How best practices in circuit protection and board layout ensure long-term safety and reliability
From smart glasses to fitness-monitoring wristbands, wearable technology has become one of the hottest trends in the consumer electronics market, and these devices now permeate nearly every part of the average consumer’s daily life. According to Business Insider, the global wearables market is projected to grow at a compound annual rate of 35 percent over the next five years, reaching 148 million units shipped annually by 2019.
With the rapid growth of wearables, device manufacturers now have a greater vested interest in protecting both the technology and user. Due to their proximity to the skin, wearables are regularly exposed to static electricity generated by the user. If a device’s battery-changing interfaces, buttons, sensor circuits or data I/Os are left unprotected, electrostatic discharges (ESD) from human touch can damage those circuits and subsystems beyond repair. Designers must work to incorporate advanced circuit protection technologies to protect consumers who are increasingly dependent on wearables, as well as safeguard the products on which they are dependent.
As consumers demand improved wearable performance and smaller form factors, providing adequate circuit protection for these devices becomes more challenging. This article examines board layout strategies and circuit protection technologies that should be implemented early in the design process, as well as highlighting four steps for selecting ESD diodes that improve the safety and reliability of wearable designs.
Robust, Compact Circuit Protection
Steady improvements in back-end assemblies and wafer fabrication processes have made it possible to have robust ESD protection in a small form factor without degradation in ESD performance. For example, Littelfuse’s general purpose 01005 ESD diode has a dynamic resistance value of less than 1Ω and can withstand 30 kV contact discharge (IEC 61000-4-2). Despite its extraordinarily small dimensions (0.4mm x 0.2mm), the diode delivers the same level of ESD robustness and low clamping performance (dynamic resistance) as its larger counterparts (e.g., SOD882/SOD723 and 0201).
As the latest chipsets used in wearable devices get smaller and faster, circuit protection component form factors and performance must do the same.
Demanding operating conditions require robust circuit protection. The Human Body Model (HBM) test level of modern integrated circuits can reach 2,000 V, while most application designers ensure that their equipment meets at least Level 4 of the IEC 61000-4-2 test standard (15 kV air discharge, 8 kV contact). Many wearables and portable device manufacturer now have their contact discharge level raised to 15 or 20 kV, with some setting it as high as 30 kV — adding a new challenge. New designs in the ESD diode structures ensure that the compact ESD devices are robust enough to meet these demanding conditions for long-term operation and reliability.
Modern ESD protection manufacturing techniques can shrink the component’s board area usage. For example, a designer can save around 70 percent in board area by moving from the most common discrete form factor for ESD diodes, the SOD882 package (1.0 mm x 0.6 mm), to the 0201 form factor (0.6 mm x 0.3 mm). Furthermore, a designer can recognize a savings of over 85% when moving from the SOD882 to the 01005. It is essential to make these design decisions early in the process when flexibility in configuration and board layout is available.
The SP1012 Series Diode Array packs five ESD diodes in a flipchip package. Less than 1 square millimeter in size, these ESD diode arrays are helping circuit designers economize on both PCB space and costs.
Low-capacitance ESD diode arrays, like the SP3022 Series, are specifically designed for consumer electronics such as fitness bands, smart watches, smartphones and tablets..
Four-Step Guide to Selecting ESD Diodes
Design engineers can optimize the protection of their wearable designs by utilizing the following selection guidelines for ESD diodes:
1. Specify unidirectional or bidirectional: digital circuits and DC circuits traditionally use unidirectional ESD diodes, including pushbuttons and switches. AC circuits use bidirectional diodes, which may involve any signal with a negative voltage waveform greater than -0.7 V. AC circuits include legacy data ports, RF interfaces, analog video, and audio.
Unidirectional diode configuration performance improves during negative-voltage ESD strikes, so design engineers should opt to use these when possible. During these strikes, the clamping voltage will depend on the forward bias of the diode, which is typically less than 1.0 V. During a negative strike, bidirectional diodes provide a clamping voltage based on the reverse breakdown voltage, which is higher than the forward bias of the unidirectional diode. Therefore, by choosing the unidirectional configuration, engineers can dramatically reduce the stress on the system during negative strikes.