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51.2 SPECIFIC QUALITY AND ACCEPTABILITY CRITERIA BY PCB TYPE
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Many unique inspection properties for PCBs are dependent on their type of manufacture. There are two major classifications of printed circuits: rigid PCBs and flexible PCBs.
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ACCEPTABILITY AND QUALITY OF FABRICATED BOARDS
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Rigid PCBs Rigid PCBs make up the bulk of what is built around the world. Rigid PCBs are used in a variety of applications and products. The materials and testing procedures for each type of PCB can be dramatically different. These manufacturing details are thoroughly covered elsewhere in this book and are only touched on here. The processing for multilayer PCBs is a multiple of that required for single- and double-sided PCBs, plus surface preparation, lay up and lamination processes are needed. The additional processes required for each additional layer introduce opportunity for anomalies to creep into the PCB. 51.2.1.1 Single-Sided PCBs. These contain no plated-through holes (PTH), eliminating the need to inspect the attributes associated with PTHs. These PCBs are much simpler to manufacture and PCB shops specializing in these types of PCBs have fewer processes to manage. Many single-sided PCBs are manufactured using a punching process to make part mounting holes and create the outline of the PCB. Single-sided PCBs are typically the least expensive type of PCBs and are usually used in simple and inexpensive applications. 51.2.1.2 Double-Sided PCBs. These differ from single-sided PCBs in that the circuitry is on both sides of the PCB and there are PTHs connecting the top to the bottom. Moving from single-sided to double-sided PCBs requires the manufacturer to keep consistency of the image from top to bottom, and also requires drilling and through-hole-plating processes. These additional processes also require increased inspection, as the attributes they represent can significantly affect acceptability and quality of the PCB. 51.2.1.3 Multilayer PCBs. These are much more complex than double-sided PCBs as they require that circuitry be embedded within the PCB. The PTHs must also make connection to internal layers as well as top and bottom layers. Multilayer PCBs also introduce the lamination process, which is where the manufacturer actually creates the composite PCB from layers of circuitry in a lamination press under high pressure and heat. 51.2.1.4 Backplane PCBs. These multilayer PCBs have connector arrays installed that hold other PCBs. These arrays must be rigid enough to ensure that insertion of PCBs does not affect their performance and reliability. They usually have many layers and tend to be large and thick. Here, again, the manufacturing processes differ from those of traditional multilayer PCBs, and additional requirements for the connector area and PTH areas are necessary. 51.2.1.5 High-Frequency PCBs. These can be any of the preceding types of PCBs and have unique properties that allow high-speed electrical signals to travel within or on the PCB without significant degradation. The materials used and geometry of the circuit are critical to produce the correct impedance for these signals to maintain integrity. Additional testing is typically performed to verify the more precise nature of the requirements for these types of PCBs, such as controlled impedance.
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Flexible PCBs Flexible PCBs must be capable of bending and flexing, either for installation or continuously throughout the product s life, and come in single-sided, double-sided, multilayer, and highspeed versions just like the rigid PCBs previously described.
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Rigid/Flexible PCBs Rigid/flexible combination PCBs combine the benefits of the rigid PCB with integrated flexible parts. This type of PCB requires evaluation of both the rigid and flexible sections for acceptability.
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METHODS FOR VERIFICATION OF ACCEPTABILITY
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Production PCB Most acceptability criteria are evaluated on the production PCB by visual or dimensional means. Visual inspection is typically done under low magnification by human operators. Although the human eye is very capable of catching inconsistencies in the circuit patterns and surface materials, it is not always consistent and can allow nonconforming attributes to slip through. Because of this, many companies have turned to using automated optical inspection (AOI) machines to check pattern integrity, consistency, and dimensions. AOI provides a much more consistent evaluation of circuit geometry than the human eye, but is a slower and more costly option. Many visual inspection attributes have dimensional attributes attached to them. It is important to note that magnification for inspection of dimensional attributes should be sufficient to make an accurate measurement to determine acceptability.
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Test Coupons and Patterns For testing purposes, there are several reasons to utilize test coupons and patterns that are representative of the PCBs rather than the PCBs themselves. The downside of using coupons is that they take up space on the manufacturing panel and therefore increase the overall cost of the PCBs. Also, the test coupons are not actually part of the PCB and are typically placed at the edges of the production panel, where production attributes may be different from the production panel. This can cause the coupons and patterns to reflect different attributes than the PCBs they are attempting to represent. It is very important to take steps to ensure that the test coupons and patterns are actually manufactured in a manner that is representative of all the attributes of the PCBs associated with them. It is important to ensure that any holes in the coupon are drilled with the same tools and parameters as those on the corresponding PCB. Coupons and patterns are typically used in situations where using a PCB for testing would either not provide the information required or the PCB would have to be destroyed in order to get the information. Coupons and patterns can be used for microsectional analysis, electrical measurements, environmental simulation, and reliability evaluation. See Fig. 51.11 for an example of test coupons taken from IPC-2221. (Unless noted otherwise, figures in this chapter are courtesy of the Association of Connecting Electronics Industries [IPC], and are taken from the publication IPC-600, Acceptability of Printed Circuits. These figures can be found in that document under the headings noted in this chapter for specific topics. Figures credited to the author of this chapter are taken from presentations made by him. Other figures are from the sources noted in the captions. For further details on design and placement of coupons, see IPC-2221.)
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Microsection Microsectioning the holes from a PCB provides a cross-sectional view of the construction of the PCB. Microsectioning a PCB with a design that removes nonfunctional pads makes it virtually impossible to inspect all the interconnecting layers in the hole, so microsections are typically done on associated and optimized coupons containing PTHs that contain pads on all the layers. Microsectioning PCBs requires great skill, as the PCB contains both hard and soft materials and the area of interest is very small. Improper microsection technique can either create or hide anomalies. For example, interconnection separation can be created or hidden by improper techniques.
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