barcode scanner in asp.net c# FABRICATION in Software

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FABRICATION
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Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.
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DRILLING PROCESSES
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Laminating Company of America, Garden Grove, California
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24.1 INTRODUCTION
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The purpose of through-hole drilling printed circuit boards is twofold: (1) to produce an opening through the board that will permit a subsequent process to form an electrical connection between top, bottom, and internal conductor pathways, and (2) to permit through-the-board component mounting with structural integrity and precision of location. The quality of a hole drilled through a printed circuit board is measured by its ability to interface with the following processes: plating, soldering, and forming a highly reliable, nondegrading electrical and mechanical connection. As with any process, the elements of the drilling process are:
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Materials Machines Methods Workers
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When workers are properly trained and educated so that they possess a sound understanding of the other three elements, it is possible to drill holes meeting the aforementioned requirements with high productivity, consistency, and yield. The goals of this chapter are as follows:
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Understanding the drilling process thoroughly Recognizing what might go wrong Locating where problems could occur Detecting whether problems do occur Finding root causes of problems Correcting undesirable conditions Striving for zero discrepancies Making improvement a team effort
Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.
PRINTED CIRCUITS HANDBOOK
MATERIALS
Materials that affect the drilling process are as follows and as shown in the fishbone diagram of Fig. 24.1.
Laminate material Drill bits Drill bit rings Entry material Backup material Tooling pins
Laminate Material A typical circuit board laminate panel consists of three basic components:
Supporting fibers Resin Copper layers
The laminate substrate material is constructed of supporting fibers (most commonly a glass fiber weave) and a resin (most commonly an epoxy compound). Finished board thickness may range from 0.010 to 0.300 in or thicker, with the most common panel thickness ranging around 1 /16 (0.0625 in). 24.2.1.1 Supporting Fibers. Generally, the larger the glass fibers in the weave, the lower the cost of the base material. However, larger fibers are less desirable from a drilling point of view because they are more likely to cause the drill bit to deflect, resulting in decreased hole
FIGURE 24.1 Materials used in the drilling process, with major variables and special considerations identified.
DRILLING PROCESSES
registration accuracy. In addition, larger fibers adversely affect drilled hole wall quality because they create larger (drilling) voids, defined as tear-out of supporting fibers. In simple terms: the larger the glass fibers, the larger the chunks torn out during drilling and thus the greater the hole wall roughness. 24.2.1.2 Resin. The most common resin system is FR-4 epoxy. However, general drilling considerations (see Fig. 24.1) are the same for all resin systems. The glass transition (Tg) rating of a material is defined as the temperature at which fully cured resin starts to soften. The Tg of FR-4 is typically around 130 C or higher. Many materials have resin systems that include additives or that are different from FR-4 resins (e.g., polyimide) in order to raise the Tg. The reason higher Tg is desired is because it implies a more stable material with lowered z-axis expansion rates. Although higher-Tg material may reduce the extent of hole wall resin smear, the compromise of a higher-Tg product is that the material is more brittle and more abrasive to the drill, resulting in increased tool wear and hole wall roughness. This increased abrasiveness to the drill bit may be offset by reducing the surface speed to reduce the spindle speed, resulting in less frictional heat being generated. Lowering drilling heat reduces drill wear. 24.2.1.3 Copper Layers. Outer and inner copper layers may be of various thicknesses, the most common being what is referred to as 1/2-oz and 1-oz copper. One-ounce copper equates to a thickness of approximately 1.4 mils (0.0014 in). The more copper layers within a laminate, the more balanced the panel from a drilling point of view, meaning reduced occurrences or extents of drilled hole defects such as voids (torn-out fiber bundles). However, in order to compensate for a greater number of copper layers, the chip load (advance per revolution) that determines the infeed rate usually needs to be adjusted to control nail-heading. Increasing the chip load reduces the amount of nail-heading. In addition, more copper layers wear the drill bit at a faster rate and may require lowering the maximum hit count per drill bit.
Drill Bits 24.2.2.1 Materials. Drill bits are made of tungsten carbide because its wear resistance (and relatively low cost) make it the most ideal material for cutting the very abrasive circuit board laminate materials. The compromise of this very hard (carbide) material is that it is also brittle and subject to damage in the form of chips if not handled carefully and correctly. 24.2.2.2 Handling and Inspection. When handling drill bits, do not permit the bits to come in contact with one another and be careful not to touch them to the sides of the tool pods. Modern drilling machines utilize drill cassettes to reduce time per load by eliminating manual tool changes.These cassettes may accommodate 120 or more drill bits and reduce drill bit handling damage. If drill bits are measured to verify diameter using a contact-type measuring device (such as a contact micrometer), take the measurement away from the point to prevent chipping of the cutting corners. Following diameter measurement, inspect the drill bit for damage using a microscope. After use, again use care when removing drill bits from the tool pods or cassettes and remember, if they are intended for repointing, to use the same careful handling practices as when they were new. 24.2.2.3 Geometric Attributes. The geometry of a drill bit very much affects the way it behaves during drilling. (See Fig. 24.2 for attribute nomenclatures.) The land is the area remaining after fluting. In order to reduce the amount of land that creates friction with the hole wall (thus generating heat), drill bits are margin relieved. The amount of land remaining in contact with the hole wall during drilling is referred to as the margin. The wider the margin, the greater the friction area and the higher the drilling temperature, resulting in higher extents of heat-related hole quality defects such as resin smear and plowing (defined as furrows in the resin).
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