Is MEMS Technology Fragile? A Guide to Caring for Your MEMS Products | Symmetry Blog
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MEMS technology is a wonderful gift endowed to us by the marvels of modern engineering. These tiny parts provide many of the miraculous features in portable devices. MEMS enable the Internet of Things industry and advancing our progress towards more advanced technologies.
As small as these parts may be, engineers need to be careful when handling MEMS parts. Due to the special characteristics of MEMS, it’s important for engineers to know the whys and hows of caring for their MEMS products.
While care should always be given for all products when handled by anyone in the electronics industry, MEMS, in particular, must be given special care when handling them. Keep reading to learn why.
What is MEMS technology?
MEMS stands for Microelectromechanical Systems, which represents the technology of microscopic devices. MEMS Exchange defines MEMS as “a technology that in its most general form can be defined as miniaturized mechanical and electro-mechanical elements (i.e., devices and structures) that are made using the techniques of microfabrication… the critical physical dimensions of MEMS devices can vary from well below one micron on the lower end of the dimensional spectrum, all the way to several millimeters.”
Many of the sensors in our smartphones and other portable devices contain MEMS parts to track environmental data. From gyroscopes and accelerometers to temperature and pressure sensors, MEMS technology is at the forefront of the Internet of Things.
What makes MEMS parts so fragile?
While MEMS are certainly small, that is not the only reason why they are fragile. Unlike the vast majority of non-MEMS ICs, MEMS devices are not "solid state,” but instead contain tiny physically moving parts that are precisely placed in a certain position and move in an exact orientation.
Due to the precision of its operation, the tiny parts in a MEMS product is easily susceptible to issues if it experiences physical trauma. Since many MEMS products are sensors that are precisely calibrated, a small bump could knock off the accuracy of the sensor. Even worse, MEMS parts can easily break apart and shatter if dropped from a moderate height.
However, once the MEMS device is soldered onto a PCB, it becomes more robust and resistant to damage. This is because the overall PCB adds bulk and protection around the MEMS part, providing shock absorption. This is not to say that the PCB is in any way resilient to damage, and like any electronic part, PCBs are susceptible to damage if given enough physical trauma.
Once a MEMS product breaks, can it be repaired?
Due to the microscopic nature of MEMS technology, repairing a MEMS sensor after it has been broken is not practical. While conventional electronic parts of the past may make it possible for repairing the part at home, MEMS parts do not allow the opportunity for home repair. If you damage a MEMS part while designing your application, you must buy a new part.
What can I do to prevent my MEMS products from breaking?
To prevent unintentional issues with MEMS products, make sure you follow a few of these guidelines provided by InvenSense’s Application Notes on MEMS Handling
Surface Mount Packages
MEMS (Micro Electro-Mechanical Systems) motion sensors are generally sensitive to mechanical stress coming from the PCB. Minimize PCB stress by following these design rules:
When using MEMS components in plastic packages, PCB mounting and assembly can cause package stress. Package stress in turn can affect the output offset over a wide range of temperature. This stress is caused by the mismatch between the Coefficient of Linear Thermal Expansion (CTE) of the package material and the PCB. Traces connected to pads should be as symmetric as possible. Maximizing symmetry and balance for pad connection will help with component self-alignment and will lead to better control of solder paste reduction after reflow.
Any material used in the surface mount assembly process of the MEMS product should be free of restricted RoHS elements or compounds. Pb-free solders should be used for assembly.
Offset shift may exist after the MEMS motion sensor being mounted onto the PCB board. It depends on individual applications to decide what degree of offset shift they may tolerate. For best consistency, it is recommended to do an onboard calibration after the accelerometer is soldered. Additional information is available upon request.
Exposed Die Pad Precautions
InvenSense products have very low active and standby current consumption. The exposed die pad is not required for heat sinking, and should not be soldered to the PCB. Failure to adhere to this rule can induce performance changes due to package thermo-mechanical stress. There is no electrical connection between the pad and the CMOS.
Trace Routing
Routing traces or vias under the device package is not recommended. Routed active signals may harmonically couple with the MEMS sensor devices, compromising the sensor’s performance. To avoid harmonic coupling don’t route active signals in nonshielded signal planes directly below, or above the sensor package in cabling or adjacent PCB. Note: For best performance, design a ground plane under the sensor to reduce PCB signal noise from the board on which the sensor is mounted. If the sensor is stacked under an adjacent PCB board, design a ground plane directly above the sensor device to shield active signals from the adjacent PCB board.
Component Placement
Do not place large insertion components such as keyboards or push buttons, connectors, or shielding boxes at a distance of less than 2 mm from the MEMS motion sensor. This is to prevent mechanical stress or large thermal sink/source to the adverse of sensor performance. Maintain generally accepted industry design practices for component placement near the device to prevent noise coupling and thermo-mechanical stress, avoiding the neighborhood of any vibration sources like vibrators, speakers, buzzers, and etc.
To read the entire Application Notes on MEMS Handling from InvenSense, click here.