Wafer-level chip scale package
A chip scale package implements solder bars to form a connection between a chip and a trace, formed in a substrate, such as another chip or PCB. Solder bars are formed by depositing one or more solder layers into the socket, or optionally, depositing a base metal layer into the socket and applying the solder layer to the base metal layer. The geometry of a solder bars may be rectangular, square, or other regular or irregular geometry. Solder bars provide a greater utilization of the connectivity footprint and increase the electrical and thermal flow capacity. Solder bars also provide a robust connection.
1. Field of the Invention
The disclosure herein pertains to materials and methods for soldering one electrical component to another, for example, in the use of specially prepared solder films to connect chips and other circuit components to printed circuit boards (PCBs) or other substrates.
2. Description of the Related Art
A wafer-level chip scale package (package) is an integrated circuit assembled at the wafer level. The package provides a physical interface between each pad of a chip and a corresponding circuit connection, e.g., a PCB, interposer, or another chip in a chip-on-chip installation. Intra-package connections may extend straight and vertically through the package from the chip to the PCB so that the package has the same width and depth as the chip, thereby reducing or eliminating wasted space. Utilizing a package saves space and weight when compared to face-up chip placement utilizing wire connections between a chip and PCB.
Solder balls may be used to connect the chip to the PCB, for example, using automated ‘flip-chip’ processing equipment as shown in U.S. Pat. No. 5,918,792 issued to Stumpe et al. Generally, each solder ball connects a chip to conductive traces on the PCB. The chip contains a socket that is made of adhesive and conductive materials bonded to the wafer in an area of the chip often referred to as the pad. Each socket retains a selectively placed preformed solder ball as an aid to solder-coupling the chip with the substrate. The pad is a conductive area on the wafer where either conductive material is applied to the chip or ion implantation renders the wafer conductive. Although silicon wafers are most commonly used, other materials such as gallium arsenide are also utilized.
With preformed solder balls positioned on the socket and the chip positioned on the PCB or other circuit connection, the preformed solder balls are heated to at least their melting point. Adhesion forces pull the molten solder over the socket and circuit connection to establish an electrical contact between the chip and the circuit connection. The process of melting the preformed solder ball to form an installed solder ball that couples the socket and the circuit connection is known as “reflow.” The socket resides in electrical contact with conductive traces on the PCB, and these traces operably connect the chip to other PCB components.
Pads include a socket area that may contain a solder receptacle and contact materials. Other portions of the pad may be protected by passivation and polymer layers. As there is difficulty in directly bonding solder to silicon or other wafer materials, the contact materials on the socket are often formed by an under-bump metallization (UBM) layer and an adhesion layer. The adhesion layer, e.g., aluminum, aluminum-copper, connects the pad to the UBM, which is capable of bonding with solder. The solder receptacle may be coextensive with the surface of the UBM, such that the socket boundaries are defined by the surface of the UBM where the perimeter of the socket is formed by voids in passivation and polymer layers. The passivation layer is a dielectric and may protect embedded circuits from contamination or physical damage. The passivation layer may, for example, be formed of polysilicon or oxinitride. The polymer layer provides additional dielectric properties and masks the package for etching, which exposes the UBM and forms the sockets. In some applications, the UBM is applied to the entire void formed in the passivation and polymer layers as a lining in the etched void. In such an application, the socket is defined by the exposed surface of the UBM.
After application of the UBM, the passivation layer may be applied to the pad and etched to form a plurality of openings exposing the UBM layer. A polymer layer, for example a benzocyclobutane (BCB) layer, is next applied to the passivation layer. The BCB layer is also etched to form a plurality of openings, each corresponding to, and aligning with, an opening in the passivation layer. Together, the adhesion layer, UBM layer, and openings in the passivation and BCB layers form a cavity or socket that may be used to contact an installed solder ball.
Prior art packages are typically assembled by placing preformed solder balls at selected socket locations on the chip. The chip is then placed in a predetermined positional alignment such that each preformed solder ball contacts the socket where it resides and conductive traces on the PCB or other circuit connection. The preformed solder balls are reflown to complete the connection between the trace and the UBM layer. Optionally, an underfill resin is applied to the space between solder balls. The resin provides shock-load tolerance beyond the limits of the solder balls alone. The chip may be reinforced by attaching the solder balls to a ceramic carrier, such as an interposer with conductive vias, for example, to enhance rigidity when the PCB is too flexible or as a manufacturing aid to increase solder ball installation alignment tolerances. The interposer is then attached to the PCB.
Packages often have different sockets that connect, for example, with low-current, data (I/O) circuits and high-current, power circuits. Both I/O and power circuits utilize solder ball connections. A single solder ball is often inadequate to carry the load of power circuits. Therefore, the load of a power circuit may be shared across a plurality of electrically parallel solder ball connections. Some installations may require additional solder ball connections to provide adequate heat dissipation from the chip.
Although the prior art solder-ball package provides many benefits, problems remain. Power connections require a greater current capacity compared to I/O connections. As a result, these high-current connections may require multiple solder balls to handle the required current flow. The use of multiple solder balls imposes additional design constraints. For example, the footprint of multiple solder balls must accommodate the plurality of solder balls and the unused space between each solder ball, e.g., the space between each socket. The resulting package is overly large, and the connection footprint contains wasted space. Additionally, the relatively small cross-sectional area of solder balls limits the chip's ability to transfer heat. Heat may accumulate within a chip and degrade performance. Safe operating temperatures are more easily exceeded, and this circumstance may impose associated operational constraints affecting frequency, current, or voltage. If thermal effects are unmitigated, damage to the chip may occur.
Incomplete reflow and stress cracking exacerbate the problems associated with the limited cross-sectional area of solder balls. As chip processing activity fluctuates, associated variable power draw and near-constant electrical resistance produce a corresponding fluctuation in heat, resulting in thermal expansion and contraction of the package and components therein. These fluctuations, known as thermal cycling, create stresses on the connections, for example, because materials of the package do not have the same coefficients of thermal expansion. Thermal stress may lead to fatigue cracks or to complete breakage of contact with a solder ball. Additionally, incomplete reflow further compounds the stress. Incomplete reflow occurs when a preformed solder ball melts to form a solder ball but does not achieve complete coverage of the UBM layer or other connection area. The resulting incomplete connection has a diminished cross-sectional area, which provides a further diminution of performance and an increased probability of cracking and breakage.
Certain features in the present disclosure may overcome certain problems outlined above by providing, for example, systems and methods of chip-to-socket connection. Preformed thick-film solder bars may have increased thermal and electrical connectivity, increased tolerance to stress, and a decreased package footprint. These packages may achieve greater performance, reliability, and efficiency.
In one embodiment, a solder bar is provided. The solder bar connects at least one socket with a conductive trace, for example on a PCB or other circuit connection. The socket is defined on a chip and provides a contact surface to at least one of the chip's circuit conduits. The socket geometry accommodates the geometry of the installed solder bar. Optionally, the solder bar may form various geometries providing the desired connection area and footprint. For example, if a chip and/or substrate design provides for an “L” shaped power connection, the socket and solder bar may have a complementary “L” shape.
Solder bars offer a greater contact surface area over the same connectivity footprint, as compared to an equivalent footprint of prior art solder balls. Chip performance may be increased by exploiting additional current and heat flow capacities over this surface area, as compared to prior solder ball connections. Furthermore, the solder bars may provide stronger connections having greater tolerance to stress, such as thermal cycling stresses. Accordingly the package of the present disclosure may have improved durability, reliability, and/or operational parameters.
In another embodiment, solder bars form geometries other than a linear bar-shape, such as rectangle or “I” geometries. The solder bar may form any contiguous geometry providing a single electrical potential. Solder bar geometry may be, for example, “I,” “E,” “U,” ring, and/or square, or any other regular or irregular geometry that is complementary to a chip pad design. The ability to pre-form solder simplifies the task of implanting solder bars for chip-bonding processes.
In one embodiment, the solder bar has a base metal layer coated with a solder layer. The base metal may be an alloy, layered, and/or doped composition of, for example, copper, gold, platinum, palladium, silver, aluminum, tin, bismuth, vanadium, tungsten, titanium, or lead. As an option, the base metal layer may be omitted increasing resistance and may prevent the solder from forming into clumps, by action of adhesion forces during reflow. The footprint geometry of the solder bar is defined by a socket, having dimensions complementary to one bump, for retaining solder during reflow.
The height of the solder bar is determined by the combined height of the base metal layer, if used, and the solder layer. The base metal layer may be comprised of zero or more layers and the solder layer may be one or more layers. By way of example, solder bars may be formed of different thick film layers, each layer having a different melting temperatures. Forming the solder layers and base metal layers may be accomplished, for example, by electroplating the socket. Other methods may be employed to form the base metal layer and/or solder layer, such as liquid chemical deposition, chemical-vapor deposition, sputtering, or screen printing. The thickness of each of the base metal layer or solder layer, as well as the composition thereof, is a matter of design where the thicknesses are adjusted to meet connection and processing requirements. Some of the determinate factors for the solder bar height and materials include providing sufficient solder volume to facilitate complete wetting of the connection surfaces during reflow, providing sufficient solder volume to bridge the gap between connection surfaces without inducing unwanted resistance, mitigation of parasitic capacitance when using heterologous conductors, and limiting the solder volume to avoid overflow and/or inadvertent contact with another circuit.
Solder materials and techniques for their deposition are known, for example, as shown in U.S. Pat. No. 5,326,453 issued to Endicott et al. Solder characteristics may be adjusted by forming alloys of various materials, such as tin, silver, gold, manganese, copper, lead, and bismuth, to provide the desired characteristics of the connection. Examples of solder design factors include melting point, resistance, thermal conductivity, flux requirements, and flow rate. The melting point for the solder layer is generally greater than that of the melting point for the base metal layer.
In one embodiment, a method is provided for constructing a solder bar. First, a pad is formed with a socket to accept the solder bar. The socket geometry is in essence a planar face on the pad onto which has been deposited an adhesion layer and an UBM layer, or another such layer as may be desirable to facilitate solder bonding and electrical contact with the pad. The socket may be formed on a region of the pad where, for example, passivation and BCB materials have been removed or where such materials have not been deposited. The socket differs in geometry from sockets designed to accept solder balls because no dimple or recess for retaining the solder ball is required. With the socket formed to an appropriate geometry for accommodating a solder bar of the type described above, a base metal, such as copper, is optionally but preferably deposited directly onto the UBM layer. Deposition is preferably made by electroplating, but may be augmented or implanted by screen printing or other deposition processes. The thickness of the base metal is a matter of connection design requirements, and the base metal may sometimes be omitted, but the thickness is generally that of a thick film. A thickness in the range from 13,000 to 17,000 Å is suitable for many design requirements. Next, a solder layer is electroplated onto the base metal, or directly to the UBM if the base metal is omitted. Sputtering, liquid chemical deposition, chemical vapor deposition, or screen printing may be used as alternatives to electroplating the solder. The application of solder and/or a base metal layer may include an overage and a subsequent removal of the overage by grinding, shaving, etching, or other removal processes. As an option, the solder layer is placed in contact with a trace and reflowed to complete the connection between a pad and trace.
Problems associated with prior art preformed solder ball placement and reflow may be reduced or eliminated. For example, adhesion forces of molten solder may be increased by the presence of a base metal layer, e.g., by capillary action, whereas in solder ball placement such forces may be insufficient to completely wet large or non-circular connection surfaces. By implementing the solder bar construction methods, disclosed herein, the base metal layer and/or solder layer may be uniformly applied to essentially any socket geometry.
BRIEF DESCRIPTION OF THE DRAWINGS
DETAILED DESCRIPTION OF THE DRAWINGS
According to the instrumentalities described herein, improved electrical conductivity is provided to circuit 30 and improved thermal conductivity is provided to the chip containing switching element 31 by implementing solder bars to establish first and second terminals 31, 35. Advantages in current or power throughput are obtained and dissipation of heat and flux N is improved by providing first and second terminals 31, 35 with larger contact surface area, whereas the footprint of chip 4 over area 36 may also be advantageously decreased.
Sockets 37A, 37B may be formed by multiple processes; for example by screen printing the adhesion layers 49A, 49B, onto wafer 50, followed by liquid or vapor deposition of passivation layer 44 to a uniform thickness at interface 51. BCB layer 45 is applied through a mask to passivation layer 44. The masked application of BCB layer 45 leaves areas of passivation layer 44 exposed, including the areas of the passivation layer covering adhesion layers 49A-C. Lithographic etching of passivation layer 44, where passivation layer 44 is unprotected by the masked application of BCB layer 45, removes the exposed portions of passivation layer 44. As a result, adhesion layers 49A-C are exposed and available to receive sputtered UBM layers 52A-52C. A discrete and isolated electric pathway is formed, for example, between wafer 50, adhesion layer 49A, UBM layer 42A, solder bar 41, trace 46A, and via 47A.
A data circuit 53 contacts to solder ball 43, which is optionally replaced by a solder bar (not shown).
Base metal application 97 or solder application 98 may, for example, be accomplished by electroplating in an electrochemical reactor using standard plating solutions. After step 98, process 90 has assembled the solder bar. Installation of the solder bar may place a trace of a substrate in contact or in close proximity with the solder bar and heated to reflow 99 the solder bond with the trace and complete the connection.
FIGS. 10A-E illustrate stages of solder bar manufacture. Structure 110 is prepared by application of an adhesion layer 81, passivation layer 83, and UBM layer 82 (
Structure 120 shows solder 86 after melting during reflow (
The objective of the comparison was to determine performance differences between a standard chip scale package having solder ball connections 3 and package 301 having plural solder bar 70 connections. Solder bar 70 connections utilized a copper base metal layer with solder applied to the copper (
Although the foregoing discussion emphasizes the bonding of a chip scale package to a PCB, it will be appreciated that the discussion also applies to bonding between other types of components. For example, a chip-on-chip structure may be created where two wafers are subjected to the process shown in
Changes may be made in the above methods and systems without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall there between.
1. A method of constructing a preformed solder bar made-ready for installing a microchip, comprising:
- forming a socket on a first surface of a microchip, such that the socket has predetermined physical dimensions complementary to those of a microchip connection pad footprint occupied by at least one contact pad area on the microchip,
- the socket presenting a conductive base capable of bonding to solder;
- bonding solder to the conductive base to place a solder bar in the socket and place the microchip in made-ready condition for installation.
2. The method of claim 1, wherein the microchip contains a silicon wafer and the step of forming the socket comprises
- depositing an adhesion layer onto the wafer, and
- depositing under-bump-metallization (UBM) material contacting the adhesion layer to complete formation of the conductive base.
3. The method of claim 2, wherein the step of depositing the adhesion layer includes depositing a conductor selected from the group consisting of aluminum, nickel-vanadium, titanium, tungsten and copper.
4. The method of claim 2, wherein the step of depositing the UMB material includes depositing a conductor selected from at least one of titanium, tungsten, vanadium, tin, copper, aluminum, gold, silver, and lead.
5. The method of claim 1, wherein the step of forming the socket includes the predetermined dimensions selected from the group consisting of rectangular, “E,” “L,” and “U” shapes.
6. The method of claim 1, wherein the step of forming the socket includes the physical dimensions selected from the group consisting of ring, square, and circular shapes.
7. The method of claim 1, wherein the step of forming the socket includes the physical dimensions being complimentary to the solder bar having a planar rectilinear configuration.
8. The method of claim 1, wherein the step of forming the socket includes the physical dimensions being complimentary to the solder bar having a planar curvilinear configuration.
9. The method of claim 1, wherein the step of forming the socket further comprising a step of forming a passivation layer on substantially all of the first surface, exclusive of an area where the socket is located.
10. The method of claim 9, wherein the step of forming the passivation layer includes the steps of:
- applying one or more layers of passivation material to the entire first surface; and
- removing selected portions of the passivation material covering the area where the socket is to be located.
11. The method of claim 10, wherein the step of applying one or more layers of passivation material includes applying at least one layer selected from the group consisting of polysilicon, silicon dioxide, and benzocyclobutane.
12. The method of claim 1, further comprises depositing a non-solder base metal in the socket after the step of forming the socket and prior to the step of bonding solder, such that the solder bar contains the non-solder base metal and the solder in respective layers.
13. The method of claim 12, wherein the step of depositing the non-solder base metal includes electroplating the non-solder base metal.
14. A method of claim 12, wherein the step of depositing the non-solder base metal includes screen printing at least one base metal layer.
15. The method of claim 14, wherein the step of depositing the non-solder base metal includes depositing the non-solder base metal layer selected from the group consisting of copper, gold, titanium, tungsten, and vanadium.
16. The method of claim 1, wherein the step of bonding solder includes electroplating one or more solder layers onto the conductive base.
17. The method of claim 1, wherein the step of bonding solder includes screen printing one or more solder layers onto the conductive base.
18. A chip scale package made-ready for installation of a microchip on a printed circuit board or chip-on-chip stack, comprising:
- a microchip including a first surface having a connection pad area;
- an adhesion layer contacting the connection pad;
- an under bump metallization layer contacting the adhesion layer;
- at least one passivation layer delineating an opening to the under bump metallization layer to define a socket; and
- a solder bar in contact with the under bump metallization layer, the solder bar having dimensions complementary to the socket and a geometric configuration selected from the group consisting of planar curvilinear and planar rectilinear configurations.
19. The chip scale package of claim 18, wherein the solder bar includes a base metal layer in contact with the under bump metallization layer and the solder layer.
20. The chip scale package of claim 18, wherein the geometric configuration includes a geometry selected from the group consisting of rectangular, “E,” “L,” and “U” shapes.
21. The chip scale package of claim 18, wherein the geometric configuration includes a geometry selected from the group consisting of ring, square, and circular shapes.
22. The chip scale package of claim 18, wherein the solder bar comprises a plurality of sublayers having different characteristics.
23. The chip scale package of claim 18, wherein the solder bar contains a base metal layer and a solder layer 24. The chip scale package of claim 23, wherein the base metal layer is selected form the group consisting of copper, gold, silver, lead, tin, titanium, tungsten, and vanadium.