PACKAGES WITH MULTIPLE UNDERFILL TYPES AND METHODS FOR FORMING THESE

Abstract

Method comprising: bonding a first package component over and to a second package component, distributing a first underfill between the first package component and the second package component, bonding a third package component over and to the second package component, wherein a second underfill is present between the third package component and the second package component, and wherein the first underfill and the second underfill are different underfill types, and forming the first package component and the third package component in a molding compound.

Description

BACKGROUNDIntegrated circuit packages may include multiple package components, such as device dies and package substrates, bonded together to increase functionality and integration level. Due to the difference between different materials of the plurality of package components, warpage may occur. As the size of the packages increases, the warpage becomes more severe.The prior art relating to the subject matter of the invention can be found, for example, in US 2013 / 0 249 532 A1, US 2004 / 0 262 774 A1 and US 2005 / 0 122 697 A1.The invention provides a method according to claim 1, a package according to claim 10 and a package according to claim 16. Embodiments are set forth in the dependent claims.BRIEF DESCRIPTION OF THE DRAWINGSAspects of the present disclosure will be best understood from the following detailed description when read in conjunction with the accompanying figures. It should be noted that, according to the standard industry process, various elements are not drawn to scale. Indeed, for clarity of explanation, the dimensions of the various elements may be arbitrarily increased or decreased FIGS. 1-12, 13A, 13B, and 13C show the cross-sectional views of intermediate stages in forming a package having multiple underflow types, in accordance with some embodiments. FIGS. 14-17 show top views of packages having multiple underflow types, in accordance with some embodiments. FIGS. 18-23, 24A, 24B, and 24C show the cross-sectional views of intermediate stages in forming a package having multiple underflow types, in accordance with some embodiments. FIGS. 25-28 show the top views of packages having multiple underflow types, in accordance with some embodiments. FIG. 29 illustrates a cross-sectional view of a package having multiple underflow types, in accordance with some embodiments. FIG. 30 illustrates a top view of a package having multiple underflow types, in accordance with some embodiments. FIG. 31 illustrates a process flow for forming a package, in accordance with some embodiments.DETAILED DESCRIPTIONThe following disclosure provides many different embodiments, or examples, for implementing various features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, forming a first element over or on a second element in the description below may include embodiments in which the first and second elements are formed in direct contact, and may also include embodiments in which additional elements may be formed between the first and second elements such that the first and second elements may not be in direct contact. In addition, the present disclosure may repeat reference numerals and / or letters in the various examples. This repetition is for purposes of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and / or embodiments discussed.Moreover, terms relating to spatial relativeity, such as "underlying", "under", "lower", "overlying", "upper", and the like, may be used herein for ease of discussion to describe the relationship of one element or structural element to another element or structural element (elements or structural elements) as depicted in the figures. The terms relating to spatial relativeness are intended to encompass different orientations of the device being used or operated in addition to the orientation depicted in the figures. The device may be otherwise oriented (rotated 90 degrees or otherwise oriented) and the spatially relative terms used herein may likewise be construed accordingly.A package having multiple underflow types and the method of forming the same are provided. According to some embodiments of the present disclosure, multiple package components, such as device dies and packages, are bonded to another package component, such as an interposer. A first underflow type and a second underflow type different from each other are distributed to be under first and second of the plurality of package components. The various underfills may be selected from a non-conductive film (NCF), a capillary underfill, a form underfill, and the like. By employing different types of underfill, the warpage of packages is reduced and the difficulty in filling gaps underlying large package components is reduced. Embodiments discussed herein are intended to provide examples to enable making or using the subject matter of this disclosure, and one of ordinary skill in the art will readily understand modifications that may be made while retaining the scope of various embodiments contemplated. Like reference numerals are used throughout the several views and embodiments to refer to like elements. Although embodiments of methods may be discussed as being performed in a particular order, other embodiments of methods may be performed in any logical order.In the discussion below, several types of underfills, such as an NCF, a capillary underfill, and a form underfill, are employed in various embodiments. The properties of some of these underflow types are discussed herein. These underfills may comprise similar materials. These differences are more distinguishable by comparison and by their use, as will also be discussed in the description of the processes discussed below. According to some embodiments, each of the underfills (the NFC, the capillary underflow and the mold underflow) may include a base material and may or may not include a filler, which may be in the form of filler particles mixed with the base material. The base material may be a resin, an epoxy resin and / or a polymer. Some examples of base materials include epoxy amine, epoxy anhydride, epoxy phenol, isocyanate ester, acrylate, methacrylate, polyester, polyacrylonitrile, or the like, or combinations thereof. The filler is dielectric and may contain silica, alumina, boron nitride or the like, which may be in the form of spherical particles.In some embodiments, capillary underfills are dispensed in the form of flowable materials and then cured to a solid. The filler particles in the capillary underfills may also have different sizes / diameters, which may range between about 0.1 μm and about 20 μm. The volume percentage of filler may range between 0 percent and about 80 percent. Capillary imperfections may have a coefficient of thermal expansion (referred to as CTE1, measured below the corresponding glass transition temperature) in the range between about 15 ppm and about 50 ppm. The capillary underfills may also have CTEs (referred to as CTE2, measured above the corresponding glass transition temperature) in the range between about 50 ppm and about 250 ppm. The modulus values of the capillary underfills (at temperatures below the respective glass transition temperature) may be in the range between about 2 GPa and about 15 GPa, while their modulus values (measured at temperatures above the respective glass transition temperature) may be in the range between about 0.01 GPa and about 0.5 GPa. The glass transition temperatures of capillary underfills may range between about 60° C. and about 250° C. The viscosity of capillary underfills (at 25° C.) may be in the range between about 2 Pa·s and about 100 Pa·s. The viscosity of NCFs (at 100° C.) may be in the range of between about 0.01 Pa·s and about 0.3 Pa·s.In some embodiments, NCFs are solid films when used. The filler particles in the NCFs may also have different sizes / diameters, which may range between about 0.1 μm and about 20 μm. The volume percentage of filler may range between 0 percent and about 60 percent. The CTE1values of the NCFs (measured below the corresponding glass transition temperature) may range between about 20 ppm and about 70 ppm. The NCFs may also have CTE2values (measured above the corresponding glass transition temperature) in the range between about 60 ppm and about 250 ppm. The modulus values of the NCFs (measured at temperatures below the respective glass transition temperature) may be in the range between about 1 GPa and about 10 GPa, while their modulus values (measured at temperatures above the respective glass transition temperature) may be in the range between about 0.01 GPa and about 0.5 GPa. The glass transition temperatures of NCFs may range between about 60° C. and about 250° C.According to some embodiments, mold underfills are flowable when dispensed to form packages, and are then cured to a solid. The filler particles in the mold underfills may also have different sizes / diameters, which may be in the range between about 0.1 μm and about 20 μm. The volume percentage of filler may range between 0 percent and about 97 percent. Capillary imperfections may have CTE1values (measured at temperatures below the corresponding glass transition temperature) in the range between about 3 ppm and about 30 ppm. The form imperfections may also have CTE2values (measured above the corresponding glass transition temperature) in the range between about 10 ppm and about 100 ppm. The modulus values of the form underfills (measured at temperatures below the corresponding glass transition temperature) may be in the range between about 5 GPa and about 30 GPa, while their modulus values (measured at temperatures above the corresponding glass transition temperature) may be in the range between about 0.1 GPa and about 2 GPa. The glass transition temperatures of form underfills may range between about 100° C. and about 250° C. The viscosity of form underfills (at 25° C.) may be in the range between about 50 Pa·s and about 1,000 Pa·s.In some embodiments, when two or more of a capillary underfill, an NCF underfill, and / or a mold underfill are used in a same package, although these underfills may (or may not) comprise the same materials, their compositions, such as types of base materials, the type of fillers, and / or the percentages of the base materials and the fillers, are different from each other. Accordingly, properties of different underfills are different from each other. For example, the CTE of an NCF may be greater than the CTE of the (cured) capillary underflow, which is further greater than the CTE of the (cured) mold underflow. The viscosity of the form underflow is higher than the viscosity of the capillary underflow. Accordingly, capillary underfills may be used to fill small gaps and the gaps underlying large package components, while a mold underfill may be used to fill larger gaps and may be used to surround package components. Since NCFs are already deposited prior to bonding (as shown in FIGS. 2 and 4 ), NCFs may also be used to fill small gaps and the gaps underlying large package components. The volume percentage of filler in the form underflow may be greater than the volume percentage of filler in the capillary underflow, which may also be greater than the volume percentage of filler in the NCF. The modulus of the form underflow may be greater than the modulus values of the capillary underflows and the NCFs.FIGS. 1-12, 13A, 13B, and 13C show the cross-sectional views of intermediate stages in forming a package having multiple underflow types, in accordance with some embodiments. The respective processes are also schematically depicted in the process flow illustrated in FIG. 31.Referring to FIG. 1, a wafer 20 is formed having multiple package components 22 therein. The wafer 20 may be a device wafer, a reconstructed wafer (with device dies packaged therein), or the like. Each of the package components 22 may be a device die, a package having a device die(s) packaged therein, a system-on-a-chip die (SoC die) having multiple integrated circuits (or device dies) integrated as a system, or the like. The device dies in the package components 22 may be or include logic dies, memory dies, input-output dies, integrated passive devices (IPDs), or the like, or combinations thereof. For example, the logic device dies in the package components 22 may be CPU (central processing unit) dies, GPU (graphics processing unit) dies, mobile application dies, MCU (microcontroller) dies, BB (BaseBand) dies, application processor (AP) dies, or the like. The memory dies in the package components 22 may include SRAM (Static Random Access Memory) dies, DRAM (Dynamic Random Access Memory) dies, or the like. For example, the package components 22 may include high-performance memory (HBM) stacks, which may include memory dies forming a die stack and an encapsulation material (such as a molding compound) encapsulating the memory dies. The device dies in the package components 22 may include semiconductor substrates and interconnect structures.Electrical connectors 24 are formed on an upper surface of the wafer 20. In accordance with some embodiments, electrical connectors 24 may include non-solder metal elements 24A, which may include metal pads, metal pillars, or the like, and solder regions 24B over the non-solder metal elements 24A. According to some embodiments, the wafer 20 is singulated, for example, by sawing through the wafer 20 by scribe lines 30, such that the package components 22 are separated from each other.In alternative embodiments, wafer 20 is not singulated at this stage. Rather, as shown in FIG. 2, an NCF 28 is mounted (laminated) over the wafer 20. The corresponding process is shown as process 202 in process flow 200 illustrated in FIG. 31. In some embodiments, the NCF 28 is a preformed solid (and flexible) film that is bonded to the wafer 20 by, for example, pressing the NCF 28 against the wafer 20. The electrical connectors 24 are thus pushed into the NCF 28. According to alternative embodiments, the NCF 28 is dispensed onto the wafer 20 as a flowable material and is then cured to a solid film. The NCF 28 may embed electrical connectors 24 therein, with the top surface of the NCF 28 being higher than the top ends of the electrical connectors 24. According to some embodiments, after attaching the NCF 28, the wafer 20 is singulated together with the NCF 28, for example, by sawing through the wafer 20 through scribe lines 30. The corresponding process is shown as process 204 in the process flow 200 illustrated in FIG. 31. Accordingly, as shown in FIGS. 1 and 2, the singulated package components 22 may or may not include the NCF 28.Referring to FIG. 3, a package component 32 is provided having package components 34 therein. The package component 32 may be a wafer. In some embodiments, package component 32 includes a substrate 36 and an interconnect structure 41 over substrate 36. In some embodiments, the substrate 36 is a semiconductor substrate, which may be a silicon substrate. According to alternative embodiments, the substrate 36 is a dielectric substrate. The interconnect structure 41 may include multiple dielectric layers and conductive elements (such as metal lines, vias, bond pads, and the like) therein. The dielectric layers and the conductive elements are respectively represented using dielectric layers 38 and bond pads 42.According to some embodiments, the package component 32 includes an interposer wafer. The package components 34 may thus also be referred to as interposers 34, which may include vias 40 extending into the semiconductor substrate 36. According to other embodiments, the package component 32 is of a different type, such as a reconstructed wafer, a package substrate fin, or the like. Accordingly, the package components 34 may be packages, package substrates, or the like. In the discussion below, package components 34 are referred to as interposers as an example, although they may also be of a different type.Referring to FIG. 4, the package components 22 are bonded to the package component 34. Although a group of package components 22 is illustrated, multiple groups of package components 22 may also be attached, each group being attached to the corresponding package component 34. Each of the package components 22 may include any of the structures and circuits mentioned above, and the structures of the package components 22 in the same group may be the same as or different from each other. Each of the package components 22 may therefore be selected from a device wafer, a package, an IPD, or the like, as mentioned above. Some of the package components (labeled 22A) have NCFs 28 mounted thereon. Accordingly, the package components 22A are pressed against the interposers 34 such that electrical connectors 24 pass through the respective NCFs 28 to contact the electrical connectors 24. The NCFs 28 may be squeezed out of the gaps between the corresponding overlying package components 22 and the underlying interposer 34. In some embodiments, some package components (labeled 22B) do not have NCFs attached thereto. Throughout the specification, letters A, B, and C may be used to distinguish package components from each other. The letters are used to distinguish which underflow types will be among them, and are not used to distinguish the types / circuits of the package components.The bonding process includes attaching package components 22A and 22B over the corresponding package components 34. Next, as shown in FIG. 5, a reflow process is performed to bond the package components 22A and 22B to interposer 34. The respective processes for bonding package components 22A and 22B are shown as processes 206 and 208 in the process flow 200 illustrated in FIG. 31. Due to the pressing and further due to the heating in the reflow process, the portions of the NCFs 28 outside the gaps may have convex / rounded top surfaces and sidewalls.Referring to Fig. 6, a capillary underfill 44 is dispensed and then cured. The corresponding process is shown as process 210 in process flow 200 illustrated in FIG. 31. Through the capillary, the capillary underflow 44 flows to the gap between the package component 22B and the interposer 34, and climbs the gaps between adjacent package components 22 when the gaps are narrow, for example less than about 300 μm. If the gaps are wide, such as wider than about 300 μm, the capillary underflow 44 may not be able to climb to the top surfaces of the package component 22B. Dashed lines 44T schematically show the top surfaces of the corresponding capillary underfill 44 Due to the shrinkage occurring during curing, the capillary underfill 44 may have concave sidewall surfaces, as may be noted with reference to FIG. 13C.FIG. 7 illustrates the encapsulation of the package components 22 in the encapsulation material 48. The encapsulation material 48 may also include a base material and a filler. In some embodiments, the encapsulation material 48 includes a molding compound that may be formed of a different material (such as a different base material and / or a different filler) than the capillary underfill 44. In alternative embodiments, the encapsulation material 48 is formed from or comprises a mold underfill discussed in the preceding paragraphs, and therefore its material and properties are not repeated. In accordance with some embodiments in which the top surfaces of the capillary underfill 44 are lower than the top surfaces of the package components 22, as shown by the dashed top surfaces 44T, the encapsulation material 48 may also fill the top portions of the gaps, where the top portions are higher than top surfaces 44T.After the encapsulation material 48 has been dispersed, a curing process is performed to solidify the encapsulation material 48. According to alternative embodiments, the capillary underfill 44 and the encapsulation material 48 are cured in a same curing process. A planarization process, such as a chemical mechanical polishing (CMP) process or a mechanical grinding process, is performed to remove excess portions of the encapsulation material 48 over the package components 22. The substrates (such as semiconductor substrates) of the package components 22 may be exposed. The resulting structure is referred to as reconstructed wafer 50.Referring to FIG. 8, the reconstructed wafer 50 is placed over a support 52. The corresponding process is shown as process 214 in the process flow 200 illustrated in FIG. 31. The support 52 may be a glass support, an organic support, or the like. A releasing film 54 is coated on the substrate 52 to attach the reconstructed wafer 50 to the substrate 52. The release film 54 may be formed of a polymer-based material (such as a light-to-heat conversion (LTHC) material) that may be removed from the reconstructed wafer 50 along with the carrier 52 in subsequent processes. According to some embodiments of the present disclosure, the release film 54 includes an epoxy-based thermal release material coated on the carrier 52.FIGS. 9 and 10 show the formation of a back side interconnect structure. The corresponding process is shown as process 216 in process flow 200 illustrated in FIG. 31. Referring to FIG. 9, in accordance with some embodiments, a backside grinding process may be performed to thin the substrate 36 until the vias 40 are exposed. Next, the substrate 36 may be slightly recessed by etching so that the vias 40 protrude from the back side of the substrate 36. A dielectric material, such as silicon oxide, silicon nitride, silicon oxynitride, or the like, is then deposited on the substrate 36, followed by a planarization process to level the top surface of the dielectric material and vias 40. The remaining dielectric material is shown as dielectric layer 56.Next, referring to FIG. 10, dielectric layer(s) 58 and conductive elements 60 (including RDLs and / or metal pads) are formed to be electrically connected to the vias 40. In accordance with some embodiments of the present disclosure, the dielectric layers 58 are formed from oxides such as silicon oxide, nitrides such as silicon nitride, or the like. Conductive elements 60 may be formed by plating or, alternatively, damascene processes. Electrical connectors 62, which may include bond pads, metal pillars, solder regions, and / or the like, are formed over and electrically connected to the conductive elements 60.Next, in accordance with some embodiments, reconstructed wafer 50 may be debondd from carrier 52. The corresponding process is shown as process 218 in the process flow 200 illustrated in FIG. 31. According to some embodiments, a light beam such as a laser beam is projected onto the release film 54, and the release film 54 is decomposed under the heat of the light beam. The reconstructed wafer 50 is detached from the carrier 52 in this way. In a subsequent process, the reconstructed wafer 50 may be placed on a dicing tape (not shown) and singulated along scribe lines 64 using a dicing process such that the reconstructed wafer is separated into separate packages 50'. The corresponding process is shown as process 220 in the process flow 200 illustrated in FIG. 31.FIG. 11 illustrates the bonding of one package 50' to another package component 66. The corresponding process is shown as process 222 in the process flow 200 illustrated in FIG. 31. In some embodiments, package component 66 is a wafer level component having multiple identical package components 66' therein. For example, the package component 66 may be a package substrate strip having a plurality of package substrates 66' therein. The package substrates 66' may be package substrates having cores including cores or may be coreless package substrates having no cores therein. According to alternative embodiments, the package component 66 may be of another type, such as an interposer wafer, a reconstructed wafer, or the like. The package component 66 may be free of active devices therein, such as transistors and diodes. The package component 66 may also be free of (or may include) passive devices therein, such as capacitors, inductors, resistors, or the like.The package component 66 includes a plurality of dielectric layers 68. In some embodiments, the dielectric layers 68 may include dry films such as Ajinomoto build-up films (ABFs), polybenzoxazole (PBO), polyimide, or the like. When the dielectric layers 68 include cores, the dielectric materials may include epoxy, resin, glass fiber, prepreg, glass, molding compound, plastic, combinations thereof, and / or multilayers thereof. Redistribution lines 70, which may include metal lines / pads and vias, metal tubes, and the like, are formed in the dielectric layers 68. Redistribution lines 70 are interconnected to form through connections in package component 66. The package component 66 may also include electrical connectors, such as solder regions 72, on its bottom.FIG. 12 illustrates the distribution of underflow 74, and the corresponding process is shown as process 224 in the process flow 200 illustrated in FIG. 31. The underfill 74 is then cured. In some embodiments, the underfill 74 is formed from a same underfill as the capillary underfill 44 or from an underfill different from it.FIG. 13A illustrates the attachment of a thermal interface material 76 to the top surface of package 50'. The corresponding process is shown as process 226 in process flow 200 illustrated in FIG. 31. The thermal interface material 76 may be distributed on the package 50' as a flowable material or as a preformed film laminated to the package 50'. An adhesive ring 78 may be dispersed on the package component 66'. A heat sink 80 (which may also be a metal lid) may then be placed and attached to the thermal interface material 76 and possibly the adhesion ring 78. The corresponding process is shown as process 228 in process flow 200 illustrated in FIG. 31. The package components 66' may be singulated from the respective package component 66 to form a plurality of packages 82, each having a package component 66', a package 50' bonded thereto, and a heat sink 80.FIG. 14 shows a top view of the package 50' in accordance with some embodiments, wherein the reference cross section 13A- 13A is shown in FIG. 14 in FIG. 13A. The package components 22A may include NCFs 28 thereunder. The package components 22B may be large package components, such as SOC dies. Accordingly, the capillary underflow 44 is distributed from one side of one of the package components 22B and flows under both package components 22B. The capillary underflow 44 may hillock the gaps between the package components 22B and their adjacent package components 22A. The encapsulation material 48 surrounds the package components 22A and 22B and may further fill the wide gaps between adjacent package components 22A and 22B.FIG. 13B shows the reference cross-section 13B- 13B in FIG. 14, in accordance with some embodiments. As shown in FIG. 13B, each of the package components 22A includes an NCF 28 underneath as shown. The NCFs 28 underlying adjacent package components 22A may be in contact with or spaced apart from each other. FIG. 13C shows the reference cross-section 13C- 13C in FIG. 14, in accordance with some embodiments. As shown in FIGS. 13A and 13C, dashed lines 44T represent the top surface of the capillary underfill 44 according to some embodiments, wherein the encapsulation material 48 is higher than top surfaces 44T and is in contact with the capillary underfill 44. According to alternative embodiments, the capillary underflow 44 extends to the same level as the top surfaces of the package components 22A and 22B, and the dashed lines 44T are not present.FIG. 15 shows the top view of package 50' in accordance with alternative embodiments of the present disclosure. This embodiment is similar to the embodiment illustrated in FIG. 14, except that the two package components 22B in combination occupy a central region of the package 50' and extend near the opposite edges of the package 50'. The capillary underflow 44 is distributed to underlying and surrounding the package components 22B. Two columns of package components 22A may be disposed on the opposite sides of the package components 22B, with the NCFs 28 located below the package components 22A.FIG. 16 shows the top view of package 50' in accordance with alternative embodiments of the present disclosure. This embodiment is similar to the embodiment illustrated in FIG. 15, except that four package components 22B in combination occupy a central region of package 50' and extend near the opposite edges of package 50'. The capillary underflow 44 is distributed to underlying and surrounding the package components 22B. Two columns of package components 22A are located on the opposite sides of the package components 22B, with the NCFs 28 located below the package components 22A. The package components 22A on the right side of the package components 22B are smaller than the package components 22A on the left side of the package components 22B.FIG. 17 shows the top view of package 50' in accordance with alternative embodiments of the present disclosure. This embodiment is similar to the embodiment shown in FIG. 14, except that a large package component 22A is located in the center with the NCF 28 underlying the package component 22A. The package components 22B are arranged to surround the package component 22A. The capillary underflow 44 is distributed to lie under and surround each of the package components 22B.FIGS. 18 through 23 and 24A show the cross-sectional views of intermediate stages in the formation of a package, in accordance with some embodiments of the present disclosure. These embodiments are similar to the above embodiments except that instead of using the combination of an NCF and a capillary underflow, a combination of a capillary underflow and a shape underflow is used while an NCF is not used. Unless otherwise indicated, the materials and processes for forming the components in these embodiments are substantially identical to the same components identified by like reference numerals in the foregoing embodiments shown in FIGS. 1-12, 13A, 13B, and 13C. The details regarding the forming process and the materials of the components illustrated in Figs. 18 to 23 and 24A can therefore be found in the discussion of the foregoing embodiments.FIG. 18 illustrates bonding of package components 22B and 22C (collectively referred to as package components 22) over package component 32. The package component 32 may be an interposer wafer, although it may also be of a different type.Referring to Figure 19, a capillary underflow 44 is dispensed. According to some embodiments, the gaps between the package component 22B and its adjacent package components 22C are large, and / or the gaps between the package components 22C and the respective underlying portions of the package component 32 are large, and therefore the capillary underflow 44 is bounded around the package component 22B and does not flow under the package components 22C. According to alternative embodiments, first package components 22B are bonded to package component 32 and capillary underfill 44 is dispensed, followed by bonding package components 22C to package component 32.FIG. 20 illustrates the dispensing of the mold underflow 84 flowing to the gaps between the package components 22C and the respective underlying portions of the package component 32. The mold underflow 84 also surrounds the package components 22B and 22C and is distributed to a level higher than the top surfaces of the package components 22B and 22C. In some embodiments, the capillary underflow 44 is cured before the mold underflow 84 is dispensed. In alternative embodiments, the capillary underflow 44 and the mold underflow 84 are cured in a same curing process. After the mold underfill 84 is cured, a planarization process, such as a CMP process or a mechanical polishing process, is performed to remove excess portions of the mold underfill 84. The substrates (such as the semiconductor substrates) of the package components 22B and 22C may be exposed in this manner. The reconstructed wafer 50 is formed in this manner.Referring to FIG. 21, reconstructed wafer 50 is disposed over carrier 52 via release film 54. In accordance with some embodiments, a backside grinding process may be performed to thin the substrate 36 until the vias 40 are exposed. A dielectric layer 56 is then formed as shown in Fig. 22. A dielectric layer(s) 58 is then(are) formed, and conductive elements 60 (including RDLs and / or metal pads) are formed in the dielectric layers 58 to be electrically connected to the vias 40. The electrical connectors 62, which may include bond pads, metal pillars, solder region, and / or the like, are formed. The details of forming these elements have been discussed in the foregoing embodiments and will not be repeated here.Next, the reconstructed wafer 50 may be debondd from the carrier 52. In a subsequent process, the reconstructed wafer 50 may be placed on a dicing tape (not shown) and singulated along scribe lines 64 using a dicing process such that the reconstructed wafer 50 is separated into separate packages 50'.FIG. 23 illustrates the bonding of package 50' to another package component 66 that may include package components 66'. Next, as shown in FIG. 24A, a thermal interface material 76 is formed over the package 50'. An adhesive ring 78 is also dispersed and a heat ring (or metal cap) 80 may be attached to the thermal interface material 76 and the adhesive ring 78.FIG. 25 shows a top view of package 50' in accordance with some embodiments. Reference cross sections 24A-24A, 24B-24B, and 24C-24C are shown in FIGS. 24A, 24B, and 24C, respectively. According to some embodiments, as shown in FIG. 25, two package components 22B are arranged side by side, wherein a capillary underflow 44 is distributed to lie under and surround the package components 22B. The package components 22C may be disposed around the package components 22B. According to some embodiments, the package components 22B are large in area and it is difficult for the mold underflow to flow through the gaps between the package components 22B and the underlying package component 34. Accordingly, the capillary underflow 44 having a lower viscosity than the mold underflow 84 is used.FIG. 26 shows the top view of package 50' in accordance with alternative embodiments of the present disclosure. This embodiment is similar to the embodiment shown in FIG. 25, except that the two package components 22B in combination extend near the opposing edges of the package 50'. The capillary underflow 44 is distributed to underlying and surrounding the package components 22B. Two columns of package components 22C are disposed on opposite sides of the package components 22B, with the mold underflow 84 extending below the package components 22C and surrounding the package components 22B and 22C.FIG. 27 shows the top view of package 50' in accordance with alternative embodiments of the present disclosure. This embodiment is similar to the embodiment illustrated in FIG. 26, except that four package components 22B are present, and the number of package components 22C on the left and right sides of package components 22B are different from each other. The capillary underflow 44 is distributed to underlying and surrounding the package components 22B. The mold underfill 84 extends under the package components 22C and may extend on multiple sides of each of the package components 22C. The right side column includes more and smaller package components 22C than the left side column.FIG. 28 shows the top view of package 50' in accordance with alternative embodiments of the present disclosure. This embodiment is similar to the embodiment shown in FIG. 14, except that a large package component 22C is located in the center of package 50' with the mold underflow 84 extending below package component 22C. The package components 22B are arranged to surround the package component 22C. The capillary underflow 44 surrounds and extends under the package components 22B. The mold underfill 84 is also distributed to surround the package components 22C and 22B.FIG. 29 illustrates a cross-sectional view of package 82, in accordance with some embodiments. In these embodiments, the NCF 28 is below the package component 22A and the capillary underfill 44 extends below the package components 22B. The capillary underflow 44 also surrounds the package components 22A and 22B. A mold underfill and / or molding compound may or may not be present in package 50'.FIG. 30 shows a top view of package 50' in accordance with some embodiments. According to these embodiments, an NCF, a capillary underflow and a form underflow are used. The NCF 28 is below the package component 22A. The capillary underflow 44 extends under and surrounds the package components 22B. The mold underfill 84 extends below the package components 22C and further surrounds the package components 22A, 22B, and 22C. There may be no molding compound present in package 50'.In embodiments illustrated above, some processes and elements are discussed in accordance with some embodiments of the present disclosure to form a three-dimensional (3D) package. Other elements and processes may also be included. For example, test structures may be included to aid in verification testing of the 3D package or the 3DIC devices. The test structures may include, for example, test pads formed in a redistribution layer or on a substrate, which enables testing of the 3D package or 3C, use of needles and / or probe cards, and the like. The verification testing may be performed on intermediate structures as well as the finished structure. In addition, the structures and methods disclosed herein may be used in conjunction with test methodologies that incorporate intermediate verification of proven defect free chips (Known Good Dies) to increase yield and decrease cost.The embodiments of the present disclosure have some advantageous features. As the sizes of packages and device dies increase, it becomes increasingly difficult to completely fill gaps between package components without causing voids. Additionally, as the volume and area of the underfill increase, the warpage of packages becomes more severe due to shrinkage of the underfill during its curing process. The embodiments of the present disclosure use a mixture of different underflow types to reduce the void problem and the warp problem. For example, an NCF is strong upon application and therefore does not contribute to shrinkage. In addition, some underfills, such as the capillary underfill, may be cured first to relieve some strain, followed by the spreading and curing of other types of underfill, such as a mold underfill.According to some embodiments of the present disclosure, a method includes: bonding a first package component over a second package component; distributing a first underflow between the first package component and the second package component; and bonding a third package component over the second package component, wherein a second underflow is present between the third package component and the second package component, and wherein the first underflow and the second underflow are different underflow types. In one embodiment, the second underfill comprises a non-conductive film. In an embodiment, the method further comprises: attaching the non-conductive film to the third package component, wherein the third package component is bonded to the second package component after the non-conductive film is attached, and wherein the non-conductive film fills a gap between the third package component and the second package component. In one embodiment, the non-conductive film is a solid film when the non-conductive film is attached. In an embodiment, the first underfill comprises a capillary underfill and the method further comprises forming the first package component and the third package component in a molding compound. In an embodiment, the second underflow includes a mold underflow, and the method further includes distributing the mold underflow between the third package component and the second package component. In an embodiment, the method further comprises attaching a non-conductive film over a fourth package component; and bonding the fourth package component over the second package component. In one embodiment, the first underflow and the second underflow have different coefficients of thermal expansion. In one embodiment, the first underflow and the second underflow are in contact with each other.According to some embodiments of the present disclosure, a package includes: a first package component; a second package component over and bonded to the first package component; a first underflow in a first gap between the second package component and the first package component; a third package component over and bonded to the first package component; and a second underflow in a second gap between the third package component and the first package component, wherein the first underflow and the second underflow are different underflow types. In one embodiment, the first underfill comprises a capillary underfill or a form underfill, and the second underfill comprises a non-conductive film. In an embodiment, the first underfill has a first extension portion outside the first gap, and the first extension portion has a concave side wall, and the second underfill has a second extension portion outside the second gap, and the second extension portion has a convex and curved side wall. In an embodiment, the package further includes a molding compound that molds the second package component and the third package component therein, the molding compound having a composition different from compositions of the first underfill and the second underfill. In an embodiment, the package further includes: a fourth package component over and bonded to the first package component; and a third underfill in a third gap between the fourth package component and the first package component, wherein the first underfill, the second underfill, and the third underfill are different underfill types. In one embodiment, the first underflow physically contacts the second underflow.According to some embodiments of the present disclosure, a package includes: an interposer; a first die bonded to the interposer via a plurality of first bumps; a second die bonded to the interposer via a plurality of second bumps; a first underfill layer including a first portion between the first die and the interposer, the first underfill layer surrounding the plurality of first bumps; a second underfill layer including a second portion between the second die and the interposer, the second underfill layer surrounding the plurality of second bumps, and a first CTE of the first underfill layer being different than a second CTE of the second underfill layer; and an encapsulation material encapsulating the first die and the second die, wherein the encapsulation material further surrounds the first underfill layer and the second underfill layer. In an embodiment, the encapsulation material comprises a material different from materials of both the first underfill layer and the second underfill layer. In an embodiment, the first underfill layer is different from the encapsulation material and the second underfill layer is the same as the encapsulation material. In an embodiment, the first underfill layer and the second underfill layer further include elongated portions that physically contact one another to form a distinct boundary. In one embodiment, the first underfill layer has a concave sidewall and the second underfill layer has a convex sidewall.

Claims

A method comprising: bonding a first package component over and to a second package component; distributing a first underflow between the first package component and the second package component; bonding a third package component over and to the second package component, wherein a second underflow is present between the third package component and the second package component, and wherein the first underflow and the second underflow are different underflow types; and forming the first package component and the third package component in a molding compound.The method of claim 1, wherein the second underfill comprises a non-conductive film.The method of claim 2, further comprising: attaching the non-conductive film to the third package component, wherein the third package component is bonded to the second package component after the non-conductive film is attached, and wherein the non-conductive film fills a gap between the third package component and the second package component.The method of claim 3, wherein the non-conductive film is a solid film when the non-conductive film is attached.The method of any preceding claim, wherein the first underflow comprises a capillary underflow.The method of claim 5, wherein the second underflow comprises a mold underflow, and the method further comprises distributing the mold underflow between the third package component and the second package component.The method of claim 6, further comprising: attaching a non-conductive film over a fourth package component, and bonding the fourth package component over the second package component.The method of any preceding claim, wherein the first underflow and the second underflow have different coefficients of thermal expansion.The method of any preceding claim, wherein the first underflow and the second underflow are in contact with each other.A package comprising: a first package component, a second package component over and bonded to the first package component, a first underflow in a first gap between the second package component and the first package component, a third package component over and bonded to the first package component, a second underflow in a second gap between the third package component and the first package component, wherein the first underflow and the second underflow are different underflow types, and a molding compound forming the second package component and the third package component therein.The package of claim 10, wherein the first underfill comprises a capillary underfill or a form underfill and the second underfill comprises a non-conductive film.The package of claim 11, wherein the first underfill has a first extension portion outside the first gap, and the first extension portion has a concave sidewall, and the second underfill has a second extension portion outside the second gap, and the second extension portion has a convex and curved sidewall.The package of claim 11, wherein the molding compound has a composition different from compositions of the first underfill and the second underfill.The package of any of the preceding claims 10 to 13, further comprising: a fourth package component over and bonded to the first package component, and a third underfill in a third gap between the fourth package component and the first package component, wherein the first underfill, the second underfill, and the third underfill are different underfill types.The package of any of the preceding claims 10 to 14, wherein the first underflow physically contacts the second underflow.A package, comprising: an interposer; a first die bonded to the interposer via a plurality of first bumps; a second die bonded to the interposer via a plurality of second bumps; a first underfill layer including a first portion between the first die and the interposer, wherein the first underfill layer surrounds the plurality of first bumps; a second underfill layer including a second portion between the second die and the interposer, wherein the second underfill layer surrounds the plurality of second bumps, and wherein a first coefficient of thermal expansion, CTE, of the first underfill layer is different than a second CTE of the second underfill layer; and an encapsulation material, encapsulating the first die and the second die, wherein the encapsulation material further surrounds the first underfill layer and the second underfill layer.The package of claim 16, wherein the encapsulation material comprises a material different from materials of both the first underfill layer and the second underfill layer.The package of claim 16 or 17, wherein the first underfill layer is different from the encapsulation material, and the second underfill layer is the same as the encapsulation material.The package of any of the preceding claims 16 to 18, wherein the first underfill layer and the second underfill layer further comprise extended portions that physically contact each other to form a distinct boundary.The package of any of the preceding claims 16 to 19, wherein the first underfill layer has a concave sidewall and the second underfill layer has a convex sidewall.

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