The membrane marvel

Smartphones are being used for evermore varied applications, so protecting the delicate components inside from the elements has become vital. Engineering Materials takes a look at how a membrane technology designed for clothing is now being used to ruggedise electronic devices.

Almost a billion smartphones were sold in 2014. As well as the ongoing trend of increasing sales, they are also being used in more extreme environments, from logging a run to filming skydives to taking underwater snaps, todays smartphones are expected to keep working regardless.

And it is not just extreme environments, most are exposed to rain at some point, many are dropped, have drinks spilled on them, or are taken to the beach. And a bizarre frequency are accidentally dropped down the toilet - reported to be as many as one in five!

One risk is that liquids and particulates can enter through a device's acoustic openings such as the microphone, receiver and speaker. In addition, if the acoustic transducers are compromised in some way, sound quality can be affected, ultimately leading to device failure.

Features such as high-resolution cameras, multiple acoustic transducers and electro-mechanical components such as enclosure gaskets and flex connectors have also made providing long-term smartphone reliability that much more complicated.

Indeed, typical causes of failure and poor performance include water and particulate ingress through ports and connectors, electrostatic discharge, liquid and particle ingress, and shock due to being dropped.

To this end, US based W. L. Gore & Associates – famous for its GORE-TEX textile material that is both breathable and waterproof – has teamed up with many electronic device OEMs to collaborate on innovative waterproofing solutions.

The company has developed vents that use expanded polytetrafluoroethylene (ePTFE) membranes. These use a node-and-fibril construction that permit gas molecules (air) to pass through while completely repelling water and other solid particles. Although non-woven materials can capture fine particles, the microscopic pores of ePTFE membranes block virtually all particles, regardless of size or shape.

The ePTFE membranes are extremely thin at just 0.24mm, or less. But, perhaps most importantly, these membranes quickly and easily respond to sound waves, converting them into mechanical vibrations. Then, on the other side of the membrane, these vibrations are converted back into high-quality sound – and all this with 100% waterproof protection against anything from light rain to complete immersion.

Particular particulates
A second scenario that commonly arises is when particulate matter ingresses in to smartphone openings. It doesn't have to be as dramatic as dropping the phone at the beach. Everyday dust and dirt can also cause problems.

OEMs may combat this by placing a protective mesh over the acoustic openings, however, these are only able to capture larger particles that are bigger than the defined pore size of the mesh.

In-house testing at Gore claims that particle shape and surface area are more critical than pore size in determining the amount of protection a material can provide. For example, particles such as a human hair with a surface area equal to, or greater than, the specified pore size can still permeate the woven material due to its shape. What's more, any particles that are caught simply sit on the surface of the mesh and block airflow and reduce venting capability.

The benefits of non-woven material

Woven materials capture particles equal to or greater than a specified pore size, whereas non-woven materials capture a greater range of particle sizes and shapes.

Specialised methods and in-house environmental testing facilities at Gore show that non-woven materials catch particles across a range of shapes and sizes thanks to its 3D structure.

When made out of this material, vents can protect sensitive electronics from contaminants while still permitting any enclosure to 'breathe'. In this way, damage or device failure due to pressure differentials or ambient conditions are avoided.

This is a common problem for sealed devices and can be split in to two main groups: rapid temperature changes caused by, for example, taking a device from a warm car into cold weather outside, and also air pressure changes such as taking off and landing in an aircraft.

When pressure builds in an acoustic cavity or chamber, it creates a bias on the transducer's compliant surfaces such as the speaker and receiver diaphragms. This can reduce acoustic output and eventually damage the transducer.

This is particularly the case if the device is equipped with waterproof transducers, as pressure vents are necessary to maintain high-quality acoustic performance. These prevent transducer bias by equalising pressure within the housing without compromising sound quality.

Early design collaboration
To be truly effective, acoustic and pressure vents have to be designed together at the early stage of a device's development. This helps to maximise acoustic performance while offering the best protection against water and particle ingress.
They should be designed specifically for portable electronics, using acoustically transparent materials that keep out external elements while relieving pressure inside the housing – and maintaining acoustic performance.
The vent dimensions are typically customised following the customer needs and requirements. However, vents are available in pre-cut shapes and sizes according to industry-standard dimensions for speakers, receivers, microphones and enclosures.

About the authors: Victor Lusvardi is global sales leader and Oliver Teller is a product specialist at W. L. Gore & Associates' Portable Electronic Vents Business Unit.

Justin Cunningham

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