Patent Description:
<CIT> Al discloses a lidless electronic package, comprising:.

The claimed subject-matter is defined by the independent claim.

Particular aspects of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. An electronic package includes multiple surfaces (e.g., in a stepped arrangement) to prevent warping of the package while also protecting the semiconductor die from damage during heat sink installation. A first portion of the stiffener can be thicker and extend above a top surface of the die to provide better protection from warping. A second portion of the stiffener, which is disposed between the die and the first portion of the stiffener, has a surface that is below the top surface of the die. The second portion provides a landing shelf for the heat sink that prevents the heat sink from first contacting the die and damaging the die, e.g., if the heat sink approaches the die at an angle with respect to the top surface of the die. By having the surface of the second portion below the top surface of the die, the heat sink can also rest flat against the die, which provides better thermal contact between the heat sink and the die than if there was space between the heat sink and the die. This provides better heat transfer between the die and the heat sink. Thus, electronic packages described herein allow for larger stiffeners that provide better protection against warping while also protecting the die from damage that could result from having a stiffener that extends above the top surface of the die.

An electronic package, which is also referred to an integrated circuit (IC) package, includes a substrate and one or more die disposed on a surface of the substrate. Each die can include integrated circuits fabricated on semiconducting material. An electronic package can also include one or more stiffeners that prevent the components of the electronic package from warping. For example, there are mismatches of coefficients of thermal expansion (CTE) in the different components of an electronic package. When the temperature changes, e.g., during the package assembly process, the CTE mismatch can cause the electronic package to warp, bend, or twist, resulting in a less flat electronic package. Flatness problems can make the soldering process more difficult, resulting in poor solder joints between the electronic package and a circuit board or other mounting surface.

A stiffener can prevent the electronic package from warping. A stiffener can be attached to the perimeter of the substrate, e.g., around the perimeter of the die. The stiffener can have a higher elastic modulus than the substrate to prevent the substrate from warping. In general, the protection against warping improves with an increase in the size of the stiffener, e.g., with an increase in the thickness of the stiffener. However, the size of the stiffener may be limited by the size of the substrate and characteristics of the package assembly process.

If the stiffener extends above the top surface of the die (i.e., the surface opposite the surface of the substrate), the stiffener can provide better protection against warping. However, this can make it more difficult to install the heat sink without damaging the die. For example, if the heat sink is installed at an angle, (e.g., not flat with respect to the top surface of the die), the heat sink can damage the die. In some cases, heat sinks are installed by people that may not be able to see how flat the heat sink is when the heat sink is approaching the die due to the geometry of the electronic package.

Stiffeners described herein have multiple surfaces and/or varying thicknesses that allow for a portion of the stiffener to extend above the top surface of the die, while also protecting the die from damage during the heat sink installation. According to claimed invention, the stiffener includes a first surface that is above the top surface of the die and a second surface that is below the top surface of the die. The second surface is disposed between the die and the first surface. In this way, the second surface can provide a landing shelf for the heat sink in case the heat sink approaches the die at an angle, while also allowing for a larger stiffener.

According to the claimed invention, a lidless electronic package includes a substrate having a surface and a die disposed on the surface of the substrate. The die has an outside perimeter, a bottom surface adjacent to the surface of the substrate, and a top surface. The electronic package includes a stiffener disposed on the surface of the substrate. The stiffener includes a first surface that is a first distance away from the surface of the substrate and a second surface disposed between the die and the first surface. The first distance is greater than a distance between the surface of the substrate and the top surface of the die. The second surface is a second distance away from the surface of the substrate that is less than the distance between the surface of the substrate and the top surface of the die.

<FIG> depict an example electronic package <NUM>. The relative dimensions in each figure are illustrative only and not necessarily to scale. <FIG> is a cross-sectional view of the example electronic package <NUM>. The electronic package <NUM> includes a substrate <NUM> and a die <NUM> disposed on (e.g., attached to) a surface <NUM> of the substrate <NUM>. Although one die <NUM> is illustrated in <FIG>, multiple die can be disposed on the surface <NUM> of the substrate <NUM>.

The electronic package <NUM> also includes interconnection pins for attaching the electronic package <NUM> to a mounting surface, e.g., to a printed circuit board. The electronic package <NUM> is a lidless electronic package that does not include a lid over the die <NUM>. For example, the electronic package can be a lidless ball grid array (BGA) package.

The electronic package <NUM> also includes a stiffener <NUM> disposed on (e.g., attached to) the surface <NUM> of the substrate <NUM>. In general, the stiffener <NUM> can be made of a material that has a higher elastic modulus than the elastic modulus of the substrate <NUM>. For example, the stiffener <NUM> can be made of a metal (e.g., aluminum, copper, or steel), a ceramic, a composite material, or another appropriate material. The stiffener <NUM> can be a single stiffener structure or multiple stiffener structures coupled to one another, as described below with reference to <FIG>.

Referring to <FIG>, the stiffener <NUM> can enclose at least a portion of the outside perimeter of the die <NUM>. The outside perimeter of the die <NUM> includes the four sides 121A - 121D of the die <NUM>. In this example, the stiffener <NUM> encloses the entire outside perimeter of the die <NUM>. In some implementations, the stiffener <NUM> encloses only a portion of the outside perimeter of the die <NUM>. For example, the stiffener <NUM> can be installed on one or more sides of the die <NUM>, e.g., on two or three sides of the die <NUM>. In another example, the stiffener <NUM> can substantially enclose the outside perimeter of the die <NUM> by having a respective stiffener on each side 121A - 121C of the die <NUM>, but some space between each adjacent stiffener.

In the example of <FIG>, the stiffener <NUM> is adjacent to each side 121A - 121D of the die <NUM>. For example, each side 121A - 121D of the die <NUM> may contact a respective part of the stiffener <NUM>. In some implementations, there is space between the stiffener <NUM> and the sides 121A - 121D of the die <NUM>.

<FIG> is a partial cross-sectional view of a portion <NUM> of the example electronic package of <FIG>. Referring to <FIG>, the stiffener <NUM> includes a first surface <NUM> and a second surface <NUM>. The first surface <NUM> is above the top surface <NUM> of the die <NUM>, while the second surface is below the top surface <NUM> of the die <NUM>. The top surface <NUM> of the die <NUM> is the surface of the die <NUM> opposite the surface <NUM> of the die <NUM> that is adjacent to the surface <NUM> of the substrate <NUM>. The term "top surface" is used for ease of subsequent discussion, but the top surface <NUM> may not always be the top portion of the die <NUM> depending on the orientation of the electronic package <NUM>. For example, if the electronic package it flipped, the top surface <NUM> would be below the surface <NUM>.

The term "above" is used to refer to the first surface <NUM> being a greater distance away from the surface <NUM> of the substrate <NUM> than the distance between the top surface <NUM> of the die <NUM> and the surface <NUM> of the substrate <NUM>. Similarly, the term "below" is used to refer to the second surface <NUM> being a shorter distance away from the surface <NUM> of the substrate <NUM> than the distance between the top surface <NUM> of the die <NUM> and the surface <NUM> of the substrate <NUM>. In different orientations of the electronic package <NUM>, e.g., if the electronic package <NUM> is flipped upside down from the illustrated orientation, the first surface <NUM> may be at a lower altitude than the top surface <NUM> of the die <NUM>. However, the first surface <NUM> will still be at a greater distance from the surface <NUM> of the substrate <NUM> than the distance between the top surface <NUM> of the die <NUM> and the surface <NUM> of the substrate <NUM>.

The additional thickness of the stiffener <NUM> between the first surface <NUM> and the surface <NUM> of the substrate <NUM> provides better protection against warping, allowing the substrate to remain flatter. As used herein, thickness of a portion of the stiffener <NUM> represents the distance between the surface <NUM> of the substrate <NUM> and the surface of the portion of the stiffener <NUM> opposite the surface <NUM> of the substrate <NUM>. The protection against warping improves with an increase in the thickness of the stiffener. In this example, the portion of the stiffener <NUM> between the first surface <NUM> and the surface <NUM> of the substrate <NUM> is thicker than the portion of the stiffener <NUM> between the second surface <NUM> and the surface <NUM> of the substrate <NUM>.

The second surface <NUM> provides a landing shelf for a heat sink when the heat sink is being installed on the electronic package <NUM>. As described in more detail below with reference to <FIG>, if the heat sink approaches the die <NUM> at an angle with respect to the top surface <NUM> of the die <NUM>, the heat sink is more likely to make contact with a portion of the second surface <NUM> (e.g., on one side of the die <NUM>) rather than the die <NUM> itself. This protects the die <NUM> from damage caused by the heat sink.

To provide the landing shelf, the second surface <NUM> is below the top surface <NUM> of the die <NUM>. That is, the second surface <NUM> is a shorter distance away from the surface <NUM> of the substrate <NUM> than the distance between the top surface <NUM> of the die <NUM> and the surface <NUM> of the substrate <NUM>. As described below with reference to <FIG>, this allows the heat sink to rest flat against the die <NUM>.

The second surface <NUM> of the stiffener can be close to being the same distance from the surface <NUM> of the substrate <NUM> as the top surface <NUM> of the die <NUM>. For example, the second surface <NUM> of the stiffener <NUM> can be <NUM> - <NUM> millimeters (mm) below the top surface <NUM> of the die <NUM>. That is, the distance <NUM> between the surface <NUM> of the substrate <NUM> and the top surface <NUM> of the die <NUM> can be <NUM> - <NUM> longer than the distance <NUM> between the surface <NUM> of the substrate <NUM> and the second surface <NUM>. The closer the second surface <NUM> of the stiffener <NUM> and the top surface <NUM> of the die <NUM> are to being the same distance from the surface <NUM> of the substrate <NUM>, the more likely it is that the heat sink contacts a portion of the second surface <NUM> of the stiffener <NUM> prior to contacting the die <NUM>. However, if the designed distance is too short, manufacturing defects can result in a second surface <NUM> that is above the top surface <NUM> of the die <NUM>, resulting in reduced thermal contact between the heat sink and the die <NUM>.

In some implementations, the first surface <NUM> of the stiffener <NUM> and the second surface <NUM> of the stiffener <NUM> are substantially parallel. That is, the surfaces <NUM> and <NUM> may be parallel or have less than a five-degree angle between the surfaces. Similarly, the surfaces <NUM> and <NUM> may be substantially parallel with the top surface <NUM> of the die <NUM>.

The stiffener <NUM> of this example also includes a lead-in draft <NUM> and a recessed groove <NUM>. The lead-in draft <NUM> provides a sloped surface from the first surface <NUM> of the stiffener <NUM> towards the die <NUM>. The lead-in draft <NUM> can help guide the heat sink towards the center of the electronic package <NUM> if the heat sink is approaching the die <NUM> near the first surface <NUM> of the stiffener <NUM>. The slope of the lead-in draft <NUM> with respect to the first surface can be less than ninety degrees, i.e., not normal to the first surface <NUM>.

The recessed groove <NUM> can help ensure that the heat sink rests flat against the die <NUM>. For example, if the second surface <NUM> of the stiffener <NUM> extended to the lead-in draft <NUM>, an end of the heat sink could rest on the lead-in draft <NUM>. This would result in the heat sink being at a non-zero angle with respect to the top surface <NUM> of the die <NUM>, resulting in less thermal contact between the heat sink and the die <NUM>.

<FIG> is a cross-sectional view of the electronic package <NUM> of <FIG> with a heat sink <NUM> being installed on the electronic package <NUM>. The heat sink <NUM> includes a base <NUM> (e.g., a copper base) and a fin array <NUM>. In this example, the heat sink <NUM> is approaching the die <NUM> at an angle, i.e., the bottom surface of the base <NUM> of the heat sink <NUM> is not parallel with the top surface <NUM> of the die <NUM>. A first side <NUM> of the heat sink <NUM> contacts a portion of the second surface <NUM> of the stiffener <NUM> before any portion of the heat sink <NUM> contacts the die <NUM>. After the first side <NUM> contacts the second surface <NUM> of the stiffener <NUM>, the second side <NUM> of the heat sink <NUM> can continue moving towards the second surface <NUM> of the stiffener <NUM> on the other side of the substrate <NUM> until the base <NUM> of the heat sink <NUM> rests flat on the top surface <NUM> of the die <NUM>. The heat sink <NUM> can be attached to the electronic package <NUM> using a mechanical device (e.g., a springloaded screw), and adhesive (e.g., a thermal adhesive), or another appropriate attachment technique.

If the heat sink <NUM> approached the die <NUM> farther to one side of the electronic package <NUM>, a side of the heat sink <NUM> may contact the lead-in draft <NUM>. The lead-in draft <NUM> can guide the side of the heat sink towards the center of the electronic package <NUM>. In this example, the side of the heat sink <NUM> that contacts the lead-in draft may rest above the recessed groove <NUM>.

<FIG> is a cross-sectional view of the electronic package <NUM> of <FIG> with the heat sink <NUM> installed thereon. The base <NUM> of the heat sink <NUM> rests on the top surface <NUM> of the die <NUM>. In this example, the heat sink <NUM> is longer than the die <NUM>. Thus, a portion of the heat sink <NUM> extends over the second surface <NUM> of the stiffener <NUM>. As the second surface <NUM> of the stiffener <NUM> is below the top surface <NUM> of the die <NUM>, there is a gap between the base <NUM> of the heat sink <NUM> and the second surface <NUM> of the stiffener <NUM>. This gap helps to ensure that the base <NUM> of the heat sink <NUM> rests flat on the top surface <NUM> of the die <NUM>. For example, if the second surface <NUM> of the stiffener <NUM> was above the top surface <NUM> of the die <NUM>, the base <NUM> of the heat sink <NUM> would not directly contact the top surface <NUM> of the die <NUM> at least in some portions of the top surface <NUM> of the die <NUM>.

The configuration of the stiffener <NUM> in the electronic package <NUM> also provides a wider window in which the heat sink <NUM> can be installed relative to designs in which the stiffener includes a single thickness. This allows the fin array <NUM> to be lowered closer to the top surface <NUM> of the die <NUM>. This also lowers the heat sink thermal resistance by minimizing the thickness of the base <NUM> of the heatsink, which allows for higher heat flux (watts/square mm) from the die <NUM>.

<FIG> is a cross-sectional view of another example electronic package <NUM>. The example electronic package <NUM> includes a substrate <NUM> and a die <NUM> disposed on (e.g., attached to) a surface <NUM> of the substrate <NUM>. Although one die <NUM> is illustrated in <FIG>, multiple die can be disposed on the surface <NUM> of the substrate <NUM>. The electronic package <NUM> also includes interconnection pins for attaching the electronic package <NUM> to a mounting surface, e.g., to a printed circuit board.

The electronic package <NUM> also includes a stiffener <NUM> disposed on (e.g., attached to) the surface <NUM> of the substrate <NUM>. Similar to the stiffener <NUM> of <FIG>, the stiffener <NUM> can be made of a material that has a higher elastic modulus than the elastic modulus of the substrate <NUM>.

In this example, the stiffener <NUM> is made of two stiffeners <NUM> and <NUM>. The stiffener <NUM> is disposed on the surface <NUM> of the substrate <NUM> and encloses at least a portion of the outside perimeter of the die <NUM>, similar to the stiffener <NUM> of <FIG>. The stiffener <NUM> has a surface <NUM>, e.g., a flat surface that is below the top surface <NUM> of the die <NUM>. That is, the surface <NUM> of the stiffener <NUM> is a shorter distance away from the surface <NUM> of the substrate <NUM> than the distance between the top surface <NUM> of the die <NUM> and the surface <NUM> of the substrate <NUM>. The surface <NUM> of the stiffener <NUM> provides a landing shelf for a heat sink, similar to the second surface <NUM> of the stiffener <NUM> of <FIG>.

The stiffener <NUM> is disposed on (e.g., attached to) the surface <NUM> of the stiffener <NUM>. The stiffener <NUM> provides additional protection against warping of the electronic package <NUM>. For example, the combination of the size of the stiffener <NUM> and the size of the stiffener <NUM> provide better protection against warping than the stiffener <NUM> alone.

The stiffener <NUM> can be disposed along the entire perimeter of the stiffener <NUM> and enclose at least a portion of the die <NUM>. For example, the stiffener <NUM> can be disposed on an outside edge of the stiffener <NUM>, as shown in <FIG>. The stiffener <NUM> can cover the outside edge of the stiffener <NUM> on each side of the substrate <NUM>. For example, if the substrate is rectangular-shaped like the substrate <NUM> of <FIG>, the stiffener <NUM> can run along all four sides and the stiffener <NUM> can be disposed on the stiffener <NUM> on all four sides.

The stiffener <NUM> can include a lead-in draft <NUM> that is sloped similar to the lead-in draft <NUM> of <FIG>. According to the claimed invention, although not shown, the stiffener <NUM> includes a recessed groove similar to the recessed groove <NUM> of <FIG>. For example, the stiffener <NUM> can include a recessed groove where the lead-in draft <NUM> contacts the surface <NUM> of the stiffener <NUM>.

Implementations of the subject matter and the operations described in this specification can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification.

Claim 1:
A lidless electronic package (<NUM>), comprising:
a substrate (<NUM>) having a surface (<NUM>);
a die (<NUM>) disposed on the surface of the substrate, the die having an outside perimeter, a bottom surface adjacent to the surface of the substrate, and a top surface opposite the bottom surface; and
a stiffener (<NUM>, <NUM>) disposed on the surface of the substrate and that encloses at least a portion of the outside perimeter of the die, the stiffener including:
a first surface (<NUM>, <NUM>) that is at a first distance from the surface of the substrate, the first distance being greater than a distance between the surface of the substrate and the top surface (<NUM>) of the die;
a second surface (<NUM>, <NUM>) that is disposed between the die and the first surface, the second surface being at a second distance from the surface of the substrate, wherein the second distance is less than the distance between the surface of the substrate and the top surface of the die;
wherein the first surface is above the top surface of the die and the second surface is below the top surface of the die; characterised in that
a recessed groove (<NUM>) is disposed between the first surface and the second surface, the recessed groove including a third surface that is closer to the surface of the substrate than the second surface.