Not every chip operates fully according to the specification. Very complex ICs contain hundreds of millions to several billions of transistors and can have several hundreds to more than a thousand bond pads. It is therefore a tough job to locate the failure somewhere inside the chip, when, for instance, one output signal fails. The relation between an incorrect signal on one of the output pins and the location of an internal failure is very vague. As discussed in the previous chapter, dedicated advanced testing techniques are already included in the design to support testing. In many cases, however, this is not enough. Logical (design) errors are relatively easy to detect and to locate during scan test and in full functional test. The cause of timing errors, e.g. due to the occurrence of noise, can be so complex that even with the most advanced failure analysis techniques, it can take months before the correct diagnosis can be made.