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R = Measured resistance in meg-ohms A = Effective area in square cm T = Average thickness of specimen in cm Surface resistivity s (meg-ohms) s = (R*P)/D (12.6)
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R = Measured resistance in meg-ohms P = Effective perimeter of electrode in cm D = Width of test gap in cm
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12.5.3.3 Dielectric Strength Breakdown. The dielectric strength of a laminate material is its ability to resist electrical breakdown. The dielectric strength defines a specific voltage that the laminate resists for a specified time whereas the dielectric breakdown voltage defines the maximum voltage at which the laminate fails. These properties can be measured perpendicular to the reinforcement (z-axis) or parallel to the reinforcement (x-y axis). The more important value is the z-axis strength because more and more thin prepreg and laminate cores are being used in high-end multilayer applications. The minimum thickness in the laminate is defined as the shortest distance between the copper treatment peaks that needs to resist the desired test voltage.The values for dielectric strength vary with test setup, temperature, humidity, frequency, and wave shape, but are, if tested under controlled conditions, comparable between materials. The test method following IPC-TM-650, method 2.5.6.2, describes the determination of the perpendicular electric strength of laminates. Four specimens should be tested. The sample size is recommended to be 4 in. 4 in. with any copper cladding removed. Unless otherwise specified, the samples need to be conditioned for 48 hours in distilled water at 50 C. After that, the samples are immersed in ambient temperature distilled water for a minimum of 30 min. to allow the samples to achieve temperature equilibrium without a substantial change in moisture content. The test is performed at ambient temperature (23 C). Relative humidity is not significant as the tests are performed under oil. Samples are inserted into the high-voltage test equipment and tested to failure at a 500volts/sec. increase. The results are reported in volts/mil. The test method (see Fig. 12.10) following IPC-TM-650, method 2.5.6, describes the determination of the parallel electric strength of laminates. Four specimens should be tested: two in machine direction, and two in the transverse direction for reinforced materials. The samples size is recommended to be 3 in. long by 2 in. wide with any copper cladding removed. Two holes 0.188 in. in diameter are to be drilled along the centerline of the 3 in. dimension and midway between the edges in the 2 in. dimension, with a spacing of 1 in. center to center. Conditioning and test setup in oil is identical. Electrodes are inserted in the holes and the samples are tested to failure at a FIGURE 12.10 Dielectric 500volts/sec. increase. The results are reported in kilovolts (KV). strength breakdown 12.5.4 Other Laminate Properties Other laminate properties to consider include flammability and water absorption. 12.5.4.1 Flammability. The flammability of laminate materials is classified according to Underwriters Laboratories (UL) specifications. All of the tests are performed using a standard test setup under an exhaust hood using a Bunsen burner as a source for the flame. The categories are as follows:
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UL-94-V-0 Specimens must extinguish within 10 sec. after each flame application and within a total of less than 50 sec. after a total of 10 flame applications. No samples are to drip
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LAMINATE QUALIFICATION AND TESTING
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flaming particles or exhibit glowing combustion that lasts for more than 30 sec. after the second flame test. UL-94-V-1 Specimens must extinguish within 30 sec. after each flame application and within a total of less than 250 sec. after a total of 10 flame applications. No samples are to drip flaming particles or exhibit glowing combustion that lasts for more than 60 sec. after the second flame test. UL-94-V-2 Specimens must extinguish within 30 sec. after each flame application and within a total of less than 250 sec. after a total of 10 flame applications. Samples may drip flaming particles or burn briefly, but no specimen may exhibit glowing combustion that lasts for more than 60 sec. after the second flame test.
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In most cases, the laminator will provide results for these tests routinely. In the case of new materials for which the laminator has not supplied any flammability data, it may be a good idea to check this property. This does not require an elaborate test setup, and you can get a first indication by igniting a sample specimen under an exhaust hood using a lighter. 12.5.4.2 Water Absorption. Depending on their specific molecular composition, every laminate material will absorb a certain amount of water. This will happen not only during the many wet processing steps in printed circuit board manufacturing but also as a result of exposure to normal environmental conditions. The absorbed moisture may change the properties of the laminate and increase the risk of blistering and delaminations during high-temperature processes such as reflow soldering. The test method, according to IPC-TM-650, method 2.6.2.1, determines the amount of water that is absorbed by a laminate material sample when immersed in water for 24 hours. The test is easy to perform and the results for different laminates are readily comparable. The test samples for this test need to be 2 in. long by 2 in. wide. The thickness is not specified but should not vary widely when more than one material is characterized. The edges of the samples need to be sanded smooth and copper cladding is to be removed using standard etching processes. The samples are preconditioned in a drying oven for 1 hour at 105 C (221 F), cooled down to room temperature in a desiccator, and weighed immediately after removal. Then the samples are placed in distilled water at 23 C. It is important to place the samples on their edge to maximize the laminate area exposed to water. After 24 hours, the samples are removed, dried with a dry cloth, and immediately weighed. The moisture absorption is reported in percent weight increase. 12.5.5 Additional Tests In addition to the previously described test methods, many more can be found in IPC-4101 and in the test methods manual IPC-TM-650. All of these tests address laminate properties that may have a significant impact on the performance of the final product. The final decision as to which qualification tests to include or exclude always needs to be made on a case-by-case basis depending on the performance requirements for the printed circuit board. At the same time, there are a number of laminate qualification tests that rarely will be performed in the printed circuit manufacturing facility. In many cases, the laminate supplier will perform a number of qualification tests itself and share the results with its customers. This may in many cases be sufficient, especially after a close relationship between the supplier and the PCB manufacturer has been established. 12.5.6 Prepreg Testing During the qualification of a new laminate material, it may also become necessary to perform prepreg-specific tests to verify its quality. The most commonly tested properties of prepregs are resin content, flow during mutilayer processing, and gel time. Details about additional properties and all corresponding test methods can be found in IPC-4101.
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