The key to LiDAR safety

Why high-performance sensor packaging is crucial in high-level advanced driver assistance systems (ADAS).

Why high-performance sensor packaging is crucial in high-level advanced driver assistance systems (ADAS).

Safety is the name of the game when it comes to the latest automotive innovations of assisted and fully autonomous driving. LiDAR (light and detection ranging) technology is one of the most important pieces to enable the realization of autonomous driving. It works with the help of a laser light beam, which constantly scans vehicle surroundings. The sensors in the system collect data and create a real-time, high-resolution 3D points-cloud map of the environment. In redundant systems, LiDAR is linked by artificial intelligence to work together with cameras and radar to enable what is known as ADAS 5: the highest level of advanced driver assistance that allows full automation of driving functions under all conditions.

Solid state LiDAR sensors for ADAS and autonomous driving.
LiDAR sensor in the system collect data and create a real-time, high-resolution 3D points-cloud map of the environment.

The safety of autonomous driving is dependent on LiDAR transmitting accurate optical signals

While significant strides have been made in developing consistently safe and reliable autonomous driving systems, there are still some issues to be conquered. One issue to address for the safest possible autonomous driving experience is the quality of optical signal transmission in LiDAR sensor systems. The challenge lies in the fact that LiDAR sensors must be extremely precise and are mostly installed on the outside of vehicles. They are exposed to harsh weather conditions such as extreme temperatures and high humidity, as well as shock and vibration. These hazards could damage or even destroy the sensitive inner components of the system, including laser diodes, MEMS mirrors, and signal-receiving photo-diodes.

Moisture intrusion, in particular, is what many developers fear most, as even small moisture droplets can cause the metals in laser diode chips to oxidize, leading to power degradation and even prematurely reaching EOL (end of life) state. Moreover, in a fully self-driving car, a lack of sensor precision could result in 3D map information of the surrounding environment being incorrect or delayed. This could lead to incorrect operation and even traffic accidents, both life-threatening safety risks.

The harsh conditions of driving environments combined with extreme optical performance requirements necessitate that sensitive LiDAR components need robust, vacuum-tight packaging. Furthermore, to establish LiDAR technology in mass-market automotive applications, absolute safety, reliability, and durability are crucial. Packages for LiDAR sensors must enable stable and high-performance optical signal transmission – not just for a few hours or days, but consistently over a time period of many years.

Glass-to-metal sealed packages for light sources.
Glass-to-metal sealed packages for MEMS mirrors.

High quality packaging components are essential for accurate optical signals

When it comes to hermetic sensor packaging, there are different types of technologies available. One of the most commonly used technologies for harsh-environment automotive and opto-electronic sensor packaging is glass-to-metal sealing, abbreviated as GTMS. Hermetic packaging using GTMS technology fully encapsulates the sensor components in a metal housing, which features glass-sealed electrical contact pins for power supply. The advantage of insulating the metal contacts with specialty glass lies in the fact that the material is impermeable and virtually non-aging. Gas and moisture cannot penetrate through the glass seal, which maintains high hermeticity levels on a near-permanent basis. In comparison, non-hermetic sealing materials, such as polymers, age naturally over time and cannot maintain a truly hermetic environment.

Using glass-to-metal sealing technology, sensor packages are typically equipped with hermetically sealed optical glass windows or lenses as an interface to allow optical signal transmission. The use of high quality glass windows or lenses is important to enable smooth and precise signal transmission.

Glass-to-metal seals deliver exceptional robustness

An important quality criteria of glass-to-metal seals is the expert selection of specialty glass types and metals with appropriate coefficients of thermal expansion (CTE), since the two materials typically expand or shrink differently in the sealing process. A careful material match and combination also enables the direct bonding of the two materials without the use of additional interface materials. Once melted together, the result is a highly durable combination of glass and metal that is so robust it can withstand even extreme pressure and temperature variations found in automotive environments.

The typical “pin-design” of a GTMS package – also referred to as “through-hole design” – allows the sensor package to be tightly connected with a PCB (printed circuit board). The electrical contact pins can be inserted into and tightly fixed to the PCB using solder, which makes the assembly highly shock and vibration resistant. An alternative to GTMS packages are ceramic surface-mount device (SMD) packages. These packages are mounted flat on the PCB, making them potentially more susceptible to mechanical impact and lateral loads in particular.

Typical design of a glass-to-metal sealed (GTMS) package.
GTMS package fixed on PCB by through-hole pin and solder makes the assemblies robust.

In this newly emerging field, a number of different LiDAR sensor technologies are currently “in the race.” Examples include mechanical spinning, MEMS scanning, flash, and mirror system technologies. The requirements for LiDAR sensor packaging are also very complex and differ across these methods. However, a consistent requirement is found in supplier expectations: LiDAR sensor companies are looking for a trusted and experienced partner capable of supporting at each step of the way, from research and prototyping all the way up to volume production with consistently high quality.

SCHOTT LiDAR application expert Georg Mittermeier says: “SCHOTT is ready to serve fast-growing LiDAR technology with a collaborative approach, high quality, and custom-designed as well as standard packaging products.” He adds: “Based on decades of expertise as a supplier to the Datacom and automotive industries, customers benefit both from our opto-electronic know-how as well as our high volume manufacturing capabilities, including worldwide IATF 16949 certified production sites.”

May 8th, 2020

Contact

Georg Mittermaier
Electronic Packaging
SCHOTT AG

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