Conformity Magazine - April 2008 - (Page 36)

temperature component was not utilized during the pneumatic hammer system testing, we duplicated the screening process using only the vibration table portion of the HALT chamber system. Efforts were made to ensure that conditions during the HALT evaluation using the skewed fixture chamber were identical to the HALT evaluation using the pneumatic hammer system. The Results The skewed fixture chamber ESS system was able to precipitate defects at a much lower overall vibration level in a much shorter time, thus increasing product throughput and reducing the cost of testing. Product # 1 – Printed Circuit Boards The HALT test using the skewed fixture chamber system was designed to duplicate the original test performed with the pneumatic hammer system, except the frequency range for vibration was controlled over the range 5 to 500 Hz with a flat profile. (The comparison unit did not go through thermal cycling with vibration when tested on the pneumatic hammer system, and was not included as part of this test.) The response accelerometer model(s) used and their locations were exactly the same in both tests. The results of the tests are listed in the Table 1. During both tests, the principal failure mode was that the same capacitor broke off. However, the pneumatic hammer system went to 35 Grms and took 70 minutes before this failure was found. Significantly, the electromagnetic shaker ESS system uncovered the problem at 5 Grms, in just 5 minutes. Note that the Grms values reported for the pneumatic hammer system are the mathematical average of the combined threeaxis Grms values, filtered to a low frequency, usually to 2000 Hz, even though most of the energy for a pneumatic hammer table is between 2,000 to 25,000 Hz. Therefore, the Grms values seen by the product on the three axes are actually much higher than the calculated Grms values for the pneumatic hammer shock method. Thus, the calculated 35 Grms value is much lower than the actual G-level input to the product. It should also be noted that the broken capacitor had an accelerometer mounted on it (see Figure 8), but another identical capacitor, mounted immediately beside the broken one, did not break loose. Another test was performed with the skewed fixture chamber system without mounting the accelerometer on the capacitor, and the capacitor did not break off. Clearly, it was mass loading by the accelerometer that caused the failure of the capacitor. In a later test, the new system found a real problem, which was a break in the Ethernet ring; however, after re-plugging the connecter, the unit recovered. Product #2 – Switching Power Supply For the HALT process on the pneumatic hammer shock system, the power supply was only subjected to vibration. So, for a direct comparison using the skewed fixture system, the power supply tested was subjected only to random vibration stress. The data from these two tests are shown in the Table 2. The same weakness was found in both tests (mechanical fatigue failure of the FET transistor); however, the pneumatic hammer ESS system required vibration steps up to 10 Grms and took 244 minutes to precipitate the weakness. The skewed fixture ESS system precipitated the same weakness at 7 Grms, and took only 32 minutes. Product #3 – Network Device This chassis, when tested on the pneumatic hammer system, went through a high/ low temperature step test, a vibration test and a combined vibration & temperature cycling test. Since the important difference between the skewed fixture ESS system and the pneumatic hammer system is in the spectrum and uniformity of the vibration imposed, only the vibration Figure 5: Exploded view of the accordion assembly when it is pulled apart 3 Conformity April 2008

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Conformity Magazine - April 2008 NewsBreaks Editor’s Note Mandatory Analog Cellular Service Ends FCC Releases Quarterly Reports on Consumer Inquiries and Complaints New Standards List Released for The EU’s Low Voltage Directive FDA Issues Notification on Unretrieved Device Fragments FDA Offers Guidance on Waivers for In-Vitro Diagnostic Devices ESD Open Forum Emerging Issues in Standards Challenges in Testing Design Fundamentals Focus on... New Products Product Reviews Premarket Approval Application Guidance Issued by FDA New Jersey Enacts Electronic Waste Recycling Regulations A New U.S. Standard for Workstation and Computer Design Might the CPSC Hold Retailers Accountable for Safe Products? CPSC Loses “Sole Toy Tester” CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - April 2008

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