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Method for minimizing underfill entrapment in solder joints

IP.com Disclosure Number: IPCOM000146385D
Publication Date: 2007-Feb-13
Document File: 3 page(s) / 311K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for minimizing underfill entrapment in solder joints. Benefits include improved functionality, improved performance, improved reliability, and ease of implementation.

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Method for minimizing underfill entrapment in solder joints

 

Disclosed is a method for minimizing underfill entrapment in solder joints. Benefits include improved functionality, improved performance, improved reliability, and ease of implementation.

Background

      During conventional flip-chip underfill processing, underfill entrapment can cause solder-joint fatigue failure and lead to electrical connection and device failures. To solve these problems, dipless die bumps have been used. However, they can cause serious pick-and-place and thermal compression bonding (TCB) yield loss due to the sliding of die bumps over substrate bumps.

      Conventionally, solid silica particles are used in no flow underfill (NUF) material. Because the density of silica particle (~2.2 g/cm3) is much heavier than epoxy resin systems (~1.1 g/cm3), the particle tends to precipitate and cannot freely flow with the epoxy resin (see Figure 1).

      During the thermal compression bonding process, the substrate solder melts and the chip is compressed. The reduction of the gap height between the die and substrate forces the underfill material to flow towards the peripheral sides of die, forming an epoxy fillet. At the same time, the surface tension of the melting solder pushes against the underfill material. The denser silica particle cannot freely flow with the epoxy resin system and its limited fluid drag force. The excessive silica particles gather around the melted substrate solder bump. The surface tension of solder melt is not strong enough to push the silica particle out. As a result, the silica particles surrounded by epoxy resins are trapped inside the solder joint (see Figure 2).

General description

      The disclosed method is the use of surface-closed porous silica filler to minimize the underfill entrapment inside solder joint during the NUF processing of microelectronic packaging.

The method is used in flip-chip packages with a NUF process, capillary underfill (CUF) process, or other encapsulating processes. The cured formulation resides between die and substrate for flip-chip assemblies.

      The key elements of the disclosed method include:

•     Use of surface-closed porous silica filler instead of nonporous silica filler in NUF material

•     Use of no NUF material containing surface-closed porous silica filler to address underfill entrapment in solder joints

•     Effective density of the surface-closed silica filler that is very close to the density of the resin that the particle is mixed into

•     Filler particle that flo...