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Successful micromolding of IR optical sensor lenses using high-performance resin

15 August 2018

Devices like smartphones have created a revolution in the way people connect with each other. The electronic devices of tomorrow are trending towards improved user experience, functionality and seamless connectivity to the world. Sensors are key enablers supporting this ever increasing trend in electronic devices, such as smart phones, tablets, drones, robots and security systems.

Categories of sensors that are widely used are for example optical sensors, and more specifically proximity sensors. A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact, by emitting an electromagnetic field or a beam of electromagnetic radiation (infrared, for instance), and looking for changes in the field or return signal. Recently sensor companies discovered so called “Time of Flight” proximity sensors where the time it takes for the light from the emitter to the receiver is used to calculate a very accurate distance in just a few nano seconds.

Withstanding High Temperatures

Proximity sensors are small and need to be used on a logic micro controller so standard soldering techniques on PCB should be enabled. This automatically means that all sensor components need to survive high solder temperatures, even up to 260°C if lead free solder paste is used. Typically, sensor components like lenses, used to collimate the light from the emitter and the receiver side, are made of thermoset polymers or glass, since not many plastics survive high solder temperatures. To address these high temperature requirements, SABIC, a global leader in the chemicals industry, recently launched its EXTEMTM resin.  The resin, with its glass transition temperature of 267°C / 513°F, withstands lead free reflow soldering process temperatures enabling cost effective and productive assembly of micronized sensor lens components.

EXTEM TPI polyimide resin features low haze, IR transparency and is therefore a candidate material to be used for optical sensor lenses due to its high refractive index, low moisture uptake and the possibility to use precision injection molding of freeform optics.

Enabling cost efficient mass production

A typical significant drawback of the use of thermoset polymers and optical glass, is the high cost of mass production, related to a need for time consuming curing steps, and, in the case of glass, the need for grinding and polishing. A thermoplastic polymer is cost efficient, since it can be injection molded into thin, precision optical lenses, enabling mass production quantities to volumes of millions of components per week.

SOPROD SA, a molding company from Switzerland, put EXTEM resin to the test. Thanks to the properties of the resin, this project – the company’s first experience micromolding a thermoplastic resin – has been a great success. The material’s high flow and low shrinkage make it well suited for mass producing small, precise parts, giving SOPROD a strong competitive advantage.

SABIC has several decades of history in supplying thermoplastic polymers in the opto-electronics industry, mostly with its ULTEM™ resin and LEXANTM Copolymer resin. These are widely used in this industry as fiber-optic components in pluggable trans-receivers and have building blocks enabling IR light transmission without degrading signal quality. EXTEMTM polyimide resin has a number of the same qualities as the ULTEM portfolio, with the added benefit of a higher glass transition temperature. Mechanically, it has good dimensional stability, excellent tensile strength and high modulus as compared to other thermoplastic resins.  Unlike most commercial polyimides, PEIs are thermoplastics which can be injection molded once heated to above its glass transition temperature and maintain transparency through the process because of its amorphous nature. Very special shapes of lenses can be molded, spheric as well as a-spheric, but also free form optics become available for optical designers, while geometry limitations occur for epoxy-based solutions.

3D sensing potential

Diffractive Optical elements (DOE) are just a new trend in the sensor market, where 3D facial recognition and Virtual Reality become available for consumer electronics. A DOE utilizes a surface with a complex microstructure for its optical function. The micro-structured surface relief profile has two or more surface levels. Until now, the surface structures are etched in fused silica. However, with SABIC’s resin customers can now micro mold the surface structures in a thermoplastic material, while still being able to withstand the high heat Reflow Soldering process for on-chip assembly. This potentially leads the way to mass production of Diffractive Optical Elements in consumer devices.

SABIC’s collaboration with SOPROD SA demonstrates the advantages and feasibility of micromolding IR optical lenses from EXTEM resin. With this new method, customers in the electronics industry can quickly produce the components required to develop true next-generation devices.

 



 
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