In-circuit IC tester?

There are too many *different* ICs of different types in a GIVEN package for this to be possible. Take something "simple" like the SOT23-3 package; 1) dual cathode diodes; 2) dual anode diodes; 3) Jfets(P and N channel); 4) MOSfets(P and N channel); bipolar transistors (NPN, PNP; single, darlington); 5) single diodes (may be various lead configurations). Now that may not be an exhaustive list, but it gives you an idea as to variety. The SOT23-5 allows even more variety; 8-pin DIP/SO-8 etc is worse yet.

And "in-circuit"?? Forgeddit. Too much other things can be connected to the DUT that could be DUDs to mess with analysis of any type; even if all is OK, other devices can cause abnormal readings (where "abnormal" means same detection scheme on an isolated known good part).

Think of the very simple case of a single logic gate, say a two-input OR gate, in circuit. How would your hypothetical tester test it? There are two inputs and one output; there's nothing else that can be measured, so you have to base your decision on that. Okay, so suppose you measure zero at both inputs and at the output. That's working, right? Well, it's correct, but maybe the chip is completely dead; you'd get the same results.

So, you say, fine, I'll inject a signal into one of the inputs. Ah, but can you do that? Suppose that the reason the inputs are zero is that something else connected to the inputs is shorted to ground. So, you can't inject enough signal to get the logic level to change. No way to test. Or, suppose that the inputs are high, but the output is zero. It could be that the gate is defective; or, it could be a short-circuit somewhere else, pulling the gate down.

And so on. The bottom line is that there are very, very many circuits where it is simply not possible to test a device in circuit, because the fact that it is in circuit interferes.

Okay, that was assuming that you could get to all the pins. But modern chips often have the pins on the bottom - Ball Grid Array. Once it's soldered to the board, there's no way to access the pins, even if you could do something useful with them.

And speaking of chips with lots of pins: they tend to be memories, microcontrollers, and the like. That is, they have software and/or data inside them. Suppose that a particular chip is a microcontroller, and one of its functions is that a particular output is supposed to either be a buffered or an inverted copy of one of its inputs, depending on a user-configurable value in flash memory. How would you possibly test that? It's not enough to know the datasheet for the chip: to know what its behavior is supposed to be, you have to know the software that it is currently running, as well as its entire history of input data.

So, that's why general-purpose IC testers in general, and in-circuit ones in particular, are so hard to find. They are only useful on an extremely limited set of cases.