EIPC Technical Snapshot: Automotive Technology
Although current circumstances have forced the postponement of its live conferences, seminars, and workshops, EIPC continues to provide a platform for the exchange and dissemination of the latest knowledge and technical information to the European interconnection and packaging industry. Its current series of technical snapshots, delivered in a webinar format, address technology challenges facing the automotive, telecom, and high-speed sectors of the industry.
The most recent session focused on automotive technology and some of the key factors driving the global automotive PCB industry. The webinar was moderated by EIPC vice-president Emma Hudson, and included presentations from three industry experts.
The first presentation, entitled “An Exploration of Advanced Semiconductor Packaging’s Effect on Automotive Electronic Hardware Design and Assembly,” came from Lenora Clark, director of autonomous driving and safety technology for ESI Automotive. Her stated objectives were to understand how megatrends were changing semiconductor package needs and design features in the automotive space and to relate these to the rest of the electronic build.
For carmakers, the choice of electronics package was driven by the megatrend, and for those involved in designing and building the electronics, it began with the semiconductor package. Although advanced semiconductor packaging had obvious applications in smartphones and data centres, it had not previously been considered a growth area for automotive. But now its growth was exponential.
The megatrends were requiring exceptional quantities of electronics in the areas of connectivity, autonomy, ride-sharing, and electrification. Advanced driver assistance systems required numerous cameras, short- and long-range radars, high-resolution lidar, and multiple ultrasonic sensors, together with the processing power to support all the electronics and the interconnection between them. Electric vehicles required semiconductor packages to support inverters, chargers, and power conversion.
Although electronics had been used in automobiles for many years, most of the packaging was legacy. Advances in technology demanded advanced packaging, and this was very new to the automotive industry. Additionally, a different architecture was evolving, where information was no longer processed locally at individual sensors but needed microcontrollers for multiple nodes.
It was necessary for multiple sensors to communicate with each other, and more advanced processing was required, with a central computer making the ultimate decision. Automotive semiconductors had higher functionality and integration at the package level and were required to work at different frequencies, voltages, and temperatures than their smartphone equivalents.
One of the biggest differences was the operational lifetime. A smartphone was only expected to work for up to three years, and a field failure rate of less than 10% was tolerable, but automotive electronics were expected to remain defect-free for up to 15 years. The applications were life-critical, so reliability was a crucial factor, and this requirement would influence package design and choice of materials.
High-density PCB designs would be essential, with more layers, finer lines and spaces, and features like stacked microvias probably unavoidable. A move to any-layer-via construction was a possibility. High-frequency applications for radar and 5G would require low-loss substrates and possibly mixed-material stack-ups with consequent fabrication difficulties. For assembly, solder alloys with enhanced reliability would be required, especially under long-term mechanical stress and thermal cycling, together with fluxes formulated to minimise electrochemical corrosion.
In her summary, Clark remarked that the automotive supply chain was unique. Existing members understood the requirements but would now need to learn new skills. New suppliers would have to learn the needs and expectations of the automotive industry.
Technology ambassador for Ventec International Group and president of EIPC, Alun Morgan gave the second presentation titled “Developing Universal Solutions to Automotive Materials Challenges.” He commented that electronics currently represented about 40% of the value of the vehicle, compared with 18% 20 years ago.
His see-through motor car illustration highlighted a series of hot-spots in automotive electric and electronic systems that required thermal management. He chose lighting as a subject for detailed discussion, commenting that automotive lighting had undergone a transition in recent years, with LEDs now predominant as they had become progressively brighter, more efficient, and less expensive. In headlights, compared with xenon or halogen-lamp, LEDs had the advantages of lower power consumption and dissipation, longer lifetime than the vehicle, colour close to daylight, and low lifetime-ownership cost. They could now be designed-in as permanent components of the vehicle.
A refresher course in thermodynamics reminded the physicists in the audience that, according to the first law, in a closed system, energy could neither be created nor destroyed but could only be converted into different forms. The second law was that heat would always flow from a region of high temperature to one of lower temperature, which was the key to thermal management.
But although LEDs had become more efficient in converting electrical energy to light, the fact remained that at least 60% of that energy was actually converted to heat. And in car headlights, that was a lot of heat to manage. Traditionally, the heat had been conducted away by attaching a heavy and bulky heat sink.
The modern way was to mount the LED on a printed circuit fabricated on an insulated metal substrate (IMS)—a laminated combination of a copper foil, a heat-conducting dielectric material, and a relatively thick metal-base plate—generally aluminium, and occasionally copper—that could then be coated with a finish known as ER1 that substantially improved the emissivity of the metal surface. Morgan demonstrated its effect with some impressively complicated mathematics.
Insulated metal substrates had become widely and successfully adopted for the thermal management of LED devices, but some challenges still had to be addressed. Morgan described a real-world case study where an automotive OEM had conducted a severe long-term thermal cycling test on an LED headlamp assembly and observed some electrical failures which had been traced to cracks in solder joints. These were shown to have been caused by shear stresses generated by differential expansion-contraction during thermal cycling in operation.
In his example, the LED package had a coefficient of thermal expansion (CTE) of 10 ppm/°C, whereas that of the aluminium base was 23 ppm/°C. The CTE mismatch was reduced by using a low-CTE aluminium alloy (19 ppm/°C), and the dielectric was re-formulated to lower its elastic modulus so that it effectively became a compliant inter-layer. An additional benefit of the re-formulation was to increase its maximum operating temperature to 155°C. The outcome was that the modified IMS material enabled the LED headlight to exceed 3,000 thermal cycles without failure.
The final presentation, entitled “Today’s Multilayer PCB Requirements for the Automobile Industry,” came from Multiline International Europa Managing Director and EIPC Board Member Paul Waldner. He also used a see-through motor car illustration to demonstrate the extent to which numerous electronic systems had become fundamental to the operation of the vehicle.
This time the focus was on how the automobile sensed its environment and how it would need to become increasingly networked: vehicle-to-vehicle, vehicle-to-cloud, vehicle-to-infrastructure, vehicle-to-pedestrian, vehicle-to-road-sign, and vehicle-to-drone. He remarked that tomorrow’s automobiles would incorporate many elements of industrial development.
Inputs, processing, and outputs would be controlled with a high-performance server, and high-frequency PCBs would be an essential requirement for vehicle radar and lidar and for supporting ultra-fast communications. There was a convergence of technologies in multilayer PCB requirements for the communications, military, and automobile industries because of the signal-speed requirements of 5G communication networks and distance-to-hazard sensing in the development of autonomous vehicles. PCBs for future automobiles would require high circuit density and high layer counts with different materials—some of which would demand very high lamination temperatures. High reliability was fundamental.
Waldner considered that there were three necessities for managing the technology convergence: better pressing technology, better layer to layer registration of multilayers, and better signal integrity for high-speed circuits. He discussed the principles and details of a proprietary lamination press, with induction heating offering uniformity of temperature throughout the stack and the benefits of accurate control of parameters and low power consumption. Although the four-slot tooling system had been established for many years, he believed that it still offered many advantages, particularly when laminating mixed materials with different expansion coefficients.
And regarding signal integrity, the track geometry of fine-line designs could be controlled to precise limits with a proprietary modified semi-additive process using an ultra-thin palladium catalyst layer to initiate electroless copper deposition. His example showed 20-micron traces on a 50-micron pitch with a square cross-section.
A question-and-answer session followed the presentations and raised points for discussion on the subjects of stacked microvia reliability, SAC alloys for automotive applications, and high-frequency challenges, to which the experts responded readily. Emma Hudson did a skillful job of moderating the session and keeping the whole proceedings within the scheduled one-hour time slot. Thanking the speakers for their contribution and the audience for their attention, she announced that further technical snapshot webinars are planned for November 18 and December 19.