Patent Publication Number: US-2023147273-A1

Title: Warpage Compensation for BGA Package

Description:
RELATED APPLICATIONS 
     This application claims the benefit of priority of U.S. Provisional Application No. 63/276,449 filed Nov. 5, 2021 which is herein incorporated by reference. 
    
    
     FIELD 
     Embodiments described herein relate to electronic packaging, and in particular to stiffener structures. 
     BACKGROUND INFORMATION 
     As microelectronic packages become thinner and larger in size, structures are also being implemented within the microelectronic packages to control warpage at room and high temperatures. For example, stiffener structures are widely used in multiple chip modules (MCMs) for warpage, reliability and thermal performance. In an exemplary implementation one or more devices are surface mounted onto a module substrate, and then optionally underfilled. A stiffener structure is then secured onto the module substrate and surrounding the device(s). Stiffener structures are commonly bonded to the package module with adhesive tapes such as urethane, polyurethane, silicone elastomers, etc. 
     SUMMARY 
     In an embodiment, an electronic assembly includes a module substrate with a first side and a second side opposite the first side, a ball grid array (BGA) package bonded to the first side of the module substrate, and a stiffener structure bonded to the second side of the module substrate. The stiffener structure may span an area directly opposite the module substrate of the BGA package, and be shear bonded to the second side of the module substrate. In accordance with embodiments, shear bonding may be performed at elevated temperature where the BGA package is intrinsically flatter, and with a suitable material such as solder material or thermoset material to accomplish shear bonding and provide rigidity and modulus to lock in a flat or near-flat surface contour. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A- 1 E  are schematic cross-sectional side view illustrations of a method of forming an electronic assembly with a stiffener structure and BGA package bonded to opposite sides of a module substrate in accordance with an embodiment. 
         FIG.  2    is a flow diagram for a method of forming an electronic assembly with stiffener structure and BGA package bonded to opposite sides of a module substrate in accordance with an embodiment. 
         FIG.  3    is a schematic bottom view illustration of a stiffener structure bonded to a module substrate in accordance with an embodiment. 
         FIG.  4    is a schematic cross-sectional side view illustration of an electronic assembly with separate lid bonded to a stiffener structure in accordance with an embodiment. 
         FIG.  5    is a schematic cross-sectional side view illustration of an electronic assembly and stiffener structure with integrated lid in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments describe electronic assemblies and methods of assembly including bonding of a stiffener structure at elevated temperature on the opposite side of a module substrate from a mounted ball grid array (BGA) package to control warpage. 
     In one aspect, it has been observed that ball grid array (BGA) packages can create local and global warpage on module substrates (e.g. printed circuit boards) due to stiffness and coefficient of thermal expansion (CTE) differences, resulting in un-flat module assemblies that can change over storage and operating temperature ranges. Moreover, such un-flat module assemblies can add to critical thickness of the electronic assembly, be problematic form mating assemblies such as thermal solutions (e.g. stiffener structures, lids), and shape changes across a temperature range can contributed to cyclic stress failures in the module substrate layers or solder joints, such as BGAs. 
     In accordance with embodiments, compensation designs and processes are described for controlling warpage of a module substrate with BGA component(s) across a range of temperatures. In various aspects, stiffener structures can be bonded to an opposite side of a module substrate from a BGA package and also be bonded at elevated temperatures locking in a near-flat surface contour of the module substrate that is fundamental for BGA design. As a result, overall electronic system design for housing the electronic assembly can be with a reduced z-height, and overall thinner product. 
     In one aspect, it has been observed that intrinsically stressed BGA packages are designed to flatten at their reflow temperatures to ensure uniform joint formation. In accordance with embodiments, the stiffener structures can also be bonded at elevated temperatures, which are below the BGA package reflow temperatures yet sufficiently high to return the BGA packages to flat or near flat shapes. The bonding materials for the stiffener structures can be selected to provide sufficient Young&#39;s Modulus (also generally referred to as modulus), stiffness and adhesion strength to provide shear bonding with the module substrate and transfer the mechanical properties of the stiffener structure to the module substrate. As a result, warpage may be controlled across a range of operating temperatures for the electronic assembly. 
     In an embodiment, an electronic assembly includes a module substrate including a first side and a second side opposite the first side, a BGA package bonded to the first side of the module substrate (for example, with a high temperature solder), and a stiffener structure bonded to the second side of the module substrate (for example, with a low-medium temperature solder) to achieve shear bonding. The stiffener structure may span an area directly opposite the module substrate of the BGA package to match the BGA package with modulus, geometry, coefficient of thermal expansion, flatness, etc. 
     In various embodiments, description is made with reference to figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions and processes, etc., in order to provide a thorough understanding of the embodiments. In other instances, well-known semiconductor processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the embodiments. Reference throughout this specification to “one embodiment” means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The terms “over”, “to”, “between”, “spanning” and “on” as used herein may refer to a relative position of one layer with respect to other layers. One layer “over”, “spanning” or “on” another layer or bonded “to” or in “contact” with another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers. 
     Referring now to  FIGS.  1 A- 1 E  and  FIG.  2   ,  FIGS.  1 A- 1 E  are schematic cross-sectional side view illustrations of a method of forming an electronic assembly with a stiffener structure and BGA package bonded to opposite sides of a module substrate in accordance with an embodiment;  FIG.  2    is a flow diagram for a method of forming an electronic assembly with stiffener structure and BGA package bonded to opposite sides of a module substrate in accordance with an embodiment. In interest of clarity and conciseness the sequence of schematic cross-sectional side view illustrations of  FIGS.  1 A- 1 E  are discussed concurrently with the flow diagram of  FIG.  2   . 
     In the exemplary embodiment, the electronic assembly includes a module substrate  102  including a first side  104  and second side  106 . For example, module substrate  102  may be a printed circuit board (PCB), which may a rigid board and be cored or coreless. A plurality of first components  160  can be mounted on the first side  104  of the module substrate  102 . A plurality of second components  140  can be mounted on the second side  106  of the module substrate  102 . In accordance with embodiments, both the first components  160  and second components  140  can be mounted onto the module substrate  102  prior to mounting a BGA package  110  onto the first side  104  of the module substrate  102 . Furthermore, one or more second packages  130  can be mounted onto the second side  106  of the module substrate  102  prior to mounting BGA package  110 . In accordance with embodiments, the BGA package  110  may be a comparatively large package that occupies a larger area of the module substrate than the individual first and second components  160 ,  140  and second package(s)  130 . Furthermore, the BGA package  110  may include an intrinsic stress, which is illustrated by the crowning shape in  FIG.  1 A . The BGA package  110  in accordance with embodiments may perform high density logic, and may for example include central processing unit (CPU) die, graphics processing unit (GPU) die, system on chip (SoC) die, various engines, etc. that includes a high performance logic area that may include a high density of devices that tend to run hot, and transfer BGA package  110  stress to the module substrate  102 . First components  160  and second components  140  may be passive devices (e.g. resistor, inductor, capacitor, etc.) or active devices, and may be chips or packages. For example, first and second components  160 ,  140  may be memory packages, such as dynamic random-access memory (DRAM) including one or more dies, which can be stacked dies, or side-by-side. First and second components  160 ,  140  can additionally be different types of components, and need not be identical. One or more second packages  130  can also be mounted on the second side  106  of the module substrate  102 . In an embodiment, a second package  130  is a package that includes a single die or a plurality (e.g. two or more) of side-by-side dies. For example, second package  130  may include a plurality of side-by-side logic, or system on chip dies. In an embodiment, second package(s)  130  are chip scale packages providing different circuit functionality (logic, power management, integrated voltage regulator, etc.). 
     Referring now to  FIG.  1 B , at operation  2010  the BGA package  110  is mounted onto the first side  104  of the module substrate  102  at elevated temperature. As illustrated by the wavy lines, heat is applied globally to reflow solder bumps  112  applied to the bottom side  114  of the BGA package  110 . For example, the solder bumps  112  may mate with corresponding solder pads  108  on the module substrate  102  to form joints. In an embodiment, the BGA package  110  is bonded to the first side  104  of the module substrate  102  with a plurality of solder joints  115  characterized by a melting temperature greater than 200° C. For example, the elevated temperature, or reflow temperature, for bonding may be approximately 230° C. Specifically, the BGA package  110  may have been designed to a have a flat or near-flat shape at the elevated bonding temperature in order to achieve uniform joint formation. 
     In accordance with embodiments, the first and second components  160 ,  140  and second package(s)  130  can have also been bonded to the module substrate  102  with one or more bonding materials  132  and solder pads  108  characterized by melting temperatures greater than 200° C. in order to withstand bonding of the BGA package  110 . 
     The partially assembled electronic assembly can then be allowed to cool to room temperature at operation  2020 . For example, this operation may be performed as general storage or transfer during assembly. Referring now to  FIG.  1 C , it has been observed that the BGA package  110  can create local and global warpage on the module substrate  102  due to stiffness and CTE differences, resulting an un-flat module substrate  102  and overall electronic assembly that can change over storage and operating temperatures. For example, the intrinsic BGA package  110  crowning illustrated in  FIG.  1 A  can be transferred to the module substrate  102  as shown in  FIG.  1 C . 
     Still referring to  FIG.  1 C , it can be seen why the plurality of first and second components  160 ,  140  and second package  130  have been pre-assembled onto the module substrate prior to bonding the BGA package  110 . As shown, the uneven surface profile caused by the BGA package  110  could result from difficulty in placement of solder pads  108  onto the module substrate, or even mounting of the additional components or second package  110 , which could result in broken or incomplete joint formation. 
     Referring now to  FIG.  1 D , at operation  2030  a stiffener structure  120  is then mounted onto an opposite side (e.g. second side  106 ) of the module substrate  102  from the BGA package  110 . In this case, bonding may be performed at a moderate bonding temperature of less than 200° C. to create a shear bonded interface with the second side of the module substrate. For example, the stiffener structure  120  can be bonded to the second side  106  of the module substrate  102  with a solder material characterized by a melting temperature between 150° C.-190° C., which can be below the reflow temperature of the BGA package  110  as well as first and second components  160 ,  140  yet above the operating temperature of the BGA package  110 . More specifically, bonding or reflow temperature may be selected where the BGA package  110  intrinsically flattens. In a particular embodiment, the intrinsic BGA package  110  flattening and bonding or reflow temperature is between 150° C.-165° C. Alternatively, with the same bonding temperatures, the stiffener structure  120  can be bonded to the second side  106  of the module substrate  102  with a one or two part adhesive, for example, such as a glass paste or cured polymer (e.g. thermoset) such as polyimide, silicone epoxy, etc. to create a shear bonded interface with the second side of the module substrate. 
     In accordance with embodiments, the bonding material  122  is selected to achieve necessary stiffness, modulus (Young&#39;s Modulus) and adhesion strength with the module substrate to provide shear-coupling. This may be achieved by selection of suitable materials such as solder, glass paste, or cured polymers (e.g. thermoset materials) that can provide stiffness and adhesion strength that is greater than traditional adhesive tapes such as urethane, polyurethane, silicone elastomers, etc. For example, solder materials may have a modulus of greater than 20 GPa, such as 30-50 GPa, and a thermoset material such as epoxy or acrylonitrile butadiene styrene (ABS) may have a modulus of approximately 1-4 GPa, whereas a traditional pressure sensitive tape may have a modulus of less than 0.5 GPa. In accordance with embodiments, the bonding material  122  may have a Young&#39;s Modulus of greater than 1 GPa, or even greater than 20 GPa. 
     In accordance with embodiments, the moderate bonding temperature and optional pressure (P) applied to the module substrate  102  and/or BGA package  110  can return the assembly, including the BGA package  110  and module substrate  102 , to a flat or near-flat state from when the BGA package was bonded. The stiffener structure  120  additionally is designed with specific materials, geometry, CTE, flatness, etc. and bonding material  122  is selected to achieve a specific modulus, stiffness, and adhesion strength with the module substrate  102  to provide shear-coupling and lock in a flat and stable electronic assembly across the operating temperature range of the electronic assembly. In an embodiment, the stiffener structure  120  can be formed of a high modulus, low CTE material to reduce stress and warpage of the module substrate  102 . In an exemplary implementation a low CTE stiffener material can be a nickel-iron alloy (FeNi36), iron-nickel-cobalt alloy (sold under the trademark KOVAR, trademark of CRS Holdings, Inc., Delaware), iron-nickel alloy (Alloy42), stainless steels (SUS410, SUS430), etc. In an embodiment, the stiffener structure  120  is formed of a low CTE 400 series stainless steel, with a CTE around 11 ppm/° C. Other notable low CTE, high modulus materials include molybdenum and molybdenum-copper alloys, both having higher thermal conductivities that traditional high modulus, low CTE materials, which can be a thermal benefit to the BGA package as well as the components within the stiffener footprint. In accordance with embodiments, the modulus, thickness, geometry, CTE, bonding temperature and bonding material all work in concert to compensate the BGA package induced warpage. 
     The electronic assembly  150  may then be allowed to cool to room temperature at operation  2040 , with module substrate  102  being flat or substantially flat as shown in  FIG.  1 E  across the operating temperature range of the BGA package  110 . In an embodiment, the module substrate  102  is characterized by a maximum curvature across an area of the BGA package  110  covering the first side of the module substrate of less than 100 μm. For example, this may represent a significant reduction in warpage, or crowning, for a similar module substrate  102  without such a stiffener structure, in which a total module height of 1.5 mm includes 200-300 μm of warpage, or crowning. Accordingly, embodiments may facilitate the assembly of an electronic assembly with reduced total z-height. 
       FIG.  3    is a schematic bottom view illustration of a stiffener structure  120  bonded to a module substrate  102  in accordance with an embodiment. Specifically, the electronic assembly  150  of  FIG.  1 E  may correspond to a cross-section taken along section X-X of  FIG.  3   . As previously described, the second side of the module substrate  102  can be populated with a plurality of second components  140 , which can be a variety of different active and passive components. Furthermore, the stiffener structure  120  can be frame-shaped including outer walls  124  (e.g. along a perimeter) and optional inner (interior) walls  126 . The outer walls  124  and optional inner walls  126  can form one or more openings  125 , which may optionally be fully enclosed. Such enclosure may optionally provide additional function of electromagnetic interference (EMI) shielding for one or more second components  140  or second packages  130  mounted to the second side of the module substrate  102 . Use of an electrically conductive bonding material  122 , such as solder material, can additionally facilitate EMI shielding. In accordance with embodiments, the bonding material  122  can be continuously or near continuously applied along the bottom surface  121  (see  FIG.  1 E ) of the frame-shaped stiffener structure  120  rather than at separate and discrete locations. More specifically, the bonding material  122  can near continuously span along the bottom surface  121  for the lengths of one or more outer walls  124  and inner walls  126 , though can be broken up to separate areas to better control distribution and uniformity of the bonding material. The more uniform and continuous the bonding, the greater and more consistent the shear coupling effect may be and the more effective the stiffener structure compensation. In an embodiment, a bond line covers approximately 90% of the stiffener structure footprint (e.g. bottom surface  121 ). 
     Referring to both  FIG.  3    and  FIGS.  1 D- 1 E , in accordance with embodiments, the stiffener structure  120  may include an opening  125 , and a second package  130  is mounted onto the second side  106  of the module substrate  102  within the opening  125 . For example, the BGA package  110  can cover a larger area of the module substrate  102  than the opening  125  in the stiffener structure and the second package  130 . Additionally components or packages can also be mounted within the opening( 2 )  125 . 
     In some embodiments, the electronic assembly can include additional structures for EMI shielding and/or thermal function. For example, a lid can complete EMI shielding for components within the stiffener structure footprint.  FIG.  4    is a schematic cross-sectional side view illustration of an electronic assembly  150  with separate lid  170  bonded to a stiffener structure  120  in accordance with an embodiment. The lid  170  may be bonded to the stiffener structure  120  with an electrically conductive adhesive or gasketing for example. In an embodiment, lid  170  is bonded to the stiffener structure  120  with an electrically conductive pressure sensitive adhesive  172  such as an adhesive tape formed of urethane, polyurethane, silicone elastomers, etc. and filled with conductive fillers, which may have a lower stiffness, modulus and adhesion strength to the stiffener structure  120  and the stiffener structure than does the bonding material  122  with the module substrate  102  and the stiffener structure  120 . The lid  170  may be formed of the same or different material than the stiffener structure  120 . For example, the lid  170  can optionally be formed of a more thermally conductive material such as copper, while still controlling warpage. Furthermore, a thermal interface material (TIM)  174  can be applied between the second package  130  and the lid  170 . TIM  174  may be applied using any suitable technique such as dispensing or tape. Exemplary TIMs  174  include, but are not limited to, thermal grease, solder, metal filled polymer matrix, etc. In an embodiment, the lid  170  spans over the outer walls  124  and the inner walls  126  of the stiffener structure. 
       FIG.  5    is a schematic cross-sectional side view illustration of an electronic assembly  150  and stiffener structure  120  with integrated lid in accordance with an embodiment. In this manner, a roof  127  can ben integrally formed with the outer walls  124  and/or inner walls  126  as a single piece. A TIM  174  may still be applied between the second package  130  and roof  127  to facilitate heat transfer. 
     In utilizing the various aspects of the embodiments, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for forming an electronic assembly with stiffener structure to compensate for BGA package warpage. Although the embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that the appended claims are not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as embodiments of the claims useful for illustration.