Electronics Protection - September/October 2012 - (Page 6)
Feature The Unknown Problem with Airtight Enclosures
Jason Zambotti, Product Specialist W. L. Gore & Associates In today’s world, sophisticated electronics are used in all types of outdoor equipment in just about every application — telecommunications infrastructure equipment, solar energy systems, outdoor lighting systems, just to name a few. This translates to electronics being exposed to the environment’s harsh conditions. A rugged environment is any environment that can potentially damage electronics. This can be outdoors, where the product is exposed to changing weather patterns, whether it’s a sudden thunderstorm or the more subtle shifts between a hot day and cold night. It can also be an environment where there’s risk of exposure to harsh chemicals, surfactants or liquids such as those used in high-pressure sprays to clean equipment. It can even include the environment inside the manufacturing facility or the internal environment of the device itself, generating a significant level of heat during operation. In most cases, electronics are designed to be deployed all over the world, so any or all of these situations may be applicable to the design. To protect the electronics, most engineers design sealed enclosures with robust housing materials, durable seals, and strong bolts to ensure a tight seal. The enclosure is effectively air-tight and waterproof, particularly if it must past Ingress Protection (IP) or National Electrical Manufacturers Association (NEMA) standards. However, once the enclosure is installed in the field, it may begin to show evidence of water and particulates inside the housing. Using watertight enclosures does not necessarily guarantee long-lasting protection and reliable performance because pressure differentials, which over time can cause leak paths, have not been addressed. experienced throughout the course of a typical day or during a sudden thunderstorm. The graph in Figure 1 shows that the temperature can differ by approximately 50°F over the course of an average day in the desert.
Figure 1. Temperature Cycle for 24-hour Period
Why do Airtight Housings Leak?
Failures often occur because the airtight seals prevent the device’s ability to equalize pressure. As the external pressure fluctuates, the enclosure tries to equalize the internal pressure by drawing in air from the outside. If the housing is completely airtight, pressure builds up inside in the form of a positive or negative buildup. Positive buildup causes the housing to bloat, while negative buildup creates a vacuum. Either type of buildup leads to stress on the seals, joints or gaskets, which in turn compromises and damages their effectiveness. The compromised seals begin to allow water and contaminants to enter the housing, which can ultimately lead to electronic failure. For example, at Gore, our engineers recently worked with a global manufacturer of wireless routers that were specifically designed to withstand extreme environmental conditions. However, after several months in the field, the housings began to leak, and the water caused corrosion and performance-related issues with the electronics. The engineering team determined that the leakage was due to failure of the enclosure’s seals. They considered using more rugged seals to eliminate the water intrusion problem, but they soon realized that this would only prolong the time before the seals failed again.
What Causes Pressure Differentials?
Temperature changes are the most common cause of failures due to pressure. The temperature change can be internal, external or both. The electronics inside the housing generate a significant amount of heat, which can result in problems if the heat cannot dissipate. Externally, temperature changes can be dramatic, as
A recent customer of Gore designs smart antennas for fleet management systems. The antennas are mounted on the exterior of commercial vehicles, exposing them to severe weather conditions as the trucks move across the country. The antenna is housed in an ASA thermoplastic enclosure with IP67-rated gaskets. The customer’s engineering team found that sudden temperature drops caused a vacuum inside the housing and put stress on the seals. The seals eventually failed and allowed moisture inside, which damaged the electronics (Figure 2). They needed a way to equalize the pressure in- Figure 2. Failed Seal in a GPS System side the housing without allowing water and particulates to enter. Altitude changes are another common cause of sudden pressure differentials. Because most shipping containers are not pressurized, transporting products around the world creates many opportunities for them to encounter significant pressure differentials. If the altitude changes are not compensated for, the resulting vacuum makes it very difficult, if not impossible, to open the container. A customer was developing a mobile tool chest to meet military specifications that required the chest to open as soon as it hit the ground after being dropped from an aircraft. The tools inside also needed to be protected from water, dirt and sand, so the customer used O-ring gaskets that provided excellent seals. As shown in Figure 3, the pressure on seals could increase from about 2 psi (137.9 mbar) when dropped from the aircraft to almost 16 psi (1,103.2 mbar) in the short period it took to reach the ground. If not equalized, this rapid pressure change would prevent the sealed case from opening. Although electronic devices such as those used in solar energy systems are permanently installed and do not have to deal with altitude changes, they are often exposed to high-pressure sprays to clean the panels. These sprays can generate pressure drops greater
September/October 2012 www.ElectronicsProtectionMagazine.com
Table of Contents for the Digital Edition of Electronics Protection - September/October 2012
Electronics Protection - September/October 2012
Table of Contents
HP Intelligent Series Racks Offer Monitoring Capabilities for Data Centers
The Unknown Problem with Airtight Enclosures
Key Drivers When Turning to Captive Panel Screws
Emerging Trends in Field Equipment Enclosures
JV Industrial Solves Power Outage Issues with Battery-Free Flywheel
The Green Data Center Opportunity
The Power-Grid and Protecting Electronic Devices
Maximizing Availability, Capacity and Efficiency with Rising Data Center Temperatures
Canyonwest Cases Releases new Enclosures for Flat-Screen TV Transportation
Ohmite Manufacturing Releases Radial-Fin Heat Sink Extrusions
Rogers Introduces Condux Plus Conductive Foams
FIP Gasketing Resin Offers Advantages over Gasketing Materials
Minmax Power Releases MSGWI06 Series of DC/DC Power Modules
New E-Line Locking Systems Reduce Security Costs and Enhance Locking Solutions
Calendar of Events
Electronics Protection - September/October 2012