How close can I place two 0603 resistors on a PCB?
I have a silk layer bounding box of 2.6mm x 1.4mm around the pads of 0603s, and I often place resistors with touching bounding boxes. Lengthwise placed next to each other this leaves 0.2mm between pads. For reflow soldering this has never caused problems. For wave soldering you will need more space, esp. in the wave direction. For hand soldering it depends on the soldering skills of the person who assembles the board. 0.2mm may be possible if you don't use too much solder.
While 0.2mm may look like very tight, remember that this is reflow soldering. When the solder paste melts it's capillarily drawn to the contact surfaces of the resistor, so it won't flow to adjacent pads.
This can vary a lot.
How is it being soldered? By hand or pick and place? What fab house are you using? What are their requirements?
These are things you should ask yourself and get answers for in order to determine how close is possible for the fab process.
You will also need to look at what signals are on each resistor. When you have high speed lines you can have issues. Also if you have high voltage you can have issues.
I know this is rather broad. If you have a specific case I will be willing to add more.
From here "The document is "IPC-7351A Generic Requirements for Surface Mount Design and Land Pattern Standard". You have to buy it from www.ipc.org." You CAD package may have it already. There are 3 variantes, so you would use the "least" (i.e. smallest one).
"If you want to look at the recommended footprints, without buying the standard, you can download a free viewer at: http://landpatterns.ipc.org/default.asp
There's more info available at: http://www.ipc.org/ContentPage.aspx?pageid=4.6"