Simple question, I hope. For a reflow soldered board, what are the rules for how close SMD parts can be to each other? Any pointers appreciated.
There are "rules" but they depend on the method the assembler uses to solder SMD parts. IR is different from hot air, for example. You need to talk to the assembler. Also, "short" parts (like an 0805) require different separations than tall ones, like a surface mount SMPS inductor.
Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net
It really depends on the size of your components and like Jim said will vary between assemblers and their methods.
One rule of thumb I use is to place them (when parallel to each other), with at least a gap of 1/2 their width. One option if you are running out of board space is also to use smaller SMD components, such as 0402 sized passives.
I typically place them as tight as I think I can while maintaining hand solderability. Most of my boards are machine soldered - but I like maintaining that ability. That normally means enough space to get some tweezers in between.
As for what is manufacturable - that's a question (as previously stated) for whoever is doing your assembly.
One rule of thumb I use is to place them (when parallel to each other), with at least a gap of 1/2 their width.
Tightest I put SMDS are 6 mil, only because that is generally the limit for pad spacing for most PCB fab places without paying extra for better specs.
For assembly, my intuition is that 6 mil spacing would be as tight as you want to get. I have gotten some assembly done in the past and maybe 2-3% of my boards have some type of solder bridge on them. The solder bridge is usually from pad to exposed via or exposed via to exposed via, as I placed my vias with 6 mil spacing, very rare is the bridge from pad to pad. Now I know you can put solder resists over vias, did not know that before, and I think with that I would be very comfortable with 6 mil spacing for assembly.
10 mils per http://www.aapcb.com/PCBAssemblyTips.asp at "Spacing Guidelines for PCB Assembly".
"Dare to be naïve." - Buckminster Fuller
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