Ball grid array rework

Embodiments of the invention relates to a method and apparatus for rework of a BGA package. Memory shape material is placed adjacent to a plurality of solder joints of the package. Stimulation is applied to the material, with the stimulation causing the material to change from a non-stimulated shape to a stimulated shape. This stimulation causes an expansion of the material. As the material expands, it exerts a tensile force on the BGA package and an adjacently positioned carrier, causing a separation of the two components, while mitigating collateral heat of adjacently positioned components.

BACKGROUND

The present invention relates to rework for a ball grid array package, including removal of the ball grid array package from an associated printed circuit board or carrier. More specifically, the invention relates to an expansion material and control of the material in a manner that applies an expansion force to adjacently positioned solder joints.

Computer systems are generally configured with a memory module(s) and integrated circuits in communication with a printed circuit board (PCB). A ball grid array package (BGA) is a type of surface-mount packaging used for integrated circuits. BGA packages are used to permanently mount devices such as microprocessors. The BGA package uses a grid of solder balls as its connectors to a printed circuit board (PCB) or carrier. Rework pertains to repair or refinishing a PCB assembly. The aspect of rework generally includes uncoupling the package or other electronic components from the substrate, and in one embodiment re-soldering components to the substrate.

BRIEF SUMMARY

The invention comprises an apparatus for a rework process that employs localized heating for expanding solder joints between the BGA and the PCB.

A BGA package is assembled on to a printed circuit board with an expansion material. Configuration of the package includes interstitial placement of the material within a matrix of solder joints. A tensile force is applied between a ball grid array (BGA) package and the printed circuit board through delivery of electrical current to the material. In one embodiment, the application of the force is controlled. The current creates a localized heating of the expansion material, and the heated material applies an expansion force to the solder joints. This expansion force separates the BGA from the printed circuit board.

Other features and advantages of this invention will become apparent from the following detailed description of the presently preferred embodiment(s) of the invention, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION

An apparatus and method for a rework process creating localized stimulation for localized heating and expansion of solder joints between a BGA and a PCB or an alternate substrate is described in detail below. The apparatus and method employs an expansion material in communication with the BGA package and the PCB. More specifically, the expansion material is interstitially placed within a matrix of BGA solder joints. The material is comprised of a memory shape alloy, or in one embodiment, a high z-axis coefficient of thermal expansion material. When subject to stimulation, such as heating, the material expands, and this expansion applies an expansion force to both the PCB and the BGA package. In one embodiment, the heated material has an exothermic reaction, and the heat softens the adjacently positioned solder joints. Accordingly, application of heat to the material both expands and softens the solder joints, thereby facilitating separation of the BGA from the PCB.

Referring now toFIG. 1, a perspective view (100) of a ball grid array package (rendered transparent) is shown assembled on to a PCB or substrate with an expansion material situated interstitially between the solder joints. As shown, the BGA package is shown with a substrate (110) and a matrix of solder balls (120). An expansion material (130) is placed within the matrix (120). In one embodiment, the matrix (120) includes a plurality of rows and columns. The material (130) is shown placed in each adjacently positioned row in the matrix (120). In one embodiment, the material (130) may be placed in alternative rows of the matrix, and should not be limited to adjacently positioned rows. The material (130) is comprised of a height and width that enables it to be received within the matrix. In one embodiment, the width of the material is limited to the width between adjacently positioned solder balls. Similarly, in one embodiment, the height of the material is limited to the height of the solder balls from the base of the substrate (110) to the top surface of the solder ball (122). The BGA package may be pre-configured with the material within the matrix of solder joints, or in one embodiment, the material may be threaded or otherwise placed adjacent to the solder joints. Accordingly, the configuration of the material is limited by the configuration of the matrix, and specifically by the dimensions and positioning of the solder balls that comprise the matrix.

The BGA package shown inFIG. 1is an illustration of the transparent ball grid array package assembled on to the PCB or substrate. Referring toFIG. 2, a cross sectional view (200) of the BGA package in communication with a PCB prior to application of force. As shown, the BGA package (210) is in communication with the PCB (230) through an array of solder balls (220). A gap (250), also referred to herein as a first gap, represents the spacing between the BGA package (210) and the PCB (230) formed by the connection of the PCB (230) and the BGA package (210) via one or more solder joints (220), also referred to herein as a solder ball connection. Specifically, the solder joints (220) hold the PCB (230) and BGA (210) in a fixed relationship. In one embodiment, the connection formed by the solder balls is referred to as a solder joint. Expansion material (not shown) is positioned between adjacently positioned solder joints. Specifically, the expansion material is arranged in parallel on both sides of the solder joint in the gap (250). Accordingly, the expansion material occupies vacant space formed between adjacently positioned solder joints.

The expansion material shown inFIGS. 1 and 2, represents the material at a rest state prior to expansion of the material and/or expansion of the adjacently positioned solder joints. The material is configured to change shape in response to a stimulus. Referring toFIG. 3, a cross sectional view (300) is provided illustrating expansion of the solder joints following application of a stimulus to the associated expansion material. As shown, the BGA package (310) is in communication with the PCB (330) through an array of solder balls (320). A gap (350), also referred to herein as a second gap, represents the spacing between the BGA package (310) and the PCB (330) formed by the connection of the PCB (330) and the BGA package (310) via one or more solder joints (320), also referred to herein as a solder ball connection. Expansion material (340) is shown between adjacently positioned solder joints. Specifically, the expansion material (340) is arranged in parallel on both sides of the solder joint. As shown herein, the expansion material (340) has an expanded form and elongated shape following receipt of a stimulus and in comparison to the material (240) shown inFIG. 2prior to receipt of a stimulus. In addition, the second gap (350) is shown larger than the first gap (250), as a result of the change in size and shape of the expansion material (140). As shown, the size and shape of the expansion material (340) changes and elongates in response to the stimulus. In one embodiment, the size and shape of the solder joints (320) also changes in response to the changes experienced by the material (340). As shown herein, in one embodiment, the solder joints (320) elongate in conjunction with, or in response to, the elongation of the material (340). Accordingly, application of stimulus to the expansion material (340) increases the size of the second gap (350) representing the space between the BGA package (310) and the PCB (330).

The focus of the expansion material is to facilitate rework of the BGA package. Rework may be required for a plurality of reasons, including replacement, upgrade, etc., of a component. InFIG. 3, the expansion of the material in conjunction with expansion of the solder joints is shown. Referring toFIG. 4, a cross sectional view (400) is provided illustrating further expansion of the material following application of the stimulus. The BGA package (410) is shown with limited communication with the PCB (430). More specifically, a gap (450) represents spacing between the BGA package (410) and the PCB (430). The gap (450) is also referred to herein as a third gap. Responsive to the expansion of the material (440), the solder joints (420) are severed. As shown, a first portion of the solder joint (422) is in communication with the BGA package (410) and a second portion of the solder joint (424) is in communication with the PCB (430). In one embodiment, the deformed solder joints remain in communication with the PCB (430) and are removed from the BGA package (410). The severing of the solder joint (420) is in response to expansion of the material (440). In one embodiment, the material (440) remains contiguous, while the solder joints are sheared. The expansion of the material (440) applies a high out of plane force on the PCB (430) and on the bottom of the package (410). Accordingly, a resulting effect of the material expansion severs the solder joints.

Referring toFIG. 5, a flow chart (500) is provided illustrating a process decoupling the BGA package and the PCB. As shown, a material is fed through a matrix of solder balls on the BGA package (502). In one embodiment, the matrix includes a plurality of rows and columns, and the material is arranged in parallel across the rows or across the columns. Similarly, in one embodiment, the material is placed in each adjacently positioned row or column, or in another embodiment, the material is placed in selected rows or columns within the matrix. In one embodiment, the material placed in the matrix includes a memory shape alloy. Following the material placement, an electric current is driven through the material (504). The current delivers heat to the material and promotes a high z-axis expansion of the material (506). In one embodiment, the material has an original expanded shape and is placed in the matrix in a reduced shape. When subject to heating or in receipt of electrical current, the material transforms from the reduced shape to the original expanded shape. In one embodiment, delivery of electricity, referred to herein as a compression mode, is controllable with the amount of electricity proportional to the expansion of the material. Similarly, in one embodiment, external heating may be applied to the solder joints (508). In one embodiment, the external heating is delivered from a hot air rework nozzle. Similarly, in one embodiment, the material receives electrical current concurrent with the external heating directed to the solder joints. Delivery of the current and the heat may be concurrent or sequential. The expansion force from the material severs the solder joints (510), and the PCB is then separated from the BGA package (512). Accordingly, delivery of current to the material transforms the material into an expanded shape, forcing the PCB and the BGA package to separate.

As shown inFIGS. 1-5, displacement growth between the BGA package and the PCB, also referred to as the carrier, is driven by expansion of the material and solder joint deformation due to displacement. In one embodiment, neither the displacement nor the solder joint deformation is uniform. However, the memory shape alloy material supports direct heat delivery to a specified location and mitigates collateral heat of adjacently positioned components. Accordingly, the heat delivery is direct and specific.

As shown and described herein, the memory shaped alloy material is placed within the matrix of the solder joints in a reduced form. When subject to heat, the material transform into an expanded shape. Expansion of the material is limited with respect to the configuration of the carrier. In one embodiment, the maximum z-axis expansion is 42.5 microns. To ensure expansion in the direction of the solder joint, the separation force may be tuned through use of anisotropic expansion materials. In one embodiment, the anisotropic expansion materials may be in a rectangular form with an elongated axis, such as a ribbon structure. Accordingly, selection of materials may be employed with respect to the structure of the carrier and/or the desired expansion for rework.

The rework illustrated and described herein is with respect to a BGA package and a carrier. In one embodiment, the expansion material may be used to separate one or more die in a multi-die stacked package. The memory shape alloy material enables the separation while mitigating collateral reflow from any adjacently positioned components. Accordingly, the application of the memory shape alloy material may be expanded to various configurations within a carrier and associated packages and die.

The BGA rework may be automated through a series of tools and associated computer readable and executable instructions. Referring toFIG. 6, a system is provided with a computer in communication with PCB. The computer (610) is provided with a processing unit (612) operably coupled to memory (614) across a bus (616). One or more tools are shown to facilitate expansion of the memory shaped alloy material placed adjacent to the solder joints of the BGA package. The tools include an applicator (624) to deliver the electrical current to the material, and a regulator (626) to control the level of current being delivered. In one embodiment, one or more computer readable instructions are processed to control the delivery of current to the material. Similarly, in one embodiment, the level of current delivered to the material is directly or indirectly proportional to the tensile force between the BGA package and the PCB, and by controlling the current, the regulator (626) controls the application of force. Rework may be directed to a specific BGA package, a multi-stack die, or in one embodiment, a combination of two or more packages or die. The applicator (624) and regulator (626) are employed to direct the rework to a designated location and associated material.

The tools shown inFIG. 6may be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices, or the like. The tool may also be implemented in software for execution by various types of processors. An identified functional unit of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions which may, for instance, be organized as an object, procedure, function, or other construct. Nevertheless, the executable of the tool need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the tool and achieve the stated purpose of the tool.

Referring now to the block diagram ofFIG. 7, additional details are now described with respect to implementing an embodiment of the present invention. The computer system includes one or more processors, such as a processor (702). The processor (702) is connected to a communication infrastructure (704) (e.g., a communications bus, cross-over bar, or network).

The computer system can include a display interface (706) that forwards graphics, text, and other data from the communication infrastructure (704) (or from a frame buffer not shown) for display on a display unit (708). The computer system also includes a main memory (710), preferably random access memory (RAM), and may also include a secondary memory (712). The secondary memory (712) may include, for example, a hard disk drive (714) and/or a removable storage drive (716), representing, for example, a floppy disk drive, a magnetic tape drive, or an optical disk drive. The removable storage drive (716) reads from and/or writes to a removable storage unit (718) in a manner well known to those having ordinary skill in the art. Removable storage unit (718) represents, for example, a floppy disk, a compact disc, a magnetic tape, or an optical disk, etc., which is read by and written to by removable storage drive (716). As will be appreciated, the removable storage unit (718) includes a computer readable medium having stored therein computer software and/or data.

In alternative embodiments, the secondary memory (712) may include other similar means for allowing computer programs or other instructions to be loaded into the computer system. Such means may include, for example, a removable storage unit (720) and an interface (722). Examples of such means may include a program package and package interface (such as that found in video game devices), a removable memory chip (such as an EPROM, or PROM) and associated socket, and other removable storage units (720) and interfaces (722) which allow software and data to be transferred from the removable storage unit (720) to the computer system.

The computer system may also include a communications interface (724). Communications interface (724) allows software and data to be transferred between the computer system and external devices. Examples of communications interface (724) may include a modem, a network interface (such as an Ethernet card), a communications port, or a PCMCIA slot and card, etc. Software and data transferred via communications interface (724) is in the form of signals which may be, for example, electronic, electromagnetic, optical, or other signals capable of being received by communications interface (724). These signals are provided to communications interface (724) via a communications path (i.e., channel) (726). This communications path (726) carries signals and may be implemented using wire or cable, fiber optics, a phone line, a cellular phone link, a radio frequency (RF) link, and/or other communication channels.

In this document, the terms “computer program medium,” “computer usable medium,” and “computer readable medium” are used to generally refer to media such as main memory (710) and secondary memory (712), removable storage drive (716), and a hard disk installed in hard disk drive (714).

Computer programs (also called computer control logic) are stored in main memory (710) and/or secondary memory (712). Computer programs may also be received via a communication interface (724). Such computer programs, when run, enable the computer system to perform the features of the present invention as discussed herein. In particular, the computer programs, when run, enable the processor (702) to perform the features of the computer system. Accordingly, such computer programs represent controllers of the computer system.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. Employment of the memory shaped material for the rework supports direct application of the rework to a specific component. Accordingly, the placement and arrangement of the shape changing material enables and supports rework to a direct location, while mitigating collateral heat to adjacently positioned components.

ALTERNATIVE EMBODIMENT

It will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. In one embodiment, the expansion material may be in the form of a wire that is fed through the matrix of solder balls. The material is shown to be placed in a parallel or relatively parallel arrangement within the matrix. In one embodiment, the material may be placed in the matrix in a non-parallel arrangement. Accordingly, the scope of protection of this invention is limited only by the following claims and their equivalents.