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FIGURE 6.15 Laminate pressing with continuous copper foil and direct current heating. (Illustration Courtesy of Cedal)
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down, while still part of a continuous roll. The prepregs are laid up in the proper sequence, and then the roll of copper foil is passed over the prepregs so that it is applied to the top of what will become the finished laminate. Typically, two rolls of foil are used to allow dissimilar copper weights or types to be bonded to either side of the laminate. A separator plate, typically anodized aluminum, is then placed on top of this sandwich, and the process is repeated so that several laminates are stacked up. At this point, the stack of laminates is loaded into a press and subjected to heat, pressure, and vacuum. However, as opposed to the conventional process, this technique involves applying direct current to the copper foil that runs throughout the stack. The current heats the foil and therefore the prepregs are adjacent to the foils. By controlling the amount of current, you control the temperature of the materials in the stack.
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Continuous Manufacturing Processes Over the years, continuous lamination processes have also been designed. Rather than cut the prepregs and foils into sheets, lay them up and press them as individual laminates, these processes use rolls of prepreg or bare fiberglass cloth and rolls of copper foil that are continuously unwound and fed together into a continuous horizontal press. One process starts with rolls of prepreg. Another starts with the bare, untreated fiberglass cloth, applies a resin to the cloth and is sandwiched with the copper foil as it is fed continuously into a horizontal press. At the back end of the press, sections of the continuous laminate can be cut into sheets, or with thin laminates, rolls of copper clad laminate can be manufactured.
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1. IPC-4101, Specification for Base Materials for Rigid and Multilayer Printed Boards. 2. IPC-4562, Metal Foil for Printed Wiring Applications.
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BASE MATERIAL COMPONENTS
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Edward J. Kelley
Isola Group, Chandler, Arizona
7.1 INTRODUCTION
Although there are many types of base materials, they all contain three components:
The resin system, including additives The reinforcement(s) The conductor
Each of these components is important in its own right, and in combination they determine the properties of the base material as well as the relative cost of the material. Environmental legislation such as the European Union s Restriction of Hazardous Substances (RoHS) directive has a profound impact on all levels of the electronics supply chain, including these components. RoHS restricts the use of lead, which is an element in the solder used for component assembly onto printed circuits. The impact on the base materials and components is primarily the result of higher assembly temperatures that are associated with lead-free assembly. Table 7.1 summarizes the key issues for base material components. RoHS issues will be discussed further in Chap. 10. The RoHS directive also restricts specific halogen-containing flame retardants. However, most base materials for printed circuits do not contain the restricted flame retardants. Nevertheless, there is growing interest in halogen-free materials, and this will be discussed further in this chapter.
7.2 EPOXY RESIN SYSTEMS
The most successful and widely used resin systems for printed circuit applications are epoxy resin systems. There are many types of epoxy resins, and this class of resin system continues to be the workhorse among printed circuit board materials. This is a result of the combination of good mechanical, electrical, and physical properties and the relatively low cost of epoxies in comparison to the higher-performance resins. In addition, epoxy systems are relatively easy to process, which aids in keeping manufacturing costs down.
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PRINTED CIRCUITS HANDBOOK
TABLE 7.1 Lead-Free Assembly Impact on Base Material Components Component Resin System Lead-Free Assembly Impact 1. Peak assembly temperatures can reach the point where resin decomposition begins. 2. Higher temperatures result in increased thermal expansion and stress on plated holes as a result. 3. Vapor pressure of absorbed moisture much higher at lead-free assembly temperatures; can lead to blistering/delamination. 4. Phenolic lead-free compatible materials often not as good for electrical performance, especially Df. Potential Solutions 1. Formulate resin system with higher decomposition temperatures. 2. Formulate for lower coefficients of thermal expansion. 3. Evaluate materials for moisture absorption/ release characteristics; drying processes in PCB fabrication and/or assembly. 4. Evaluate non-dicy/ non-phenolic laminate materials. Related Considerations 1. Reformulation can adversely affect electrical properties and manufacturability. 2. Can also impact mechanical properties and manufacturability. 3. PCB storage conditions during manufacturing and prior to assembly are much more important, especially humidity conditions. 4. Cost/performance tradeoff. New materials now available that offer more choices. Loss of resin-to-glass adhesion through thermal cycling could impact CAF resistance. Roughness will impact signal attenuation, especially at high frequenciesi.
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