Making the Connections in Electronic Circuitry- The Continuing Evolution of Joining Technologies
Author: Tetsuro Nishimura Company: Nihon Superior Co., Ltd. Date Published: 1/25/2016
Pan Pacific Symposium
Abstract: The first generation of lead-free solders were very different from their tin-lead predecessor in that they were essentially a dispersion of intermetallic compounds in a pure tin matrix and the first round of innovation was the introduction of elements such as nickel that were targeted at those intermetallics. It was found that the level of additions required to achieve quite dramatic changes in behaviour were much smaller than those usually used with conventional alloying elements and “microalloying” became one of the new tools of the solder alloy developer with additions such as aluminium, zinc and manganese offering measureable benefits. More recently observations on the failure mechanism in lead-free solders have prompted the addition of elements such as bismuth and indium that provide solid solution strengthening of the tin matrix. Now as electronics is finding its way increasingly into severe environments such as the engine compartment of motor vehicles approaches similar to those employed in alloys for jet engine turbine blades are being applied in solder development. The microstructure is packed as densely as possible with finely dispersed intermetallic phases while the matrix is strengthened with solute elements and attempts are made to control the number and orientation of the tin crystals of the matrix. Now the optimum operating temperatures of the power semiconductor required for electric and hybrid vehicles and renewable energy generation are moving beyond what tin-based solders can survive. In a process sometimes known transient liquid phase sintering (TLPS) most of the low melting point tin phase reacts during the bonding process to form high melting point intermetallic. The ease with which nanoparticles of high melting point metallic materials such as silver can be sintered at normal process temperatures is providing another option for the creation of joints for elevated temperature service. In this paper the author will review these developments in joining methods and the recent advances that have been made in low pressure lowtemperature sintering of nano-silver materials for power semiconductor die attach.