# Patent

## Title
Stackable low-profile lead frame package

## Abstract

In an embodiment, an IC assembly comprises an IC having a top surface comprising a plurality of input/output terminations, a plurality of leads arranged around the IC, a plurality of bond wires, and an encapsulant. Each lead has a first surface and a second surface opposite the first surface, and has a feature protruding from the first surface proximate an inward end of the lead nearest the IC. The feature extends from the first surface to approximately a plane that includes a bottom surface of the IC. Each bond wire connects a respective lead to a respective I/O terminal on the IC. The encapsulant seals the bond wires, the IC, and a first portion of the leads that includes the feature. The feature creates on offset from the bottom of the IC to permit the encapsulant to surround the first portion.

## Background
BACKGROUND
1. Field of the Invention
This invention is related to the field of integrated circuit packaging and, more particularly to stackable integrated circuit packaging.
2. Description
Over the course of the development of the electronics industry, there has been an endless effort to increase both compactness and the performance of electronics products. The semiconductor devices have increased in terms of the numbers of transistors that can be created in a given space and volume, but it is the semiconductor package that has largely established the lower limits of the size of devices. So called chip scale and chip size packages have served well to meet this challenge by creating input/output (I/O) patterns for interconnection to the next level circuits, which are kept within the perimeter of the die. While this is suitable for making interconnection at near chip size, desire for even greater functionality in the same foot print and area has lead in recent years to increased interest in and to the development of stacked integrated circuit (IC) devices and stacked package assemblies. One area of specific interest and need is in the area of stacked chip assemblies for memory die. Particularly, the cost effectiveness of such solutions is of interest.
Crowley, et al. in U.S. Pat. No. 7,045,396 disclose a lead frame package that is stackable. Moreover, it discloses the use of a paddle which adds to package thickness and increases the package cross section profile. This is not suitable for applications wherein height standards must be met while stacking larger numbers of die (for example an eight high package stack), while still allowing for sufficient airflow between assemblies for cooling. Another example of a stackable lead frame package is described by Haba in United States Patent Application 2006/0118933 titled: “Stackable frames for packaging microelectronic devices”. Again the structure is not suitable for low profile stacking.
Huang discloses in U.S. Pat. No. 6,630,729 another approach to creating a stacked lead frame structure wherein the thickness of the lead frame establishes the thickness of the assembly and the lead frame is stacked.
Chye, et al in U.S. Pat. No. 6,951,982 disclose another approach to for a two stack lead frame integrated circuit (IC) package wherein the lower package has long leads and the upper package has short leads for connecting to the upper surface of the longer leads of the lower lead frame package.
Fee, et al. in U.S. Pat. No. 6,943,450 also disclose a stackable lead frame IC package.
Lyu, et al. in U.S. Pat. No. 6,878,570 disclose a method for stack assembly of lead frame packages.
Beyond the desire to provide for stacking, a feature for lead frame packages having small I/O terminals is that they have a design element such as lead features which allow for reliable capture of the lead in the resin and which will prevent the inadvertent removal of the leads from the encapsulant. An example of such is the rivet like contact is described in U.S. Pat. No. 6,001,671.
Methods used in the fabrication of lead frame packages having small terminals are known by those skilled in the art. For example, typical four sided flat or two sided flat type semiconductor packages, such as bottom lead type (e.g. quad flat no-lead (QFN)) or lead end grid array type semiconductor packages, can be fabricated using a method which may involve, for example, a sawing step for cutting up a semiconductor wafer having a plurality of semiconductor ICs into individual die. This is followed by a semiconductor die mounting step where the semiconductor die is joined to the paddles of lead frame die site and integrally formed on to the lead frame strip by means of a thermally-conductive adhesive resin. This step is followed by a wire bonding step where the innermost ends of the lead frame (i.e. closest to the die) are electrically connected to an associated I/O terminal of the semiconductor die. Next a resin encapsulation or molding step is performed to encapsulate each semiconductor die assembly including bonding wires for the semiconductor die and lead frame assembly. Next is a singulation step where the I/O leads and paddle connections of each lead frame unit are cut proximate to the lead frame to separate the semiconductor package assemblies from one another. These separated devices can be marked, tested and burned in to assure their quality. Depending on the lead frame design, the leads may be formed into a so-called “J-lead” or “gull wing” configuration. However when fabricating a bottom lead type or short peripherally leaded type semiconductor packages, the lead forming step is omitted. Instead, the lower surface or free end of each lead is exposed at the bottom of the encapsulation and the exposed portion of each lead may be used as an external I/O terminal for use with a socket or for attachment to a PCB with joining material such as a tin alloy solder. A semiconductor package structure created by the process just described can be seen in FIG. 1.
FIG. 1 also identifies the most basic elements of such a semiconductor IC package. The semiconductor IC package 100 includes a semiconductor die 101 bonded to a paddle 102 by means of a thermally-conductive epoxy resin 103 and a plurality of I/O leads 104 are arranged at each of either two or four sides of the paddle. The arrangement of the leads is laid out such that the leads are spaced apart from the side of the paddle while extending perpendicularly to the associated side of the paddle. The semiconductor package also includes a plurality of conductive wires 105 for electrically connecting the inner lead bond locations 107 to the semiconductor die bond sites 106, respectively, and a resin encapsulate 108 for encapsulating the semiconductor die and conductive wires. The semiconductor package further includes outer leads extending outwardly from the inner lead bond locations, respectively. The outer leads may have a particular shape such as a “J-lead” shape or a planar bottom lead shape, as shown. These outer leads serve to make interconnection to the next level assembly such as a PCB.
FIGS. 2 A-D shows various lead frame package configurations specifically designed for stacking or slightly modified to allow for stacking. FIG. 2A shows an example of a lead frame with a J or C shape allowing soldering from one lead to the other in the same foot print. FIG. 2B shows an example of a straight lead semiconductor package in stacked form. The leads could also be shaped in a “gull wing” form if desired. FIG. 2C shows another example of a lead frame structure where the lead frame is accessed from top and bottom at offset points. This allows for stacking at lower profile, however the foot print is different on the two sides. FIG. 2D shows yet another stacking structure.
FIGS. 3A-3C show example solutions for stacking semiconductor die themselves rather than stacking the assembled packages. Often there is a preparatory step involving the creation of a redistribution circuit layer (RDL), especially in the cases where the die terminations are in the center, such as DRAM die. The RDL is a layer of circuits which interconnect native and primary semiconductor die I/O terminals to secondary I/O terminal locations distal from the original I/O locations.
FIG. 3A shows an example of such a stacked die assembly 300A construction where the central I/O terminals of the die 301 have been redistributed to the edge 302 using a redistribution circuit 303. A connection to each of the die is made at the edge contact using a conductive material 304. Such assemblies could be mounted directly on to PCBs, however they would very difficult to standardize.
FIG. 3B shows a stack die assembly construction 300B designed to overcome this limitation by assembling the stacked die on an interposer 305 to make possible interconnection to a standard registered outline, such as those published by JEDEC (Joint Electronic Device Council a division of the American Electronics Association).
FIG. 3C shows another example of a stacked die assembly package 300C where the semiconductor die 306 are interconnected to a common base substrate 307 by means of wire bonds 308. The semiconductor dice are separated by spacers 309, which add height to prevent the wires from touching the die above. The stacked semiconductor die are assembled on an interposer having a standard or registered I/O footprint or one that can be easily registered or made standard.
FIGS. 4A and 4B show additional stacked semiconductor die packaging solutions wherein the semiconductor die are stacked into an assembly and interconnected to one another through holes filled with a conductive material. This allows interconnections to be made through the silicon (or other base semiconductor material). For practical reasons, the semiconductor die are commonly stacked in wafer form. This approach, however, increases the probability that there may be a bad die in some quantity of the final stacked die assemblies. Even with high yields, the factorial effect can have a significant impact on overall assembly yield. (E.g., with 98% yield per wafer, the maximum statistical yield is 83% for an 8 high stack).
FIG. 4A shows an example of such an assembly 400A with metal filled conductive vias 401 making interconnection from one die to the next through each semiconductor die from top to bottom. On one (or possibly both) surfaces the I/O are redistributed over the surface of the die face to facilitate assembly at the next interconnection level such as a module or PCB.
FIG. 4B shows another example of such a stacked die assembly with interconnections made from die to die using metal filled conductive vias. The stacked semiconductor die assembly is shown mounted onto an interposer 402 which can have a standard or registered I/O footprint or an I/O footprint that can be easily registered or made standard.
A difficulty for stacked die semiconductor package constructions is that burn in of the bare die is difficult and such die if available can be expensive. Another reason is that semiconductor die of different generations and/or from different suppliers will normally be of slightly different size and shape and often have slightly different I/O layout. Another concern for any stacked die semiconductor package solution, which does not employ known good die, is that the assembly yield is not knowable until the final assembly is tested and burned in. This is a potentially costly proposition.



BRIEF DESCRIPTION OF THE DRAWINGS
The following detailed description makes reference to the accompanying drawings, which are now briefly described.
FIG. 1 illustrates in cross section a lead frame package for surface mounting.
FIGS. 2A-2D illustrate in general cross section lead frame packages designed for stacking.
FIGS. 3A-3C illustrate in general cross section stacked semiconductor die assemblies having edge of die connections.
FIGS. 4A and 4B illustrate in general cross section stacked semiconductor die assemblies having interconnections made through the semiconductor by means of holes filled with a conductive material.
FIGS. 5A and 5B illustrate in top and cross section views a first process step for manufacturing an embodiment of a lead frame package.
FIGS. 6A and 6B illustrate in top and cross section views a second process step for manufacturing an embodiment of the lead frame package.
FIGS. 7A and 7B illustrate in top and cross section views a third process step for manufacturing an embodiment of the lead frame package.
FIGS. 8A and 8B illustrate in top and cross section views a fourth process step for manufacturing an embodiment of the lead frame package.
FIGS. 9A and 9B illustrate in top and cross section views a fifth process step for manufacturing an embodiment of the lead frame package.
FIG. 10 illustrates in cross section view an embodiment of the lead frame package.
FIG. 11 illustrates in cross section view an assembled embodiment of several of the lead frame packages stacked together.
FIG. 12 illustrates in cross section view a process step for manufacturing an stacked embodiment.
FIG. 13 illustrates in cross section view a completed assembled stacked embodiment.
FIG. 14 illustrated one embodiment of several stacked packages assembled on a dual inline memory module (DIMM).
FIGS. 15A-15B illustrate top and cross section views of another embodiment with etch resist applied.
FIGS. 16A-16B illustrate top and cross section views of another embodiment after etching.
FIG. 17 is a cross section view of another stacked embodiment.
FIG. 18 is a flowchart illustrating one embodiment of a manufacturing process.



While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
DETAILED DESCRIPTION OF EMBODIMENTS
In the following description and in the accompanying drawings, specific terminology and images are set forth to provide a thorough understanding. In some instances, the terminology and images may imply specific details that are not required to practice all embodiments. Similarly, the embodiments described and illustrated are representative and should not be construed as precise representations, as there are prospective variation on what is disclosed that will be obvious to those of average skill in the art. Thus this disclosure is not limited to the specific embodiments describe and shown but embraces all prospective variations that fall within its scope. For brevity, not all steps will be detailed, where such details will be known to one of skill in the art having benefit of this disclosure.
Stacked IC packages, and especially memory packages, should have as many of the following qualities as possible: 1) It should be not significantly greater in area than the IC; 2) It should allow for the stacking of die of substantially the same size but should also be amenable to stacking of die of nominally different sizes as might be the case when using die from different fabricators; 3) It should be of a height no greater than the IC die including protective coatings over the active surface of the die; 4) It should be easily tested and burned in to allow for sorting for infant failures; 5) It should allow for the creation of a stacked package assembly; 6) It should be easy to inspect for manufacturing defects; 7) It should be reliable and resistant to lead breakage during handling; 8) It should be inexpensive to control costs; 9) It should offer good thermal conductivity to provide efficient heat removal; and 10) It should offer reasonable capability to perform rework and repair if needed.
A low profile IC package is disclosed herein. In some embodiments, the low profile package is suitable for stacking in a very small volume. Various embodiments may be tested and burned in before assembly. The package may be manufactured using existing assembly infrastructure, tested in advance of stack assembly and require significantly less raw material, which may help to control manufactured cost, in some embodiments. FIGS. 5A-5B show in top view 500 and cross section view 500′ at line 502 in FIG. 5A, respectively, of a portion of one embodiment of a lead frame. FIGS. 5A-5B illustrate an early manufacturing step of the lead frame. The lead frame may be one site in a lead frame strip containing multiple sites, each of which can be used to package an IC. The lead frame shown in FIGS. 5A-5B has a plurality metal I/O leads 501 which extend inwardly from an outer connecting portion 503.
The leads 501 form an opening 504 within the leads that is approximately the size of the IC that is to be packaged with the leads 501. The opening 504 may be slightly larger than the IC to provide tolerance for manufacturing variations in the size of the IC, to provide an insulating gap between the leads 501 and the IC, etc. As can be seen in FIGS. 5A-5B together, the lead frame may be generally planar. A top surface, as viewed in FIG. 5A, and a bottom surface opposite the top surface, may be approximately parallel to the plane of the lead frame. The plane may be referred to as the major plane of the lead frame, and the top and bottom surfaces may be referred to as in-plane.
The lead frame may be formed of any conductive metal. For example, the lead frame may be stamped from a sheet of the conductive metal (or from a strip of the conductive metal as one lead frame site in the strip), etched into the sheet/strip, etc. Exemplary materials may include copper, iconel, alloy 42, tin, aluminum, etc. Furthermore, metal alloys may be used, or metals may be plated subsequent to the etching steps described below.
While FIG. 5A illustrates a generally square opening 504, the opening 504 may have any shape (e.g. rectangular) dependent on the shape of the IC that is to be packaged.
FIGS. 6A-6B show a second step in the manufacturing process in which one embodiment of the lead frame is again shown in top view 600 and cross section view 600′ at the line 602 in FIG. 6A. At the point shown in FIGS. 6A-6B, an etch resistant material (more briefly “etch resist”) has been applied on a top surface (etch resist 601a) and on a bottom surface (etch resist 601b) of the lead frame to prepare it for etching. The top surface and bottom surface are relative to the top view 600 shown in FIG. 6A. However, the labels 601a and 601b are relative and may be reversed in other embodiments.
The etch resist 601b is applied proximate the inward end of the each lead, while the etch resist 601a is applied further from the inward end than the etch resist 601b. 
FIGS. 7A-7B show a third step in the manufacturing process after an etching process has been performed and the etch resist removed, for one embodiment The lead frame is again shown in top view 700 and cross section view 700′ through the line 702, respectively in FIGS. 7A-7B. As illustrated in FIGS. 7A-7B, the leads have been etched away except for the portions covered by the etch resist, thus creating “bump” features 701a and 701b on the top and bottom surfaces, respectively, of the etched lead. The bump features are generally protrusions that extend a distance from the corresponding surface. Consistent with the locations of the etch resists 601a and 601b in FIGS. 6A-6B, the bump feature 701b is proximate the inward end of each lead and the bump feature 701a is further from the inward end than the bump feature 701b. An exploded view is provided in FIG. 7A to reveal greater detail. Phantom lines are used for the bump features 701b to indicate that they are on the far side relative to the viewer.
FIGS. 8A-8B show a fourth step in the manufacturing process, at which the IC is inserted into the package assembly. The lead frame is again shown in top view 800 and cross section view 800′ through the line 806 in FIG. 8A for one embodiment. A semiconductor die 801 is placed centrally into the opening defined by the I/O leads. Interconnections are made from the leads (e.g. shown at 804) to the I/O terminals 802 on the die using metal bonding wires 803 of gold, aluminum, copper or other suitable conductors. The I/O terminal areas to be wire bonded are commonly provided with a finish that is suitable for assuring reliable wire bonding (e.g., gold, silver, palladium, etc.) In some embodiments, bonding wires 803 may be insulated (e.g. with a polymer). An example is the bonding wire technology developed by Microbonds, Inc., of Markham, Ontario, Canada. Insulated bond wires, when employed, may help to prevent shorting of the bond wires to the die surface or edge.
An alternative approach to interconnection involves the use of a redistribution layer which routes the die I/O terminals to near the edge of the die to reduce the length of the wire bonds. Such an embodiment may have an increased package thickness, but also shorter wire bond length which may improve electrical performance and specifically lead inductance.
The I/O terminals on the semiconductor may optionally be prepared with bumps to facilitate stitch bonding of the wires. Generally, the I/O terminals may be any connection point on the IC die for bonding to the leads. For example, peripheral I/O pads may be used instead of the terminals on the die area as shown in FIG. 8A. Furthermore, the I/O terminals need not be only in the center, as shown in FIG. 8A, but may be spread out over the area of the die, as desired.
The semiconductor die, may, in one embodiment, be thinned to a thickness suitable for meeting product reliability requirements, such as those related to charge leakage for deep trench features. For example, the die may be less than 200 μm and may even be less than 100 μm. In comparison, the lead frame may be 150 μm to 200 μm thick, in one embodiment, and thus the semiconductor die may be thinner than the lead frame in one embodiment. That is, the assembled and stackable low profile semiconductor die package may have a thickness that is not substantially larger than the thickness of the lead frame. For example, the assembled and stackable package may have a thickness that is less that 250 μm, or even less than 200 μm.
The package may be fabricated without the use of a paddle, which would otherwise increase the profile height of the assembled package, as illustrated in the figures.
FIGS. 9A-9B show a fifth step in the manufacturing process for one embodiment, related to the encapsulation of the package assembly such as by a molding process. The lead frame assembly is again shown in top view 900 and cross section view 900′ through the line 904 in FIG. 9A. In the illustrated embodiment, an encapsulant such as a resin is used to form over-molded encapsulation 901. The insulating encapsulant material has been dammed off in the mold so as to prevent the encapsulant from covering the entire length of the leads, while still allowing the encapsulant to flow under the lead to mechanically lock the lead into the encapsulant. That is, the bump features 701b may provide an offset from the bottom of the IC 801 to the lead surface, so that the encapsulant can surround the lead. Furthermore, in one embodiment, the bump features 701b serve to provide the mechanical lock for the leads. On the other hand, a remaining portion of the leads, including the bump features 701a, are outside of the encapsulant.
As can be seen in FIG. 9B, the bump features 701b may extend from the bottom surface of the leads to approximately a plane that includes the bottom side of the IC (reference numeral 903). Thus, the bump features 701b provide the offset as mentioned above.
FIG. 9B also shows that the bottom side 903 of encapsulated semiconductor die is exposed and is without a paddle in an effort to keep the profile of the assembly as low as possible, in this embodiment. Again, it is noted that the bottom side and the top side of the IC are relative. The bottom side 903 is opposite the top side of the IC, which has the I/O terminals of the IC.
FIG. 10 shows a cross section view of an embodiment of the assembled semiconductor die package structure 1000 including the IC 801 and a lead 501 having an encapsulated end 1002a proximate to the die edge for wire bond attachment and a distal end 1002b which is not covered by encapsulant 901 and which has a bump feature. The excess lead frame has been trimmed away for the embodiment of FIG. 10. Within the package, the lead is encapsulated on all surfaces for the length of the lead defined by the lead frame etching process previously described, to improve lead capture by the encapsulant, using the bump feature as shown. The structure further includes bond wires 803. The bump feature on the outer lead frame is optional but may provide a connection site for stacking the packaged ICs. That is, the bump feature may provides a shape suited to limiting the amount of solder required to make interconnection between low profile semiconductor IC packages when they are stacked. The bump may also serve to improve contact of the leads during test and burn in.
FIG. 11 shows an embodiment of a plurality of low profile semiconductor IC packages in a stack 1100. In one embodiment, each of the individual low profile semiconductor IC packages 1102 have been tested and burned in prior to assembly to improve assembly yield. That is, by testing and burning in the individual low profile semiconductor IC packages 1102, test and burn in failures may be sorted out prior to stacking the IC packages and thus may potentially improve yield. The low profile semiconductor IC packages are joined together both mechanically and electrically using a suitable joining material 1101 (e.g., tin alloy solder) while not contributing to the assembly thickness, in some embodiments. For example, assembly may be performed by reflow of solder balls or paste in a heating source such as a convection oven. Alternatively, the devices can also be stack assembled by pulse heating with a laser.
In some embodiments, a package assembly will have a total height that will not exceed limits defined by cooling airflow needs for the next level assembly while at the same time the stack low profile semiconductor IC packages may reach higher counts. For example, in an embodiment in which the ICs are memory chips and the stacked devices are to be included on a DIMM, stacks as high as eight low profile semiconductor IC packages may be formed while still providing a gap between DIMM modules. For example, the eight high stack of semiconductor IC packages may be less than 2.5 mm and may be approximately 2.0 mm in total height or less when assembled. That is, the height of the stack may not be substantially greater than a number of the IC packages multiplied by a height of the IC package. While an 8 high stack is illustrated, any number of IC packages may be stacked in other embodiments. For example, more than 4 IC packages may be stacked, or at least 8 may be stacked.
In one embodiment, a DIMM having stacked IC assemblies as described herein may allow for minimum DIMM connector spacings. The actual minimum spacing depends on a variety of factors, such as the amount of airflow available in a given system design, the amount of heat generated during use, the devices that will be physically located near the DIMMS, the form factor of the system itself, etc. The minimum spacing may be, for example, the width of the connectors themselves (e.g. about 10 mm currently, although it is anticipated that the connector width may be narrower in the future). Such a DIMM may address one or more factors that are prevalent in the electronic system industry. While memory capacity requirements are increasing (e.g. due to the increasing address capabilities of processors, such as the 64 bit processors currently available from many vendors), memory bus speeds are also increasing. To support higher speeds, DIMM connectors are often closely spaced (to minimize wire lengths to the connectors) and also the number of connectors may be limited to limit the electrical loading on the bus. Furthermore, small form factor machines such as rack mounted servers limit the amount of space available for all components. It is difficult to cost effectively provide dense, high capacity DIMMs using monolithic memory ICs, as the size of the IC dramatically increases its cost. A DIMM using lower cost ICs stacked as described herein may provide dense, high capacity DIMMs more cost effectively, in some embodiments.
FIG. 12 shows in cross section a simplified view of one embodiment of a tool 1200 for encapsulating a stack 1100 of low profile semiconductor IC packages. Gaps may form between the low profile semiconductor IC packages, and the encapsulation may help assure that the gap is filled (e.g. with an insulating resin which may be thermally conductive) to allow for more effective thermal transfer of heat through the stack 1100. The encapsulation of the stack 1100 may prevent hot spots and provide for more efficient and uniform heat flow throughout the assembly. Returning to FIG. 12, a mold cavity 1201 receives the stack 1100 and an encapsulant 1205 is injected under pressure through a pipe 1202 and a valve 1203 into the chamber 1201. To improve flow and fill of the gap a vacuum may be applied to the chamber and preclude the creation of voids. Alternatively, pressure sufficient to compress and diffuse any entrapped gasses could be applied.
FIG. 13 shows one embodiment of an assembled and encapsulated structure 1300 comprised of a stack 1100 of the low profile semiconductor IC packages electrically and mechanically joined together using a suitable conductor material and having an over molded encapsulant 1301 to yield a stacked fully encapsulated package assembly suitable for mounting on to the surface of a PCB such as a DIMM module PCB. As can be seen, the solder connections at the bottom of the assembled and encapsulated structure 1300 may be exposed for connection to the DIMM module.
FIG. 14 shows one embodiment of a DIMM module PCB assembly 1400 with a plurality of assembled and encapsulated structures 1401 of low profile IC packages mounted on the PCB.
FIGS. 15A-15B, 16A-16B, and 17 illustrate another embodiment of the packaging techniques described herein. FIGS. 15A-15B are similar to the step shown in FIGS. 6A-6B for the above embodiments. FIGS. 16A-16B are similar to the step shown in FIGS. 7A-7B for the above embodiments. Generally, the embodiment shown in FIGS. 15A-15B, 16A-16B, and 17 may include a third bump feature on the bottom side of the lead, located a similar length from the inward end of the leads as the second bump feature on the top side of the lead. Thus, a nearly continuous connection may be possible using the second and third bump features in a stack, which may permit the use of a conductive film between the ICs to form a stack.
FIGS. 15A-15B show one embodiment of the lead frame in top view 1500 and cross section view 1500′ at the line 1502 in FIG. 15A, respectively. At the point shown in FIGS. 15A-15B, an etch resist has been applied on a top surface (etch resist 1501a) and on a bottom surface (etch resists 1501b and 1501c) of the lead frame to prepare it for etching. The etch resists 1501a and 1501b are similar to the etch resists 601a and 601b in FIGS. 6A-6B, respectively. Additionally, the resist 1501c is applied in approximately the same location of the bottom surface of the lead as the resist 1501a is applied, with respect to distance from the inward end of the lead.
FIGS. 16A-16B show an embodiment at the step in the manufacturing process after the etching has been performed and the etch resist removed. The lead frame is again shown in top view 1600 and cross section view 1600′ through the line 1602, respectively in FIGS. 16A-16B. As illustrated in FIGS. 16A-16B, the leads have been etched away except for the portions covered by the etch resist, thus creating bump features 1601a, 1601b, and 1601c. An exploded view is provided in FIG. 16A to reveal greater detail. Phantom lines are used for the bump features 1601b to indicate that they are on the far side relative to the viewer.
The remainder of the packaging process for a single IC may be similar to the above described embodiments. When stacking the ICs, solder may be used as described above. Alternatively, since the bump features 1601a and 1601c form a nearly continuous connection from top to bottom of the IC, a conductive film may be used to make the connections. For example, FIG. 17 illustrates an embodiment in which an anisotropic conductive adhesive film 1701 is used to connect between stacked ICs. The film 1701 may provide both thermal and electrical connection between the stacked ICs and may permit the soldering and injection encapsulation steps to be eliminated for this embodiment.
Turning now to FIG. 18, a flowchart is shown illustrating one embodiment of a method of manufacturing a stacked IC or DIMM embodiment. The lead frame may be created (block 1802). For example, the lead frame may be part of a lead frame strip and may be stamped into the strip, etched, etc. Etch resist may be applied to the lead frame (block 1804). The etch resist may be applied in one or more locations, in various embodiments. For example, the etch resist may be applied to the bottom surface of the leads proximate to the inward ends of the leads, and optionally to the top surface of the leads further from the inward ends (and still further optionally to the bottom surface further from the inward ends). The lead frame is etched, creating one or more bump features on each lead below the etch resists (block 1806) and the etch resist is removed (block 1808). The IC to be packaged is inserted into the opening between the leads, and bonding wire is used to attach the IC pads to the leads (block 1810). The IC and wires bonds are encapsulated, along with the inward ends of the leads (block 1812) and the excess lead frame (e.g. beyond the optional second and third bump features, in some embodiments) is removed (block 1814). The ICs may then be tested and/or burned in, to eliminate failures prior to stacking (block 1816). The stack may then be created from two or more ICs (block 1818), and the stack may be encapsulated in some embodiments (block 1820). One or more stacked ICs may be attached to a DIMM (block 1822).
In one embodiment, a lead frame for an integrated circuit (IC) comprises a plurality of inward extending leads formed of a conductive metal. The leads have a first surface and a second surface opposite the first surface. Each lead has a first feature on the first surface proximate an inward end of the lead, and the plurality of leads form an opening within the leads into which the IC is insertable. The opening is approximately (e.g. not smaller than) a size of the IC.
In an embodiment, an IC assembly comprises an IC having a top surface comprising a plurality of input/output terminations, a plurality of leads arranged around the IC, a plurality of bond wires, and an encapsulant. Each lead has a first surface and a second surface opposite the first surface, and has a feature protruding from the first surface proximate an inward end of the lead nearest the IC. The feature extends from the first surface to approximately a plane that includes a bottom surface of the IC. Each bond wire connects a respective lead to a respective I/O terminal on the IC. The encapsulant seals the bond wires, the IC, and a first portion of the leads that includes the feature. The feature creates on offset from the bottom of the IC to permit the encapsulant to surround the first portion.
In one embodiment, a method comprises creating a lead frame comprising a conductive metal having a plurality of inwardly projecting leads. An opening formed within the leads is approximately a size of an integrated circuit (IC) to which the leads are to be connected. The method comprises applying an etch resist proximate the inward ends of the leads on a first surface of the leads; etching the lead frame subsequent to applying the etch resist; and removing the etch resist subsequent to etching the lead frame. The etched lead frame comprises leads having a feature protruding from the first surface proximate the inward ends of the leads.
In another embodiment, a dual in-line memory module (DIMM) comprises a plurality of stacked memory assemblies electrically coupled to a DIMM printed circuit board (PCB). Each of the plurality of stacked memory assemblies has a total height that permits a minimum DIMM connector spacing with DIMMs in adjacent connectors. Each of the plurality of stacked memory assemblies comprises a plurality of integrated circuit (IC) assemblies stacked vertically.
Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.

## Claims

1. A lead frame for an integrated circuit (IC), the lead frame comprising:
a plurality of inwardly extending leads formed of a conductive metal, each lead having:
a first surface substantially parallel to a common plane and having a first feature proximate an inward end of the lead, wherein the first feature protrudes from the first surface, and
a second surface opposite the first surface having a second feature further from the inward end of the lead than the first feature, wherein the second feature protrudes from the second surface and wherein the plurality of leads form an opening within the leads into which the IC is insertable, wherein the opening is approximately a size of the IC.

2. The lead frame as recited in claim 1 wherein each lead further comprises:
a third feature protruding from the first surface, wherein the third feature is approximately the same distance from the inward end as the second feature.

3. The lead frame of claim 1, wherein the thickness of the lead frame is between 150 μm and 200 μm.

4. The lead frame of claim 2, wherein the third feature is configured to couple the lead frame to a plurality of integrated circuits.

5. The lead frame of claim 1 further comprising:
an area of the second surface adapted to be wire bonded to a terminal on a die,
wherein the area of the second surface is provided with a finish that is suitable for assuring reliable wire bonding.

6. The lead frame of claim 5 wherein the finish is formed from gold, silver, or palladium.

7. The lead frame of claim 5 wherein the area of the second surface adapted to be wire bonded is at a location on the second surface opposite the first feature.

8. The lead frame of claim 1 wherein the first feature is a bump feature that extends a distance from the first surface and the second feature is a bump feature that extends a distance from the second surface.

9. The lead frame of claim 1 wherein the first feature and the second feature have shapes that are substantially identical to each other.