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Will wearable devices be reliable?

17 Oct 2014  | Greg Caswell and Craig Hillman

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Vibration, mechanical shock and tumble test

Vibration is not typically associated with the human body, but outliers can occur (especially with tools and transportation). Examples of this would be jackhammers and reciprocating saws, which have been known to induce failures in rigid medical devices.

Similarly, mechanical shock due to a product being dropped can reach 1,500g for a mobile phone. This level is likely to be lower for wearables due to their lighter weight, but could be repeated (i.e., applications for shoes).

Wearable electronics are much more susceptible to being either dropped or tumbled onto a surface. It has been documented that multiple drops are much more damaging to the electronics than a single drop. The issue with wearables is that the product moves with the user and can encounter multiple types of drops that are unconstrained in nature. How do the products survive?

Tumbling, or as some call it, clattering is a result of an object being dropped on one corner, rotating, striking another corner and so on. These multiple shocks to a wearable system can be devastating.

Clearly, an understanding of the methodologies for providing support for the electronics as well as understanding the failure modes that may be encountered is vital.


Drop in water/toilet

How many times have we heard of someone leaning over their toilet and their cell phone falling out of their pocket into the toilet? We then hear of putting the phone and battery into rice to remove as much moisture as possible. Some cell phone manufacturers have addressed this issue by coating the circuit board with either a conformal coating or a superhydrophobic coating to protect the electronics. Doing so allows an assembly to be submerged as shown in Figure 6 for prolonged periods of time without failure.

 Cell phone being dropped in water

Figure 6: Cell phone being dropped in water

This issue of exposure to water and rain must be addressed for wearable electronics to survive.


Repeatable bending

Cyclic bending has been used as a technique for assessing the mechanical durability of the solder joints of parts mounted on circuit boards, or even the strength of the PCB itself as this issue is considered one of the biggest risks to wearables. For example, certain human movements that induce bending (flexing of the knee) can occur over 1,000 times a day.

Typical testing causes the PCB to deflect either one or two millimetres, and the cycle count can be in the hundreds of thousands. Today with wearable electronics, this issue is even more prevalent because the application must feel like a textile rather than the normal rigid or flexible circuit board.

Some OEMs are creating products where the electronic circuit is placed on a stretchable material. The issue involves the long-term performance of the stretchability and connectivity. Wearable electronics will involve large numbers of stretching cycles. There is also an indication that next generation substrate materials may experience a change in electrical properties after exposure to bending, particularly at elevated temperatures. Will degradation or ageing occur? What is the impact on the conductors and the connectivity of the device?


Copper bend and torque

To offset the cyclic bending issue, some systems are implementing a Silicon Nanowire structure so that the interconnect can be stretched rather than be part of a rigid circuit board construction. Again, what is the durability of this new technology? Can the nanowires handle multiple bend and torque operations?


Sweat

The main constituents after water are chloride, sodium, potassium, calcium, magnesium, lactate and urea. In addition, several other chemicals such as iron, copper, urocanate and other proteins, metals and enzymes are present. However, the main issues are with chloride and sodium with the source of chloride being the primary concern.

It has been documented in blogs that many Apple iPod Nanos have shorted out due to sweat. Users are resorting to placing their units in baggies to protect them while in use. Similar issues have been encountered with cell phones coming in contact with sweat. What is occurring is that openings in the products enclosures are letting the moisture penetrate to the electronics and sweat, as it is also salty, is the worst combination possible for causing shorting or dendritic growth.

It will be necessary for designers/manufacturers of wearable electronics to address this issue, otherwise product life expectancies will not be achieved and warranty issues will abound.

Conformal coating is a possible solution but adds expense to the product. DfR believes that the expense far outweighs the long-term warranty issues that may be encountered if coatings are ignored.

Figure 7 illustrates the increases in contamination levels with respect to various handling environments. Clearly, sweat is an issue that all wearable manufacturers must address.

 Contamination exposure in different environments

Figure 7: Contamination exposure in different environments


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