ELIMINATING SUBSTRATE METAL CRACKS IN A BALL GRID ARRAY PACKAGE

An apparatus includes a substrate including a planar surface. A die is attached to the planar surface of the substrate with an interposed die attach material. A solder mask is interposed between the die attach material and the planar surface. The solder mask includes a recessed portion extending beneath a periphery of the die, and the recessed portion is filled with a molding underfill.

FIELD

This disclosure relates generally to packaging of electronic components, and more specifically to the reduction of stress-induced cracks in a Ball Grid Array (BGA) package.

BACKGROUND

Electronic component packaging includes the use of dissimilar materials, each with different temperature coefficients of expansion, thermal conductivity and the like. During component qualification and ultimately field operation, these various materials may expand and contract in ways that will introduce cracks leading to component failure. In particular, die attach epoxy may be used to attach a die (e.g., semiconductor die), to a substrate. The die attach epoxy will exhibit differential stress across the width of a die and expand or contract at a different rate to the die or substrate, when the package in temperature cycled. Such stress may lead to cracks forming in the epoxy, which may further lead to cracks in the substrate and electrical connections therein.

DETAILED DESCRIPTION

Embodiments described herein provide for the elimination, or substantial reduction, of substrate metal cracks in a BGA package as a result of die attach epoxy cracks propagating downward through the metallization patterns of the underlying substrate. Epoxy cracks due to thermal stress are prevented from propagating by forming a recessed portion beneath the die periphery and filling this portion with molding underfill.

FIG.1shows a cross-sectional view of a BGA package10, in accordance with an embodiment of the present disclosure. The BGA package10includes a die (e.g. a semiconductor die, a III/V compound die, a GaN die and the like), attached to a substrate14with a die attach material16. The substrate14further includes a substrate core18, a plurality of copper interconnect (or traces)20connected to vias22and solder balls24. A solder mask26is applied to both sides of the substrate core18. The die12is bonded with bond wires28to bond fingers30(shown with optional plating). A recessed portion32is formed under the periphery of the die12and filled with molding underfill. In one embodiment, a plan view of the recessed portion32follows the same shape as the periphery of the die12. In another embodiment, the recessed portion32includes chamfered or rounded corners.

FIG.2shows a plan view of an embodiment of a BGA package40, in accordance with a manufacturing step of the present disclosure.FIG.2with continued reference toFIG.1shows a first layer42of a solder mask applied to a BGA substrate (e.g., a portion of the solder mask26ofFIG.1).FIG.2shows the first layer42of the solder mask applied to a top surface of the BGA substrate. In one embodiment, the first layer may also be applied to a bottom surface (not shown) of the BGA substrate.

FIG.3shows a plan view of an embodiment of a BGA package50, in accordance with a manufacturing step of the present disclosure.FIG.3with continued reference toFIG.2shows a bonding area52exposed by removing part of the first layer42of the solder mask. In one embodiment, the bonding area52is exposed by a photolithographic process.

FIG.4shows a plan view of an embodiment of a BGA package60, in accordance with a manufacturing step of the present disclosure. Subsequent to the exposure of the bonding area52, a second layer of the solder mask is applied to the first layer52to form a combined solder mask54.FIG.4with continued reference toFIG.3shows the bonding area52exposed again while also forming (and then filling) a filled recessed portion62in the solder mask. In one embodiment, the filled recessed portion62has a same depth as a thickness of the second layer. In another embodiment, the depth of the filled recessed portion62is different than the thickness of second layer but less than the total thickness of the solder mask, including the first layer and second layer of the solder mask. In one embodiment, a centerline64of the filled recessed portion62may align with a periphery of a die (not shown) to be affixed to the BGA package60.

FIG.5shows a cross-sectional view of a preceding manufacturing step to the embodiment ofFIG.4taken along line A-A′, in accordance with the present disclosure. The embodiment70ofFIG.5includes a substrate core72and metallization74(e.g., copper interconnect), similar to the substrate core18and metallization20ofFIG.1. A combined solder mask54is formed on a top surface of the embodiment70. In one embodiment, the combined solder mask54is also formed on a bottom surface of the embodiment70. The combined solder mask54includes a recessed portion78formed in the combined solder mask54, prior to filling. The combined solder mask54includes the first layer42having a first thickness80. The combined solder mask54including the application of the first layer42and the second layer has a combined thickness82.

FIG.6shows an embodiment90of a cross-sectional view ofFIG.4taken along line A-A′, in accordance with another manufacturing step of the present disclosure.FIG.6, with continued reference toFIG.4andFIG.5shows the filling of the recessed portion78with a dispenser92, to form the filled recessed portion62. In one embodiment, the filled recessed portion62includes a same molding underfill, subsequently used to encapsulate the embodiment90. In another embodiment, the filled recessed portion62includes a different molding underfill, subsequently used to encapsulate the embodiment90.

FIG.7shows a plan view of an embodiment100of a BGA package, in accordance with another manufacturing step of the present disclosure.FIG.7, with continued reference toFIG.4, shows a die102placed over the filled recessed portion62. In the embodiment100, a periphery104of the die is preferably aligned to be coincident with the centerline64(illustrated inFIG.4) of the filled recessed portion62. In one embodiment, the filled recessed portion62is formed with a minimum keep out distance108from the bonding area52to ensure adequate room is available for bonding a bonding wire to the bonding area52.

FIG.8shows an embodiment110of a cross-sectional view ofFIG.7taken along line B-B′, in accordance with another manufacturing step of the present disclosure.FIG.8, with continued reference toFIG.6andFIG.7shows the die102attached to the combined solder mask54with a die attach material112. The die attach material112includes a bleed out area114, where the die attach material112extrudes or bleeds out from under the die102. In one embodiment, the filled recessed portion62has a width to ensure a minimum keep out distance108from the bonding area52. The width of the filled recessed portion62is further defined by a bleed out distance116. In another embodiment, the die102may be slightly misaligned with respect to the centerline64due to manufacturing tolerances, as long as the width of the filled recessed portion62is sufficient to satisfy the minimum required keep out distance108and minimum required bleed out distance116.

FIG.9shows a plan view of an embodiment120of a BGA package, in accordance with another manufacturing step of the present disclosure. In the embodiment120, a plurality of bond wires122electrically connect circuitry on the die102to bond fingers in the bond area52. The bond fingers further connect to solder balls, similar to solder balls24ofFIG.1, through the metallization74(seeFIG.8).FIG.10shows an embodiment130of a cross-sectional view ofFIG.9taken along line C-C′, in accordance with another manufacturing step of the present disclosure. The embodiment130shows a finalized BGA package, with attached solder balls132on a bottom surface of the BGA. In one embodiment, the same molding underfill134used to fill the filled recessed portion62is used to encapsulate the BGA. In another embodiment, a different molding underfill134used to fill the filled recessed portion62is used to encapsulate the BGA. In another embodiment, the bonding area52may further include plating136to facilitate attaching bond wires122to the bond fingers in the bonding area52.

FIG.11shows an embodiment140of a method to eliminate substrate metal cracks in a BGA, in accordance with an embodiment of the present disclosure. With continued reference toFIG.2,FIG.3,FIG.5,FIG.6andFIG.7, at142a first solder mask42, (seeFIG.2) is applied to a substrate. At144, a bonding area52is opened on the first solder mask42, (seeFIG.3). At146, a second solder mask is applied to the first solder mask, thereby forming a combined solder mask54, (seeFIG.5). At148, the bonding area52is reopened while forming a recessed portion78under a die periphery104, (seeFIG.5andFIG.7). At150, the recessed portion78is filled with a molding underfill to form a filled recessed portion62, (seeFIG.6).

As will be appreciated, at least some of the embodiments as disclosed include at least the following. In one embodiment, an apparatus comprises a substrate comprising a planar surface. A die is attached to the planar surface of the substrate with an interposed die attach material. A solder mask is interposed between the die attach material and the planar surface, wherein the solder mask comprises a recessed portion extending beneath a periphery of the die, the recessed portion filled with a molding underfill.

Alternative embodiments of the apparatus include one of the following features, or any combination thereof. The solder mask comprises a first layer applied to the planar surface and a second layer applied on the first layer. The recessed portion comprises a depth equal to a thickness of the second layer. The recessed portion extends in a direction parallel to the planar surface and comprises a width greater than a bleed out distance of the die attach material. The width ensures the recessed portion is formed in the solder mask at least a minimum distance from a bond finger configured to bond with a wired connection to the die. The periphery of the die bisects the recessed portion. The die attach material comprises epoxy. The molding underfill filling the recessed portion is a same type of molding underfill used to encapsulate the die. The apparatus is a Ball Grid Array (BGA). The BGA is a Molded Array Process BGA.

In another embodiment, a method to eliminate substrate metal cracks in a ball grid array package comprises applying a first solder mask on a planar surface of a substrate. A bonding area of the first solder mask is opened to expose a bond finger configured to bond with a wired connection to a die. A second solder mask is applied on the first solder mask. The bonding area is opened to expose the bond finger and form a recessed portion under a periphery of the die. The recessed portion is filled with a molding underfill.

Alternative embodiments of the method to eliminate substrate metal cracks in a ball grid array package include one of the following features, or any combination thereof. The die is attached to the second solder mask with a die attach material, wherein the periphery of the die bisects the recessed portion. The die is encapsulated with the molding underfill. A minimum width of the recessed portion is determined by a bleed out distance of the die attach material. A maximum width of the recessed portion is determined by defining a minimum distance of the recessed portion from a bond finger configured to bond with a wired connection to the die.

In another embodiment, an apparatus comprises a substrate comprising a first surface opposing a second surface, wherein the second surface comprises a plurality of solder balls arranged in a ball grid array. A die is attached to the first surface of the substrate with an interposed die attach material. A solder mask is interposed between the die attach material and the first surface, wherein the solder mask comprises a recessed portion extending beneath a periphery of the die, the recessed portion filled with a molding underfill used to encapsulate the die.

Alternative embodiments of the apparatus include one of the following features, or any combination thereof. The periphery of the die bisects the recessed portion. The recessed portion extends in a direction parallel to the planar surface and comprises a width greater than a bleed out distance of the die attach material. The width ensures the recessed portion is formed in the solder mask at least a minimum distance from a bond finger configured to bond with a wired connection to the die. The solder mask comprises a first layer applied to the first surface and a second layer applied on the first layer, wherein the recessed portion comprises a depth equal to a thickness of the second layer.