Solder reflow is an important step in both IC and MEMS packaging processes. And, in a growing number of MEMS devices, these problems are more pronounced as the size of the MEMS packages tend to be smaller leading to a very tight thermomechanical coupling in these structures.
This directly impacts the structural integrity of the packages and hence is very important to characterize the behavior of the package accurately. A whole list of functional parameters affects the integrity of these devices; from the material properties of the solders and coppers to the reflow temperature cycle.
Physical prototyping may help in getting an accurate picture of the package but is a very time consuming and expensive process leading to a smaller sample size of design variants that are put to test.
OnScale’s Digital Prototyping approach powered by flexible Multiphysics solvers and on-demand Cloud HPCs enable engineers to run lots of simulations in parallel.
In this example, we will run a thermomechanical analysis of the typical solder reflow cycle of a typical MEMS package and look at the stress results. We will also parametrize the thermal expansion coefficient of the solders to compare four different material properties.
Why is packaging important?
Microelectromechanical Systems (MEMS) requires specialized complex packaging schemes to achieve the required metrological performance of these devices. MEMS Packaging is a monumental task for several reasons:
- Packaging is often application specific, so a wide range of devices require unique packaging processes.
- MEMS devices often consist of components which need to interface with an external environment as well as components that need to be protected from this environment.
- Due to the size and intricacy of these devices, they are generally very fragile and susceptible to thermal residual stresses induced during the fabrication and packaging processes.).
Packaging can affect the reliability and long-term stability of these devices, so it is extremely important to get the packaging right. Packaging dominates the whole production process in terms of cost and time so optimizing this process can improve cost, yield and device performance.
MEMS Package Thermomechanical Example
This MEMS Package model has been setup to run with domain decomposition on the cloud. The model consists of ~20 million DoF.
The total number of CPUs to run on is set by the variable nptot. This is set to 256 for this model.
We will parametrize the thermal expansion coefficient of the solders used in the model for the given reflow temperature cycle.
The temperature source begins at a peak temperature and slowly decrease over time this affects the structural integrity of the solder bumps. This will be analysed using Von Mises stress.
What is Von MIses Stress?: Von Mises stress is a value used to determine if a given material will yield or fracture. It is mostly used for ductile materials, such as metals.
The thermal expansion coefficient describes how the size of an object changes with temperature as you can see the stress in the solder increases as the thermal expansion coefficient increases.
As expected, the maximum Von-Mises stresses are concentrated on the corner joints. Further analysis is required to optimize the structure of the package to keep all the joints within the commonly accepted nominal value of 70 MPa for the SAC solders.
The estimated CH cost of this simulation is 320CH run on 256 CPUs
If you would like to try this example for yourself just download the files and submit a simulation, we have also attached the paraview results file for you to view!
Note: This simulation requires 320 core-hours. Check your core-hour account balance before running the simulation. Please get in contact with via our forum if you need any assistance.