Reliability of electronic systems, Text 17

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With the growth in complexity of electronic systems, perhaps the most crucial problem to be faced, and one particularly critical in military applications, is that of reliability.

The probability that any complete electronic system will function as intended is found by multiplying together the probabilities of the individual components and connections making up the system. Indeed, with the present state of development of reliable electronic devices, the soldered connections in a system—which outnumber the components many times over — are probably more of a collective hazard than are the components themselves.

As a simple numerical example, suppose a system is composed of only seven components and connections, each having a probability of survival of 90 per cent. Connected together, these seven probabilities give an average probability of survival for the entire system of less than 50 per cent  (0.9×0.9×0.9×0.9×0.9×0.9×0.9=0.48).

Of course, in practice, the individual probabilities will be higher than 90 per cent. But when the calculation is extended to the thousands of components and tens of thousands of soldered joints composing the electronic systems of modern aircraft or missiles, it is obvious that these individual probabilities must be extremely high if the entire system is to have any real chance of carrying out its intended function. Some idea of the magnitude of the problem can be gained from this comparison: the B-17 or B-29 of World War II used some 2,000 individual electronic components; today, the B-58 requires nearly 100,000 in systems of much greater complexity.

In recent years, therefore, a great deal of attention has been given to this problem of increasing the reliability of electronic components and circuit construction. However, the probability formula shows that a reduction in the number of components and connections raises the over-all probability of system survival much faster than does improvement in the probabilities of the individual parts of the system. For this reason, modern electronics research has as one of its major objectives a reduction in the number of components and connections required to perform a given function. In this respect, molecular electronics is particularly attractive. If a given function is performed within a single, solid block of material, interconnections are eliminated completely.

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