Methods and apparatus for an improved integrated circuit package

In a described example, an integrated circuit (IC) package includes an IC die disposed on a die attach pad; a plurality of leads electrically connected to terminals on the IC die, the leads including a base metal; and molding compound material encapsulating portions of the IC die, the die attach pads, and the plurality of leads; the plurality of leads having a solder joint reinforcement tab. The solder joint reinforcement tabs include a first side, a second side opposite to the first side, a third side, a fourth side opposite to and in parallel to the third side, a fifth side forming an end portion of the solder joint reinforcement tab, the solder joint reinforcement tabs including a solderable metal layer on the second, third and fourth sides and on portions of the fifth side.

TECHNICAL FIELD

This disclosure relates generally to integrated circuit packaging, and more particularly to formation of solder joint reinforcement tabs on an integrated circuit package.

BACKGROUND

Lead frame strips are used in manufacturing packaged integrated circuits. Lead frame strips are comprised of multiple individual lead frames connected together by saw streets. An embedded lead frame strip is a lead frame strip to which integrated circuit dies are attached and embedded in molding compound. Lead frame strips include lead frames with conductive lead frame leads and a die attach pad for attaching an integrated circuit die. The conductive lead frame leads can be mechanically connected for stability in processing, but are separated electrically prior to completion of the packaging process. Terminals on the integrated circuit dies are electrically connected to the lead frame leads prior to completion of the packaged integrated circuit. In some packaged integrated circuits, bond wires are used to couple the terminals on the integrated circuit dies to the leads of the lead frames. In other packaged integrated circuits, the terminals on the integrated circuit dies are directly mounted to the leads of the lead frames using conductive solder balls or columns.

Packaged integrated circuits (PICs) such as Small Outline No-Lead (SON) and Quad Flat No-Lead (QFN) PICs are packaged by attaching IC dies using die attach material to a metal lead frame strip, making electrical connection from the IC dies to the lead frame leads, encapsulating the IC dies and lead frame strip in molding compound, and then singulating the individual SON or QFN packages by separating the individual packages by cutting through the saw streets by sawing or laser cutting, an operation referred to as “singulation.” The singulated SON or QFN PICs may then be soldered or connected electrically by other means to leads on a printed circuit board (PCB) that goes into a piece of electronic equipment. SON or QFN packages are referred to as “no lead” packages because the leads of the lead frame used for electrical connections between the PIC and a printed circuit board are located at the periphery of the PIC, and do not extend away from the package, as in prior packages such as J-lead and dual in-line (DIP) packages, saving area on the circuit board. Typically solder connections are formed between the terminals of the PIC and traces on the printed circuit board. QFN and SON packages are only slightly larger in board area than the integrated circuit die and are sometimes referred to as “near chip scale” packages, enabling smaller circuit board sizes and higher integration in end equipment.

Lead frame strips are typically made of a base metal such as copper or a copper alloy that is plated with a layer of solderable metal. The lead frame strips are sometimes referred to as “pre-plated” lead frame strips because the solderable metal layer can be provided on the lead frame strip by a lead frame manufacturer. One such solderable metal layer is an electroplated layer of nickel followed by an electroplated layer of palladium. Electroless and electroplated metal layers are sometimes used. Other solderable metals include silver, tin, gold, platinum, solder, and alloys thereof. The layer of solderable metal is intended to prevent oxidation of the lead frame base metal, which occurs when the base metal is exposed to air. Solder is not able to properly wet any oxidation layer that forms, and the solder cannot form a reliable solder joint to the oxidized base metal.

During singulation of the packaged integrated circuits, the layer of solderable metal and the base metal of the lead frame strips are cut through. The base metal of the lead frame is exposed on the sidewalls of the PICs during the singulation process, and may oxidize when exposed to air. Improvements are therefore desirable.

SUMMARY

In a described example, an integrated circuit (IC) package includes an IC die disposed on a die attach pad; a plurality of leads electrically connected to terminals on the IC die, each of the plurality of leads including a base metal; and molding compound material encapsulating portions of the IC die, the die attach pads, and the plurality of leads; the plurality of leads having a solder joint reinforcement tab extending from a periphery of the IC package. The solder joint reinforcement tab includes a first side extending longitudinally in a first direction, a second side opposite to and in parallel to the first side, a third side that is oriented in a second direction perpendicular to the first direction, a fourth side opposite to and in parallel to the third side, a fifth side forming an end portion of the solder joint reinforcement tab, the solder joint reinforcement tab including a solderable metal layer on the second, third and fourth sides and on portions of the fifth side.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are not necessarily drawn to scale.

FIG. 1Ais a cross section showing a lead frame strip108. In the figures herein, the lead frame strips are shown with a frontside surface, which is the surface the IC dies are mounted on, facing upwards. The opposite surface, a backside surface of the lead frame strip, portions of which can form external terminals of the IC packages, is shown facing downwards in the figures. However, the orientation of the devices shown in the figures is for explanation, and does not affect the operation of the embodiments. InFIG. 1Alead frame strip108is shown having two IC dies110attached to a frontside surface of the lead frame strip108. The ID dies are bonded by die attach material114to lead frame die mount pads102. Bond wires122electrically connect terminals116of the integrated circuit dies110to lead frame leads104. Portions of the IC dies110, the bond wires122, and the lead frame strip108are encapsulated in molding compound112. The backside surface of the die mount pads102and the backside surface of lead frame leads104are not encapsulated with molding compound112. Lead frame strip108includes solderable metal layer120on a base metal. Lead frame strip108can be provided to a packaging process including a pre-plated material that includes the solderable metal layer120. Alternatively, the solderable metal layer120can be applied to the base metal of lead frame strip108at a prior step (not shown) in the packaging process. The integrated circuit dies110are spaced from one another by a saw street112. Saw street112is a defined area of the lead frame strip108and the molding compound112where the integrated circuit devices can be separated in a later process step by cutting through the saw street. The exposed solderable metal layer120on the backside surface of the lead frame leads104and the die mount pads102form external PIC terminals124and126(seeFIG. 1B). Solder joints (not shown inFIG. 1A) will be used to electrically connect these external PIC terminals124and126to leads on a printed circuit board (PCB) when the PICs are mounted for use.

FIG. 1Bis a cross section showing the two PICs100ofFIG. 1Aafter the saw street118(FIG. 1A) is removed by sawing or laser cutting or other methods to singulate (singulation is the process of separating the plurality of molded IC dies mounted to the lead frame strip into single packaged ICs) the PICs100. The singulation process cuts through the molding compound112, the solderable metal layer120and the base metal of the lead frame strip108, forming a sidewall on the PICs100. Mechanical saws, laser cutting tools, and other methods can be used to singulate the PICs100. The singulation can be done with one tool or alternatively, the singulation can be done with more than one type of tool or by a method performed in multiple steps.

FIG. 2Ais a cross section showing two integrated circuit dies210that are “flip chip” mounted (the integrated circuit dies are mounted with terminals216facing down) to lead frame leads204of a lead frame strip208using ball bonds214. InFIG. 2, similar reference labels are used for similar elements shown inFIG. 1, for clarity. For example, molding compound112inFIG. 1corresponds to the molding compound212inFIG. 2. In flip chip mounting the integrated circuit dies are described as “flipped” when compared to a wire bonded package, where the terminals116are arranged facing away from the lead frame108surface (seeFIG. 1A). Portions of the lead frame strip208, the IC dies210, and the ball bonds214are encapsulated in molding compound212. The two PICs200are joined together by saw street218. Note that in a flip chip mounted package as shown inFIG. 2A, there is no die pad and no attachment to the die pad, the dies210are supported by the lead frame208at the terminals216. Heat slugs or other thermal conductors can be used to make a thermal path from the dies210. In another approach, thermally conductive filler particles can be incorporated into the molding compound212to improve heat conduction from the dies210.

FIG. 2Bis a cross section showing the two PICs200ofFIG. 2Aafter the saw street218(FIG. 2A) is removed by sawing or laser cutting to singulate the PICs200. The sawing cuts through the molding compound212, the solderable metal layer220and the base metal of the lead frame strip208to form sidewalls on the singulated PICs200. The bottom surfaces of the lead frame pads204are not encapsulated with molding compound212. The solderable metal220on the bottom surfaces of the lead frame pads204remains exposed and forms external PIC terminals224and226(seeFIG. 2B) that are used to attach the PICs200to leads on a PCB (not shown).

Lead frame strips108(FIG. 1A) and208(FIG. 2A) are typically made of a base metal such as copper or a copper alloy that is plated with a layer of solderable metal, shown as120and220. One such solderable metal layer is an electroplated layer of nickel followed by an electroplated layer of palladium. Other solderable metals include silver, tin, gold, platinum, solder, and alloys thereof. ENIG (electroless nickel/immersion gold), ENEPIG (electroless nickel/electroless palladium/immersion gold) and other electroless and electroplated metal layers are used. Multiple layers of alternating materials are sometimes used in the layer of solderable metal to reduce the possibility of migration of copper ions from the base metal through the electroless or electroplated solderable material. The layer of solderable metal,120and220, prevents oxidation of the lead frame base metal which occurs when the base metal is exposed to air. When oxidation occurs, solder is not able to properly wet the oxidation layer and form a reliable solder joint when the PICs are later installed to a printed circuit board (PCB) using a solder mounting process.

As is illustrated in the cross sections inFIGS. 1B and 2B, during singulation of the lead frame strips and the molding compound, the layer of solderable metal,120and220, and the base metal of lead frame strips,108and208, are cut through by sawing or laser cutting. The base metal of the lead frame,108and208inFIGS. 1 and 2, is then exposed on the sidewalls of the PICs100and200during singulation, and the exposed base metal may subsequently oxidize when exposed to air.

FIG. 3is a cross section of a PIC300soldered to a printed circuit board (PCB)334with solder joints328and330. Note that inFIG. 3, similar reference labels are used for similar elements that are shown inFIG. 1, for clarity. For example, the integrated circuit die310inFIG. 3corresponds to the integrated circuit dies110inFIG. 1. Integrated circuit die310includes terminal316. Die attach314attaches the IC die310to die pad302of lead frame strip308. Bond wire322electrically connects the terminal316to the lead frame lead304. Molding compound312partially encapsulates the integrated circuit die310, the bond wire322, the lead frame strip308including the die pad302and the lead frame lead304. Solder joints328and330connect external PIC terminals324and326on the bottom of the PIC300to conductive leads336on the surface of the PCB334. The layer of solderable metal320covers the backside surfaces of the lead frame strip307and the terminals324,326include solderable layer320. As shown inFIG. 3, an oxide layer332may form on the vertical side of the lead frame lead304where the base metal is exposed during singulation. Solder330does not wet the oxide layer332, preventing a strong, reliable solder joint330from forming.

FIG. 4Ais a perspective view of a PIC400.FIG. 4Ashows the sidewall of the PIC400that is exposed during singulation. The singulation cut forms the sidewall (see PIC100and PIC200inFIGS. 1B and 2B) which includes a vertical surface in molding compound412and in lead frame leads404. A layer of solderable metal420encases the lead frame leads404, except for the sidewall of the base metal of the lead frame leads404that is exposed during singulation. The sidewalls of the lead frame leads404that are exposed on the sidewall of the PIC400include base metal which oxidizes when exposed to air.

FIG. 4Bis a perspective view that shows the PIC400ofFIG. 4Aafter solder joints430are formed between the external terminals on the bottom of the lead frame leads404and conductive leads on a PCB434. (The external terminals and the conductive leads on the PCB434cannot be seen inFIG. 4Bas the solder430covers the conductive leads in this view). The solder430wets the solderable metal420that covers the external PIC terminals, but does not wet or adhere to the oxidation layer (see332inFIG. 3, for example) formed on exposed base metal on the sidewall of the lead frame leads404. This solder joint, shown as330and430inFIGS. 3 and 4B, respectively, may be weak and may delaminate causing the PCB integrated circuit to fail. The delamination that can occur while the PCB is in use can cause field failures of the end equipment, necessitating repair or replacement of the end equipment.

FIG. 5is a flow diagram of the major steps in the manufacture of PICs using an embodiment having solder joint reinforcement tabs (SJRTs).

In step502, a first cut is made in the saw street from the frontside of the embedded lead frame strip. The first cut extends through the molding compound in the saw street and into the frontside surface of the lead frame strip in the saw street. In one example embodiment, the cuts were made using a wide singulation saw blade. (In an alternative embodiment, laser cutting may also be used, or in another alternative embodiment, multiple passes with a narrower saw blade can be used to form a wide cut. In addition, other methods to perform the first cut can be used.) Typically when cutting from the frontside of the lead frame strip, the backside of the embedded lead frame strip can be supported. One such means of support is a tape.

FIG. 6Ais a cross section illustrating the results of step502. Note that the reference labels used inFIGS. 6A-6Bare similar for similar elements shown inFIGS. 1A-1B, for clarity. For example, the molding compound612inFIG. 6Acorresponds to the molding compound112inFIG. 1A.FIG. 6Ashows two PICs600joined together by a saw street618. The molding compound612and the lead frame608are first cut in the saw street618from the frontside of the embedded lead frame strip608, leaving a thin band621of saw street material618. In this example embodiment, thin band621of the saw street618has a thickness “T” of approximately 40 μm+/−10 μm that remains after the first cut, connecting the bottom of the two PICs600. In additional alternative embodiments, the thickness T of the band621can vary. In an example embodiment the first cut is made by a singulation saw blade, in one example the singulation saw blade used is 300 μm wide. The thin band621includes a thin portion of lead frame material (visible inFIG. 6A) and also molding compound material (not visible in the cross section ofFIG. 6A) that is left after the first cut in saw street618. The cut from the frontside of the lead frame strip exposes the base metal of the lead frame strip in the thin band621in the saw street618, because the cut into the lead frame strip608removes the solderable metal620from the frontside of the lead frame strip where the cut is made. InFIG. 6AIC dies610are attached by die attach614to lead frame die mount pads602. Terminals616on the dies610are electrically connected to lead frame leads604using wire bonds622. The surfaces of the base metal of lead frame608are coated with solderable metal layer620. Portions of the IC dies610, lead frame die mount pads602and lead frame leads604, and wire bonds616are encapsulated in molding compound612to form the embedded lead frame strip with PICs600joined together by saw streets618. The backside surfaces of the die mount pad602and the lead frame leads604are not encapsulated by molding compound612, and these exposed portions form external PIC terminals624and626. Solder joints that will be made between these external PIC terminals624and626and leads on a PCB (not shown inFIGS. 6A-6B) are used to mount the PICs600on a PCB.

Returning toFIG. 5, in step504a second cut is made through the thin band of saw street621. This step completely separates the PICs600; that is, it is a singulation step. A thin ledge of saw street material664remains attached to and surrounds the bottom of the PICs600. In an embodiment, the second cut is made from the backside of the lead frame strip. In one approach a singulation saw is used. In an alternative approach a laser cutting operation is used.

The results of step504inFIG. 5is illustrated in the cross section ofFIG. 6B. The center of the thin band621of the remaining saw street618is cut through in the second cutting step, completing singulation of the PICs600. The second cut is narrower than the first cut in step502. Following the singulation, the thin ledge664of saw street material remains attached to and surrounds the bottom of the PICs600. In an example embodiment a singulation saw was used to make the second cut. In an example embodiment, a thin singulation blade used to make this cut is about 200 μm wide. In an alternative embodiment, laser cutting can be used. After this second cutting, the ledge664of the remaining saw street material attached to each PIC has a length shown as L inFIG. 6Bthat is about 50 μm+/−20 μm long measured from an outside boundary of the PIC and the ledge664has a thickness T shown inFIG. 6Athat is about 40 μm+/−10 μm thick. The frontside (the upper surface as drawn in the figures) of the embedded lead frame strip is typically supported when the thin remaining band623is cut through from the backside (the lower surface of the leadframe strip as drawn in the figures). One such means of support is a tape.

As seen inFIG. 6B, the embodiments include a solder joint reinforcement tab formed in ledge664for the leads626of the PICs600. As is further described hereinbelow, the solder joint reinforcement tabs provide additional area of the solderable metal layer620to provide increased wettable area for solder to form a reliable solder joint between the external terminals of the PIC and a PCB. Note that the portions of the solder joint reinforcement tabs in ledge664that form surfaces due to the first cutting operation and the second cutting operation of the embodiments are not covered by solderable metal layer620, as the cutting operations expose the base metal of the lead frame strip608.

FIGS. 7A-7Eillustrate in a series of views the method of forming an embodiment. Note that inFIGS. 7A-7E, similar reference labels are used for similar elements shown inFIGS. 6A-6B, for clarity. For example, molding compound712corresponds to molding compound612inFIGS. 6A-6B.

FIG. 7Ais a perspective view showing the sidewall of the PIC700after the singulation process. The ledge764of saw street material surrounding the base of the PICs700is comprised of solder joint reinforcement tabs (SJRTs)725with portions of the molding compound723left between the SJRTs725.

FIG. 7Billustrates in a perspective view one embodiment after performing the step506inFIG. 5. The remaining molding compound material723(seeFIG. 7A) between SJRTs725may be removed by laser cutting or may be removed by applying pressure to the molding compound portions723and breaking them off.

InFIG. 7B, the SJRTs725are shown with the solderable metal layer720on portions of the SJRTs. This is further shown in a right side view and a front side view of the PIC700illustrated inFIGS. 7C-7D, respectively.

InFIG. 7C, the PIC700in the perspective view ofFIG. 7Bis shown in a right side view. InFIG. 7C, the view looks onto the end portions of the SJRTs725(seeFIG. 7B). As shown inFIG. 7C, the SJRTs725have a first side labeled725-I that corresponds to the frontside portion of the lead frame strip where base metal was exposed by the first cutting operation in step502. The first side extends longitudinally in a first direction away from the periphery of the base of PIC700. An opposing second side725-II is parallel to the first side and is covered by the solderable metal720, the second side corresponds to the backside surface of the lead frame strip708. A third side725-III is oriented in a second direction perpendicular to the first direction of the first side725-I and is covered by the solderable metal layer720, and an opposite fourth side725-IV is parallel to the third side725-III and also covered by the solderable metal layer720. A fifth side of the SJRTs725, the end portion of SJRT725formed by the second cutting operation of step504, is labeled725-V in the front side view inFIG. 7D. As shown inFIGS. 7B and 7C, this fifth side is partially covered by the solderable metal layer720, and also includes base metal of the lead frame strip708that is exposed by the second cutting operation. Because the SJRTs725extend away from the PIC in a direction parallel to the base of the PIC700, these SJRTs725can be described as “horizontal SJRTs” relative to the PIC700.

FIG. 7Eshows in a projection view the PIC700including the SJRTs725after it is soldered to a PCB734. The bottom surface and sides of the SJRTs725are coated with solderable material (not visible inFIG. 7E), so solder readily forms solder joints to the bottom and sides of the SJRTs725. The solder joints730connect conductive leads (not visible as obscured inFIG. 7E) on the PCB734to the external terminals on the bottom of the PIC700(obscured inFIG. 7E, not visible, see624,626inFIG. 6Bfor example) and also to the bottom and sides of the SJRTs725. The enlarged solder joint730between the lead on the PCB734and the bottom and sides of the SJRTs725significantly strengthens the solder joints730(when compared to the solder joints formed on PIC leads made without use of the embodiments) and significantly reduces failure of the PCB mounted packaged integrated circuits. These advantages are accomplished with little additional cost.

Again referring toFIG. 5, an optional step508(illustrated in a series of perspective views inFIGS. 8A through 8D) may be performed. InFIGS. 8A through 8D, similar reference labels are used in for similar elements shown inFIGS. 6A-6B, for clarity. For example, molding compound812corresponds to molding compound612inFIGS. 6A-6B.FIGS. 8A-8Dillustrate in a series of perspective views a process in which the SJRTs825are bent from an initial horizontal position to a vertical position, to end up alongside and against the sidewalls of the PICs800. This approach reduces the footprint (board area of the PCB) of the PIC800and also increases the strength and reliability of the solder joint830(seeFIG. 8D.) Because the SJRTs are bent upwards in the process, the SJRTs825can be described as “vertical SJRTs” with respect to PIC800.

InFIG. 8Athe PIC800with the ledge864of saw street material surrounding the base of the PIC800is placed on a brake table854. The ledge864of saw street material is comprised of SJRTs825with portions of molding compound823left between them.

InFIG. 8Bthe brake856which is connected to the brake table854by a hinge852is raised to apply upward pressure to the ledge864of saw street material. This bends the SJRTs825from a horizontal orientation to a vertical orientation against the vertical sidewall of the PIC800. The upward pressure also causes the portions of molding material823remaining between the SJRTs825to break off. In another alternative embodiment, if desired the portions of molding material823may be removed using laser cutting prior to bending the SJRTs825.

InFIG. 8C, the PIC800is shown after the bending operation is complete. The SJRTs825are shown now oriented vertically and lying alongside the sidewall of PIC800. The vertical bottom surface and sides of the SJRTs825are coated with the solderable layer820.

As is illustrated inFIG. 8D, when the PIC800with the vertical SJRTs825is soldered to leads on a PCB834, a significantly taller solder joint830is formed. The increased height of the solder joint830provides increased strength and reliability.

The described examples presented herein to illustrate the embodiments show QFN integrated circuit packages. However, the embodiments are not so limited and the methods and arrangements of the embodiments can be applied to other package types where increased solder joint reliability is desired.

Modifications are possible in the described embodiments, and other alternative embodiments are possible within the scope of the claims.