Fine pitch stencil printing of Sn/Pb and lead free solders for flip chip technology

Abstract

This paper presents a flip chip technology based on an electroless Ni/Au bumping process which has been developed by IZM/TUB. Nickel bumps offer a surface with very good suitability for flip chip soldering. In the following an interconnection method is described which uses ultra fine pitch stencil printing of solder paste on wafers, ceramic and organic substrates. The eutectic Pb/Sn solder alloy is by far the most commonly used solder in industry. Facing the ecological challenge and federal legislation the paste suppliers are developing lead free solder pastes. Due to the fact that the variety of solder pastes is still growing it is necessary to find an ideal alloy for a specific application. Therefore, in comparison to eutectic Sn/Pb solder different alloys, e.g. Bi/Sn, Sn/Bi/Cu, Sn/Ag, Sn/Cu, Au/Sn are investigated in this paper. In the first part of this paper a low cost flip chip technology based on chemical Ni/Au bumping and solder printing is presented. For this the basic process steps and key aspects are described in detail. The experimental results of an ultra fine pitch technique on wafers and on substrates are shown as well. The second part of this paper presents a comparison of the properties of different solder pastes concerning the usability for flip chip technology. For this, flip chip soldering using dies with Ni/Au bumps was performed on ceramic and FR-4 substrates. The quality of the flip chip joints were investigated by metallurgical cross sections and electrical and mechanical measurements. Finally, the reliability results of these joints after thermal cycling are presented. A comparison of underfilled and nonunderfilled flip chip devices will complete the investigations.