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While creating new Altium component for our company's library I started to wonder: why one would place a designator on an assembly drawing instead of value of a component? Wouldn't it be easier to assembly a board with drawing that contains actual values? If you have designators you need to lookup values in BOM to find out what component you need to place on a board. If you had values you know that straight away.

Maybe I am missing something? Please explain.

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    \$\begingroup\$ Testing, maintenance, understanding. Replace R17, it seems to be broken. (OK). Replace that 10K resistor, it seems to be broken. (Which one? There are 40 of them). Design changes. (Yes I know the PCB says it's 10K. But we changed it to 20K because the customer wanted more gain). \$\endgroup\$
    – user16324
    Commented Feb 26, 2016 at 15:20
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    \$\begingroup\$ And also: Which value do you think is important enough to make the cut? Resistance? Accuracy? Thermal Drift? Parasitic inductance? Maximum surface voltage? Power rating? Material composition? These all get very neatly tied to "The one with designator R1 has these parameters:...". You'll run into many of these once you go to high-reliability and/or high frequency designs. \$\endgroup\$
    – Asmyldof
    Commented Feb 26, 2016 at 15:31
  • \$\begingroup\$ On assembly drawings, which can be to any scale, there may be value in having both designator AND value. \$\endgroup\$
    – user16324
    Commented Feb 26, 2016 at 18:42

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You are missing several somethings:

  1. Values are generally longer than the designator (R1 as opposed to 100K, or U1 as opposed to dsPIC30F3013) so the designators take up less space.
  2. Values may change though the part designator is the same. Say you discover that R1 needs to be 90K instead of 100K. If you have values on the board, you now get to scrap all your boards and have new ones made with the correct value.
  3. As Brian Drummond mentioned in a comment, you may have 30 parts with the same value. How do you locate the correct one when comparing the board with the schematic? How do you tell a technician to replace a particular resistor with a different value? Saying "replace R17 with a 15k" is much easier than saying "that 10K resistor over there, beside the 5 capacitors and lower down than the inductor needs to be replaced with a 15K."
  4. As Asmyldof mentions in comment: The value itself often isn't worth much. The designator refers you to the parts list where you see that C1 isn't just a 1nF capacitor as you could see from the value. It is also (and much more importantly) an X1 rated capacitor safe for use in power lines. Which value would you print on the board? The capacitive value, or the voltage it is rated to safely handle?
  5. A final thing that probably doesn't weigh as much as it used to is manually inserted parts. You are asking a bit much of an uneducated factory worker for them to understand the values. They used to have manual insertion stations where there was a tray of parts marked by the designator. The workers looked at the board and saw, say, R5 and pulled a part out of the R5 hole in the tray and put it in the board - and never knew the difference between a resistor and a capacitor and didn't care. Tab A in Slot A and on to the next part.

Number 4 is one I hadn't really thought of until Asmyldof mentioned it, but consider a capacitive drop power supply with a 33nF capacitor providing the drop, and another 33nF capacitor used in the low voltage section. How do you represent that in your assembly drawing when you use just the value? That 33nF in the dropper had better be safety rated, and you'd better not mix it up with that other 33nF that isn't safety rated. The designator saves your bacon again by referring you to the BOM where you see that C1 is 33nF X1 and C10 is 33nF and rated for 25V.

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  • \$\begingroup\$ These are good points. But: \$\endgroup\$
    – aadam
    Commented Feb 26, 2016 at 15:37
  • \$\begingroup\$ These are good points. But: 1. It is not an issue. We print our assembly drawings (with values) on A3 paper. It is possible to read everything. 2. This is the reason why we never put values on silkscreen, just designators. Assembly drawing on the other hand is easy to reprint. You can even change revision number so there is no mistake it is another product. 3. This is good. Assembly drawing with designators is good for cross-reference with schematics. 5. I would like to see that. Let's say 100 caps each 100n, each have different designator. So workers have 100 trays, each tray with 1 cap? \$\endgroup\$
    – aadam
    Commented Feb 26, 2016 at 15:44
  • \$\begingroup\$ 5: They had a pigeon hole in the tray marked "C1 through C100, C205" \$\endgroup\$
    – JRE
    Commented Feb 26, 2016 at 15:46
  • \$\begingroup\$ 4. This is very interesting. I never though about it, because we never had to deal with very high frequencies where specific passive from specific manufacturer is required, because of some of its properties are just a little better. \$\endgroup\$
    – aadam
    Commented Feb 26, 2016 at 15:51

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