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64 The PCB Magazine • April 2016 original testing of the control material by the supplier and UL. Control materials were select- ed so that at least two temperatures/data points overlapped with the materials under evalua- tion. Multiple test materials were used to ad- dress the range of test temperatures for the ma- terials being tested and included Control 1, 2, and 3. Control materials were included in each oven being used for thermal aging testing. Con- trol material data was compared to existing data from suppliers and UL to prove out the PWB test validity. All samples (controls, baselines, and ther- mally aged test vehicles) were pre-conditioned following ASTM D618: Standard Practice for Conditioning Plastics for Testing, 48 hours at 25°C and 50% relative humidity. Dielectric breakdown voltage was determined following ASTM D149: Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commer - cial Power Frequencies [4] . The test was performed at a frequency of 60Hz and a voltage ramp rate of 500V/sec using a Hipotronics 750-2D149 AC Dielectric Analyzer. The fixture used for hipot testing the test vehicles is shown below in Figure 2. Each test vehicle was tested in the X-Y (PTH to PTH), Z-Core (laminate core), and Z-Fill (pre- preg). Dielectric failure or dielectric breakdown consists of an increase in conductance, limiting the electric field that can be sustained. Randomization of samples was carefully considered through all test phases. This includ- ed sample selection based on sample ID which long-term thermal reliability of pCb materials contained Lot #, Panel #, and Panel Position, A–P (Figure 3). It is theorized that similar col- ored circuits will have reasonably similar elec- trical performance. Oven location was also planned and noted on all oven sample logs. Samples and controls were mixed within a rack (typically no more three in a row of either). Rack number and posi- tion of rack in oven were meticulously tracked. It should also be noted that bromine contain- ing laminates were thermally aged in separate ovens than the non-bromine containing lami- nate systems. This was done to prevent any cross contamination that may arise due to out gassing during the thermal aging tests. A total of twelve Sun Electronic System, Inc. ovens, Model EC16HA-LM were used for thermal ag- ing. All ovens were continuously monitored for temperature stability using an Agilent 34972A LXI Data Acquisition/Switch Unit with an Agi- lent 20 Channel Multiplexer Model 34901A. All ovens had two thermocouples for redundancy. Figure 4 illustrates sample racking and sample location in oven. The test plan followed is presented in the flow diagram in Figure 5. It illustrates the pre- conditioning, weighing, aging, post condition- ing, final weighing, and dielectric breakdown test sequence for controls and test vehicles. Results and Discussion 500-Hour Pre-Screen Testing The 500-hour pre-screen at four fixed tem- peratures was executed following IEEE98 A.1 [3] Figure 2: Hipot test fixture. Figure 3: Circuit location on panel.