of a prior art sensor package assembly <NUM>. The sensor package comprises a sensor <NUM>, package <NUM>, wire bonds <NUM>, conductive tracking <NUM>, and a lid <NUM>.

In order to protect a sensor <NUM> from external influences that may damage or destroy it when in use, the sensor <NUM> is placed in a protective package <NUM> that surrounds the sensor <NUM>. The sensor <NUM>, after being placed in the package <NUM>, is wire bonded to the package <NUM>, in order for electrical connections to be made from the sensor <NUM> to the external world. The protective package <NUM> comprises conductive tracking <NUM> which allows an electrical connection to be made from outside the package <NUM> to the sensor <NUM>. In order to fully seal the package <NUM> from external influences, a lid <NUM> is bonded to the package <NUM>. This sealing of the internal space in the package <NUM> prevents any external influence directly affecting the sensor <NUM>.

However, extremely stressful events, for example very high accelerations, may cause failures or malfunctions within the sensor package assembly <NUM>. Typically, the first failure within the sensor package assembly <NUM> due to such high stress events is a failure of one or more wire bonds <NUM>. The failure of a wire bond <NUM> may occur as a short circuit of one wire bond <NUM> to another, or the entire wire bond <NUM> may collapse. A collapse may occur as a joint failure, i.e. where the wire bond <NUM> is joined to the sensor <NUM> or the metal tracking <NUM>.

<FIG> shows a cross sectional schematic of a sensor package <NUM> in accordance with an example of the present disclosure. The sensor package <NUM> comprises a sensor <NUM>, a base <NUM>, an interposer <NUM>, wire bonds <NUM>, conductive tracking <NUM>, <NUM>, a lid <NUM>, sealed to the external walls <NUM> and optional adhesive <NUM> between the interposer <NUM> and the external walls <NUM>.

In this example, an interposer <NUM> is inserted between the sensor <NUM> and the external walls <NUM>, resting on the package base <NUM>. Conductive tracking <NUM> is deposited on the interposer <NUM> before it is added to the sensor package <NUM> to allow an electrical connection to be made from one side of the interposer <NUM> to the other. The sensor <NUM> can then be wire bonded <NUM> to the conductive tracking <NUM> of the interposer <NUM>, and the conductive tracking <NUM> of the interposer <NUM> can be wire bonded <NUM> to the conductive tracking <NUM> on the external walls <NUM>. This creates an electrical connection from the sensor <NUM> to the outside of the package <NUM>, via the interposer <NUM>.

The present inventors have discovered that the failure rate of a wire bond <NUM> is correlated to the weight of the wire bond <NUM>, and therefore its length. Whilst it is envisaged that other changes such as reducing wire bond <NUM> diameter or changing the wire bond <NUM> material may also affect the wire bond <NUM> weight, the present disclosure is focused on wire bond <NUM> length, as this approach also provides other effects explained in more detail below.

The insertion and fit of the interposer <NUM> in this example allows for extremely short wire bonds <NUM> within the package <NUM>, whilst maintaining the electrical connection between the sensor <NUM> and the conductive tracking <NUM> on the package <NUM>. These short wire bonds <NUM> are significantly more resilient to high stresses than the long wire bonds <NUM> of <FIG>. This allows the sensor package <NUM> to be more resilient to shocks and/or damage, and maintain an increased operating range.

Furthermore, the addition of an interposer <NUM> allows use of a pre-existing package <NUM> design, together with the pre-existing package <NUM> manufacturing method. Thus, the interposer <NUM> can be retrofitted into such packages <NUM>, removing the need to redesign the entire package <NUM> to achieve the same reduced wire bond <NUM> length. In this example the interposer <NUM> is adhesively bonded to the package <NUM> by an adhesive layer <NUM>. This may increase the resilience of the sensor package <NUM> by ensuring that the interposer <NUM> stays in place under high stress.

<FIG> show perspective views of an interposer <NUM>, and a sensor package <NUM>, respectively, in accordance with examples of the present disclosure.

<FIG> shows an interposer <NUM> in accordance with an example of the present disclosure. The interposer <NUM> is patterned with conductive tracking <NUM>, which allow for connections, for example wire bonds, to be made to it. As shown, the interposer <NUM> comprises cut-outs <NUM> which allow adhesive to be dispensed without the adhesive coming into contact with the conductive tracking <NUM>. This prevents any potential contamination of the conductive tracking <NUM>, which could further prevent any potential wire bonding process from taking place. The adhesive also bonds the interposer <NUM> to one or more externals walls of a package, increasing the resilience of a sensor package , as mentioned above.

<FIG> shows a sensor package <NUM> in accordance with an example of the present disclosure. The sensor package <NUM> comprises the interposer <NUM> of <FIG> fitted inside its external walls <NUM> and a sensor <NUM> fitted inside or surrounded by the interposer <NUM>.

The package <NUM> is of a pre-existing, mass produced design, and so the interposer <NUM> is designed to fit specifically within the package <NUM>. As mentioned above, this avoids a redesign of the package <NUM>.

As shown, the interposer <NUM> is inserted into the package <NUM>, surrounding the sensor <NUM>, occupying the majority of the space between the sensor <NUM> and the package <NUM>. It can be seen how the interposer <NUM> is shaped to fit the geometry defined by the external walls <NUM> of the package <NUM> and sits in contact with the external walls <NUM> at least in the corners of the square geometry in this example.

Adhesive may be dispensed into the cut-outs <NUM> as described above to secure the interposer <NUM> in place within the package <NUM>. The cut-outs <NUM> prevent the interposer <NUM> from moving up past the cured adhesive, which increases the resilience of the sensor package <NUM>.

As shown, the sensor <NUM> is wire bonded <NUM> to the conductive tracking <NUM> on the interposer <NUM>. The conductive tracking <NUM> on the interposer <NUM> is further wire bonded <NUM> to the conductive tracking <NUM> carried by the external walls <NUM> of the package <NUM>. As described above, this enables an electrical connection to be made between the sensor <NUM> and the package <NUM>. As previously mentioned, the decreased length of the wire bonds <NUM>, <NUM> increases the resilience of the sensor package <NUM> under high stress.

Thus it will be seen that, in accordance with the present disclosure, a sensor package is provided that may facilitate wire bonds of a decreased length. The present inventors have realised that a decreased wire bond length corresponds to a reduced chance of breakage under high stresses. As the interposer is disposed in the space between the external wall and the sensor, the wire bonds typically joining the sensor directly to the external wall can be shortened into two wire bonds between the sensor and the interposer, and the interposer and the external wall. These shortened wire bonds may be less susceptible to high g-force impacts or shocks, increasing the resilience and operating range of the sensor package. The present inventors have realised that there is a correlation between the weight of a wire bond, and its susceptibility to breaking under high stress. Thus, by reducing the length of a wire bond, its weight is also decreased, and therefore its chance of breakage under high stress. Whilst it is envisaged that other changes such as reducing wire bond diameter or changing the wire bond material may also affect the wire bond weight, the present disclosure is focused on wire bond length.

Use of the present disclosure may further enable the use of a pre-existing package design, together with a pre-existing package manufacturing method. Thus, the interposer can be retrofitted into such pre-existing packages, without any need to redesign an entire package. This saves both the costs and time of such a redesign.

Use of the present disclosure may also allow for existing sensors to be used in emerging markets that require extreme robustness, where such devices may not previously have been suitable.

The interposer comprises conductive tracking, the sensor is wired bonded to the conductive tracking, and the conductive tracking is wire bonded to the at least one external wall. This may allow the sensor to be electrically connected to the at least one external wall, via the conductive tracking of the interposer. This may allow signals from the sensor to be accessed from outside the package. This may also allow for further reduction in the length of the wire bonds, as the conductive tracking may span the width of the interposer. The wire bonds may then be joined to the interposer at the edges. Furthermore, the tracking may be designed to optimise the distance between where a connection is made from the sensor and where a connection is made from the interposer. The tracking may also be designed to optimise the distance between where a connection is made from the package and where a connection is made from the interposer.

In some examples, the first or second material is ceramic, for example alumina. In some examples, the second material is plastic. In some examples, the second material is FR-<NUM>.

According to one or more examples of the present disclosure, in addition or alternatively, the interposer is formed by injection moulding. This may allow for existing manufacturing techniques to be utilised, for example Moulded Interconnect Device technology, streamlining manufacture and reducing costs. In some examples, the interposer is a Moulded Interconnect Device (MID).

According to one or more examples of the present disclosure, in addition or alternatively, the interposer is in contact with the at least one external wall. This means that the interposer may be in physical contact with the at least one external wall as well as being wire bonded to the at least one external wall for electrical contact. This physical contact of the interposer and the at least one external wall may increase the robustness of the package, by reducing the chance of the interposer moving or becoming misaligned within the sensor package during use. For example, the interposer may be fitted inside the package in contact with the at least one external wall. Preferably, the interposer is shaped to fit the geometry defined by the at least one external wall. In some examples, the sensor package comprises at least four external walls forming a square, rectangular or polygonal geometry. In such examples, the interposer may be shaped to have a corresponding square, rectangular or polygonal geometry such that the interposer fits inside the package at least partially in contact with the at least four external walls. In such examples, the interposer is preferably in contact with the external walls at least in the corners of the square, rectangular or polygonal geometry.

According to one or more examples of the present disclosure, in addition or alternatively, the interposer is adhesively bonded to the at least one external wall. This adhesion may further improve the stability and resilience of the sensor package under stress.

According to one or more examples of the present disclosure, in addition or alternatively, the interposer is shaped to accommodate the adhesive used to adhesively bonded the interposer to the at least one external wall. In some examples, the interposer may comprise cut-outs which are shaped to accommodate the adhesive. In some examples, the adhesive accommodated by the cut-outs may increase the resilience of the sensor package under stress. In those examples wherein the interposer is fitted inside the package in contact with the at least one external wall, for example fitted in contact with the corners defined by the external walls, the cut-outs may be arranged between areas of contact, for example between the corners. In such examples the sensor package can benefit from a robust fit with the interposer and the bonding provided by the adhesive.

According to one or more examples of the present disclosure, in addition or alternatively, at least two of the interposer, the sensor, and the at least one external wall are in the same plane. This may allow for a further reduced wire bond length between the sensor and the interposer. This may also allow for a further reduced wire bond length between the interposer and the at least one external wall. In some examples, the interposer, the external wall and the sensor will all be in the same plane.

According to one or more examples of the present disclosure, in addition or alternatively, the interposer surrounds the sensor. This may allow for decreased length wire bonds to be made between the interposer and the sensor. In some examples, the at least one external wall surrounds the interposer. This may allow for decreased length wire bonds to be made between the at least one external wall and the interposer.

According to one or more examples of the present disclosure, in addition or alternatively, the interposer fills at least <NUM>% of the space between the sensor and the at least one external wall. This may allow a decreased length wire bond to connect the interposer and the sensor, and/or the interposer and the at least one external wall. In some examples the interposer fills at least <NUM>% of the space between the sensor and the at least one external wall.

According to one or more examples of the present disclosure, in addition or alternatively, the sensor is a MEMS sensor. In some examples, the MEMS sensor is silicon-based. In some examples, the MEMS sensor is an inertial sensor. In some examples, the MEMS sensor is an accelerometer, a gyroscope, or a pressure sensor.

According to one or more examples of the present disclosure, the sensor package further comprises potting or a lid to seal the package. In this way, the interposer does not seal the package. The sealing of the sensor package by potting or a lid may protect the sensor from any direct external influences that may damage or destroy the sensor. Sealing of the sensor package further allows control of the internal environment of the package, which may be optimised depending on the sensor.

In one or more examples the method mentioned above further comprises: arranging at least two of the interposer, the sensor, and the at least one external wall in the same plane.

In one or more examples the method mentioned above further comprises: adhesively bonding the interposer to the at least one external wall.

It will be appreciated by those skilled in the art that the present disclosure has been illustrated by describing one or more specific examples thereof, but is not limited to these examples; many variations and modifications are possible, within the scope of the accompanying claims. Features of any aspect or example described herein may, wherever appropriate, be applied to any other aspect or example described herein.

Claim 1:
A sensor package (<NUM>; <NUM>) comprising:
a sensor (<NUM>; <NUM>);
at least one external wall (<NUM>; <NUM>);
an interposer (<NUM>; <NUM>), arranged between the sensor (<NUM>; <NUM>) and the at least one external wall (<NUM>; <NUM>);
wherein the interposer (<NUM>; <NUM>) comprises conductive tracking (<NUM>; <NUM>);
wherein the sensor (<NUM>; <NUM>) is wire bonded (<NUM>; <NUM>) to the conductive tracking (<NUM>; <NUM>);
wherein the at least one external wall (<NUM>; <NUM>) is made from a first material, the interposer (<NUM>; <NUM>) is made from a second material, and the first and second materials are different; characterized in that
the conductive tracking (<NUM>; <NUM>) is wire bonded (<NUM>; <NUM>) to the at least one external wall (<NUM>; <NUM>) and in that
the second material is plastic.