Lead Free Paste with under 3um Solder Particles for Fine Pitch C4 bumping and pre-coating solder applications

Abstract

The work to be detailed in this paper is our development of 96.5mass%Sn-3.0mass%Ag-0.5mass%Cu fine solder particles with an average particle size of under 3um (D50), using a chemical reduction method. An evaluation was conducted on the properties of the particles. The average size of particles appeared to be under 3um with a higher yield compared to particles using the conventional gas atomization method. The melting temperature of fine solder particles using this method was its eutectic temperature, which is same as using the gas–atomized particles. 120um pitch solder bumps from the solder paste using the above mentioned fine solder particles were created on the substrate. As a result of property evaluation, it was turned out that the solder paste created a superior printing shape and coplanarity compared to the conventional paste with gas-atomized particles. In order to investigate the superior printing property generated by the paste with fine solder particles, the rheology of the paste was evaluated.It was verified that the anisotropic shape of particles has contributed to prevent the printed paste from slumping, which has resulted in the improvement of printed shape. It also shows that the filling characteristic of the paste was improved by the smaller particles and the better coplanarity was observed. The importance of finer solder particles for finer pitch assembly will be presented.