Browse Prior Art Database

Method for a SC-BGA

IP.com Disclosure Number: IPCOM000021246D
Publication Date: 2004-Jan-07
Document File: 5 page(s) / 216K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a selective columnar ball grid array (SC-BGA). Benefits include improved reliability, improved performance, and improved ease of implementation.

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Method for a SC-BGA

Disclosed is a method for a selective columnar ball grid array (SC-BGA). Benefits include improved reliability, improved performance, and improved ease of implementation.

Background

         BGA solder joints in surface mount technology (SMT) packages conventionally have the same spherical shape (see Figure 1). Under stressful conditions, they fatigue and crack. This problem is conventionally solved using the following techniques:

•         Removing or sacrificing solder joints in high-risk locations

•         Reducing mechanical preload on the package

•         Adding under-fill to the BGA (which substantially increases assembly costs)

General description

The disclosed method is the placement of columnar (such as non-spherical) solder joints in locations that are highly subject to fatigue, such as substrate corners, edges, and under the die shadow. The method uses via-in-pad technology on the board side to form columnar solder joints.

         The key elements of the method include:

•         Columnar solder joints

•         Via-in-pad features on a printed circuit board (PCB)

Advantages

         The disclosed method provides advantages, including:

•         Improved reliability due to improving fatigue resistance by placing columnar solder joints in locations that are highly subject to fatigue

•         Improved reliability due to reducing the sheer stress on the columnar solder joints and the substrate/board pads

•         Improved performance due to enabling the full use of all solder joints on the package for electrical connections (no sacrificial joints are required)

•         Improved ease of implementation due to not impacting substrate routing and minimally impacting board routing

•         Improved cost effectiveness due to not increasing substrate cost and incrementally increasing board cost

Detailed description

The disclosed method is the placement of columnar solder joints using via in pads on a PCB (see Figure 2). The cross-sectional view of the columnar solder joint illustrates the highest stress interface (see Figure 3). The corner and die shadow solder joints are formed into columns. They are the joints that are conventionally under the highest stress during temperature cycle loading. The remaining BGA joints must maintain the conventional spherical shape so that the package does not collapse onto the board due to its weight. The highest stress interface is in the middle of the columnar solder joint, because that is the location that has the smallest cross-sectional area.

The columnar BGA geometry can be achieved through plugless via-in-pad technology. The BGA pad has a plated through hole (PTH), which is not plugged. When the BGA is surface mounted to the PCB, solder paste is added to the BGA pad prior to component placement. When the assembly is in reflow, the solder paste and BGA ball melt. The molten solder is drawn by capillary action into the PTH, creating the columnar joint appearance. If the PTH is sized properly, solder drip on the backside of the board is minimized.

Model        

A quarter...