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March 2014 • The PCB Magazine 45 CAF Preconditioning and Test Parameters Test Methods 1. Testing was carried out in accordance with IPC-TM-650 Method 2.6.25, Conductive Anodic Filament (CAF) Test. 2. 100 volts DC bias was applied to each cir- cuit through a 1 mega-ohm series resistor. Volt- age drop across the resistor was measured daily in each sample with a high-impedance voltme- ter, without removing or altering the applied bias. 3. The test method allows for two humidity levels. The more aggressive environmental con- dition option of 85°C (+/-2°C) and 87% (+3/- 2%) relative humidity was chosen for this test. In all, 30 coupons were tested. No coupons were tested for material 'A' and 'D'. For materi- als 'B', 'C', 'E', 'RA' and 'RB' three coupons each for grid size 40 mm (16 mil) and 50 mm (20) were tested in accordance with IPC-TM-650, method 2.3.25. Below is a summary of the re- sults for the 30 coupons tested. Figures 12 and 13 show the typical CAF fail- ures seen on failed coupons. This CAF data shows that the CAF failures increase as the pitch reduces. Coupons at 16 mil pitch had 16% more failures than those 20 mil pitch. The failure rate is influenced by both the material and the drilling process. Since this ex- ercise was to compare different materials under the same manufacturing conditions, we evalu- BEHAVIOR OF MATERIALS IN THE MANUFACTURING ENVIRONMENT continues table 5. Figure 11, 12 and 13: Failure due to surface contamination.