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Method for hourglass-shaped BGA solder balls

IP.com Disclosure Number: IPCOM000021033D
Publication Date: 2003-Dec-17

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for hourglass-shaped ball-grid array (BGA) solder balls. Benefits include improved functionality, improved reliability, and improved performance.

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Method for hourglass-shaped BGA solder balls

Disclosed is a method for hourglass-shaped ball-grid array (BGA) solder balls. Benefits include improved functionality, improved reliability, and improved performance.

Background

         Conventionally, BGA solder balls are round. A sharp angle forms in the ball structure during the solder reflow process (see Figure 1). One of the main defects that occurs during this process is cracking. When the solder cracks, the joint fails. Causes include mishandling and externally induced or residual mechanical stress. Cracking may occur on the top side or bottom side of the solder ball where the joints are weakest because of the ball’s structure.

         Solder joint cracking/failure is conventionally solved by the following methods:

•         Specifying the ratio between the pad size and the BGA ball solder resistance opening (SRO-to-pad ratio).

•         Specifying the ratio of the solder paste to the pad

•         Ensure the strain-to-BGA hot spot with process control and equipment design

General description

         The disclosed method is the formation an hourglass-shaped structure during the solder reflow process. A stencil with an embedded retaining ring surrounds the BGA ball to form the shape.

Advantages

         The disclosed method provides advantages, including:

•         Improved functionality due to ensuring a consistent solder ball shape because of the use of a stencil, which guides the solder volume upward and downward during reflow process

•         Improved reliability due to reducing the mechanical stress on the solder ball and preventing cracking and joint failure

•         Improved reliability due to improving the strength of the solder ball because of the hourglass shape rather than the conventional round ball shape

•         Improved performance due to providing support to maintain the parallelism between the BGA package and the PCB

Detailed description

         The disclosed method is the formation an hourglass-shaped structure during the solder reflow process. The reliability of the solder connection is determined by the amount of pressure on the solder. As the amount of pressure increases, the failure rate also increases. The amount of pressure can be expressed using the following equation:

Pressure=Force/Area

         The disclosed method can be verified using model simulation. Assumptions include the following:

•         Shear force is applied uniformly to the interface between the BGA ball and the pad of the package.

•         Volume of the ball is the same for an hourglass-shaped ball and a barrel-shaped ball after the reflow process.

         For the simulation, a uniform shear force of 1 kg is applied to the interface between the BGA ball and the pad on the BGA pad. The bottom of the BGA ball is anchored on the PCB pad. The top surface is attached to the BGA package but is...