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I came across solder bridge in the schematics for STM32F4 Disocvery board and I don't think I fully understand the design here. For example in schematics they have a lot of solder bridge symbols like this(SB18, SB19 and SB20): enter image description here

But for example R56 is just 0 ohm jumper resistor.
On the board solder bridges are either not connected or have component(0 ohm resistor?).

My original idea was that solder bridge means that for different revisions they can leave it unconnected, but this idea is probably wrong since there are a lot of R## resistor footprints without component meaning that they could have simply use 0R symbol instead of SB. The question is - why use SB symbol instead of 0 Ohm resistor and why use 0 Ohm resistor in SB locations instead of you know... solder bridge? And if it is not 0 Ohm resistor as I guess - what is it? There is no markings and I don't want to desolder it from board.

Or even better question - why not just have a traces? Especially for example in 8MHz crystal connection to uC pins(like on example above), why not just connect it with copper?

enter image description here

enter image description here

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Jumpers are used to make different connections for different configurations. Solder bridges can be replaced with 0 ohm resistors or jumper wires -- They all do the same thing.

In your example the solder bridges are used for different configurations of the oscillator. The ones that are populated are a default, but you have the option of removing the jumpers and configuring it to work in a different way. I'm not going to take the time to read the datasheet, but it's probably all in there.

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  • \$\begingroup\$ But why not save money on 30 zero ohm resistors and use solder blobs? Are blobs hard to make in manufacturing? \$\endgroup\$
    – ScienceSamovar
    Commented Oct 2, 2016 at 12:46
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    \$\begingroup\$ In production it's much easier to apply 0 ohm resistors using a pick & place machine. Solder blobs would need to be applied by hand, which would cost a lot more than just using 0 ohm resistors. \$\endgroup\$
    – DerStrom8
    Commented Oct 2, 2016 at 12:50
  • \$\begingroup\$ Ah... I thought there should be some process of making solder connections in manufacturing, apparently not. OK then. \$\endgroup\$
    – ScienceSamovar
    Commented Oct 2, 2016 at 12:52
  • \$\begingroup\$ Also, in this specific case, they may use an actual resistor to tune some parameter of the crystal circuit, just like R57 is 220 ohms. (this is just a guess, though...) \$\endgroup\$
    – ricardomenzer
    Commented Oct 3, 2016 at 12:32
  • \$\begingroup\$ @ricardomenzer I have indeed seen this on some products, but those types of resistors are generally eliminated (or set to a fixed value) after the prototyping phase when all of the testing has been completed. I suppose in some high-sensitivity products they are "tuned" but usually this is done with variable resistors. \$\endgroup\$
    – DerStrom8
    Commented Oct 3, 2016 at 19:39

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