Simulation models have played, and will continue to play an important part in the predictive aspects of lead-free reliability. Thermo-mechanical modeling of the visco-elastic properties of lead-free solders including creep fatigue, elastic and plastic strain, CTE, elastic modulus, and stress relaxation, are critical details in the extrapolation of performance for a given set of environmental conditions. Presently, the ability to predict reliability solely on lead-free material attributes and behavior is not sufficient to warrant confidence in a result without verification of the mode of failure and the trigger mechanism responsible for the effect. A concurrent method of mathematical and FEM modeling, along with the important failure analysis mechanism, can be used until enough field reliability data can be accumulated to verify the accuracy of either method. Given the number of variables at numerous levels, this prospect is still a work in progress.

Modeling Approaches:
There are several types of models which differ based on the assessment of the failure apparatus responsible for creating the fatigue crack and fracture along the solder joints. They include strain, stress, fracture mechanics, and energy based models. The most widely used energy based model relies on calculated strain energy density accumulated during cycle times to calculate the fatigue life of solder joints. The various constituent elements of steady state creep and strain rates during cycling are used to calculate the number of cycles to crack initiation. Then the solder crack growth rate is computed and the fatigue life of the solder joint is determined. The Darveaux model has worked well as an indicator for joint failure on various BGA (Ball Grid Array) packages, but has limitations on smaller wafer level chip scale packages, where solder ball geometry is much smaller.

There are a number of FEA packages that offer various
approaches to modeling. Some offer advantages in analyzing the behavioral responses of lead-free solder joints. They incorporate stress and strain effects of adjacent materials or layers, and integrate them by a series of constraint equations. Others focus on a Physics of Failure approach, which incorporates a combination of stress/strain and fracture mechanics.

When deciding on the nature of the model that is appropriate for lead-free solder joint life time reliability modeling, there are some consistent aspects that must be taken into consideration. It is very important to know which prediction models are applicable to the specific electronic packages whether using an energy density model, or a strain range method.

Some general considerations for assessing lead free
reliability modeling include:

• Choosing the proper design and substrate can play an equally important part as the selection of solder alloys.
  
  
• Consider the characteristic visco-elastic properties, such as primary and secondary creep, elastic modulus, and elongation, in the selection of a lead-free alloy. Attempt to predict the application of strain on the various locations of the package, and mitigate excessive strain by adaptation of restraints or stress decoupling interfaces.
  
  
• Components should be chosen on the basis of their suitability to the design constraints imposed by the selection of the lead-free solder. Low strain versus high strain applications may warrant differing device geometries.
  
  
• Minimize the CTE (Coefficient of Thermal Expansion) mismatch between adjacent materials to moderate the effect of high elastic modulus solder alloys.
  
  
• Limit the selection of alloys to binary or tertiary compositions, since many intermetallic formations have not been identified as being innocuous to joint integrity.
  
  
• SAC is suitable for designs that exert low strain rates, where SnPb fares better at designs where higher strain rates are expected.
  
  
• Substrates should match the solder CTE, if possible. Since many substrates designed for low CTE may have inferior copper adhesion, choose the copper tooth profile suitable to the substrate selection.
  
  
• Identifying where failures occur after environmental stressing is an important aspect of assessing the value of experimental results.
  
  
• Design experiments incorporating a robust range of factors and levels, while minimizing runs.

At the EMPF, classes are offered to assist engineers in making decisions that will help mitigate the
unknown reliability factors associated with the implementation of a lead free manufacturing process. The classes offered cover the various aspects of reliability and design, as well as material selection and manufacturing.

For more information on this class or any others, please call the Training Center Registrar at (610) 362-1295.