Patent Application
20140252073
1. Technical Field
The present disclosure is directed to the electrical coupling of two circuit boards in back-to-back configuration and, more particularly, to a ball grid array reballing template and method of making and using.
2. Description of the Related Art
In an effort to reliably connect integrated circuits or “chips” to printed circuit boards, manufacturers have resorted to a variety of methods, including wire bonding, chip carriers with beam leads, and direct chip connections. Some manufacturers have experimented with soldering as one form of direct chip connection, but the results were less than favorable due to solder bridging and variations created in the spacing between the chip and the substrate.
One early method of direct chip connection involved the use of a copper micro-ball in which copper spheres were bonded to pads of the solid state device, such as a transistor, using, for example, solder. This resulted in the first arrangement of balls in a grid pattern, known now as the Ball Grid Array (BGA). As Surface Mount Technology (SMT) became more available with the BGA connection, other problems began to arise. Thermal expansion of the substrate sometimes caused a mismatch between the silicon device and the substrate, causing high stress in the structure and resulting low reliability. While the use of underfill and new materials addressed this problem, the move to solder balls in more densely arranged arrays increased the occurrence of solder bridging.
The solder “bump” or ball serves as the interconnection point between a device and a board in what is sometimes known as flip chip technology. In order for the solder joints to be formed, the solder ball must be constructed of fully reflowable material, such as eutectic Sn/Pb material, which enables a well-controlled process to provide bump heights with a standard deviation, typically less than 2.5 μm.
Referring initially to
In high density BGA applications, the close proximity of the solder balls can result in bridging when the solder ball expands outward during the reflow process. Another potential issue is “solder balling,” in which very small spherical particles of solder separate from the main body of the solder ball and form a bridge or joint between adjacent solder balls. This is a primary concern inasmuch as the artificial bridge between two adjacent leads can create functional problems in the associated electrical circuit.
Typical approaches to dealing with this problem include: (1) Selecting a reflow process that best fits the components used in the BGA array, including the paste used for the balls to hold them to the pad; and (2) minimizing solder paste exposure to high temperatures and humidities whenever possible. Of course, many electric circuits are used in high temperature and high humidity environments, which can result in solder bridging. Moreover, solder bridging can occur during the initial manufacturing process or during a repair or replacement operation in which components that have been separated are reattached.
Hence, there is a need for a new approach that offers a robust and functional interconnect solution in which the integrity of the connection is preserved, especially in the miniaturization and high density BGA environment, and that preserves the high performance operation of the circuit in all temperature and humidity environments.
In accordance with the present disclosure, a device is provided for achieving a reliable and robust electrical connection between a ball grid array assembly to a printed circuit board, the ball grid array assembly having a plurality of conductive pads arranged in a pattern on a bottom side, and the printed circuit board having a plurality of conductive pads arranged in a mirror of the pattern on the bottom side of the ball grid array assembly. The device includes a non-rigid body formed of a thermal resistant material having a top side and a bottom side, the bottom side having an adhesive layer. An array of openings are formed in the body, the array of openings arranged in a pattern that matches the pattern of the conductive pads on the ball grid array assembly, each opening having a circular plan form shape that is sized to enable a single solder ball to be slideably received in the opening. Each opening is spaced from adjacent openings by a distance that prevents adjacent solder balls from electrically connecting to each other when subjected to a temperature sufficient to reflow the solder balls.
In accordance with a further aspect of the present disclosure, the non-rigid body is formed of a flexible, compliant material having a thickness in the range of 0.001 inches to 0.005 inches. More preferably the thickness is in the range of 0.1 mm to 0.9 mm.
In accordance with another aspect of the present disclosure, a method for reballing and attaching a ball grid array assembly is provided. The method includes forming a pattern on a template, the pattern reflecting the footprint of the ball grid array assembly that indicates at least a size of a solder ball and the locations for solder balls on the bottom side of the ball grid array assembly. The method also includes creating a plurality of apertures on the template coincident with the locations for the solder balls on the bottom side of the ball grid array assembly, affixing the template to the bottom side of the ball grid array assembly with the apertures in the template aligned with the locations for the solder balls on the ball grid array assembly. The method further includes applying solder paste to the locations for the solder balls on the ball grid array assembly, adhering a plurality of solder balls on the locations for the solder balls, placing the reballed ball grid array assembly onto a printed circuit board with the solder balls aligned with a plurality of pads on the printed circuit board, and reflowing the printed circuit board together with the reballed ball grid array assembly.
In accordance with another aspect of the present disclosure, an apparatus is provided that includes a first board having a plurality of conductive pads arranged in a pattern, a template comprising a non-rigid body having a top side and a bottom side, the bottom side having an adhesive layer and an array of openings in the body, the array of openings arranged in a pattern that is coincident with the pattern of conductive pads on the first board, each opening having a circular plan form shape, each opening spaced from adjacent openings by a distance. The template is mounted on the first board by adhering the adhesive layer to the first board and each opening in the array of openings in the body aligned with a respective conductive pad on the first board. The apparatus also includes a solder ball slideably received in each opening in the template to extend past the bottom side of the template to contact the respective pad on the first board and to extend above the top side of the body of the template, the solder ball electrically coupled to the respective pad on the first board and electrically insulated from adjacent solder balls by the template body.
In accordance with a further aspect of the present disclosure, the apparatus may further include a second board having a plurality of conductive pads formed therein and arranged in a pattern that is a mirror image of the pattern formed on the first board and the pattern of the array of openings formed in the template, the respective pads of the second board electrically coupled to a respective solder ball on the first board.
In accordance with yet another aspect of the present disclosure, the template is configured to have a thickness that prevents each solder ball from flowing into adjacent solder balls when reflowed for attachment to the first and second boards. Ideally, the adhesive may be formed of a material that allows the template to be removably adhered to the first board. Preferably, the solder ball is formed to reflow and electrically bond to the respective pad on the first and second boards when subjected to a thermal flow process.
The foregoing and other features and advantages of the present disclosure will be more readily appreciated as the same become better understood from the following detailed description when taken in conjunction with the accompanying drawings, wherein:
In the following description, certain specific details are set forth in order to provide a thorough understanding of various disclosed embodiments. However, one skilled in the relevant art will recognize that embodiments may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures or components or both associated with ball grid array mounting systems and methods, including but not limited to the apparatus for reflowing the solder balls, have not been shown or described in order to avoid unnecessarily obscuring descriptions of the embodiments.
Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open inclusive sense, that is, as “including, but not limited to.” The foregoing applies equally to the words “including” and “having.”
Reference throughout this description to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
In
Working with the bottom of the ball grid array assembly 24 facing up, the exposed pads 32 are coated with solder paste to prepare them to receive solder balls. In a preferred embodiment, a metal cut-out (not shown) is used to place the plurality of solder balls onto the pads 32. The metal cut-out has a plurality of apertures, and when placed over the bottom of the ball grid array assembly 30, the plurality of apertures coincide with pads 32 on the bottom of the ball grid array assembly 30, each aperture having a diameter sufficient to slidably receive one solder ball therethrough. A plurality of solder balls are poured over the surface of the metal cut-out and a subset of the plurality of solder balls slide through the apertures 38 of the template 36 and onto the pads 32. Residual solder balls are removed from the surface of the metal cut-out.
The ball grid array assembly 24 then undergoes a reflow process with an appropriate thermal profile to secure the solder balls onto the ball grid array assembly 24. In a preferred embodiment, the ball grid array assembly 24 undergoes the reflow process with the metal cut-out in place to help the solder balls on the pads 32 remain in place. The template 36 acts as a barrier between adjacent solder balls on the pads 32 on the carrier board 30 during the reflow process, preventing shorts due to bridging between melting solder balls. Once the solder balls are secured, the ball grid array assembly 24 is considered reballed.
The reballed ball grid array assembly 24 can now be assembled onto a printed circuit board.
The various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.
These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.
| Number | Date | Country | |
|---|---|---|---|
| 61772915 | Mar 2013 | US |
