How critical is it to declare the manufacturer part number for a component within the BOM?
It obviously depends from the component and what it is used for in a circuit. Take for example a simple 1k resistor in a certain package.
A manufacturing facility would have component engineers that deal with alternative parts, and since they cannot know what the resistor does in a circuit, they also have no clue what kind of resistor they should select. So if you don't specify at least one specific resistor part or multiple different specific parts, they don't take the risk and put just something there, they will ask you for instructions suggest alternatives that you must choose from.
Sometimes, it could be that any 1k resistor with 10% tolerance for a LED is fine, but if it needs to be a precision 1% resistor for some sensitive thing, it makes a difference. Also for high speed AC circuits the resistor material makes a difference, a wirewound resistor won't work while a film resistor would. For low noise a carbon resistor may be out of the question. For high pulse current applications a standard resistor is not acceptable. There are just so many different kinds of 1k resistors, even if you did state the tolerance, package, wattage, and many other things.
And no, the part numbering does not mean that chips with identical part from different manufacturers behave identically under all circumstances. There might be a reason why you chose a specific manufacturer. Or if there is not, then indicate it that it should not matter. But then you should also be sure that any manufacturer is fine, perhaps by looking at datasheets and/or testing the component in the circuit before accepting the use in design.
Sometimes even a same chip but from different batch or with different silicon revision fails to work as intended in a circuit.
Sometimes it matters, sometimes it doesn't. If it matters, and your customer finds out before you do through failing product, then it matters a lot.
Most parts are generic, some parts are not.
The most egregious instance of different performance semiconductors sharing the same marking I've come across recently is the TLV431. The TI part only goes up to 6 V, whereas the OnSemi and the DiodesInc parts go to 16 V.
The other biggy is high value, high K ceramic capacitors, with their horrendous voltage coefficient of capacitance. There's no point researching and standardising on a particular manufacturer and package size of capacitor, if your buying department then buy the cheapest rubbish in the same nominal size.
What my company did was to specify a list of acceptable manufacturers for each part. Let's say it took less testing to get a new resistor onto that list than a new semiconductor.
Sometimes it matters greatly, sometimes it matters not at all.
I've been on the receiving end of "it matters greatly" a couple of times.
- I was a technician in a Motorola factory that built the CQM6000. The command board (microprocessor and audio signal processing) could be placed in a test mode by attaching a pull-up to a specific accessory connector pin at power on. The text jigs all used a pull-up to the 12V power supply. This worked fine for thousands of radios - until one day it didn't. That digital signal went to the input of a 74XX something or other latch operating at 5V. The sudden change from "test mode works" to "WTF?" was that somebody, somewhere, bought that latch from a different supplier. The new ones didn't like that 12V pull-up on a 5V input. The old ones shouldn't have worked, either, but they were more tolerant. I was the lucky fellow who got to track down the cause of the test-mode failures. I knew how the test-mode worked because I had a test rig that made use of it. All the radios with test mode failures worked on my rig, but not in other ones. It turned out that mine used a pull up to 5V like it should have where other test rigs used a pull up to 12V. The pull up to 12V was used because the input pin and the 12V pin were right next to each other in the accessory connector. The test rig plug had a 1206 resistor soldered to the pins to activate the test mode. Once I figured out what was going on, all the test rigs were modified to use a proper pull up to 5V. Problem solved -but production was slowed for several hours while we looked for the cause and got it fixed.
- A small company I worked for had a custom controller board made for a repeater built of a couple of MC Micro (M110) mobile radios. The existing controller for the repeater used 9V digital logic, and the stuff we needed to connect to it used 5V digital logic. The board that the company had made by an independent contractor used some 4000 series CMOS ICs in the interface between the two systems to bridge the 9V/5V gap. The prototype worked, so we had the whole series manufactured. The finished boards didn't work. The ICs in the protoype and the ICs in the final product were from different manufacturers. The ones in the prototype had different voltage levels for high/low than the ones in the final product. We had to buy a bunch of ICs (from the correct manufacturer) and replace them in all of the finished boards because we didn't have time to have the controller redesigned.
Another case for "it matters greatly" is when selecting coupling capacitors for radio frequency circuits. A 100pF capacitor isn't just a 100pF capacitor - if the frequency is high enough. At high frequencies, you have to look at the inductance and self resonance of the capacitor. Two optically identical capacitors with the same value can be totally different at high frequencies.
A case for "it doesn't matter" are the typical pull-up resistors in digital circuits. Pretty much any 10k (or whatever) resistor will work. You don't need high precision or low inductance or any thing else special. Just (approximately) the correct resistance and the circuit is "happy."
You need to have some idea of what's critical and what's not.
Take the examples above.
The problem in the factory wasn't something that should have been anticipated. The service manuals all said "pull up to 5V for service mode." The guy who wired the test rigs saw the 12V handy (and 5V more difficult to arrange) and wired it the easy way. It worked, so nobody complained.
The repeater problem should have been designed to be more robust, though, so that it wouldn't matter which IC was used. It depended on the "typical" voltage levels for high and low rather than being designed with the maximum variations in mind (which the datasheets also give.) 5V was right at the edge of the low end of "high" for the ICs used, so the design should have considered it to be a critical point and either specified a specific IC - or better, taken a different route for converting the logic levels.
To get back to your question, take the example of the 555. If you are using it in a typical circuit, then it probably doesn't matter whether it's an LM555 from TI, a UA555 from Fairchild, or whatever. If you are going for very long timer periods or low power operation then you might need to specify a TLC555 (a CMOS variant of the 555 designed for low power consumption.)