barcode scanner programming asp.net INTRODUCTION TO BASE MATERIALS in Software

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INTRODUCTION TO BASE MATERIALS
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the hole formed.This happens because an undercured resin system, and the resulting lower than normal Tg, will result in excessive softening of the resin when it is subjected to the heat generated during the drilling process. If this smeared resin is not completely removed, it can prevent electrical connection when the hole is plated with copper. In addition, finished circuits in which the resin system is not fully cured can exhibit greater amounts of Z-axis thermal expansion.This Tg1 Tg2 adversely affects circuit board reliability due to Temperature the increased stress on the plated through holes as the circuit expands. FIGURE 6.2 Measuring the degree of cure by Because increased cross-linking will require measuring Tg values. greater amounts of energy (heat) to weaken the bonds in the molecular structure of the resin system, measurements of Tg can be used to determine the degree of cure of the resin system. For example, a measure of the degree of cure can be obtained by performing two thermal analyses on the same sample. After the first analysis, the sample is subjected to a thermal cycle designed to promote any additional cross-linking available within the resin system, and then a second thermal analysis is performed. The degree of cure is measured by comparing the difference in measured Tg between the two analyses (see Fig. 6.2). If the material is fully cured, the difference between Tg2 and Tg1 will be very small, typically within a few degrees Celsius. Negative differences in these measured values or negative delta Tg values where Tg1 is greater than Tg2 are also indicative of full cure. However, great care must be exercised when using these methods to assess degree of cure. An understanding of the resin system is important because not all resins will behave the same way when performing sequential tests such as this. The degree of cure for some advanced resin systems is not easily assessed by these techniques. In addition, sample preparation methods, particularly sample drying prior to testing, can also influence results. 6.3.1.3 Advantages and Disadvantages of High Tg Values. Implicit in many discussions of Tg is the assumption that higher values of Tg are always better.This is not always the case.While it is certainly true that higher values of Tg will delay the onset of high rates of thermal expansion for a given resin system, total expansion can differ from material to material. A material with a lower Tg could exhibit less total expansion than a material with a higher Tg, due to differing resin CTE values or by incorporating fillers into the resin system that lower the CTE of the composite material. This is illustrated in Fig. 6.3. Material C exhibits a higher Tg than material A, but material C exhibits more total thermal expansion because its CTE value above Tg is much higher. On the other hand, with the C same CTE values above and below the Tg, the higher-Tg material B exhibits less total thermal expansion than material A. Finally, although the Tg values are the same, material B exhibits less total expansion than material A C due to a lower CTE value above Tg. This will be discussed further in Chap. 10 in the B context of the impact of lead-free assembly temperatures on base materials. There are other considerations as well. Most notably, many standard 140 C Tg FR-4 materials exhibit higher decomposition temTemperature peratures, Td s, than the standard 170 C Tg FR-4s.Td is an important property for lead-free FIGURE 6.3 Tg versus thermal expansion.
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assembly, with higher values generally preferred. Advanced FR-4 resin systems can combine both high-Tg and high- Td. This will be discussed in detail in Chap. 10. In addition, higher Tg resin systems can be harder and more brittle than lower Tg systems. This can adversely affect productivity in the printed circuit manufacturing process. In particular, the drilling process can be affected as slower drilling, reduced drill bit life or reduced stack heights can be required for high-Tg materials. Lower copper peel strength values and shorter times to delamination can also be correlated with higher Tg values, though other factors influence these properties as well. The time to delamination is a measure of the time it takes for the resin and copper, or resin and reinforcement, to separate or delaminate, and can be correlated to Td. This will also be discussed in Chaps. 8 and 10, in the context of lead-free assembly. This time-to-delamination test utilizes TMA equipment to bring a sample to a specified temperature and then measures the time it takes for failure to occur. Failure is typically delamination between the resin and copper foil, or resin and glass within the laminate. Temperatures of 260 C (T260) and 288 C (T288) are commonly used for this testing.
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Decomposition Temperature, Td As a material is heated to higher temperatures, a point is reached where the resin system will begin to decompose. The chemical bonds within the resin system will begin to break down and volatile components will be driven off, reducing the mass of the sample. The decomposition temperature, Td, is a property which describes the point at which this process occurs. The traditional definition of Td is the point where 5 percent of the original mass is lost to decomposition. However, 5 percent is a very large number when multilayer PCB reliability is considered, and temperatures where lower levels of decomposition occur are very important to understand, particularly with respect to lead-free assembly. To illustrate this, consider Fig. 6.4. In Fig. 6.4, you can see curves for two FR-4 materials.The traditional FR-4, a 140 C Tg material in this case, has a decomposition temperature of 320 C by the 5 percent weight loss definition. The enhanced FR-4 has a decomposition temperature of 350 C by the 5 percent weight loss definition. Many standard high-Tg FR-4 materials actually have decomposition
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Typical Lead-free Temperature Sn/Pb assembly assembly temperature temperature range range 240 C-270 C 210 C-245 C
320 350 C
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