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Why Is Plastic Package News?

A new device available in a plastic package. Big news! Wow, no one has ever done that before, right?

In so many other cases, this might be a reasonable reaction to a press release announcing a new plastic package. Except when the topic is MEMS. You can never take anything for granted with MEMS, it would seem. Least of all packaging.

So why is this news? It was a pressure sensor from STMicroelectronics. And here’s the deal: pressure sensors have to be open to the environment. That’s how they access the pressure, through a hole or port. If you take such a device, with its delicate diaphragm, and you subject it to the plastic molding process –which occurs under high pressure – you’re very likely to damage the pressure sensor itself.

Some folks have apparently addressed this issue by encasing the inner workings with a gel that can absorb the strain – in some cases even covering the sensor, meaning the pressure measurement has to act through the gel.

What ST did was to make a rather substantial change on the sensor die itself: they’ve decoupled the sensing element from the rest of the die, suspending the sense element by silicon springs. The springs handle the molding pressure without interfering with the sensed pressure. (Well, it would seem that the springs would have to be calibrated into the measurement, but they’re not blocking in the way a gel would.)

Rather a lot of work to accommodate something as silly as a simple plastic package. Which, of course, is neither silly nor simple… You can find out more in their announcement.

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