Whereever possible, I refer to a real standard. Like, for example BS EN 140401-801..804 for regular SMT resistors. There are more of them than one might suspect. And you can weaken it up again by wordings like 'or equivalent' which leaves much open, but still sets a framework regarding the set of parameters which can be incomplete, but not entirely omitted.
Of course I am part of an industry, where most component entries are followed by 'established reliability' etc. and where the use of 'or equivalent' happens very responsibly for the most part, such as allowing the contractor to use the series of components which are designed for, but not qualified according to the relevant MIl- or CECC or younameit-standard. If you look closely at the major and specialist manufacturers offers, there is such a series along every fully qualified one.
Also, brevity should not be overdone. For example, there is no reason to omit wattage of a resistor just because you specified SMT size. Technology, value, tolerance, power, TC and max. voltage should be in a minimum spec. Also, a footnote in the BOM should reference the sizes to IEC, JEDEC, EN, or IPC.
Beyond that, especially in the commercial domain where quality is still sought but pennies are counted, it does pay really well to invest some time in a component selection guideline. There you can elaborate on the parameters above by, for example, calling up a test standard or setting tolerances for the secondary parameters too.
Also, you can name distinct manufacturers and product series which do not come under a standard.
(for example, give them a cercap series or two from AVX, Kemet, TDK, Vishay and Murata and they will need a very good reason to deviate from this)
Further, this is the place where you can hone the 'or equivalent' expression into something usable, if you
specify that
- selected equivalents must undergo a qualified datasheet crosscheck by the contractor
- manufacturers other than listed must have a QMS system in place
- datasheets shall have their values referenced to normals and/or shall state uncertainties
As this is work that you will do only ONCE it will become a very powerful tool over time. And a lot of this work can be contributed by someone from purchasing or the team secretary. Industry and regulator standards can find their way into that but you can also name components or companies which work well and which you want to be used without further explanation.
When you just think about it for a day, you will have countless entries. Like having coax connectors crimped or soldered and not 'stuck on', which surface treatment you like to have on your mounting hardware or not, that structural composites need to fulfill UL 94 V-0 as minimum and so on. Then come test and inspection and labels and markings and that the produced objects must be accompanied by their test reports in whatever form you agree on and and and...
Then you stipulate adherence to this document in your contracts and Bob is your uncle.
Of course, some quirky parameter can always lead to unexpected consequences, but this should keep the worst shit away from your products. And you can use it to achieve selective results. You could push towards use of leading edge components or keep a manufacturer (and your designers) deliberately away from them.