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Method of fabrication of solder balls and attachment to package/board substrates

IP.com Disclosure Number: IPCOM000009293D
Publication Date: 2002-Aug-14
Document File: 6 page(s) / 90K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method of fabrication of solder balls and attachment to package/board substrates. Benefits include improved ease of manufacturing.

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Method of fabrication of solder balls and attachment to package/board substrates

Disclosed is a method of fabrication of solder balls and attachment to package/board substrates. Benefits include improved ease of manufacturing.

Background

        � � � � � Manufacturing issues/concerns associated with adapting current ball grid array processes for fine pitch (< 0.5 mm) ball grid arrays (BGAs) and when BGA sizes are <0.3 mm. Low yields and associated higher cost are the main problems with developing small joint BGAs. Solder-joint reliability is a major reliability issue. Low throughput-time underfill process is the conventional solution.

        � � � � � Evolutionary improvements in the BGA process are ongoing to improve yields and reliability. However, BGA reliability, also known as solder-joint reliability, is becoming one of the dominant failure mechanisms in package/board reliability. The conventional solution is BGA underfill.

        � � � � � The conventional process steps are as follows (see Figure 1):

1.        � � Manufacture of spherical balls of solder

2.        � � Place solder spheres on a vacuum grid chuck

3.        � � Flux package land surface

4.        � � Align and place solder spheres on lands and reflow solder

5.        � � Flux motherboard lands

6.        � � Place package on board and reflow

7.        � � Underfill BGA joints if necessary

        � � � � � This process is tedious, time consuming, costly, and facing many capability issues for fine-pitch small-standoff BGA joints required for non-CPU applications.

General description

        � � � � � The disclosed method uses small solder balls formed and handled using the mesh concept. The solder mesh is designed to result in the right size of the solder ball and pitch between solder balls when the mesh disintegrates to form the solder balls during the reflow process (see Figure 2).

        � � � � � A grid or mesh of solder material is fabricated from a sheet of rolled solder material, such as tin solder. The design of the solder mesh has the appearance of circular solder discs joined together by thin solder ligaments. Flux material is applied to the package substrate lands, and the solder mesh is placed on the substrate with the solder discs aligned to the lands. Reflow heating of the solder causes the solder mesh to melt into solder balls that form a metallurgical joint with the land metallization. The solder mesh first breaks at the solder ligament between the solder discs, and the discs form into solder balls. Methods to fabricate the solder mesh include patterned roll forming, embossing, and imprinting.

        � � � � � By placing solder mesh between the package and the motherboard prior to reflow, package ball attach and package surface mount to the board can be performed in one process. By applying underfill/no-flow underfill on the solder mesh, the underfilling of the BGA joints can be achieved during the reflow process. This process has significant benefits over conventional processes, especially for new applications that require smaller ball diameters, heights,...