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Method for removing the diffused layer of copper from tin-silver bumps

IP.com Disclosure Number: IPCOM000012929D
Publication Date: 2003-Jun-11
Document File: 4 page(s) / 105K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for removing the diffused layer of copper from tin-silver bumps. Benefits include improved performance and improved reliability.

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Method for removing the diffused layer of copper from tin-silver bumps

Disclosed is a method for removing the diffused layer of copper from tin-silver bumps.� Benefits include improved performance and improved reliability.

Background

        � � � � � The conventional DIRECT LASER LAMINATION� (DLL3) process starts with a copper sheet. An organic build-up layer is formed on the surface of the sheet. A solder bump geometry is formed by a combination of CO2 laser drilling of the build-up film, which is followed by etching. Solder material (AgSn) is plated into the copper and eventually fills the void volume. After completion of the successive build-up layers, the copper is etched away. During processing, however, some of the Cu from the copper sheet diffuses into the surface region of the AgSn bumps. The etching of the copper sheet does not completely remove the copper-rich skin from the surface of the solder bumps. An additional etch procedure removes the copper rich skin from the surface of the joint. The conventional etch solution takes a relatively long time to complete and does not remove the copper rich skin completely. This process must be carefully controlled to avoid undercutting and removing some of the bumps. Test samples produced using the conventional etch process indicate that some parts have up to 40% missing bumps (see Figure 1).

        � � � � � The use of the copper sheet results in copper diffusing into the solder bump. The skin region of the bump with enriched copper has a higher melting temperature than the bulk of the solder bump. As a result, the solder joint melts nonuniformly and mandates the use of unacceptably high reflow temperatures (>260ºC).

General description

� � � � � The disclosed method ensures uniform melting of the SnAg solder bumps of a DLL3 substrate. This procedure increases yield for the controlled collapse chip connect (C4) assembly.

        � � � � � Copper diffuses into the AgSn solder bumps, forming a skin layer that has a higher melting temperature than rest of th...