Patent Application
20050077080
1. Field of the Invention
This invention relates generally to integrated circuits. More particularly, it relates to an improvement of the underside structure on what has become known as a plastic ball grid array (Plastic BGA, or PBGA) or overmolded pad array carrier (OMPAC).
2. Background of Related Art
The ball grid array (BGA) or overmolded pad array carrier (OMPAC) package has seen a rapid increase in popularity due to its motherboard space efficiency as well as good thermal and electrical performance. Originally developed in the late 1980s, the pin density (i.e., the number of electrical connections that can be made between the printed circuit board (PCB) and the integrated circuit carried in the BGA package) has grown tremendously.
The Joint Electron Device Engineering Council (JEDEC) adopted standard PBGA body sizes in August of 1993, ranging from 7 to 50 mm with pitches of 1.00, 1.27 and 1.50 mm. These pitch and body size combinations result in pin counts ranging from 16 to 2401 pins.
With PBGA packaging comes a new challenge for circuit board manufacturers installing integrated circuits contained within the PBGA packages. In particular, the placement and reflow of solder ball preforms to form bumps on the package bottomside is a new required assembly step with PBGA packages.
In particular, as shown in
A conventional Plastic BGA package 500 is based on a glass reinforced organic substrate 520 made of several possible materials, the most common of which is Bismalemide Triazene (BT) epoxy resin developed by Mitsubishi Gas Chemical. Because of its excellent high temperature properties (high Tg, excellent heat resistance, high temperature stability), BT resin is well suited for a semiconductor packaging application where various high temperature excursions are associated with the required assembly processes. These high temperature processes may include die-bonding and cure, wire-bonding, encapsulation and cure, solder ball attach reflow, cleaning and drying. Elevated temperature exposure following packaging may also be encountered during high temperature testing, burn-in, repeated baking cycles to remove moisture, and finally surface mount assembly to a desired PCB.
The BT/glass substrate 520 is generally processed in a strip which can consist of from three to twelve or more package sites. The nominal strip thickness currently used is 0.36 mm (including BT/glass core, solder mask and copper feature thicknesses) for double-sided substrates and 0.60 mm for four layer substrates.
A silicon die (i.e., the integrated circuit or IC) 514 is die-bonded and wire-bonded to the die pad on 520 or flag at each die site on the strip similar to chip on board (COB) manufacturing procedures. This is followed by an encapsulation of the sites with a filled epoxy overmold 510.
The encapsulation may be performed with a cavity mold resulting in a formed molded body shape or with a glob-top liquid encapsulant that is possibly preceded by the application of a dam for containment of the glob-top material. Whichever process is used, careful selection of encapsulant materials to minimize substrate and therefore package warpage is desired.
Once the BGA package 500 is overmolded, solder ball preforms 504 with a typical diameter of, e.g., 0.75 mm (for 1.27 and 1.50 mm pitch) are placed on the solder pads 506 on the strip bottomside, and reflowed to form bumps. These melting solder bumps 504 formed on the PBGA package 500 are sometimes referred to as a controlled collapse chip carrier connection, or C5 joint.
After solder ball reflow, it may be necessary to clean the BGA package 500 to remove flux residue. Finally, following cleaning, the individual BGA packages 500 are singulated from the strip by any one of several methods.
The JEDEC standards organization has promulgated standards regarding BGA packaging. With respect to warpage, the JEDEC standards indicate that the worst-case co-planarity, which is largely a direct result of any warpage, is currently preferably no more than 0.2 mm independent of component body size. Additionally, thicker substrates than those discussed above have been proposed to further increase package flatness, especially for larger packages.
As shown, solder balls 504a located closer to the edges and/or corners of a conventional BGA package 500 tend to be more compressed beyond that which is desired, pressing the PBGA substrate 520 of the GBA package 500 too close to the PCB 502 onto which it is ultimately mounted, causing a danger of, or resulting directly in, an electrical short 507 produced between two adjacent, excessively compressed solder balls 504a. Such shorts in packages lowers yields and lowers product reliability, particularly through the manufacturing stage, increasing costs and rework.
Conventional techniques to prevent warpage generally center around the development of new mold compounds having reduced warpage characteristics. Nevertheless, to date all conventional mold compound formulations currently available nevertheless warp to some extent. What the present inventors have appreciated is that this warpage results in corner BGA ball shorting, or at a minimum reduces manufacturing reliability because of an excessive number of devices in which corner PBGA ball shorting occurs.
There is a need for a BGA package design that better avoids or even prevents altogether solder-ball shorting caused by warpage.
In accordance with the principles of the present invention, in a ball grid array (BGA) package, the solder ball side of the BGA package comprises a plurality of solder balls, and at least one tab support extending in a direction toward a PCB to which the BGA package will ultimately be mounted. The at least one tab support prevents excessive compression of a solder ball in a vicinity of the at least one tab support.
In accordance with another aspect of the present invention, a method of preventing solder ball shorting in a BGA package comprises providing a BGA substrate for attachment of a plurality of solder balls to a BGA package. At least one tab support is provided on a solder ball side of the BGA substrate. The at least one tab support has a height corresponding to a desired distance between the BGA substrate and a PCB to which the BGA package will be mounted, in a mounted state. The at least one tab support prevents excessive compression of ones of the plurality of solder balls in a vicinity of the at least one tab support.
Features and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings, in which:
In the real-world, overmolded BGA packages warp during high temperature manufacturing processes, e.g., particularly due to the high temperatures associated with a solder reflow process intended to melt each of the solder balls and cause an electrical path to be formed with respect to each individual ball. This warpage increases with higher reflow temperatures and is severe enough to cause the corner BGA balls to bridge together, causing electrical solder shorts between the BGA balls.
The present invention includes supporting features built onto the underside, or solder ball side of the BGA package substrate. In an alternative embodiment, the tab supports may be mounted to the PCB to which the BGA package will be mounted, preferably in areas corresponding to the corners of the BGA package when mounted, and/or to the edges of the BGA package. The tab supports are preferably sized (h in
The physical pressure of each of the solder balls, working against the upward pressure caused by the use of the tab supports, tend to prevent future compression of the solder balls during high temperature processing.
In particular,
The exemplary tabs in the given embodiments comprise small standoff tabs 100 placed in all four corners of the lower substrate 120 of a BGA package 110.
The corner tab supports 100 are preferably incompressible at high temperatures such as is experienced during a solder reflow process. The corner tab supports 100 prevents a warped GBA package body (which using current component materials ALWAYS warps, at least to some extent) from contacting the printed wiring board 502. This in turn prevents the solder balls 504, once heated and in a melted form, from compressing excessively to the point that they touch or otherwise short together.
Use of corner tab supports 100 on an underside, or solder ball side of a BGA package 110 in accordance with the principles of the present invention will prevent compression of solder balls 504 in a vicinity of the tab support 100 from compressing beyond a given amount equal to the height of the corner tab support 100, i.e., the length of the protrusion of the tab support 100 from the underside of the BGA substrate 120 of the BGA package 110.
The particular shape of the tab supports 100 is insignificant, so long as they allow reflow processes to take place there around. For instance, the exemplary tab supports 100 are shown to have a square cross-section. Rectangular, circular, oval, or polygonal cross-sectional shapes are equally possible, as is the use of a glob of hardening material, e.g., a glob of epoxy.
Moreover, tab supports 100 in accordance with the present invention may be made from any suitably heat resistant material. Exemplary materials include, but are not limited to, dispensed adhesive, molded plastic tabs, corner or edge overmolded plastic tab supports, attached metal or plastic tab supports, or metal tabs which are extruded or otherwise incorporated on a BGA product to extend beyond the substrate to support the substrate in the event of warpage, e.g., caused during the reflow or other high temperature process.
Significant advantages results from the use of corner or edge tab supports in a BGA package 110 in accordance with the principles of the present invention. For instance, solder ball interconnects on BGA packages will not solder-ball-short as a result of warpage. Moreover, depopulating or non-use of solder balls nearest to corners and/or edges to increase reliability is no longer necessary, increasing connection density in a given BGA package.
Over molded tabs 100 can be designed into mold fixtures and easily incorporated into the over molding process. Furthermore, external plastic or metal tabs could be attached to the substrate 120 before or after attachment of the solder balls 504.
In particular, as shown in
While the invention has been described with reference to the exemplary embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments of the invention without departing from the true spirit and scope of the invention.
