Lead frame strip with molding compound channels

A lead frame strip has a plurality of unit lead frames. Each of the unit lead frames has a periphery structure connecting adjacent ones of the unit lead frames, a die paddle inside of the periphery structure, a plurality of leads connected to the periphery structure and extending towards the die paddle, and a molding compound channel in the periphery structure configured to guide liquefied molding material. The lead frame strip is processed by attaching a semiconductor die to each of the die paddles, electrically connecting each of the semiconductor dies to the leads, and forming a liquefied molding compound on each of the unit lead frames. The liquefied molding compound is formed such that the liquefied molding compound encapsulates the semiconductor dies and flows into the molding compound channels thereby forming molding extensions that extend onto the periphery structures.

TECHNICAL FIELD

The instant application relates to lead frame strips, and more particularly to physical supporting encapsulated semiconductor dies during processing of lead frame strips.

BACKGROUND

A lead frame forms the base or skeleton of an IC package, providing mechanical support to semiconductor dies during assembly into a finished package. A lead frame typically includes a die paddle for attaching a semiconductor die, and leads providing the means for external electrical connection to the die. The die can be connected to the leads by wires, e.g., through wire bonding or tape automated bonds. Lead frames are typically constructed from an electrically conductive material, such as copper or aluminum. The electrically conductive material may be provided in the form of a flat sheet metal. The features of the lead frames may be defined by forming openings in the flat sheet metal. The flat sheet metal can be patterned with a plurality of identically openings so as to form lead frame strips, i.e., interconnected strips used to package a number of semiconductor dies in a common process. Each lead frame strip includes a number of unit lead frames, with each unit lead frame having the die paddle and lead construction described above.

After completion of the assembly process, semiconductor dies attached to the die paddles are usually tested after separation of the unit lead frames from the lead frame strip, e.g., by punching. In other words, the semiconductor dies can be individually tested after singulation of the unit lead frames. Alternatively, the packaged semiconductor dies may be tested while still being physically supported by the lead frame strip using tie bars. This is commonly referred to as lead frame strip testing. In this technique, separation of the unit lead frames from the lead frame strip occurs after lead frame strip testing. However, the tie bars are formed from the same material as the die paddle, and are part of the unit lead frames. This is problematic for applications in which the die paddles serve an electrical connection function, e.g., in DSO (dual small outline) packages in which the exposed die paddles provide an electrical connection to the backside of semiconductor dies attached to the die paddles. In this case, the tie bars electrically short the die paddles to the lead frame strip and to other die paddles attached to the same lead frame strip, complicating the electrical testing process. Electrical isolation is also required for other lead frame processing such as partial plating and electrical charge processes.

SUMMARY

A method of processing a lead frame strip having a plurality of unit lead frames is disclosed. Each of the unit lead frames has a periphery structure connecting adjacent ones of the unit lead frames, a die paddle inside of the periphery structure, a plurality of leads connected to the periphery structure and extending towards the die paddle, and a molding compound channel in the periphery structure configured to guide liquefied molding material. According to an embodiment, the method includes attaching a semiconductor die to each of the die paddles, electrically connecting the semiconductor dies to the leads, and forming a liquefied molding compound on each of the unit lead frames. The liquefied molding compound is formed such that the liquefied molding compound encapsulates the semiconductor dies and flows into the molding compound channels thereby forming molding extensions that extend onto the periphery structures.

A method of forming a lead frame strip for packaging a plurality of semiconductor dies is disclosed. According to an embodiment, the method includes forming a plurality of connected unit lead frames, each of the unit lead frames including a periphery structure connecting adjacent unit lead frames in the lead frame strip, a die paddle inside of the periphery structure, and a plurality of leads connected to the periphery structure and extending towards the die paddle. The method further includes forming a molding compound channel in each of the periphery structures, each of the molding compound channels being configured to guide liquefied molding material so as to form molding extensions that extend onto the periphery structures.

A semiconductor device packaging assembly is disclosed. According to an embodiment, the semiconductor device packaging assembly includes a lead frame strip having a plurality of unit lead frames. Each of the unit lead frames include a periphery structure connected to adjacent ones of the unit lead frames, a die paddle inside of the periphery structure, a plurality of leads extending between the periphery structure and the die paddle, and a molding compound channel in the periphery structure. The molding compound channel is configured to guide liquefied molding material onto the periphery structure.

DETAILED DESCRIPTION

Embodiments disclosed herein include a lead frame strip with a plurality of unit lead frames. Each unit lead frame includes a periphery structure (e.g., a ring-like structure) connecting adjacent ones of the unit lead frames, a die paddle inside of the periphery structure, and a plurality of leads connected to the periphery structure and extending towards the die paddle. Molding compound channels are formed in the periphery structure of each unit lead frame. According to an embodiment, the periphery structure of each unit lead frame includes tabs extending towards the lead frame, and the molding compound channels are formed along these tabs. The molding compound channels have a trench structure such that liquefied molding material is guided through the channels. During encapsulation of the semiconductor dies, liquefied molding material flows into the channels thereby forming molding extensions that extend onto the periphery structures. These molding extensions may have a finger like structure, for example.

Advantageously, the lead frame strip configuration and methods described herein provide a mechanism to physically support the die paddles and correspondingly attached semiconductor dies using the molding material. This allows for further processing steps (e.g., lead frame strip testing) to be performed after the lead trim and before singulation of the encapsulated semiconductor dies. According to an embodiment, the encapsulated semiconductor dies are only physically connected to the periphery structures by portions of the molding compound that include the molding extensions. That is, the die paddles are not connected to the periphery structures by a tie bar and there is no electrical connection between the die paddles and the periphery structures. Thus, strip-level testing of multiple encapsulated semiconductor dies can be performed, and this testing can be applied to each terminal of the semiconductor dies, including the terminal that is connected to the die paddles.

Referring toFIG. 1, a plan view of a lead frame strip100is depicted, according to an embodiment. The lead frame strip100includes a plurality of unit lead frames102, two of which are depicted inFIG. 1. Each of the unit lead frames102has a periphery structure104(e.g., a ring-like structure) connecting adjacent ones of the unit lead frames102together. Each of the unit lead frames102additionally includes a die paddle106inside of the periphery structure104. In the event that the periphery structure104is formed as a closed loop, the die paddle106is completely surrounded by the periphery structure104. The die paddle106may have a rectangular shape, for example. The unit lead frames102further include a plurality of leads108connected to the periphery structure104and extending towards the die paddle106. Some of the leads108may be connected to the die paddle106as well.

The configuration of the unit lead frames102(e.g., the number and dimensions of the leads108, size of the die paddle106, etc.) may vary, depending upon the desired configuration of the finalized package design. Exemplary package designs include the SCT595 and SOT223 packages. The unit lead frames102may be formed along a single plane. Alternatively, the unit lead frames102may be formed along more than one plane. For example, the die paddle106may be vertically offset from the periphery structure104.

The lead frame strip100may be formed by providing a sheet layer of electrically conductive material (e.g., copper, aluminum and the like) and by forming openings110in the sheet metal, e.g., by stamping or etching. According to an embodiment, the openings110are formed by photolithography. The geometry of the openings110defines the features of the unit lead frames102, including the die paddle106, the periphery structure104, and the leads108. A plurality of lead frame strips100may be formed from a single sheet, and the individual strips100may be singulated (i.e., separated) from this sheet.

According to an embodiment, each unit lead frame102includes one or more tie bars112connecting the die paddle106to the periphery structure104. The tie bar112may be part of the sheet metal used to form the lead frame strip100. Thus, the tie bar112provides a metallic connection between the die paddle106and the periphery structure104that is separate from the leads.

The tie bars112are represented by dashed lines because they are optional. According to an embodiment, the tie bars112are not included in the unit lead frames102. In this embodiment, the leads108provide the only connection between the die paddles106and the adjacent periphery structure104. According to the embodiment ofFIG. 1, the die paddle106is physically connected (and supported) by two leads108on opposite sides of the die paddle106. These leads108may form the power supply leads108in the packaged device, for example.

Each of the unit lead frames102includes a molding compound channel114in the periphery structure104. According to an embodiment, the molding compound channels114are formed on a pair of opposite facing tabs116of the periphery structure104that extend towards the die paddles106. The arrangement of the tabs116and/or molding compound channels114may vary.

Referring toFIG. 2, an enlarged view of the tabs116having the molding compound channels114is depicted.FIG. 2Adepicts a plan-view of the tabs116and molding compound channels114andFIG. 2Bdepicts a cross-sectional view of the tabs116and molding compound channels114along the line A-A′ depicted inFIG. 2A. The molding compound channels114are configured to guide liquefied molding material. For example, the molding compound channels114may be formed as trenches or grooves extending from an outer surface118of the periphery structures104in each unit lead frame102. These trenches or grooves are deep and wide enough such that liquefied molding material (e.g., a thermosetting plastic) will enter the molding compound channels114and will be guided sidewalls of the channels114. Furthermore, the molding compound channels114increase the available surface area of the periphery structure104thereby improving adhesion between the molding material and the unit lead frames102. The number, length and depth of the molding compound channels114may be adjusted according to these considerations. The molding compound channels114are intentionally formed into the unit lead frames102, and are larger than naturally occurring impressions or indentations. Specific cross-sectional configurations of the molding compound channels114, and techniques for forming the molding compound channels114, will described in further detail with reference toFIGS. 6-10.

Referring toFIG. 3, a method of processing the lead frame strip100is shown. According to the method, a semiconductor die120has been attached to the die paddles106in each of the unit lead frames102. The semiconductor dies120may be attached by a solder material or adhesive, for example. After die attachment, the semiconductor dies120are electrically connected to the leads108. For example, wire bonds or tape automated bonds may be provided between a top side of the semiconductor dies120and the leads108. In the embodiment ofFIG. 3, the lower sides of the semiconductor die120is electrically connected to two of the leads108via the die paddle106to provide a source connection.

The method further includes forming a liquefied molding compound122on each of the unit lead frames102such that the liquefied molding compound122encapsulates the semiconductor dies120. This may be done using any of a number of known encapsulation techniques that utilize liquefied molding material. The molding compound122is an electrically insulating material, and may be a thermosetting epoxy resin or a thermoplastic, for example. According to an embodiment, encapsulation of the semiconductor dies120is done using a transfer molding process. In this technique, a mold cavity is placed on each unit lead frame102such that the die paddle106and the corresponding semiconductor die122attached to the die paddle106are arranged inside of the mold cavity. Thereafter, the liquefied molding compound122is transferred into the mold cavity (e.g., through a mold gate). The mold cavity adjoins the molding compound channels114so that the liquefied molding compound122may enter the mold cavity. According to an embodiment, the liquefied molding compound122enters the mold cavity and fills up the molding compound channels114.

An enlarged view of the lead frame strip100in region “B” is depicted inFIG. 4. The molding process is performed such that the liquefied molding compound122flows into the molding compound channels114thereby forming molding extensions124that extend onto the periphery structures104.

FIG. 4provides a detailed view of the molding extensions124. The molding extensions124correspond to portions of the molding compound112that are guided by the molding compound channels114, and remain in the molding compound channels114after the molding process.

FIG. 4Adepicts an embodiment in which the mold cavity is configured such that (within process tolerances) edges130of the molding compound122do not extend over the tabs116. That is, the only portions of the molding compound122that are arranged on or extend over the unit lead frame102are the molding extensions124.

FIG. 4Bdepicts an alternate embodiment in which the edges130molding compound122overlap with the tabs116. That is,FIG. 4Bdepicts an overmold configuration. In this embodiment, the mold cavity may be placed over the tabs116of the periphery to form a rectangular portion of the molding compound122that is supported by the tabs116.

After forming the liquefied molding compound122in the manner described above, the molding compound122is hardened. For example, if the molding compound122is a thermosetting epoxy resin or a thermoplastic, the lead frame strip100is cooled to cause the liquefied molding compound122to transition to a solid state or a partially solid state. As a result, the semiconductor dies120are encapsulated by an electrically insulating molding structure126. Furthermore, the encapsulated semiconductor dies120are secured to the adjacent periphery structures104by sections of the hardened molding structure126including the molding extensions124. That is, the die paddles106and correspondingly attached semiconductor dies120are physically coupled to the adjacent periphery structures104by sections of the hardened molding structure126that adhere to coupling points, i.e., the tabs116of the periphery structures104that include the molding compound channels114. The presence of the molding compound channels114and the corresponding molding extensions124provides a high degree of adhesion between the hardened molding compound and the unit lead frame102. Furthermore, the molding compound channels114and the corresponding molding extensions124provide an extended surface area to distribute the weight of the encapsulated semiconductor dies120. As a result, a secure and reliable connection between hardened molding structure126and the unit lead frame102is formed.

According to an embodiment, the hardened molding structure126has a rectangular portion. That is, the molding structure126has first edge sides128that are parallel to one another and second edge sides130that are parallel to one another and perpendicular to the first edge sides128. The leads108may be perpendicular to the first edge sides128. The second edge sides130extend at least to the edge side of the tabs116that include the molding compound channels114.

In the configuration ofFIG. 4A, the molding extensions124provide the exclusive physical support mechanism between the unit lead frame102and the molding structure126. In the overmold configuration ofFIG. 4B, the overlap region provides further physical support (in addition to the physical support provided by the molding extensions124) between the unit lead frame102and the molding structure126. However, the tabs116are not necessarily part of the finalized package structure. When the encapsulated semiconductor dies120are eventually singulated to form individual packaged semiconductor devices, the molding structure126can be cut along a scribe line S that is spaced apart from the tabs116, between the tabs116and the die paddle106. In other words, the process can be controlled so that the molding structure is devoid of any metallic components at the edge sides corresponding to remnant portions of the tabs116.

According to an embodiment, the molding extensions124are configured as fingers that extend away from the second edge sides130of the rectangular shaped portion of the molding structure. That is, a plurality of rectangular shaped molding extensions124that are parallel to one another extend outside of the rectangular shaped portion of the molding structure126, perpendicular to the second edge sides130.

Referring toFIG. 5, after hardening of the molding compound122, the connections between the leads108and the periphery structures104in each of the unit lead frames102are severed. According to an embodiment, every continuous metallic connection between the die paddles106and the periphery structures104in each unit lead frame102is severed. In other words, there is no tie bar112or lead108physically coupling the die paddles106to the periphery structure104. These connections may be severed by a lead trim process. If the unit lead frames102include the optional tie bar112, this connection may be severed as well. As a result, the die paddle106and leads108are electrically disconnected from the periphery structure104. Further, portions of the molding structure126including the molding extensions124provide the only physical support mechanism between the encapsulated semiconductor dies120and the periphery structures104in each unit lead frame102. That is, the die paddle106is electrically insulated from the periphery structure104, but remains physically supported by the periphery structure104.

After severing the connections, further processing steps can be performed on the lead frame strip100. These processing steps may include lead frame strip100testing, partial plating, and electrical charging, for example.

Advantageously, because the tie bar112can be eliminated, electrical access to each of the device terminals is possible, including any terminals connected to the die paddle106, during these further processing steps. That is, the portions of the hardened molding structure126that include the molding extensions124provide the necessary physical support of the encapsulated semiconductor dies120such that a tie bar112is no longer needed to perform strip-level testing.

A further advantage of eliminating the tie bar112is that the finalized packaged device is less susceptible to corrosion and delamination. In a tie bar design, when the finalized package is singulated by cutting the molding structure along a splice line (e.g., in the manner described above), the finalized package includes a remnant of the tie bar extending from the die paddle to an edge side of the package. This creates a path for chemical corrosion between the exterior and the interior of the package and therefore increases the likelihood of failure. In addition, this path increases the possibility that the molding structure126will delaminate (i.e., separate). By contrast, the lead frame strip100design disclosed herein allows for the hardened molding structure126to be cut along the splice line S (shown inFIGS. 4A and 4B) between the tabs116and the die paddle106such that an edge side of the finalized package where the hardened molding structure126has been spliced is substantially devoid of metallic material (e.g., from the tie bar112or the tabs116). In other words, the path for chemical corrosion described above can be eliminated. Further, this this can be done without forfeiting the benefits of lead frame strip testing.

According to an alternate embodiment, at least one continuous metallic connection between the die paddles106and the periphery structure104remains in each unit lead frame102after severing the connections between the leads108and the periphery structures104. For example, each unit lead frame102may include one or more tie bars112, and these tie bars112are not severed during the trimming of the leads108. After severing the connections between the leads108and the periphery structures104, the tie bar112forms the only continuous metallic connection between the die paddle106and the periphery structure104in each unit lead frame102. In the alternative, one of the leads108may not be trimmed. In either case, the portions of the hardened molding structure126including the molding extensions124advantageously provide additional physical support and adhesion between the hardened molding structure126and the periphery structure104in the manner previously discussed.

FIGS. 6-9depict possible cross-sectional geometries of the molding compound channels114.FIGS. 6-9each depict the unit lead frames102along the cross-sectional line A-A′ shown inFIG. 4.

As shown inFIG. 6, the molding compound channels114may have a v-shape cross-sectional geometry. As a result, the counterpart molding extensions124formed in the channels114act as teeth that engage with the unit lead frames102.

As shown inFIG. 7, the molding compound channels114may have a u-shape cross-sectional geometry. That is, the molding compound channels114include linear sidewalls and a bottom section that forms an obtuse or perpendicular angle with the linear sidewalls.

FIG. 8depicts an embodiment in which the molding compound channels114are formed on opposite facing surfaces118of the periphery structure104such that the molding extensions124extend over both of these opposite facing outer surfaces118. This configuration provides additional adhesion between the hardened molding structure126and the unit lead frame102. In addition, this configuration allows the lead frame strip100to be tilted while maintaining physical support of the encapsulated semiconductor dies120. The u-shaped geometry for the molding extensions124is used as an example. However, alternate geometries are possible for the molding extensions124, including the exemplary geometries disclosed herein.

FIG. 9depicts and embodiment in which the molding compound channels114have two cross-sectional regions132,134. In a vertical direction (V) perpendicular to an outer surface118of the periphery structure104, the molding compound channels114narrow in the first cross-sectional region132. In the same vertical direction (V), the molding compound channels114widen in the second cross-sectional region134. Consequently, the molding extensions124interlock with the corresponding molding compound channels114. That is, the molding extensions124engage with the sidewalls of the molding compound channels114such that the sidewalls resist physical forces applied by the molding extensions124in the vertical direction (V). As a result, the adhesion and resistance to mechanical forces provided by the molding extensions124is increased. According to an embodiment, at a transition between the two cross-sectional regions132,134, a sidewall portion136of the molding compound channels114is substantially parallel to the outer surface118.

The molding compound channels114may be formed by a punching process, for example. For example, after providing the sheet layer of electrically conductive material and forming openings110in the sheet metal that define the features of each unit lead frame102, the outer surface118of the electrically conductive material may be punched in each unit lead frame102. A pair of opposite facing molding compound channels114may be formed along opposite facing surfaces118(e.g., the molding compound channels114depicted inFIG. 8) by punching both of the opposite facing surfaces118, either sequentially or simultaneously. As a result, pairs of opposite facing molding compound channels114that are spaced apart from one another by a thinned portion of the electrically conductive material that remains after the molding compound channels114are formed. As an alternative to punching, other techniques may be utilized to form the molding compound channels114, e.g., etching, stamping, coining, etc.

FIG. 10depicts a two-step punch process that may be used to form the molding compound channels114. This two-step punch process may be used to form the cross-sectional configuration ofFIG. 9, for example.FIG. 10Adepicts a first punching step of the process in which the outer surface118is punched to form channels with linear sidewalls138extending away from the outer surface118to a bottom of the channels.FIG. 10Bdepicts a second punching step in which the sidewalls138are modified so as to form angles between the outer surface118and the bottom of the channels. This may be done by performing a wider and shallower punch in the second punch step than in the first punch step. Subsequently, an interlocking molding extension124may be formed in the channel114in the manner previously discussed.

The term “substantially” encompasses absolute conformity with a requirement as well as minor deviation from absolute conformity with the requirement due to manufacturing process variations, assembly, and other factors that may cause a deviation from the ideal. Provided that the deviation is within process tolerances so as to achieve practical conformity, the term “substantially” encompasses any of these deviations. For example, “substantially parallel” surfaces may be deviate from one another by up to five degrees.