barcode reading using c#.net Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use. in Software

Creator QR in Software Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.

Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.
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PRINTED CIRCUITS HANDBOOK
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METHODS OF INCREASING CIRCUIT DENSITY
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There are basically three ways to increase printed circuit density:
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Decrease conductor line widths and the spacings between them Increase the number of circuit layers in the PCB Reduce via and pad sizes.
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Decreasing conductor line widths requires very-low-profile copper foils for high yields in circuit etching processes. However, other things being equal, lower profiles result in decreased foil adhesion to the dielectric. Balancing the copper surface profile for both adhesion to the dielectric and the ability to etch fine circuit features, not to mention the impact of surface roughness on electrical performance at high frequencies, is an important consideration. Copper foil manufacturers continue to research methods to improve the chemical adhesion between the foil and the various dielectric materials in use, relying less on a rough surface profile for mechanical adhesion, and allowing for very low profiles for circuit etching and lower conductor losses at high frequencies. Increasing circuit layer counts have resulted in both greater overall multilayer thicknesses and thinner individual dielectrics, making thickness control and thermal reliability more important than ever. Adding layers to a PCB also demands improved registration capabilities. One of the critical variables in controlling registration is the dimensional stability of the laminate material, which can become more challenging with the thinner laminates generally used as layer counts increase. Reducing via and pad sizes also requires improved laminate dimensional stability for registration of high layer count circuits.
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One obvious method to increase printed circuit functionality is to put more circuitry per unit area of the circuit. Printed circuit densification has driven several improvements in copper foil technology. One of the first developments was high temperature elongation (HTE) foils. Other advances include low- and very-low-profile foils, thin foils, and foils for high-performance resin systems.
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HTE Foil HTE or Class 3 copper foil exhibits improved elongation properties at elevated temperatures as compared to standard electrodeposited or Class 1 copper foil. Typical elongation values for HTE copper foil range from 4 10 percent at 180 C. The growth in multilayer printed circuits has resulted in HTE becoming the most commonly used foil, since its excellent ductility at elevated temperatures helps prevent inner-layer copper foil cracking. As a printed circuit experiences a thermal cycle, the base materials will expand. The z-axis expansion applies stress to the connection of the inner-layer foil and the plated hole. With HTE foil, the reliability of this connection is improved. This property is particularly important in thicker circuits and high resin content constructions where increased zaxis expansion occurs.
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Low-Profile and Reverse-Treated Copper Foils Three classifications describe the profile of the copper foil surface as shown in Table 9.1. Copper foil profile is important for etching of fine-line circuits. Figures 7.23 through 7.26 illustrate the difference between standard- and low-profile foils.As can be seen in those photos,
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BASE MATERIALS PERFORMANCE ISSUES
TABLE 9.1 Copper Foil Profiles Foil Profile Type S Standard L Low Profile V Very Low Profile X No Treatment or Roughness Max. Foil Profile (Microns) N/A 10.2 5.1 N/A Max. Foil Profile (m Inches) N/A 400 200 N/A
the tooth profile of the standard-profile foil is much more pronounced. The etching of the lower-profile foil results in more control of the geometry of the circuit trace. In addition, in very thin laminates, the large tooth structure of the standard profile foil can result in inconsistent dielectric thickness, making impedance control more difficult, and can even result in electrical failures if the tooth structures from the opposing sides of the laminate protrude sufficiently. Reverse-treated foils (RTF) take this concept a step further. When copper foil is manufactured, there is a very smooth, shiny side and a rougher matte side. Conventional technology involved treating the matte side and laminating this side to the base material. Reverse-treated foil, as its name implies, involves putting the treatments on the smooth, shiny side of the foil and laminating this side to the base material. This has two important effects. First, the side bonded to the base material has an extremely low profile that aids in etching very fine circuit traces. Second, the rougher matte side, which is now on the surface of the laminate, can improve photoresist adhesion. This enables the removal of surface roughening processes during PCB manufacturing and can also improve inner-layer imaging and etching yields. Figure 9.1 compares laminates made with conventional and RTF foils.
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