星期三, 4月 11, 2012
星期二, 1月 10, 2012
The humidity test is designed to determine the effects of humidity on the product and assure that the product will meet published specifications while being subjected to high humidity conditions. This is an accelerated test to uncover failure modes in components, assemblies and process, which may appear in the field during normal service, while keeping test time at a minimum. Performance tests are made at various intervals of the test duration and each test must pass for the design to meet the requirements of this section.
Hygroscopic materials (e.g., printed circuit boards, molded plastics, sealed components, etc.) will absorb moisture proportional to the applied vapor pressure and time of application. Excessive moisture penetration may cause swelling of material or contamination actions, destroying functional ability as well as causing leakage paths between sensitive circuits, result when moisture reacts with trapped chemicals from construction processes causing corrosion or severe etching of metal surfaces. In some cases dendritic growth can cause short circuits or high voltage flashover between closely spaced traces or pin spacings. The following humidity test is designed to accelerate the occurrence of these failure mechanisms if they exist.
Failure from humid environments can be classified by two separate mechanisms; adsorption (surface assimilation of moisture) and absorption (moisture penetration into a hygroscopic material). This test is designed to provide the two conditions separately and at a more accelerated rate than the expected end-use environment. Because of the cumulative effects, adsorption (Clause 4.3) and absorption (Clause 4.4) tests are scheduled consecutively. A low humidity test (Clause 4.2) is include to ensure products are capable of performing at low humidity, common during winter months in many parts of the world. The additional “Five-Day Humidity Cycle” test is described in Clause 4.6. Good engineering judgment will determine when this test may be needed or should be used in addition to the required tests.
While performing humidity testing precautions must be taken to prevent condensation on the chamber ceiling or walls from dripping onto the product. No contaminating materials (wood products, chemicals) shall be present or exposed within the chamber while making these tests.
The condensation must be drained from the chamber and not recirculated. Water used to produce the required humidity shall be distilled, de-mineralized or de-ionized having a pH value between 6.0 and 7.2 at 23°C. Minimum specific resistance of the water should be 150,000 ohm centimeters.
Products which use humidity sensitive recording media in their design (e.g., magnetic tape and disc drives, printers and other paper handling products, etc.) shall be tested to the selected environmental class. This may require the use of mylar tape and special high grade magnetic materials, or a modification of the test procedure operating humidity test may be conducted with the recommended recording media in place in the worst case environment allowed by the manufacturer of the media. It is expected that this product limitation caused by the media will be identified to the customer in the technical data sheets published.
Select the appropriate humidity profile from Figure 1,2 or 3 for the selected environmental class of the product to be tested. Record performance test measurements at each period indicated in the figure. Before performance tests are made at each new humidity level, allow one hour of stabilization or the minimum specified warm-up period, whichever is greater. Starting the test in the early morning allows performance testing to be made during normal work hours over the period of the complete test (notice the times given for performance tests in the figures).
Complete performance tests for some products can be very lengthy. It may be necessary to apply abbreviated performance tests at certain steps during the humidity tests. When that is necessary, good engineering judgment must be used to evaluate those parameters of the product’s performance most likely to be influenced by the humidity condition.
4.1 Initial Performance Test.
To start the tests, place the product in a temperature/humidity controlled chamber and record the performance test results to verify conformance to specifications. Starting conditions for the test are 25°C±3°C and 50%RH±10% (approx. room ambient).
4.2 Minimum Operating Humidity
The low humidity test is set to 15% RH ±10% depending on the product’s market requirements. This is a 24 hour test. Products such as CRT’s, printers, plotters, magnetic recorders and the like, may be sensitive to this environment. Record the performance test results at the beginning and the end of this conditioning.
4.3 Maximum Operating Humidity
This test is to measure the effects of adsorption (surface leakage). The EUT (Equipment Under Test) is subjected to the maximum operating humidity environment before and after the non-operating period. The first period is for 17 hours and the second period is for 11 hours. This test is intended to accelerate failures caused by moisture building up on the surfaces where leakage and electrolysis effects may occur. Dendrite growth is also facilitated during the power-on time of this test. Record the performance test results as indicated in the figures.
4.4 Non-Operating Humidity
This test is to measure the effects of absorption (moisture penetration into hygroscopic materials).
Important: Remove recording media from the product using such consumables.
Upon successful completion of the first operating humidity test (first 48 hours), the product is turned off and a non-operating profile is initiated. The non-operating period varies from 12 hours to 72 hours depending on the environmental class (See Figures 1, 2 and 3). This high humidity environmental will accelerate corrosion of metals and deterioration of hygroscopic or porous materials.
At the end of the non-operating humidity cycle, the chamber will be returned to the maximum operating humidity conditions of Clause 4.3. Record the performance test results at the beginning of the maximum operating humidity test (after conditions stabilize) as shown in the figures.
4.5 Dry-Out and Final Performance Test
At the end of the second maximum operating humidity test the temperature and humidity are returned to ambient conditions and the performance test is repeated. Record the performance test results. Compare the test results to the results recorded at the initial performance test. Variations in the two tests may provide insight into potential weaknesses in the design.
Remove the product from the chamber and visually examine it fro signs of corrosion, etching, distorted parts and any other anomaly. Carefully record observations. If a component or assembly no longer meets electrical or mechanical specifications, a failure will be recorded. Cosmetic anomalies should be evaluated by the project team with focus on customer expectations.
Continue operating the product for the final 24 hour period at ambient conditions. Any failure occurring in that period is to be evaluated to determine if it was caused by the humidity testing just completed.
This test is designed to measure the recovery rate of products exposed to condensation which occur during normal use where rapid changes in ambient temperature take place (e.g., moving the product from a cool area to a warmer area.) Class C1 and C2 products are not tested for condensation since they are intended for use in a more normal environment where temperature and humidity are not changing in a way that causes condensation.
The recovery rate is a function of the product application. Unless otherwise specified on the product data sheet, the following recovery times will be required:
Life Support Products – one minute
Portable Products – five minutes
All Others – fifteen minutes
Recovery shall be judged as the time required for the product to conform to specifications.
Some products are not capable of performing their major functions in a condensing environment without degradation due to media limitations. These products need not be tested to this specification provided the product data sheet clearly specifies this non-condensing limitation to the customer.
5.1 Method 1
The non-operating product shall be allowed to stabilize at laboratory ambient conditions (approximately 25°C, 50% RH). The product shall be quickly placed in a chamber stabilized at 40°C and 95% RH and after five minutes turned on. Refer to Clause 5 for recovery requirements.
5.2 Method 2
When the product’s physical size will not allow movement, the non-operating product shall remain in the temperature/humidity controlled chamber. Reduce the temperature to -10°C and hold at this temperature until the thermal mass of the product is stabilized.
Set chamber temperature controller to 25°C and 50% RH. The rapid rise in chamber temperature will cause condensation on the product, since the product’s thermal mass will cause the internal temperature to lag below the dew point temperature (condensation will also be observed on the window and walls of the test chamber). Refer to Clause 5 for recovery requirements.