Medical Design Briefs - August 2024 - 21

Sensors
See the slide show to learn more about MEMS packaging.
* Other selection considerations include choosing materials
that provide protection for the sensor while also ensuring its
optimal performance. Environmental conditions in which
the sensor will ship, store, and operate must be considered.
Factors such as humidity, temperature extremes, rate of ambient
temperature change, and exposure to chemicals should
influence material choice. For example, hermetic sealing can
be employed to protect sensors from moisture and other environmental
contaminants, ensuring long-term reliability.
Hermeticity is needed for medical devices, especially implantables,
to protect the device from the various fluids in the human
body that could cause corrosion and shorting. It also
protects the patient from toxic or harmful materials being
released from the device.
Handling During Manufacturing
* During manufacturing, MEMS sensors require careful handling
to prevent damage. The devices are often flexible but
must be kept rigid during processes like ultrasonic interconnect
attach via flip chip or wirebond and custom pickup tools,
fixtures, and jigs. Proper handling of the MEMS is required to
reduce and eliminate damage and maintain sensor integrity.
Manufacturing processes should be designed with the sensor's
fragility in mind to avoid mechanical stresses that could
impair its performance.
* MEMS sensors often require air cavity packages or exposed
silicon on a board to function correctly. These configurations
provide the necessary environmental exposure for the sensors
to operate. However, protecting the wirebonding interconnect
to these delicate structures is paramount. Encapsulation
using techniques such as dam and fill or film-assisted
overmolding can protect the wirebonds from contaminants
and mechanical damage while preserving an opening for the
sensor to function properly. This process involves creating a
Medical Design Briefs, August 2024
barrier around the sensor to shield it from epoxy material,
used to protect the wirebond interconnects, while ensuring
that the sensor's functionality is not compromised. The encapsulation
process must be meticulously designed to maintain
sensor functionality while providing robust protection.
Process Development and Validation
* Interconnect technology can play a significant role in MEMS
sensor packaging. Flip chip and wirebond are two common
interconnect methods used to establish electrical connections
between the sensor and the package. Each method has its advantages
and considerations. Flip chip interconnects offer
higher density and better electrical performance in a smaller
package size, while wirebonding is a more established technology
and offers more architectural flexibility, making it popular
for MEMS and sensor devices. The choice of interconnect
method depends on the specific requirements of the sensor
application and the desired performance characteristics.
* Wafer processing poses specific challenges for MEMS sensors,
particularly due to the fragile nature of MEMS sensors and
process steps inherent in wafer singulation. Designing sensors
with manufacturing processes in mind can mitigate these
risks. This includes considering the mechanical stresses and
selecting suitable singulation methods that minimize damage
during wafer cutting.
* The methods and workflows of a MEMS saw singulation process
should also be considered; different singulation methods
should inform design and process flow, such as when to singulate
devices, how to process the MEMS die, and whether saw
or laser singulation is required. Saw singulation involves wafer
tapes and the continuous spraying of deionized (DI) water
due to high friction. Both elements of traditional saw singulation
run a high risk of causing significant damage to
MEMS devices because of their varying complexity and fragilwww.medicaldesignbriefs.com
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Medical Design Briefs - August 2024

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