The present invention relates generally to lead frame based semiconductor packages. More particularly, arrangements that enhance the reliability of bonding wires electrically coupled to a die attach pad are described.
Many semiconductor packages utilize a metal lead frame to provide electrical interconnects between an integrated circuit die and external components. Very small electrical wires referred to as “bonding wires” are often used to electrically connect I/O pads (frequently referred to as “bond pad”) on the die to corresponding leads in the lead frame. Typically, the die, the bonding wires and portions of the lead frame are encapsulated in plastic for protection, while leaving portions of the lead frame exposed to facilitate electrical connection to external devices.
Many lead frames include a die attach pad (DAP) that supports the die during assembly of the package. In some packages, the die attach pad is exposed on a surface of the package (typically the bottom surface). An exposed die attach pad can help with the thermal management of the package because the die attach pad provides a good thermal conduction path for dissipating excess heat generated by the die. In some exposed DAP packages, the die attach pad is also used as an electrical contact for the package. Most commonly, the die attach pad is used as the ground pad, although in a few packages it may be used as a power pad and theoretically it could alternatively be used as a signal pad.
In some applications, as for example when the die attach pad is used as an electrical contact, bonding wires are used to electrically connect one or more ground I/O pads on the die to the die attach pad (a process frequently referred to as “down bonding”). Most commonly, very fine gold wires are used as the bonding wires and the lead frame is formed from copper or a copper based alloy. Since gold does not adhere well to copper, the die attach pad (and other relevant portions of the lead frame) are typically plated with a thin film of silver which adheres much better than copper to the gold bonding wires. A problem that occasionally occurs is that the die will sometimes delaminate from the die attach pad during use of the device. When die attach pad delamination occurs, movement of the die relative to the die attach pad can sometimes detach the down bonding wires from the die attach pad or otherwise break the down bonding wires.
Although existing down bonding techniques work well, there are continuing efforts to further improve down bonding reliability in a cost effective manner.
A variety of semiconductor package arrangements and packaging methods are described that improve the reliability of bonding wires that down bond a die to a die attach pad. In one aspect, selected portions of the top surface of a lead frame (which may be in panel form), including portions (but not all) of the die attach pad, are plated (e.g., silver plated) to facilitate wire bonding. In some preferred embodiments, the plating on the die attach pad is arranged as a peripheral ring on that surrounds an open central region of the die support surface. In other embodiments, bars or other geometric patterns may be selectively plated on any desired and appropriate locations on the die attach pad. Unplated portions of the die support surface are roughened to improve the adherence of the die to the die attach pad and the molding compound to the die attach pad, thereby reducing the probability of die attach pad delamination and the associated risks to down bonded bonding wires. The described lead frames may be used in a variety of packages. Most commonly, a die is attached to the die support surface of the die attach pad and electrically connected to the lead frame leads by bonding wires as appropriate. At least one of the die's bond pads (typically the ground bond pad(s)) is down bonded to the die attach pad. The die, the bonding wires and at least portions of the lead frame are then typically encapsulated with a plastic encapsulant material while leaving a contact surface of the die attach pad exposed to facilitate electrically coupling the die attach pad to an external device.
The described lead frames may be used in a variety of packaging processes. Most commonly, a die is attached to the die support surface of the die attach pad and electrically connected to the lead frame leads by wire bonding as appropriate. At least one of the die's bond pads (typically the ground bond pad(s)) is down bonded to the die attach pad. The die, the bonding wires and at least portions of the lead frame are then typically encapsulated with a plastic encapsulant material. In exposed die attach pad packages, a contact surface of the die attach pad is left exposed to facilitate electrically coupling the die attach pad to an external device. When the die's ground pad(s) is/are down bonded to the die attach pad, the exposed die attach pad becomes the package's ground contact.
In various apparatus aspects of the present invention, a variety of novel package designs are described. In some embodiments, a die is mounted on a lead frame die attach pad. A first portion of the die attach pad is plated with a conductive plating material. A second, non-plated portion of the die attach pad is roughened in a manner that improves adhesion of the die to the die attach pad, thereby reducing the probability of the die delaminating from the die attach pad. In some preferred embodiments, the conductive plating is a silver based plating material formed as a ring that surrounds a roughened second portion of the die support surface. In other embodiments, bars of plating material may be formed at appropriate locations on the die attach pad. Selected bond pads on the die (typically ground pads) are down bonded to the plated portion of the die attach pad.
The invention and the advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:
a) is a diagrammatic top view of a lead frame panel having silver plated die attach pad rings in accordance with one embodiment of the present invention.
b) is a diagrammatic top view of single device area in the lead frame panel illustrated in
a) is a flow chart illustrating a method of preparing a lead frame panel and packaging integrated circuit devices in accordance with one embodiment of the present invention.
b) is a flow chart illustrating a method of preparing a lead frame panel and packaging integrated circuit devices in accordance with another embodiment of the present invention.
a)-3(g) are diagrammatic cross sectional views of a single device area illustrating several steps in the formation of the lead frame panel of
In the drawings, like reference numerals are sometimes used to designate like structural elements. It should also be appreciated that the depictions in the figures are diagrammatic and not to scale.
The present invention relates generally to packaging methods and arrangements that enhance the reliability of bonding wires electrically coupled to a die attach pad. In general, selected portions of the top surface of a lead frame are plated to facilitate wire bonding. The plating covers some, but not all of a die attach surface of the die attach pad. In some preferred embodiments, the plating on the die attach pad is arranged as a peripheral ring that surrounds an unplated central region of the die support surface. In other embodiments, bars or other geometric plating patterns may be formed at suitable locations on the die attach pad. Unplated portions of the die support surface are roughened to improve the adherence of the die to the die attach pad, thereby reducing the probability of die attach pad delamination and the associated risks to down bonded bonding wires.
Referring initially to
The specific configuration of the device areas will vary in accordance with the needs of any particular package. By way of example,
Referring primarily to
A portion, but not all, of the top surface of the die attach pad 110 is plated with the conductive plating material 120. The geometry of the plating on the die attach pad may vary widely in accordance with the needs of any particular design. The most important point being that plating should be present in regions of the die attach pad that will be down bonded to. In the illustrated embodiment, the conductive plating material on the die attach pad is arranged as a peripheral ring 122 that surrounds a non-plated central region 123 of the die attach pad. This plating geometry works well in many applications because it allows down bonding to any outer portion of the die attach pad. The plating 125 on the lead contacts 113 covers at least the regions of the lead contacts that are intended to serve as points of attachment for wire bonding.
An alternative plating geometry is illustrated in
Several different plating processes may be used to plate the lead frame panel 100 illustrated in
Referring next to
After the lead frame panel has been patterned, the pattern lead frame is mounted on a carrier 130 (step 203,
Either before or after the carrier or has been applied, selected portions of the top surface of the lead frame are silver plated using conventional silver plating techniques. The silver plating material is applied to wire bonding landing regions of the lead contacts in a conventional manner (e.g. electroplating) to form lead plating 125. Simultaneously, portions, but not all of, the die attach pad are also plated to form die attach pad plating 122. (Step 204,
It should be appreciated that a variety of plating techniques may be used to plate the desired portions of the lead frame. In the illustrated embodiment, a mask (not shown) is used to cover portions of the top surface of the lead frame that are not to be plated. The mask may take any suitable form. By way of example, conventional mechanical masks (e.g., rubber stamps) work well for this purpose, although other appropriate masks may be used as well.
In the described embodiment, silver is used as the conductive plating material because the gold bonding wires adhere better to silver than the copper lead frame. Although silver is used in the described embodiment, it should be appreciated that the primary function of the conductive plating material is to improve the attachment of the bonding wires to the lead frame. Accordingly, in alternative embodiments, appropriate portions of the lead frame may be plated with other suitable conductors that improve the adhesion of the particular material used as the bonding wires to the specific material used as the lead frame.
After the lead frame has been silver plated, the lead frame is subjected to a lead frame treatment that “textures” or “roughens” exposed surfaces of the lead frame that are not silver plated. (Step 206,
The roughened region(s) (e.g., central region 123) of the top surface of the die attach pad 110 improve adhesion of the die to the die attach pad when the die is subsequently attached to the die attach pad. The roughened side surfaces of the die attach pads and the leads help improve adhesion of the molding material to the lead frame when the die is subsequently encapsulated.
In some exposed die attach pad plastic molded package, it may not be desirable to roughen surfaces of the lead frame that are intended to be exposed and used as contacts because molding material may penetrate into vias in the textured surface during encapsulation thereby significantly reducing the electrical performance of the contact surface. However if the die attach pad is not exposed it may be acceptable and/or desirable to treat the bottom surface of the die attach pad so that the molding material adheres better to the die attach pad. In such embodiments, it may be desirable to forego attachment of the carrier 130 (step 203) or remove the carrier prior to the lead frame surface treatment. Similarly, if the surface treatment that is used is not prone to significant molding material penetration concerns, the surface treatment may readily be performed without the carrier 130 attached.
As will be appreciated by those familiar with the lead frame arts, there are several commercially available copper lead frame surface treatments that may be used to texture exposed portions of a lead frame. Preferably, a lead frame surface treatment that does not significantly attack the silver plating is selected. Therefore, the silver plating is not roughened or otherwise adversely affected by the treatment. Most commercially available lead frame treatments meet this requirement, so any of a variety of commercially available lead frame treatments may be utilized for lead frame treatment step 206. By way of example, the L1 surface treatment process marketed by Possehl Electronics, the Microetch process performed by Dynacraft or the copper oxide process performed by ASM are all suitable lead frame surface treatments that work well for the intended purpose.
After the lead fame treatment has been completed, the lead frame may be used to package integrated circuits using a variety of different packaging techniques. By way of example, a die 140 may be mounted on and adhesively secured to each die attach pad 110 in step 208 using conventional die attach techniques (
After the die 140 has been attached to the die attach pad 110, it may be electrically connected to the lead frame using conventional wire bonding techniques. (Step 210). Typically one end of each bonding wire is attached to an associated bond pad 141 on the die and a second end of each bonding wire is attached to a silver plated portion 120 of the lead frame as illustrated in
After the dice have been attached and wire bonded to the lead frame, the dice and bonding wires together with portions of the lead frame may be encapsulated in step 212 using conventional encapsulation techniques such as plastic molding, as shown in
After encapsulation and any other desired panel level processing, the panel may be singulated in step 214 to form individual semiconductor packages. The singulation may be arranged to sacrifice the tie bars 116. A singulated package is diagrammatically illustrated in
An alternative plating process is illustrated in
After the lead frame blank has been selectively plated, the lead frame is patterned (step 202) to define the desired features of the lead frames (e.g., leads, contacts, die attach pads, etc.). As discussed above, any suitable technique, as for example, stamping or etching may be used to pattern the lead frame. After patterning, the entire lead frame panel may be subjected to a lead frame treatment (step 206). Either before or after, the lead frame treatment, the lead frame panel may be mounted on an appropriate carrier (step 207). In the embodiment illustrated in
After the lead frame treatment and carrier attachment, the packaging process of
A representative singulated package formed in accordance with the either of the described processes is illustrated in
In the embodiments described above, down bond failures are substantially reduced primarily due to the fact that the surface treatment of much of the die support surface of the die attach pad 110 (e.g., roughened central region 123) significantly reduces the probability of die attach pad delamination, which is believed to be the primary cause of down bond failure. If desired, additional efforts may also be made to improve the adhesion of the down bonded bonding wires 146 to the die attach pad 110. For example, the ground I/O pads on the die 140 may be coupled to the grounded die attach pad 110 using a reverse ball bond with wedge stitched on ball (RBSOB) technique. In this approach, a wire bonding bump is initially created on the appropriate ground I/O pad on the die 140. The bump is made by using a standard wire bonding capillary to ultrasonically deposit a ball bond onto the selected bond pad. Rather than continuing the extrusion of the wire, the capillary truncates the wire near the top of the ball bond, such that only a wire bonding “ball” or “bump” remains atop the bond pad.
A reverse wire bond is then used to electrically couple the ground I/O pad to the die attach pad. During the wire bonding process, a second ball bond is formed on the die attach pad, and a stitch bond is formed on top of the bump on the ground I/O pad. With this arrangement, the down bonding wires are electrically coupled to the ground I/O pad via a stitch bond located atop the bump. The height of the bumps may vary in accordance with the needs of a particular embodiment. By way of example, in some embodiments, a bump having a height that is approximately one-third the height of a conventional ball bond works well. The described reverse wire bonding with stitch on ball technique improves the adhesion of the ground bonding wire to the lead frame. This is because the ball bond adheres to the silver-plated portion of the die attach pad better than a stitch bond would.
Although only a few embodiments of the invention have been described in detail, it should be appreciated that the invention may be implemented in many other forms without departing from the spirit or scope of the invention. Although the formation of an LLP type package has been illustrated, it should be appreciated that the invention can be used with any package that incorporates down bonding to a die attach pad. Further, although specific steps and a specific order were articulated in describing the illustrated embodiments, it should be appreciated that in many instances, the specific steps used as well as their order may be varied without departing from the scope of the invention.
It is believed that in many applications the described die attach pad surface treatment technique will adequately eliminate delamination problems between the die and the die attach pad so that no further processes will be needed to improve down bond reliability. However, when desired, in addition to the stitch on ball technique described above, other complementary techniques down bond enhancement techniques can be employed in parallel with the described approaches to further improve down bond reliability. Therefore, the present embodiments should be considered illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.