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Method for semisolid solder in electronic packaging

IP.com Disclosure Number: IPCOM000101678D
Publication Date: 2005-Mar-16
Document File: 5 page(s) / 118K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for semisolid solder in electronic packaging. Benefits include improved functionality, improved performance, and improved reliability.

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Method for semisolid solder in electronic packaging

Disclosed is a method for semisolid solder in electronic packaging. Benefits include improved functionality, improved performance, and improved reliability.

Background

      Conventional solder balls are made using gas atomization, which is a process that involves extremely high cooling rates. As a result, conventional solder balls have a very fine dendritic structure and little composition variation within the solder ball. When heated to a specific temperature, conventional solder balls fully liquefy. The solder ball collapses and bridging can occur, causing electrical shorting.

      Conventional solder paste contains organic flux binders that hold the solder paste together before reflow. However, binders decompose and vaporize during reflow, which can lead to voids within solder balls after reflow.

      Conventional solder balls melt and solidify with a dendritic structure (see Figure 1). This structure increases solder-ball rigidity and inhibits stress relaxation in the solder joint during cooling (after reflow). This rigidity can cause cracking in the silicon die, which can lead to device failure.

      When a conventional solder paste melts, it fully liquefies at a single temperature. When the paste is liquid, surface tension forces can outweigh viscosity forces in the solder. Tombstoning and billboarding of passive components can occur.

              Solder-ball bridging and collapse are conventionally minimized by optimizing the reflow profile. Voids in the solder after reflow are minimized by optimizing the reflow profile and using a minimal amount of volatile solder paste binders.

      The capability to cast create semisolid metals with an intermediate viscosity was discovered several decades ago. However, only in recent years has this technology made the transition from a laboratory curiosity to industrial mass production. The main industrial use has been the semisolid casting of aluminum alloys for automobile parts. Casting aluminum in the semisolid state has advantages, such as reduced latent heat extraction during solidification, which makes expensive die-casting tools last longer. Addition, the parts have little, if any, entrapped porosity, which enables them to be heat-treated to increase part strength.

      Two techniques are used industrially in semisolid aluminum casting, rheocasting and thixocasting (not to be confused with thixomolding). In rheocasting, an aluminum alloy is melted, cooled, and stirred to make a mushy semisolid material. It is directly fed into a die caster and formed into a structural part. Thixocasting is a similar process. An ingot of semisolid material is created and cooled to room-temperature in a factory. The ingot is shipped to another factory and reheated to the mushy condition prior to casting.

General description

      The disclosed method includes a semisolid solder with a viscosity that is fully adjustable be...