Design trends of PCBs in handheld electronics --- small and smaller
IoT has brought extra convenience to our daily life. For electronic products to be easier to carry and use, product design engineers have been taking on challenges to produce a greater range of functions for the devices on smaller PCB space and higher electronic circuit integration density. Products we see every day, such as cell phones, smart wearables, wireless ear pieces, electronic cigarettes and even AR goggles that are making a fuzz about, are designed with small 0201 package as the mainstream for electronic PCB design. Even more, System-in-Package (SiP) with much greater integration density are not unheard of.
ESD/Surge protection needs of Battery Power Rail
Every time we turn the power of an electronic product on, there is often a power spike on battery power rail. Or, if a user is replacing batteries, the battery power rail of the electronic product is exposed to the risk of electrostatic interference or damage. That’s why product R&D engineers are constantly working their best to come up with appropriate protection against such surges (ESD, surge spikes) for greater product reliability when designing their products. For this, electrostatic protection test according to IEC61000-4-2 and surge test according to IEC61000-4-5 are performed during product engineering stage, as harsh surge conditions are introduced for simulation, as to ensure that the product is well equipped against what can be thrown at it in the environment where the user is using the product.
Figure 1: Example of surge spile when the power is switched on
Electronic business is dealing with challenges coming from the various climates and environments around the world
BBrand-name manufacturers all introduce rigorous test conditions to simulate the types of surges that can be present in the various user environments in order for the electronic products sold around the world to adapt to the local climates. As an example, it is often considered for a cell phone product if it is sold in an environment of dry climate or where the power supply is somewhat unstable. Therefore, the rigorous surge test of IEC61000-4-5 is performed on the battery power rail and USB charger VBUS during product verification, as to prevent the products from excessively returning for repair in a region where the environment is harsh. To pass the high-level IEC61000-4-5 test, product design engineers usually choose to place TVS on battery power rail and USB charger VBUS, so that ESDs and surge spikes are stopped by TVS at the moment they come in. However, considering the limited PCB size in a handheld electronic product, the small-package TVS elements available in the market are often unable to withstand high surge current due to the limitation of die size. For this, product design engineers have no choice but select a larger-package TVS element in order to pass the excessive electric stress test, which means they have to choose between product PCB size and high ESD/surge robustness. These poor engineers scratch their heads during product design for how to achieve high ESD/surge robustness with as little PCB area as possible.
ESD/Surge Protection Device for Battery Power Rail --- Design Scenario
The ideal TVS for battery power rail has to have high surge withstanding current, and the leakage current is an important indicator when it comes to lower power consumption for longer battery life in electronic products. In addition to the robustness and leakage current of TVS itself, the clamping voltage is another important parameter for TVS. The first priority in selecting a TVS is to make sure the clamping voltage is low enough for TVS to do its best for protection. This eliminates the possibility that the main chip under protection is still damaged by electrostatic shock while TVS loses its function as the ESD protection element even though it is not damaged.
A lot of engineers go for TVSs with greater VRWM for additional voltage redundancy when choosing a TVS for battery power rail, as to prevent the risk of TVS mis-conduction due to unstable battery voltage. On the other hand, greater VRWM means greater performance in terms of VBD (breakdown voltage) and Vclamp (clamping voltage). Engineers are often caught between a rock, which is having ample voltage redundancy, and a hard place, which is good TVS clamping voltage, when it comes to spec design.
Last but not the least is the directivity of TVS. Ideally, a unidirectional TVS is preferred as it brings faster negative conduction rate and better negative clamping voltage, since it is DC voltage on the battery power rail. However, early electronic products were bothered by occasionally the reverse mounting of small-package 0201 elements in large quantity for mass production, which forced engineers to compromise with bi-directional TVSs in the design stage to prevent such a risk. Also because of this development, bi-directional elements are still the dominant small-package 0201 TVS elements in the market. Still, as small-package elements are taking the center stage in recent years and the need for 0201 small-package Schottky diodes is desperate, the SMT mass-production techniques are maturing and the trouble of small-package elements in terms of SMT directivity is gradually overcome thanks to the directional markings on PCBs and the markings on IC tapes and IC units. When it comes to selecting a TVS for battery power rail, many engineers insist in unidirectional elements for the best protection even if they are 0201 small-package elements.
Figure 2: unit-directional TVS trend --- Optimized Negative Clamping Voltage
The table below provides some design key points for ESD/surge protection devices for battery power rail:
| Characteristic |
Description |
| VRWM |
A little higher than voltage of power rail for safety margin |
| Directivity | Unidirectional for optimized negative clamping voltage |
| Leakage Current | Low leakage current for low power consumption |
| ESD Clamping Voltage | Low clamping voltage for efficient protection |
| Surge Ipp (8/20µs) | High Surge robustness for low field return rate |
| ESD IEC61000-4-2 | High ESD robustness for most of strict test criteria |
| Package Size | Small package for compact device |
Amazing Optimized ESD Protection Solution for Battery Power Rail --- AZ5A16-01M
By summarizing the above, Amazing Microelectronic has come up with a perfect TVS solution for battery power rail design --- the AZ5A16-01M, with the following features:
‧Package: MCSP0603P2Y (0201)
‧Directivity: Unidirectional
‧Reverse Stand-off Voltage: 6V
‧ESD Clamping Voltage at 8kV: 9V
‧System Level ESD Robustness (IEC61000-4-2): 30kV
‧Surge Ipp (8/20µs): 20A
‧Reverse Leakage Current: < 100nA
AZ5A16-01M not only provides good voltage redundancy at 4.5V and 5V for battery power rail, but also features low reverse negative leakage current for longer battery life in products. While having both of the above, no trade-off is made in terms of clamping voltage despite of the higher VRWM. Compared with 0201 unidirectional 5V – 6V TVS products, AZ5A16-01M features the lowest clamping voltage for the greatest protection available. Its unidirectional feature ensures optimized negative protection.
Figure 3: AZ5A16-01M TLP Diagram --- ESD Clamping Voltage
Figure 4: AZ5A16-01M Surge Clamping Voltage
AZ5A16-01M adopts the small-package MCSP0603P2Y design for more flexible PCB space arrangement for small handheld devices, and is equipped with the highest surge robustness in the industry at 20A (8/20µs) in order to ensure the product’s battery power rail passes the IEC61000-4-5 surge test. This helps engineers meet the needs of both small package and extreme surge robustness, thus eliminates the trade-off between small PCB space and high surge robustness.
Figure 5: AZ5A16-01M Package & Pin Configuration
Finally, AZ5A16-01M adopts the special MCSP package design. A wafer-level package similar to the traditional CSP packaging, MCSP features 6-sided protective coating for stronger packaging architecture and push strength on SMT, which helps AZ5A16-01M pass the rigorous SMT push strength test requested by many manufacturers with flying colors while maintaining the advantage of small package.
Figure 6: MCSP package architecture
Thanks to the tailored design specifically for the needs of battery power rail, AZ5A16-01M provides greater system-level ESD robustness for the increasing number of IoT wearable devices in the market, while preventing the large number of returns for repair that were seen in the past due to EOS damage, not to mention satisfying the need for good battery life and PCB design for compact design. This solves the possible dilemma for engineers in the design stage while speeding up the schedule of product introduction to the market.