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FIGURE 12.7
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Typical DMA scan.
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E = loss modulus E = storage modulus
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Figure 12.7 shows an example of a typical DMA scan. The material has a Tg of 193.17 C. 12.5.2.2 Coefficient of Thermal Expansion (CTE). The CTE describes the property of materials to expand as they are being heated. Because of the reinforced composite construction of most laminates, the CTE in x and y direction is usually different from the CTE in z direction. By convention, the x direction corresponds to the warp direction of the reinforcement, whereas the y direction corresponds to the fill direction of the fabric. The z direction is the direction perpendicular to the plane of the laminate. The difference between the values is caused by the reinforcement, which severely restrains the expansion in x and y, whereas the resin can expand unrestrained in the z direction. For copper-clad laminate materials used in printed circuit boards, the CTE in x, y, and z direction is of great importance. The x and y direction is critical because of all the components that will be mounted on the final printed circuit board. The greater the mismatch between the laminate material and the soldered components, the higher the risk that temperature changes will lead to solder fatigue and eventually to a reduction in the reliability of the board. The z direction is of equal importance because the expansion of the laminate during thermal cycles will lead to stresses in the copper plating (CTE of 17 ppm/K) of the plated through or buried holes in the board. A low z-axis CTE and high Tg are therefore generally desirable for increased through hole reliability. A number of different measurement techniques are available to determine the CTE of copper-clad laminates. The most commonly used is TMA following IPC-TM-650, method 2.4.24C, as described in Section 12.2.1. When determining the CTE of a laminate, the temperature scan must start at a temperature sufficiently lower than the specified temperature range
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LAMINATE QUALIFICATION AND TESTING
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for which the CTE is being determined to allow the heat rate to stabilize. The typical scan rate is also 10 C (18 F) per min. and the scan should be continued to at least 250 C (482 F). Because the expansion properties of the material change at Tg, there are usually two CTE values that are being reported: the CTE below Tg is commonly referred to as a1, and the CTE above Tg as a2 (see Eq. 12.4). CTE:a = L/ T ppm/K where L = change in length T = change in temperature K = degrees Kelvin (12.4)
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In the TMA scan example (Fig. 12.5), the material has a z-axis CTE(a1) of 65ppm/K and a z-axis CTE(a2) of 150 ppm/K. The CTE in x and y direction can also be measured by TMA, although care must be taken during the preparation of the sample to avoid any influence of reinforcement material on the TMA probe. The results are strongly dependent on the properties of the reinforcement fabric. Another method to determine the x- and y-axis CTE of laminate materials employs strain gauges. This method is described in IPC-TM-650, method 2.4.41.2. Many details need to be taken into consideration when making use of this method. The strain gauges need to be calibrated for the specific temperature range and the adhesive used to attach them needs to be stable over the whole range. Special care needs to be taken during specimen preparation and gauge attachment. It is recommended to run one heat cycle prior to the actual measurement to remove any residual stresses. More details can be found in IPC-TM-650. 12.5.2.3 Thermal Resistance. The thermal resistance of laminate materials is one of the key properties, especially in light of the lead-free processing conditions. It is the most important indicator of the performance of printed circuit boards during assembly operations. As already mentioned in the Sec. 12.1, most printed circuit boards will be required to withstand at least five reflow cycles with a peak temperature in the range of 260 C, but that number may increase to six and above for more complex assemblies. During exposure to these process temperatures, the laminate cannot delaminate or begin decomposition. Another important factor that can be assessed with thermal resistance tests is the performance of the laminate in printed circuit board applications that expose the board to high-operating temperatures. Several different test methods should be considered when qualifying laminates. 12.5.2.3.1 Solder Dip Resistance. The test method is described in IPC-TM-650, method 2.4.23.The original purpose of this test technique was to assess the solderability of the laminate surface, but today it is often used to assess the ability of laminate materials to withstand the temperatures in a molten solder bath. It is important to note that the original IPC methodology has not been revised in a long time and the current version may not reflect the actual temperature requirements of lead-free processing. The authors still feel that this method is a very good assessment and differentiation technique for laminate materials and recommend that either solder pot temperatures or exposure times are increased if the laminates are intended for use in lead-free assembly processes. Performance factors that are being considered are resistance to softening, loss of surface resin, scorching, delamination, blistering, and measling. The material will be tested in three different surface configurations: (1) a surface upon which no metal cladding was ever applied (if possible), (2) a surface with the metal cladding removed by standard etching processes, and (3) a surface with metal cladding as received. The sample size for all specimen is 1 1/4 in. 1 1/4 in. thickness and three samples are required for each surface configuration. All samples are tested using the same procedure. Preclean the samples by immersing them for 15 sec. in 10 percent hydrochloric acid (HCl) (by volume) and then rinse in water. The HCl should be at a temperature of 60 C (140 F). Dry the specimen quickly to avoid excess oxidation of the sample. Dip the sample into a flux agent and allow draining for 60 sec. before proceeding with the solder dip. Stir and then skim the surface of the molten solder with a clean stainless steel paddle to ensure that the solder
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