Ultra-fine SAC305 Solder Paste Interconnection Solution for Advanced Packaging

Ultra-fine SAC305 solder is widely used as lead-free interconnection in advanced packaging because of its advantages such as good joint strength, thermo-mechanical fatigue behavior and creep resistance. Stencil printing remains the technology of choice for assembling because of its ease of process control, flexibility of usage, wider process window, and economic usage. Advanced package involves integration of different components, copper (Cu) pillar, flip-chip devices, and chips in one package to achieve multiple functions in one system. The need to accommodate ever increasing demands for reduced footprints requires constant inventions of flip chips with Cu pillar diameter of
$\mathbf{55}\boldsymbol{\mu} \mathbf{m}$
and smaller. Many mobile communications consist of 6 or more flip chips in a single package. Conventional flip-chip attach uses flux and Cu pillar solder cap for interconnect formation, it is a challenge to print solder paste of fine geometries below
$\mathbf{70}\boldsymbol{\mu} \mathbf{m}$
. Heraeus's solder paste makes use of ultra-fine spherical shaped solder powder (T7 particles size) to create strong metal coalescence for bonding during reflow process of Cu pillars onto substrates. Reduction in flip-chip Cu pillar diameter to
$\mathbf{55}\boldsymbol{\mu} \mathbf{m}$
requires further reduction in stencil opening to
$\mathbf{55}\boldsymbol{\mu} \mathbf{m}$
, thus limiting the usage of T7 paste. Present study reports stencil printing application of new version of T7 solder paste with reformulated flux system. The application evaluations using
$\mathbf{55}\boldsymbol{\mu} \mathbf{m}$
stencil opening (SO) revealed good results by successfully printing
$\mathbf{35}\boldsymbol{\mu} \mathbf{m}$
line spacing (LS) without defects such as bridging, missing & inconsistent dots. It also achieved average paste release volume of ~30% up to 12hours of continues printing.