Process Considerations for Latest Developments in Automotive Vision and Detection Systems

Posted 12/04/2013 by cderama

For many years the expanding scope of electronic systems in vehicles has been built on a very conservative design ethos, with a considerable focus on reliability.  It is well known that Automotive Electronics was by far, the largest electronics market segment to be exempted from 2006’s EU RoHS legislation which included the removal of lead (Pb) from electronic assemblies.  The core argument during the successful lobbying of the EU, and subsequent exemption for the automotive electronics companies was that there was insufficient reliability data for the Pb-free alternatives.  Now that these exemptions are coming to an end, this growing segment is rapidly employing the most common alloy group which the majority of other segments had adopted before 2006. This solder alloy group is the standard Tin-Silver-Copper alloys, based close to the ternary eutectic point at Sn Ag3.8 Cu0.7. 

Added complexity in material selection comes from the ever increasing array of applications in Automotive Electronics.  One of the newer applications is the deployment of Vision and Detection systems for driver assistance.

The Evolution of Vision & Detection Systems – 4 major applications

Parking sensors have become commonplace in the last 10 years, making the process of getting your car into a tight parking space a much less risky proposition.  This is based on relatively simple radar technology and provides the driver with an increasingly audible alarm as the vehicle comes closer to the object to be avoided.

In more recent years a number of vision and detection systems have started to play a much more critical role in the driving experience.  One of these examples is the Lane Departure Warning System (LDWS).  This system uses AOI vision technology to track the edge of the road or the lane markings, and provides the driver with a warning if the vehicle veers over the “line” without indicating. The system interprets the absence of purposeful use of the indictors as a likely result of the driver loosing concentration, or in its extreme form, falling asleep. The first generation of LDWS activated a significant vibration alert in the seat in order to alert the driver to imminent danger.  The second generation took things closer to automatic intervention with a low-torque nudge of the steering wheel to put the vehicle back “on-course”.  The reliability of such a system is clearly more critical than the simple parking sensors, as the operation is designed to work at speed and prevent high speed collisions.

An increasingly common feature in vehicles is cruise control.  This has been available for decades and like many features which entered the market on luxury vehicles, has become standard for any vehicles intended for distance driving.  The new generation of cruise control is emerging, and is known as “Active Cruise Control”.  This technology not only regulates the speed, but also maintains a minimum distance between the vehicle in front, automatically reducing the throttle, and in some cases activating the brakes automatically, and then only returning to the set-speed when the distance to the vehicle in front allows. This radar based technology is integrated with the braking and throttle controls to achieve its function, and so its performance is absolutely critical to safety.

The forth and final area we will consider is the new generation of parking aids.  Supplementing the radar based parking sensors is the integration of rear view cameras with superimposed guidance lines to help the driver with reversing into a parking space.  This passive parking technology is already being advanced with some active systems which will actually assess a parking space and automatically steer the vehicle into a parking space requiring the driver to only operate the throttle and the brake pedals.  This technology uses a combination of radar and cameras to provide this advanced parking function.

Design & Manufacturing Constraints and Considerations 

So what does all of this mean?  It means that a complex array of cameras and sensors are being integrated into the vehicles periphery which poses two major challenges to the assembly process.  The first challenge is that these units need to be small and discreet to meet both weight and design integration requirements, and the second is that that they need to be robustly reliable.

Automotive vision and detection systems typically utilise camera technology which has come straight from the mobile device market, which by definition is highly miniaturised.  This is convenient for successful and discreet integration into the vehicle, but also poses a challenge to meet the extended lifetime demanded by increasing vehicle warranty terms.  To achieve these goals the assembler must consider the following aspects of material and process selection.

- Mechanical reliability

- Electrochemical reliability

- Process performance requirements

- Cost of product failure vs. cost of process

Taking each of these factors in turn :

Mechanical reliability : There can be no universal mechanical reliability test for automotive electronics as there is great variability in operating conditions from one electronic system to the next.  For example an engine control unit sees much harsher conditions than an in-car audio system.   For vision and detection systems the solder joints need to be able to provide adequate strength and creep resistance to survive the temperature and vibration conditions to which the unit is exposed.  This would require that the solder alloy and all solderable surfaces be characterised to ensure performance is met.  It is of particular importance with small area array bottom terminated lead-less components.

Electrochemical reliability : Material selection and process parameters are of paramount importance for such sensitive applications. There are two major factors which bring this aspect of reliability into sharp focus for vision and detection systems : (i) firstly, the small gaps between conductors require that any residues left on the assembly from fluxes or PCB fabrication chemicals to be extremely benign and (ii) the harsh and variable climatic conditions under which the modules need to operate. While there is no one test which can characterise materials for these applications,  a broad range of SIR, Electromigration and Corrosion tests exist, which in combination can give a strong indication of material suitability.

Process Performance Requirements  :  A lot of expertise in assembling miniaturised electronics exists in the hand-held devices sector, but for automotive this is a relatively new experience.  Process performance will directly affect product reliability in several ways.  It starts with a robust and repeatable solder paste printing process to ensure that solder joint volume is maintained on critical small footprint devices such as cameras, active and passive devices. Solder paste formulation and stencil design know-how is the key to a repeatable print process.  Beyond printing a major focus needs to be on thermal processes to ensure that both solder joint formation and correct activation of the any flux has been achieved.  This is most critical in the use of any liquid fluxes in a wave/selective process typically used for connector attach.

Cost of Product Failure vs Cost of Process : Total Cost of Ownership : There is continued pressure on Automotive tier 1 suppliers to continually reduce costs.  At the same time it is widely understood that the choice of assembly materials can be the difference between a product surviving its warranty period or not.  Assembly materials typically cost approximately 1% of the BOM for a vision detection unit, and as a result, it is somewhat surprising that compromises are sometimes made on alloy choice and material selection.  The cost of product failure in the field is the full cost of recall and unit replacement, which is of course >100% of the original BOM costs.

Summary

Automotive Vision and Detection systems are a very key growth sub-segment in Automotive Electronics.  The integration of hand-held device component technology, along with the demands created by the harsh environments under which the systems need to reliably operate provides a unique challenge for the assembler.

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