High aspect ratio printing of sintering pastes using layered stencils

Abstract

Three dimensional integrated package constructions are a very active area of development in the electronics packaging industry. In many of these constructions, creating vertical interconnections between packaging elements presents a particular challenge. This problem is exacerbated as package footprints and pitches become smaller and exacting interconnection height is needed to accommodate nested elements. Transient liquid phase sintering (TLPS) pastes have previously been shown to stencil print at very aggressive stencil geometries and also have been formulated to create dispensed column structures. TLPS pastes offer a unique combination of versatility of deposition, maintenance of deposition footprint during reflow, lack of remelt, platability, and solderability after processing that can produce a wide variety of interconnect structures. Features formed from TLPS pastes may be deposited by a number of techniques including stencil printing and dispensing. Stencil printing using multiple stencils with relief features in the secondary stencils offers the opportunity to create multiple, high aspect ratio features in a mass operation for high throughput and versatility. The study presented in this paper explores the capability of TLPS pastes in conjunction with state-of-the-art stencil and stencil printing technology. Multiple electroformed stencils will be used to form ‘layer cake’ constructions with increasingly high aspect ratios at 400 micron pitch. A base pattern at 400 micron pitch will form the foundation for the high aspect ratio constructions. The degree of processing required to form a stable platform for subsequent prints will be investigated and the soundness of the interconnection at the junctions between printed deposits will also be characterized. One or more layers will be deposited onto the base layer using a secondary stencil that has a relief feature for the printed base so that additional height can be built. The constructions that are printed and reflowed will be characterized with laser profilometry to determine manufacturability and preliminary design rules for the various constructions.