BY OLAF KURTZ, CHRISTIAN WUNDERLICH, ROBERT RÜTHER, JÜRGEN BARTHELMES, Atotech Deutschland GmbH, DIN-GHEE NEOH, Atotech S.E.A. Pte Ltd

With the development of environmentally friendly ICs at reduced thickness, the manufacturing of thermally reliable products becomes more demanding. To achieve an equivalent reliability standard for state-of-the-art ICs like quad flat no-lead (QFNs), expressed in terms of achievable moisture sensitivity level (MSL), new technologies have to be introduced. One such technology, a patented process characterized by an extended bath life, which provides high metal holding capability and contains an effective smut removal agent for use with the silicon containing C7025 base material, was developed to address this. The resultant surface topography ensures enhanced bonding between the leadframes and the mold compound used in IC assembly. This process provides an improvement in achievable MSL for all package types.

Why Leadframes Need Adhesion Enhancements

Since the 2006 EU legislation imposed lead-free production for most electrical and electronic equipment, the IC/leadframe and connector industries have been searching for lead-free solderable alternatives. A consequence of this change is the need for higher reflow temperatures due to higher soldering melting points, while the miniaturization trend results in package designs that generate more heat per area. On the basis of the IPC/JEDEC J-STD-20, the electronic industry has therefore defined a standard for the MSL classification.¹

To keep cracking from occurring when the device is heated, several substrate treatment processes have been developed to provide a firm bond between the copper alloy and polymeric material surfaces. Generally, etchants are more effective on pure copper substrate, providing better adhesion characteristics compared with copper alloys. When peroxide-based etchants are used for C194 copper alloy (containing 2-3% Fe), dissolved iron will rapidly degrade the hydrogen peroxide, dramatically (and irreversibly) reducing the product life.²

Alloy C7025 (containing <1% Si), also creates further problems due to Si smut residues that may contaminate the silver surface after processing, resulting in wire bond failure and reduced adhesion. It is imperative to remove this smut using a post-treatment operation, although current post-dip technology to clean C7025 surfaces causes loss to surface roughness with the ensuing adhesion loss.

Adhesion Enhancing Process

The principle treatment step in the leadframe adhesion enhancement process* has the capability of dissolving high levels of copper. In the activator step, the exposed copper is treated with a preparation additive that provides the desired uniform brown, organo-metallic coating, resulting in a surface topography that ensures enhanced bonding between the leadframes and the mold compound used in IC assembly.

During the copper etching process, a soluble adhesion layer is formed, attaining a maximum thickness when equilibrium exists between formation and dissolution.^3,4 The combination of increasing roughness and the formation of a Cu(II)-organic compound results in a color change to the leadframe as etching time progresses.

The proprietary organic additive forms a porous layer on the substrate surface. The density of this layer controls the diffusion of etchant to the surface, directly influencing the resulting etched microstructure.^5,6,7 The additive provides the preferred surface topography by controlling inter-granular etching (IGE) of the copper crystals grain boundaries.

Bonding to the mold compound is believed to occur due to a combination of mechanical and chemical adhesion. Mechanical interlocking is achieved due to a surface area increase, while chemical interaction occurs due to polar and covalent bonding between the molding compound and the organo-metallic coating. The physical and chemical characteristics of this surface result in excellent MSL performance of IC packages.

Process Sequence

The sequence begins with the pre-treatment operation consisting of a mild etch and an alkaline cleaner, the purpose of which is to remove any difficult surface residues from preceding processes, e.g. stamping. Prior to entering the main etching stage, leadframes are immersed into the activator to minimize drag-in of contamination and providing a light surface oxidation.

The leadframe then enters the acidic micro-etch stage, which provides controlled etching and dramatically increased surface area morphology due to the presence of organic additives. Etch depths depend on application requirements and may range between 0.8 and 1.6