Electronics Protection - Winter 2014 - (Page 18)

Feature Common IP Testing Failures and How to Avoid Them Erik Rohr, Senior Test Technician Trace Laboratories A product that is used outdoors, in a dusty factory, or near your morning latte will often require some level of tested and certified protection against the ingress of dust and water. Companies want their products to meet specific Ingress Protection (IP) ratings, for reasons of safety, functionality, and even as a marketing technique to demonstrate superior performance over the competition. This article will discuss ways that water and dust find their way into various types of products, and design techniques to ensure the best possible test. Ingress protection tests check a product's resistance to water, dust, and foreign objects. Depending on the type of test or rating, water is applied by misting, dripping, spraying, jetting, pressure washing, steam cleaning or immersion. Dust is applied by placing the sample in an enclosed chamber with a test dust that is agitated by compressed air. In both cases the sample under test is inspected for ingress, and safety and functionality are determined. A gasket is a common way to form a watertight seam between the body of an enclosure and its cover panel. The key to a good gasket seal is firm, consistent compression all the way around the gasket, but when the cover is made from a flexible plastic this is easier said than done. Water or dust may breach the gasket if the cover is bolted down with an incorrect or unspecified torque, so be sure the screws or bolts are tightened precisely before sending your product out for testing. If the torque applied to the screws is too low, the gasket will not be firmly compressed. If the torque is too high, this can cause the cover panel to flex and bend away from the surface between the screws. The fewer screws there are to hold down the cover, the more likely this is, and either situation can result in an uncompressed gasket and a leak into the enclosure. Now that the gasket is firmly compressed, it may withstand most ingress protection tests, but jetting and pressure spray tests where water is sprayed at the enclosure at high speeds may still be a challenge. These types of tests are relevant for automotive or military parts that would be cleaned by a pressure washer or high volume hose. The water can force its way in to even a well seated gasket. A simple method of protecting the gasket from this spray is adding a shield in the form of a lip that extends 1/4 inch to 1/2 inch over the gasket and blocks the majority of the direct spray, greatly easing the burden on the gasket. There are other ingress tests that seem mild, but are more difficult than they appear at first glance; tests that induce pressure and temperature differentials to simulate the internal pressures that an enclosure might create during operation. Temperature cycling from the powered electrical components is an example of this. RTCA DO-160's drip test requires that the sample temperature be stabilized above the temperature of the water immediately 18 Winter 2014 * www.ElectronicsProtectionMagazine.com before the water begins to drip. As soon as the water contacts the enclosure, the temperature of the air inside the enclosure drops and the air contracts, causing a negative pressure in the enclosure. Depending on the product's application, this can be counteracted by installing a small drain or vent at the bottom of the enclosure to equalize the inner and outer pressures. A vent at the bottom can also be useful if you expect water to enter from the top, and simply need a path for it to escape before pooling up to the electrical components. Most specifications allow some water ingress into the enclosure as long as it does not affect operation or impair safety. GoreTex is a material that is both breathable and waterproof, and useful in venting pressures as well. Enclosures with components that need to allow air flow while blocking water can be an engineering challenge with respect to ingress protection. Microphones and gas sensors are a couple examples. The key is to block the water flow by taking advantage of the water's surface tension. Plastic or wire mesh, or a dense set of bristles or wire strands are an effective design that blocks most spraying or splashing water, and still allows air or sound to pass through. The wire strands that pass through an enclosure can also be a liability in certain applications. The IP6X rating of IEC 60529 induces this negative pressure inside the enclosure by use of a vacuum pump set to 20 mbar (2kPa). This type of test is very effective in exposing any pathways from the outside of the enclosure to the inside. A path that is often overlooked lies in the gaps inside of cable jacketing or heat shrink tubing. If your enclosure has this type of cable running through the wall, you may want to ensure that the cable is sealed at the ends or an appropriate connector is used at the wall to prevent air and dust from traveling through this unexpected path. Most specifications recommend that the product be mounted or oriented in a way that simulates actual use in the field when it is tested. This may take some extra time to setup and the engineer or manufacturer may not have all the exact adjoining parts on hand, but an enclosure may react very differently to the test depending on how it is mounted or positioned. The product under test tends to be more likely to pass when it has a fixture that simulates its actual use in the field, even if the fixture is rather rudimentary. This can be a big help in moving through the testing phase with as few bumps as possible. Product designs all have their advantages and disadvantages. Anticipate what challenges lie ahead for your product in field use, and know what your testing goals are and why. The ideas in this article should help to give you a better idea of what to expect during testing. For additional guidance or a quote, please contact Trace Laboratories at www.tracelabs.com. http://www.tracelabs.com http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - Winter 2014

Editor's Choice
EMI Compliance: Choosing the Right Shielding and Gasketing
Thermal-Fluid Modeling for Flat Thin Heat Pipes/Vapor Chambers
Increase Rack Cooling Efficiency and Solve Heat-Related Problems
Seven Essential Cabinet Design Considerations for Protecting 19 Inch Electronics
A Better Alternative to Heat Pipes: Integrating Vapor Chambers Into Heat Sinks
Common IP Testing Failures and How to Avoid Them
Enclosures
EMI/EMC/RFI
Thermal
Power
Hardware
Contamination
Industry News

Electronics Protection - Winter 2014