The co-author of this article is Lutz Rauscher from Bosch Sensortec.
Up until an hour ago this blog was entitled “Trends in MEMS orientation sensor form factor reduction.” That was until we ran the draft past a colleague for proof reading, and in her feedback she summarised: “So, what you are really questioning is does size matter, and if so why does it matter?”
Having realized what she had said, and the three of us blushing past the inevitable smirks and associations that the question raises, we settled on this title simply because it questions the basic assumption within MEMS industry community that smaller actually equates to better. Now, for those to which the question conjures up negative connotations, we offer a small apology, but we do hope we will continue to have your attention.
Industry observers will not argue that the explosion in the adoption of motion sensors, principally accelerometers, was enabled by reducing the form factor to the point where the sensors could easily be integrated into mobile applications. Bosch now ships more than three million sensors a day, a large proportion of which are discrete 2 mm x 2 mm accelerometers, but the roots of the MEMS technology evolution originate in automotive industry where accelerometers were developed to trigger airbag deployment in motor vehicles, a working environment not overly subject to size constraints and in fact there were solid technical reasons not to shrink. A colleague here at Bosch Sensortec who previously worked in the automotive division commented, “None of us believed size reduction of an accelerometer to something smaller than 10 mm x 10 mm was even possible…”
It turned out to be a real challenge, but once the initial hurdles were overcome it became clear that size reduction in this context equated to cost reduction, which eventually drove the sensor footprint down to the point where entry into the mobile space became feasible.
The tipping point for mobile adoption appears to be around 3 mm x 3 mm, a point where cost, size and performance became acceptable for the consumer electronics sector and inertial sensors ubiquitously present in mobiles. That drive for smaller sensors has continued through to today, with Bosch now offering an industry leading accelerometer portfolio with a 2 mm x 2 mm and below footprint.
Customer feedback highlights the advantage: for a typical 80 mm2 smartphone PCB (available area), moving from a 9 mm2 to a 4 mm2 footprint increases placement options by 225%.
A great example are also pressure sensors – being available only in rather large and (maybe even more importantly) thick packages, they have been ignored by the industry for many years. Now being offered in a 2 x 2.5 mm2 without sacrificing performance their use has seen 3-digit YoY growth rates.
For mobiles the 2 mm x 2 mm seem to be a sweet spot for inertial sensors, establishing a good compromise between placing flexibility, ease of handling, solder pitch, performance and of course cost.
So, most of the recent effort went into combining inertial, magnetic sensors and much more intelligence into housing of the same or a just slightly increased footprint. These technological marvels have been pushing the limits of MEMS and assembly technology alike.
But what of the future? Is there real demand for smaller, and can the significant R&D investments be justified? Maybe not from a purely cost driven perspective and there are also drawbacks in further miniaturization. For example, reducing footprint also limits pin-out options: a given area and pad pitch (industry standard is 0.4 mm) dictate the total number of pin options. Also handling becomes more difficult the mechanical sensitivity of ever smaller devices may cause additional effort for the application engineering.
However, the rise of the smartphone and tablet markets has delivered small, low cost sophisticated components which in turn are enabling wearable technology applications. These products are still in their infancy, and looks to redefine the concept of personal accessory in terms of functionality and applications. Unlike the current smartphone footprint, this market is highly diverse in terms of construction and footprint. One common denominator however is that the working area for the electronics components is just a fraction of what is available in a mobile.
Last year Bosch Sensortec Bosch unveiled Consumer MEMS including the 12-bit BMA355 accelerometer in 1.2 x 1.5 mm2 Wafer Level Chip Scale Package (WLCSP) being the smallest sensor of its kind on the market. Indeed this comes as close to a bare die sensor as MEMS will ever get, enabling the next level of system integration.
Interest in the device from the smartphone and tablet sector has been limited, but this is no surprise as the current focus for discrete devices is either low cost or highest performance, currently supported by our 2 mm x 2 mm portfolio.
For wearable applications, interest is extremely strong and this is a clear indicator that small size has a clear value in the new and emerging applications. Working within a 100 mm3 volume, the BMA355 increases placement options by 215% over 2 mm x 2 mm devices – similar to what the 2×2 mm sensors did for mobiles.
In two recent customer engagements, integrating accelerometer functionality into their products (fitness and hearing aid applications) would not have been possible without the BMA355.
So what do we conclude? From a supplier's perspective, the customer vote on the question of “does size matter” is a clear yes. While some areas of the mobile consumer market are pushing for pin compatibility and standard form factors, the new applications are enabled and are driving further innovation and reaping the benefits. Quite exciting really.