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How venting solutions help extend the lifetime of electronic components in cars

How venting solutions help extend the lifetime of electronic components in cars

Technology News |
By eeNews Europe



Electronic components are playing a vital role in today’s vehicles. It is therefore all the more important that sensitive electronics function reliably despite the extreme temperature differences and rough environmental conditions they have to withstand during a drive. The most effective solution is a membrane that protects electronic housings against contamination and fluids and provides air exchange and pressure equalization at the same time.

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 their service life. These can arise when the component’s housing heats up in operation 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. Damaged or defective components usually have to be replaced, leading to high warranty and repair costs for automakers and their suppliers.

Electronic parts facing higher challenges in electric and hybrid vehicles

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 equalize 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 500 kilograms per square meter. Lightweight housings are scarcely able to withstand such pressure.

Membranes provide air and pressure equalization

OEMs generally deal with these problems in one of three ways. The first option is to pot the electronic components. While this solution creates a perfectly 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. The drawback of this system, however, is that it makes components more expensive and unnecessarily heavy.

A common and much more sensible solution is to incorporate a membrane that equalizes the air inside the housing while at the same time preventing the ingress of liquids and dirt particles. Figure 1.

Figure 1: Pressure over times in vented vs unvented housing

(Captions) The graph shows the continuous buildup of negative pressure within a hermetically sealed housing. In unvented housings, as little as 7 kPa of pressure can be enough to cause seals to fail after several temperature cycles. Vented housings equalize pressure and avoid seal leakage.

Use case: Vacuum in an inverter housing in the car wash

The following example explains the pressure conditions in an inverter housing during a drive through the car wash: Assuming that the inverter’s dimensions are 40 cm x 20 cm x 20 cm, which is equivalent to 16 l, the housing contains 4 l free air volume. While driving the inverter operates at a temperature around 70 °C. The underbody of the vehicle is sprinkled with 8 to 10 °C cold water in the car wash. Within only a few minutes the inverter is cooled down to about 40 °C. Without a venting solution this temperature difference leads to an vacuum of 90 mbar. With every drive through the car wash this vacuum stresses the seals and leads to leaks in the long term. Oils, cleaning agents and other liquids can penetrate into the housing and threaten to damage or even destroy the sensitive electronics inside.

A venting solution can equalize the vacuum within a very short time and avoids pressure peaks. After only six minutes, the pressure in the housing and the ambient pressure are already in equilibrium.

Airflow and water entry pressure determine the membrane’s performance

As indicated in the example above, the two main characteristics for membrane solutions are airflow and water entry pressure. 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 equalize 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.

A major challenge in the automotive industry is the trend toward increasingly compact electronic components. This means venting components must also become smaller if they are to be integrated into smaller housings as effectively as possible. This in turn requires greater airflow per membrane surface area, resulting in a lower water entry pressure, figure 2.

Figure 2: Membrane pore size affects airflow and water entry pressure

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 is defined by two digits: IPXY. The first digit (X) indicates the protection rating against ingress of solid foreign objects; the second digit (Y) indicates the level of protection against ingress of liquids. 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 150 mm, at angles of 0, 30, 60 and 90 degrees. The airflow rate is kept between 14 and 16 l/min, water pressure maintained at between 8,000 to 10,000 kPa and temperature at 80 °C.

ePTFE membranes provide a unique microstructure for venting applications

A material that is ideally suited to venting applications because of its unique microstructure is PTFE (polytetrafluoroethylene). The PTFE raw material is 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 hydrophobic (water resistant) thanks to its low surface tension, which means that 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, such as oils. ePTFE’s oil-repelling properties are particularly important for applications in the automotive industry, as the likelihood is very high that vehicle components will come into contact with motor oil, cleaning agents or other automotive fluids.

In order to test how well venting solutions are able to withstand up to 20 different chemicals (in accordance with the ISO 16750-5 standard) the vents are exposed to each test liquid and then left for 24 hours at room temperature (21 to 23 °C) or heated for 96 hours in an oven. Airflow and water entry pressure are measured before and after the test. Both results must lie within prescribed specifications, figure 3.

Figure 3: Values below the black horizontal line indicate that applying the chemicals has impaired the membrane. The test membrane has only limited suitability for wetting with corrosion protection (removal) agents, but lies within the prescribed standards for all other chemicals tested. For full resolution click here.

Testing the temperature resistance of membranes

Another advantage of PTFE is its extremely high resistance to extreme temperatures. Its ability to withstand temperatures ranging from -150 °C 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 in this way 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.

There are several proven methods of testing 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 5 % sodium chloride, a solution designed to simulate the salt water that electronic housings are likely to come into contact with in the winter. This procedure is repeated ten to twenty times, with venting properties measured before and after the test.

Different applications require individual membrane solutions

Choosing the right membrane to suit each particular application and its requirements is vitally important. In order to cover all application areas, membrane manufacturers offer adhesive and weldable vents as well as molded parts. Adhesive Vents are coated with a high-performance adhesive that adheres strongly to various kinds of metal and plastic. They are supplied on a lightly adhesive backing for manual or automated installation. The adhesive is long-lasting and capable of withstanding harsh conditions. 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 by the customer using ultrasonic welding. At the weld seam a small section of the housing material melts and flows into the porous structure of the membrane, thereby guaranteeing that the join is sealed and solid. Since the melting point of PTFE is much higher than the welding temperature, this process does not compromise the membrane. These vents provide a long-lasting, reliable solution even when exposed to high temperatures and strong chemicals. However, the welding process is very complex and requires specialized 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.

Molded parts represent a solution that is able to withstand the most challenging environmental conditions as well as being easy to integrate. This can be achieved by adding a step to the manufacturing process. For example, insert molding is a process that integrates the membrane directly as part of the plastic injection molding process. The molded parts can then be attached by simply snapping them 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. Furthermore, integrating the vent does not call for special machines or qualified experts: the vent is simply fitted in the housing via the “plug & play” method.

Customized venting solution for different applications

The rising number of electronic components in cars increases the need for reliable protection measurements against environmental impacts. Especially high-performance electronics and batteries in electric and hybrid vehicles present an enormous challenge for automotive manufacturers and suppliers. The most effective solutions are automotive vents with membranes that protect sensitive electronics against contamination and fluids and provide air exchange and pressure equalization at the same time. Since every application has specific requirements, it is important to collaborate with the membrane manufacturer at the earliest stage in the design process. This ensures that the selected technology undergoes the necessary testing, meets the correct requirements and provides the best venting solution.

About the author:

Robert Chamberlain is Sales Manager Automotive Electronics, W. L. Gore & Associates, Ltd.

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