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Will wearable devices be reliable?

17 Oct 2014  | Greg Caswell and Craig Hillman

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Reliability goals

The desired lifetime of a product is defined as "when the customer will be satisfied." This parameter should be actively used in the development of the product and during its qualification. Product performance can be defined in several ways. It may be the number of returns during the warranty period, it may be the survivability over a specific time period at a set confidence level, and it may be through MTBF or MTTF calculations (which DfR feels should be an administrative activity rather than a true reliability assessment.)

The desired lifetime of wearables? Let's look at rough equivalents: clothes, shoes – three months to five years (600 miles); watches – three to 20 years; glasses – two to five years; cell phones – 12 to 36 months. With a new technology, such as wearables, you the designer have a chance to influence customer expectations.

Warranty returns for consumer electronics range from 5 per cent to 25 per cent while low-volume non-hi-reliability applications are 1 per cent to 2 per cent; industrial controls range from 500ppm to 2000ppm and automotive applications 1 per cent to 5 per cent in the first year of use. Medical electronics, however, where reliability is paramount, typically have a failure range between 0.1 per cent to 0.5 per cent, clearly indicating the focus on reliability for that market segment.

Figure 4 from Square Trade shows the 12-month malfunction rate for several consumer products. Are you willing to absorb this level of product return?

 Malfunction rates for consumer products

Figure 4: Malfunction rates for consumer products


Failure inducing loads

There are many factors that can induce failures in electronics that need to be explored and assessed during the design stage of product development.

  • Temperature Cycling – Tmax, Tmin, dwell, ramp times
  • Sustained Temperature – T and exposure time
  • Humidity – Controlled, condensation
  • Corrosion – Salt, corrosive gases (Cl2, etc.), UV
  • Power cycling – Duty cycles, power dissipation
  • Electrical Loads – Voltage, current, current density; Static and transient; Electrical Noise
  • Mechanical Bending (Static and Cyclic) – Board-level strain
  • Random Vibration – PSD, exposure time, kurtosis
  • Harmonic Vibration – G and frequency
  • Mechanical shock – G, wave form, # of events

These stresses can be in combination creating higher failure rates for wearables. As such, the designer must completely understand the various environments that can impact their product.


Issues

Wearable electronics being a "Next Generation Technology" means that we need to be aware of the pitfalls and what actions need to be taken to assure that the new technologies are reliable. As typically occurs with new markets of electronics, there are several issues that need to be addressed from a reliability perspective to assure these new applications are safe and reliable.


Packaging approach

Due to the small form factor requirements of wearable electronics packaging approaches like QFNs, uCSP and CSP and MEMs sensors are popular solutions for these products. DfR has published numerous papers and articles on the issues associated with these packaging technologies and the methodologies that must be implemented by the OEM to assure long-term reliability.

Similarly, System in a Package (SiP) will see wider usage as the functions of power management, microcontrollers and the various sensors will be comingled into a single package. Smaller passives will also typically be utilised. Again, there are several known failure modes associated with both these packaging and passive configurations. However, with the advent of wearable electronics there are more issues to consider.


Temperature exposure

Electronics that can come in contact with the skin must maintain an operating temperature that will be at or below the core body temperature of 98.6°F . Above that temperature will result in the wearer being uncomfortable and if the temperature goes higher may cause pain and a high level of discomfort. Figure 5 illustrates this issue. However, very cold temperatures (below -20°C) could be a challenge especially in combination with a mechanical load. As such, wearable electronics, in some cases, must be manufactured to medical electronics standards including Pb-free requirements and also the ability to minimise tin whiskers.

 Core body temperature variations

Figure 5: Core body temperature variations


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