The life prolonger of automotive electronics
25 April 2014
Protecting electronics and batteries in hybrid and electric vehicles is an increasing challenge for carmakers. OEMs and suppliers need to find a way to handle the pressure in electronic housings and venting is increasingly important, explains Robert Chamberlain, WL Gore.
More effective protection is required for the in-vehicle electronics against environmental impacts, dirt and automotive fluids. All electronic components – whether part of compressors, pumps, motors, control units or sensors for increasingly popular active security systems – are subjected to huge temperature fluctuations throughout the service life. These can arise when the component’s housing heats up and then comes into contact with cold spray from the road or at the carwash. These fluctuations in temperature can cause a significant vacuum to develop inside the electronics housing. The resulting pressure differential can be so strong that the seals and sealing components protecting the sensitive electronics can be seriously compromised, letting in dirt particles and liquids that can corrode the component and shorten its service life.
One major challenge facing the automotive industry is the thermal management of high-performance electronics and batteries in electric vehicles, since these components need to operate in a certain temperature range in order to achieve optimum performance. They get very hot when running and need to be cooled using fluids. This can cause such huge temperature differentials within the electronic unit itself that condensate can form at the coldest point in the housing, which can lead to corrosion or cause a short circuit. For large battery housings, this problem can be so extensive that it is difficult to solve without effective measures to equalise temperature and pressure. Given the housing’s size, even minor temperature differentials can put enough pressure on the housing to cause deformation. In certain circumstances, driving a car out of a warm garage into the cold winter air can produce an interior vacuum that exerts a negative pressure of 500kg per square meter. Lightweight housings are scarcely able to withstand such pressure.
OEMs generally deal with these problems in one of three ways. The first option is to pot the electronic components. While this creates a sealed system, the unit ends up significantly heavier and cannot be reopened and repaired if it fails. Another way to achieve a hermetically sealed system is to use high-quality seals and thicker housing walls. However, this makes components more expensive and unnecessarily heavy.
A common and much more sensible solution is to incorporate a membrane that equalises the air inside the housing while at the same time preventing the ingress of liquids and dirt particles (Figure 1).
In unvented housings, as little as 7kPa of pressure can cause seals to fail after several temperature cycles.
Airflow and water entry pressure are the two fundamental characteristics that determine a membrane’s performance. Airflow describes how much air can pass through the membrane in a given period, at a given differential pressure. This defines how long it would take to equalise a pressure differential. Water entry pressure is the minimum hydrostatic pressure that the membrane must be able to withstand before it leaks. Both parameters are influenced by the pore size of the membrane, among other factors. It is the membrane supplier’s job to provide the ideal combination of airflow and water entry pressure for each individual application.
The use of compact electronic components means venting components must become smaller if they are to be integrated into smaller housings effectively. This in turn requires greater airflow per membrane surface area, resulting in a lower water entry pressure (Figure 2).
Typically, a system’s imperviousness is determined by ascertaining its IP protection rating (according to DIN 40050-9). The IP test determines the electronics housing’s protection level against solid objects and liquids. The IP protection rating (IPXY) is defined by two digits, the first (X) indicates the protection rating against ingress of solid foreign objects; the second digit indicates the level of protection against ingress of liquids. For example, IPX9K shows how well the housing with integrated membrane is able to remain watertight when exposed to steam jets. The IPX9K test is carried out in a testing chamber in which the housing, including its integrated membrane, is exposed to a steam jet from a distance of 100 to 150mm, at angles of 0, 30, 60 and 90 degrees. The airflow rate is kept between 14 and 16l/min, water pressure maintained at between 8,000 to 10,000kPa and temperature at 80°C.
Microstructure for venting
Gore uses PTFE (polytetrafluoroethylene) stretched in a specially designed process to create a membrane with very fine pores and in which the nodes are interconnected by fibrils. This material, called expanded polytetrafluoroethylene or ePTFE, is extremely water resistant thanks to its low surface tension. Any water droplets on the surface are unable to penetrate the membrane structure. The membrane is also oleophobic (oil resistant) and repels liquids with low surface tensions. ePTFE’s oil-repelling properties are particularly important for applications in the automotive industry.
Gore tests how well its venting solutions are able to withstand up to 20 different chemicals (in accordance with the ISO 16750-5 standard).This is done by exposing the vents to each test liquid and then leaving it for 24 hours at room temperature (21 to 23°C) or heating it for 96 hours in an oven. Airflow and water entry pressure are measured before and after the test (see Figure 3).
Another advantage of PTFE is its high resistance to extreme temperatures. Its ability to withstand temperatures ranging from -150 to 240°C is a property that is important given the current trend toward reducing engine size while maintaining or even improving performance. Shrinking the size of engines usually pushes temperatures above the 125°C threshold that electronic housings have been designed to cope with until now. It is no longer uncommon for temperatures to reach 150°C and above.
The company tests vents to determine their ability to withstand extreme temperatures (ISO 16750-4). In the temperature resistance test, the vent is exposed to a maximum temperature of up to 150°C for 2,000 hours, or a minimum of -40°C for 1,000 hours. In the ice dunk test, the vent is placed in a sealed housing and heated at a temperature between 80 and 120°C in an oven for 40 to 60 minutes. The housing is then rapidly cooled to between 0 and 4°C by placing it in iced water containing five per cent sodium chloride, a solution designed to simulate the salt water in winter. This procedure is repeated 10 to 20 times, with venting properties measured before and after the test.
Choosing the right membrane to suit each particular application and its requirements is vitally important. There are adhesive and weldable vents as well as moulded parts. Adhesive vents are coated with a high-performance adhesive that adheres to metal and plastic. However, as the adhesive has only limited resistance to extremely high temperatures and strong chemicals, these vents are less suitable for under-the-hood use. They are designed to be used with vehicle components that are less likely to come into contact with liquid chemicals, such as automotive lamps.
Weldable vents can be made of different material combinations and in different sizes to suit the specific requirements of the intended application. They are mainly used in applications with plastic housings, and are attached using ultrasonic welding. At the weld seam a small section of the housing material melts and flows into the porous structure of the membrane, guaranteeing that the join is sealed and solid. The melting point of PTFE is higher than the welding temperature, so this process does not compromise the membrane. These vents are long-lasting and reliable even when exposed to high temperatures and strong chemicals. However, the welding process is very complex and requires specialised welding tools and qualified experts to complete the process. In addition, protective walls have to be integrated into the housing design to protect the vent from steam jets and mechanical loads, which can be an expensive and complicated process.
Moulded parts withstand the most challenging environmental conditions and are easy to integrate. Insert molding integrates the membrane directly as part of the plastic injection molding process. The molded parts can then be attached by simply snapping into place in an opening in the housing. This protects the membrane from mechanical loads without the need to integrate expensive and complex protective walls into the housing. Integrating the vent does not call for special machines or qualified experts.
A close collaboration between carmakers, automotive suppliers and membrane provider from the earliest stage in the design process is key. Involving the membrane supplier ensures that each venting solution undergoes the necessary testing for its intended application and meets the correct requirements.