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Should the metal trace from the dividing resistor for setting the output voltage to the FB pin be made thin or thick? There should be almost no current flowing, so it seems that rather than worrying about wiring resistance, it should not be made thicker than needed, in order to reduce the parasitic inductance and capacitance.
In general, this wiring is almost always made narrower than power lines. In addition to parasitic inductance and capacitance, narrower wiring is also effective in the sense of reducing the effect of crosstalk from surrounding circuits.