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Addressing PCB size limits in wearables

Posted: 14 Sep 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Wearables? integrated circuits? LED? PCB? WLCSP?

Driven by the market, consumer electronics products become progressively more powerful, compact, and power efficient. Wearables in particular demand that a portable, battery-powered, integrated device provide everything from high-precision analogue measurement to intuitive human interfaces. Wearable device developers must carefully partition the product's feature requirements among a cluster of integrated circuits (ICs), juggling sometimes conflicting priorities.

Let's consider the requirements for a wearable design that pushes the boundaries of what is possible in terms of size, battery life, and functionality. Our example device falls into the "does one thing well" categorya screenless, coin cell-powered step counter that alerts users when they need to move around while also keeping track of the number of steps throughout the day. A simple capacitive touch interface enables user input, and a tri-coloured LED provides just enough output to provide the user with useful and necessary information. This product's design shows how powerful ICs squeezed into small packages help to facilitate innovation and product differentiation.

Product requirements
Our product is a step counter designed to be as simple and small as possible, offering no screens, buzzers, or iPhone apps, with a similarly simple and small user interface. Basic design requirements include:

Smallest achievable size. The product with case should be as close in size as possible to a CR2032 battery so that a user can carry the device in a pocket or attach it to a keychain.

User input. On one side of the coin cell-shaped case, provide a capacitive touch interface that recognises the following inputs:
???Swipe: Disable the alarm indicating that the user needs to stand
???Tap and hold: Start a new day (reset the step counter)
???Tap: Check the step count for the day

Simple output. An LED on the case provides the following output:
???Red: A periodic, short flash signals that the user has been stationary for too long
???Green double-flash: Occurs when a user starts a new day by tapping and holding the device
???1 second red/orange/green output: Indicates 33 per cent, 66 per cent and 100 per cent of steps counted for the day, persisting for a few seconds after a tap on the touch surface

The size of a CR2032 battery is 20 mm x 3 mm. How small can we make the device? Let's assume that the product's plastic case can be made thin enough so that it adds no more than about 5 mm in diameter while still supporting easy battery replacement. That leaves the depth.

In the product's stack up, its depth is composed of four components: the battery, the printed circuit board (PCB), any components on the PCB, and the product's plastic case. PCB thickness could be as small as 0.5 mm for a four-layer PCB. Minimising the depth of the components to be soldered to this PCB requires careful part selection. This is where finding high-performance chip-scale package devices becomes crucial to our design.

Wafer-level chip-scale package benefits
The wafer-level chip-scale package (WLCSP) is the culmination of years of incremental advances in manufacturing and chip assembly technology. In WLCSP packaging, the silicon is directly connected to solder balls on one side of the package, as opposed to older technologies that route silicon port pads to package pins through bond wires. This newer design enables the design of packages with a width and height that is nearly as small as the interior silicon itself.

Figure 1: Incorporating an 8Cbit EFM8SB1 into a wafer-level chip-scale package (WLCSP) leaves room for other functions necessary in space constrained modules used in wearable applications.

For our step counter an eight-bit MCU is the best choice because while functionally extremely dense, it already fits into small packages such as a 3 mm x 3 mm QFN package or a WLCSP package measuring only 1.78 mm x 1.66 mm in size.

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