Software Design, Engineering, and Testing

PCB design software Summary

2008-03-12T09:02:00.000-07:00

Normally an electrical engineer creates the circuit, then a technician makes the PCB design. PCB designing is a specialized skill. There are lots of tricks and standards, and, of course, some high-quality software tools (DipTrace, Eagle, etc.) in designing a PCB that are not easy to manufacture yet small and quite inexpensive. (see PCB layout guidelines).

Some printed circuit boards for high frequency RF use plastic with special characteristics to avoid detuning the radio. PC boards in vacuum or spacecraft frequently have solid copper or aluminum cores in order to carry away heat from components.

The conductors' width and spacing on a PCB is also very important. If conductors are placed too close, solder can shorten adjacent connectors, and the resulted PCB will be difficult to mend. If they are placed too far apart, the board may appear too large and thus expensive.

Removing big areas of copper increases pollution. Also, PCBs etch more consistently if all its regions have the same or at least similar average ratio of copper to plastic. Therefore, PCB designs can make connectors wider, leave unconnected copper , or cover large areas of plastic with arrays of electrically isolated copper squares or diamonds.

Most PCBs contain one to sixteen conductive layers laminated together. In more complicated PCBs, two or more layers are providing ground and power. Those ground planes and power planes are dedicated to detune accidental antennas, and, therefore, provide efficient power distribution. Multi-layer printed circuit boards enable creation of complex digital circuits.

Ground and power planes are rectangular conductor sheets occupying entire layers (except small holes to avoid unneeded connection to vias and throughhole components). They distribute electrical power and heat much better than narrow traces. Specialized conduction-cooled designs are used to rely on the PCB to conduct away the waste heat, unlike the air-cooling method which is still used more commonly.

Multilayer printed circuit boards have alignment marks and holes (also called fiducials) that are used to align layers and to permit the PCBs to be mounted in the equipment automatically placing and solding components. Some PCB designs place alignment and etch testing patterns on break-off tabs which can be removed before the installation.

Layers can be connected through drilled holes that are called vias. These holes are electroplated or sometimes small rivets are inserted. Some high-density PCBs have blind vias, that are visible on one surface only, or so-called buried vias, visible on neither side, but those vias are expensive to build and also difficult or completely impossible to inspect after the manufacture.

Expert designers minimize the quantity of vias in order to reduce the drilling cost. On older, two-layer PC boards, soldering a wire through the hole was a common method.

Holes are drilled either with small carbide drill-bits or by lasers. The drilling is now performed by drilling machines featuring computerized placement using "drill tapes" or "drill files." A drill file is a file describing the location and size of every drilled hole. Their another name is numerically controlled drill (NCD) files. You can also see them as Excellon files.

Component leads are either inserted in holes or simply mounted on the surface pads, then electrically and mechanically fixed to the PCB with a molten metal solder.

A solder mask is a layer that resists liquids by solder (that solder is claimed to "bead up"), and keeps solder islands from running together. It protects the outside conductor layers from corrosion and abrasion. (Without the mentioned solder mask, the fiberglass-reinforced epoxy becomes a translucent off-white. Most solder masks are green, but they are also available in other colors).

A silkscreen legend located on the top or bottom surface of the printed circuit board provides information about component part numbers and placement that helps in manufacturing and repair. New technology allows the component designators to be printed right onto the board surface, which saves time and money by putting away expensive silkscreens. This is normally done by a huge inkjet printer. Similar processes can be used for solder masks, but they should be considered developmentally.

Software testing

2008-03-12T09:00:00.001-07:00

Software testing is the process used to help identify the correctness, completeness, security and quality of developed computer software.Testing is a process of executing a program or application in the intent of finding errors. With that in mind, testing can never completely establish the correctness of arbitrary computer software. In other words, testing is criticism or comparison, that is comparing the actual value with an expected one.

There are many approaches to software testing, but effective testing of complex products is essentially a process of investigation, not merely a matter of creating and following rote procedure. One definition of testing is "the process of questioning a product in order to evaluate it", where the "questions" are things the tester tries to do with the product, and the product answers with its behavior in reaction to the probing of the tester. Although most of the intellectual processes of testing are nearly identical to that of review or inspection, the word testing is connoted to mean the dynamic analysis of the product—putting the product through its paces. The quality of the application can, and normally does, vary widely from system to system but some of the common quality attributes include reliability, stability, portability, maintainability and usability.A good test is the one which finds an as yet undiscovered error. Refer to the ISO standard ISO 9126 for a more complete list of attributes and criteria.

Software engineering

2008-03-12T08:57:00.000-07:00

Software engineering (SE) is the profession of people who create and maintain software systems by applying technologies and practices from computer science, project management, engineering, application domains and other fields.

Software engineering deals with matters of cost and reliability, like traditional engineering disciplines. Some software applications contain millions of lines of code that are expected to perform properly in the face of changing conditions, making them comparable in complexity to the most complex modern machines. For example, a modern airliner has several million physical parts[1] (and the space shuttle about ten million parts[2]), while the software for such an airliner can run to 4 million lines of code[3].

As of 2004, the U. S. Bureau of Labor Statistics counts 760,840 software engineers holding jobs in the U.S.; for comparison, in the U.S. there are some 1.4 million practitioners employed in all other engineering disciplines combined[4]. There are estimated to be about 1.5 million practitioners in the E.U., Asia, and elsewhere[citation needed]. SE pioneers include Barry Boehm, Fred Brooks, C. A. R. Hoare, and David Parnas. There is extensive debate about what SE is, who qualifies as a SE, who sets the standards, etc.

Software Design, Engineering, and Testing

2008-03-12T08:54:00.000-07:00

software Design, Engineering, and Testing

The term "software engineering" was first used as the title of a convention held in Garmisch, Germany, in 1968 that was convened to address an emerging "software crisis." NATO called the conference because of its concern with the state of the software industry, which had begun to have great difficulty providing the large, complex software systems that governments and businesses required, on time and within budget. Some advocated an "engineering" approach to the software crisis, bringing the discipline of time-proven engineering methods to the relatively new field of software in order to put the new field on a more secure footing.

In the early 1970s software engineering came to be recognized as a field of study and practice that was distinct from hardware engineering. However, early software development models were based on hardware development models. In 1976 Barry Boehm published a paper titled "Software Engineering" in which he defined the term as "The practical application of scientific knowledge in the design and construction of computer programs and the associated documentation required to develop, operate, and maintain them." A later author, Bruce Blum, built on Boehm's definition to define software engineering as "the application of tools, methods, and disciplines to produce and maintain an automated solution to a real-world problem."

In his 1976 paper Boehm described a software life-cycle model that came to be known as the waterfall development model. The waterfall model is made up of a series of stages that are completed one after another, in order. Similar to the stages of hardware development, the stages of the waterfall model are:

* Requirements analysis and definition--Developers and users of the proposed system determine its purpose and functions
* System and software design--Developers determine how the system and software will actually perform the required functions and meet its purpose.
* Implementation and unit testing--Developers build and test the units of the system.
* Integration and system testing--Developers integrate the system's units and test the entire system.
* Operation and maintenance--Developers and/or users put the system into operation and maintain it.