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Method for self-aligning pads for CSPs

IP.com Disclosure Number: IPCOM000020312D
Publication Date: 2003-Nov-12
Document File: 4 page(s) / 414K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for self-aligning pads for chip-scale packages (CSPs). Benefits include improved functionality and improved cost effectiveness.

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Method for self-aligning pads for CSPs

Disclosed is a method for self-aligning pads for chip-scale packages (CSPs). Benefits include improved functionality and improved cost effectiveness.

Background

         CSPs packages do not self-align to the printed circuit board (PCB) pads on the motherboard during the surface mount technology (SMT) reflow process. As a result, solder balls misalign to the PCB copper solder pads and create less-reliable solder joints, which have a propensity to propagate solder cracks. This condition can cause shorts and opens during CSP assembly on the motherboard.

         Very accurate placement equipment is required to place .5-mm pitch CSP components because the CSP process does not have the same self-alignment capability as larger-pitch ball-grid arrays (BGAs). They can be as much as 50% off the PCB land pad and still self-align to the PCB land pad during solder reflow.

         The CSP SMT process differs from the BGA SMT process in the self-aligning capability. The BGA assembly process uses solder paste as an interface between the solder ball and solder pad. During solder reflow, the surface tension of the flowing solder paste causes the BGA ball to self-align on the solder pad. Because of their reduced pitch size (0.5-mm pitch and smaller), CSPs use flux instead of solder paste as the interface material between the package solder ball and the PCB solder pad. Solder paste cannot be used because the solder printing process cannot screen print solder paste at the aperture sizes required.

         Sticky flux is printed onto the PCB pads of the CSP land grid array or CSP solder balls are dipped into flux before placement. This sticky flux provides sufficient tackiness between the solder ball and PCB pad to prevent the CSP from moving during solder reflow. However, flux does not provide enough wet surface tension to pull a misaligned CSP solder ball onto its pad.

         Because CSPs do not conventionally have a self-alignment capability, new placement equipment that is more accurate is required to place these parts. Additionally, packages are not designed with smaller-pitch designs because conventional assembly placement equipment cannot accurately place these parts, resulting in larger package sizes.

General description

The disclosed method is triangular PCB land pads on three corners of a CSP grid array. The method promotes alignment of the CSP component. The triangular pads create a horizontal pull on the CSP when the solder balls are not aligned to the PCB land pads. The horizontal pull is created by the wetting action of the solder ball to the large surface area presented by the hypotenuse of the triangular pad.

         The key element of the method is triangular pads at three corners of a CSP grid array.

Advantages

         The disclosed method provides advantages, including:

•         Improved functionality due to enabling conventional placement equipment to be used in...