Simulation of Microstructural Evolution in Lead-free Soldering Systems

Physics of Lead-free Soldering
During soldering, interfacial interactions with the substrate result in the formation and growth of complex intermetallic (IMC) phases at the substrate/ solder interface. In general, the IMC reaction consists of several complex processes. At first, the
substrate partially dissolves into the molten solder alloy once they come into contact. Close to the substrate/solder interface, the liquid becomes supersaturated with the dissolved metal. This supersaturation leads to the eventual precipitation
and subsequent growth of intermetallic compounds. This stage is critical for the evolution of the IMCs, in that initial intermetallic compound growth and interfacial morphology determine in great measure the reliability of soldered assemblies obtained through a given soldering process.

Microstructural Evolution with Computational Tasks
Phase-field models are used for calculating morphology of solid-liquid interface as well as solute redistribution during the solidification of a material system. Parameters used in phase-field models can be obtained by matching classical balance equations across a zero-thickness interface with approximate solutions to the phase-field equations across the interfaces. In classical models in solidification, each phase has their own governing equations and conservation equations (mass, energy) to be solved for. Additionally, supplemental conditions are also required to couple with neighboring phases. If there is a system that has more than 4 phases, for example, 8 governing equations for each phase and 6 supplemental conditions are demanded to solve the solidification behaviors of the system. Realistic simulations involving multiple phases and components involve the coupling of dynamic and conservation equations within and between phase interfaces, making the simulation of complex systems undergoing extensive topological changes in their microstructure a very challenging problem. Contrary to the classical models, phase field models are free from those restrictions by introducing the concept of conserved or non-conserved field variables ϕ which are smooth and continuous throughout the simulation domain. However, implementation of realistic phase-field descriptions of phase transformation phenomena involve the consideration of problems not present in alternative models, such as the dimensions and structures of interfaces separating neighboring domains with different values of a particular conserved or non-conserved field variables.

Soldering Technology in Electronic Materials
1. Metallurgical life cycle of solder joint
  - Solidification without defects
  - Withstanding external stresses, fatigue, thermal shock, vibration, impact and so on
2. Solidification on soldering processes
  - Nucleation, precipitation, interdiffusion, coarsening, reaction with substrate, and so on
3. Selection of soldering alloys
  - Pb-based alloys    v.s.    Pb-free alloys

Lead-free alloy Selection
1. Why “Lead-Free” Solder ?
2. Selection Criteria for lead-free solder
 - Ternary alloy (or less), if possible: Control difficulty in manufacturing
 - An alloy should be eutectic or very close to eutectic (e.g. no large pasty range during cool-down)
 - Melting point should be close to Sn-Pb eutectic as possible
 - Avoid using currently existing patent (industry freedom of action is guaranteed)
 - An alloy should be equal and better than eutectic Sn-Pb in reliability (in electronic assembles)
 - An alloy should create minimal product cost comparing with Sn-Pb eutectics
 - The best knowledge available, do not choose an alloy that will have environment issue
3. Candidate lead-free solders
 - Sn-Ag-Bi system
     Less thermal stress in SMT application, Fillet-lifting problem in through-hole joints
 - Sn-Ag-Cu system
     Very close reliability, solderability, melting behavior to Sn-Pn Eutectic alloy,
     Many patents in the alloy family, with considerable overlap
 - Sn-Ag alloy
    Less Patent, Good performance for wave soldering, High melting temperature
 - Sn-Cu alloy
    Good performance for wave soldering, Good resistance to fillet lifting
    Low cost, High melting temperature 
Phase field simulations of intermetallic compound (IMC) growth during soldering reaction
1. Interface reaction occurs when a molten metal contact with a compatible solid metal
2. The reaction involves interdiffusion betweezential to ensure the metallurical bonding at the solder joint.
3. The formation of an intermetallic layer is essential to ensure the metallurical bonding at the solder joint.
1. Sn/Cu solder, IMC layer and a Cu substrate during isothermal reactions
2. The simulation takes into account for 
   a) Fast grain boundary layer diffusion in IMC layer
     b) Concurrent IMC grain coarsening along with the IMC layer growth
     c) Dissolution of Cu from the substrate and IMC layer
Results (Click a figure you want to see as a movie)
Case 1: Fast GB Diffusion             
Case 2 : Norrnal GB Diffusion          Case 3: low GB diffuion
Case 4: Low interfacial Energy         Case 5 : high interfacial Energy  
Case 6 : Pure Sn liquid solder          Case 7 : Cu-supersaturated liquid solder