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Simulating Packaging Induced Thermal Stress in MEMS

By Chloe Allison 20 November 2019

In this article, we will discuss the issues the MEMS industry faces with packaging and how OnScale can be used to simulate a wide range of packaging effects.

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.

Simulating Packaging Effects

Cost and manufacturability of packages seriously limits to what can be achieved.
But, therein lies the opportunity to maximize the design margins that can result in a competitive advantage. Simulation is a key enabler for achieving those while minimizing cost and risks.

Figure 1: Web of Possibilities for Packaging Optimization

Packaging optimization is a multivariable problem. Each of the indicated performance metrics is influenced by many factors. For example, reliability of the package is dependent on materials, thermal load, mechanical load, etc.

Besides, each of those metrics are inter-dependent which means, a holistic system-aware approach is critical and requires thousands of design iterations to achieve the maximum efficiency across the board.

Stresses in MEMS Devices

Stresses directly affect the performance of MEMS devices, potentially causing out of spec components. Stress can come from a range of sources:

  • Residual stresses
  • Stresses induced in fabrication
  • Thermal stresses

Simulation allows engineers to assess the impact of stress, and to help design more stress tolerant devices. In OnScale, it is possible to simulate induced stresses and analyze their effect on device performance.

Figure 2: Film Stress FBAR Simulation in OnScale

Figure 3: Thermal Stress PCB Simulation in OnScale

Plastic Deformation

Plastic deformation is a permanent deformation that occurs when a material is exposed to stresses that exceed its yield stress. Even though, plastic deformation is a recognized failure mechanism in MEMs devices, some manufacturers are now beginning to exploit this phenomenon to tune the performance of devices. Plastic deformation can be simulated in OnScale to aid designers in design optimization.

Figure 4: MEMS Cap Under Load Undergoing Plastic Deformation During a Heating Cycle

Conclusion

The complications are severe in MEMS packages, it is a long and expensive process choosing the right packaging materials, developing a cost-effective interconnect technology and determining how efficiently heat is being managed. Simulation is key to achieving effective MEMS packages with minimized risk and cost.



Chloe Allison
Chloe Allison

Chloe Allison is an Application Engineer at OnScale. She received her MA in Electrical and Electronics Engineering from the University of Strathclyde. As part of our engineering team Chloe assists with developing applications, improving our existing software and providing technical support to our customers.