Patent Application
20050051604
This invention relates generally to the field of integrated circuit packaging and, more specifically, to a method and system for forming ball grid array (“BGA”) packages.
Packaging of integrated circuits, such as ball grid arrays (“BGAs”), include encapsulating semiconductor chips and their associated components within a molding. A mold press compresses a mold compound between mold press die to allow the mold compound to cure. After curing, the mold press die are removed to obtain the completed, or partially completed, packages. However, current molding techniques, combined with the fact that integrated circuit packaging is resulting in thinner packages, often lead to warpage of the mold compound after removal of the mold press die due to residual stresses that build up during the cure process from the coefficient of thermal expansion mismatch. This warpage leads to many problems, such as a coplanarity problem that leads to poor quality, poor reliability, and difficult testing of the integrated circuit packages.
According to one embodiment of the invention, a system for forming ball grid array packages includes a substrate and a stencil engaging a first surface of the substrate. The stencil includes a center region having a first set of generally circular holes formed therein and a first outer region disposed radially outwardly from the center region having a second set of generally circular holes formed therein. The diameter of each of the generally circular holes of the second set is greater than the diameter of each of the generally circular holes of the first set. The system further includes a solder paste disposed outwardly from the stencil and a squeegee operable to spread the solder paste over the stencil to fill the first and second set of generally circular holes, thereby creating a plurality of solder paste regions.
Some embodiments of the invention provide numerous technical advantages. Other embodiments may realize some, none, or all of these advantages. For example, BGA package warpage is considered in advance when designing the stencil. The stencils for depositing the solder paste are designed such that the hole diameters generally increase as their position moves from the center of the stencil to the edges. This compensates for any warpage that may take place during the molding process due to coefficient of thermal expansion mismatch, thereby leading to proper dimensions and tolerances for the completed BGA packages. This improves productivity and yield, and eliminates rework at the ball inspection process.
Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.
For a more complete understanding of the invention, and for further features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:
Example embodiments of the present invention and their advantages are best understood by referring now to
In the illustrated embodiment, stencil 100 includes a plurality of generally circular holes 102 formed therein that function to define the configuration of solder paste regions 208. Shapes other than circles may be utilized for generally circular holes 102; however, because of the nature of solder balls used in BGA packages, the shape is generally circular. Generally circular holes 102 may be formed in stencil 100 with any suitable pattern; however, a typical pattern for generally circular holes 102 is an array in which generally circular holes 102 are aligned in rows and columns with a pitch 104. However, the pattern and spacing of generally circular holes 102 depends on the type BGA package being fabricated. For example, the type of BGA package that is formed using stencil 100 is a Texas Instruments, Inc. MicroStar BGA™, model no. GGU (S-PBGA-N144). As such, there are 13 rows and 13 columns of generally circular holes 102 formed in stencil 100. Pitch 104 for this particular BGA package is approximately 0.8 mils. For other BGA packages, pitch 104 is typically between approximately 0.5 millimeter and 1.0 millimeter. However, depending on the type of BGA package being formed, generally circular holes 102 may have any suitable pitch 104 and any suitable arrangement.
One of the most important components of BGA packages is solder ball coplanarity. Coplanarity is generally defined as the unilateral tolerance zone measured upward from the seating plane of the solder balls. Poor coplanarity leads to poor quality and poor reliability, which reduces productivity and yield for BGA packages. In addition, it may require a rework at the ball inspection process and reduce cycle time. Poor coplanarity is often caused by BGA package warpage due to the coefficient of thermal expansion mismatch involved during the molding process. Two-sided integrated circuits such as dual in-line packages typically do not have this warpage problem. Therefore, in accordance with the teachings of the present invention, stencil 100 is designed to account for package warpage by increasing the volume of solder paste for solder paste regions 208 at locations where the most warpage is expected. This is facilitated by generally increasing the diameters of generally circular holes 102 as the location of generally circular holes 102 extends farther from a center 108 of stencil 100.
As one example, referring to
In an embodiment where stencil 100 is used to fabricate a MicroStar BGA™ package (Model No. GGU (S-PBGA-N144)) as described above, generally circular holes 102 associated with center region 112 have a diameter of approximately 0.28 millimeters, the generally circular holes 102 associated with first outer region 114 have a diameter of approximately 0.32 millimeters, and the generally circular holes 102 associated with second outer region 116 have a diameter of approximately 0.34 millimeters. This arrangement of generally circular holes 102 forming stencil 100 compensates for the majority of the warpage for this particular type of MicroStar BGA™ package, which happens to occur at the corners of this BGA package.
Although stencil 100 as illustrated in
The term “generally increasing” as used herein does not mean that a particular generally circular hole 102 that is farther from center 108 than another particular generally circular hole 102 has a larger diameter. It merely means that, on average, the diameters of generally circular holes 102 increase as the location of generally circular holes 102 move further away from center 108 and towards a perimeter 110 of stencil 100. For example, a particular generally circular hole as referenced by numeral 103 may be slightly further away from center 108 than a particular generally circular hole as referenced by numeral 105, yet have a slightly smaller diameter.
Thus, any package warpage that may take place during the molding process due to coefficient of thermal expansion mismatch is already compensated for in the present invention by designing sensor 100 with generally circular holes 102 as described above. This leads to proper dimensions and tolerances for the completed BGA package, which greatly improves productivity and yield and eliminates any rework at the ball inspection process.
Although embodiments of the invention and their advantages are described in detail, a person skilled in the art could make various alterations, additions, and omissions without departing from the spirit and scope of the present invention, as defined by the appended claims.
This application claims the benefit of Ser. No. 60/501,346 titled “Multi-Dimensioned Stencil Design for Ball Grid Arrays and Flip-Chip Packages,” filed provisionally on Sep. 9, 2003.
| Number | Date | Country | |
|---|---|---|---|
| 60501346 | Sep 2003 | US |
