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The resin systems discussed above will typically contain a variety of additives that either promote curing of the resin system or modify the properties in some way. Some important types of additives include the following.
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Curing Agents and Accelerators Each resin system contains organic components that must be reacted together to promote polymerization and cross-linking. Curing agents and accelerators are used to promote these reactions. Amine-based curing agents are commonly used to cure epoxy resins. Some of these, such as aliphatic diamines, are used to cure epoxies at room temperature. Others, such as aromatic diamines, require elevated temperatures. Figure 7.9 shows an example of curing epoxy with an aromatic amine. Note the OH group formed on the new molecule.As shown in Fig. 7.4, the OH can also cross-link with other epoxide groups. Historically, the most common curing agent used in epoxy resin systems for printed circuit base materials has been dicyandiamide, or dicy. Figure 7.10 illustrates the polymerization of
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O O CH2 HC CH2 + H2N
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FIGURE 7.9 Curing epoxy with an aromatic amine.
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OH O CH2 CH CH2 NH
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CH3 CH CH2O C CH3 OCH2 CH CH2 O OH
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Br O CH2 CH CH2 O Br OH N
CH3 C CH3
O O CH2 CH CH2
M Br
+ H2NCNHC N NH Dicyandiamide or l-Cyanoguanidine
Catalyst Dicy Difunctional Epoxy
FIGURE 7.10 Curing epoxy with dicyandiamide (dicy).
epoxy and dicy. However non-dicy systems have been developed to promote faster curing, reduce moisture sensitivity, and improve thermal stability. The choice of curing agent and accelerator is also driven by the resin type. Non-dicy-cured epoxy materials will be discussed further in Chap. 6, as they are often used in lead-free assembly applications. Figure 7.11 illustrates
Dicyandiamide Cure NH2 C NH C N Polymer Segment H2C R1 HC CH2 + R2 O CH2 HC O Curing Agent R1 CH CH2 O CH O Resin Polymer Segment R2 Epoxy Homopolymerization R1 OH CH CH2 NH C NH2 N C N
CH2 + HN O
Resin
Curing Agent
Hydroxyl Cure HO R1 HC CH2 + R2 O OH CH O CH2 R2
Resin
FIGURE 7.11
Curing Agent
Epoxy curing mechanisms.
R1 Polymer Segment
BASE MATERIAL COMPONENTS
O O CH2 HC CH2 + HO
FIGURE 7.12 Phenol curing mechanism.
OH O CH2 CH CH2 O
some of the curing reactions that occur as the resin system polymerizes, and Fig. 7.12 illustrates a phenol curing mechanism. 7.4.2 Flame Retardants While once the subject of little attention, flame retardants used in the resin system have become a more significant consideration. This is due primarily to legislative initiatives and the increasing focus on the toxicity and environmental impact of some of these compounds. While scientific evidence shows that some of these compounds truly pose a risk, the scientific community also generally agrees that other flame retardants are generally safe. Unfortunately, politics often clouds the decision-making process, and sometimes marketing efforts promoting environmental friendly or green products are based more on perception than scientific reality. Some have even argued that specific alternative flame retardant chemistries could even be worse, on balance, than those currently used. Nevertheless, the subject of flame retardants has become, and is expected to remain, an area of continued research. 7.4.2.1 Legislative Issues. The European Union s RoHS and WEEE directives (the WEEE directive addresses waste electronic equipment and recycling requirements) affect not only the lead used in printed circuits, but the flame retardants used in the resin system as well. The RoHS directive restricts the use of specific types of brominated flame retardants. The restricted class of compounds consists of polybrominated biphenyls (PBBs) and polybrominated biphenyl oxides (PBBOs), also called polybrominated diphenyl ethers (PBDEs). The generic structure of these compounds is shown in Fig. 7.13. Specific compounds within these classes of flame retardants can vary in their toxicity, and given the dynamic nature of legislative initiatives, it is important to check the current status of these compounds when making decisions on what materials to use. In the case of standard epoxy materials used in printed circuits, flame retardancy has commonly been achieved by brominating the epoxy resin. This normally involves manufacturing
(Br)x
(Br)x
Polybrominated Biphenyl Oxide (PBBO or PBDE)
(Br)x
(Br)x
Polybrominated Biphenyl (PBB)
FIGURE 7.13 Restricted brominated flame retardants.
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the epoxy resins with tetrabromobisphenol A (TBBPA), which contains bromine within the chemical backbone (see Fig. 7.2). The RoHS directive does not restrict TBBPA. Because TBBPA is reacted into the epoxy resin itself, it is not available for release into the environment. Under excessive exposure to heat, the bromine is released and retards burning. TBBPA has been used successfully for many years as a flame retardant and is still used in the overwhelming majority of materials. However, while the RoHS directive applies only to specific brominated flame retardants, the WEEE directive may require separation and special handling of materials containing any brominated flame retardant. The separation and special handling relate to concerns about the by-products of incineration, especially if incineration is done at too low a temperature. Furthermore, individual countries continue to look at introducing their own legislative initiatives in regard to these flame retardants, and thus is always important to check the status of these efforts. 7.4.2.2 Flame-Retardant Chemistry. Polymer combustion occurs in a continuous cycle, as highlighted in Fig. 7.14. Heat generated in the flame is transferred back to the polymer surface, producing volatile polymer fragments that constitute fuel for further burning. These fragments diffuse into the flame zone, where they react with oxygen by free-radical chain reactions. This in turn produces more heat and continues the cycle. Flame retardancy is achieved by interrupting this cycle. There are two basic ways to interrupt this cycle. One method is called solid phase inhibition and involves changes in the polymer substrate. Systems that promote extensive polymer crosslinking at the surface form a carbonaceous char upon heating.The char then insulates the underlying polymer from the heat of the flame, preventing production of new fuel and further burning. Other systems evolve water during heating, cooling the surface and increasing the amount of energy needed to maintain the flame. The second method is called vapor phase inhibition and involves changes in the flame chemistry. Reactive species are built into the polymer that are transformed into volatile freeradical inhibitors during burning. These materials diffuse into the flame and inhibit the branching radical reactions. As a result, increased energy is required to maintain the flame and the cycle is interrupted. For many materials, both solid and vapor phase inhibition occur. Polymers, including the various types of epoxy resins, differ in their inherent flammability. The types of resins and curing agents selected can impact the fundamental flammability of the resin system and determine how much additional flame-retardant components are needed to achieve the desired flammability rating. For example, polymers with a high concentration of
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