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I found that usually on BGA package and on a lot of QFN datasheets are provided IC and PAD-IC dimensions and not advised PCB-PAD dimensions and soldermask dimensions. Can someone tell me a good rule between IC-PAD dimensions to PCB-PAD dimensions and Soldermask dimensions? For BGA and QFN.

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Usually solder mask expansion of 2-4 mils is appropriate, but the minimum really depends on the process used.

Silk screen solder mask would be the crudest.

LPI is intermediate.

Laser direct imaging of LPI might allow the solder mask to have zero expansion.

Similarly, the minimum solder mask sliver will depend on the process and the PCB manufacturer, so check their design rules. This comes into play for the sliver of solder mask between two pins of a package with small lead pitch. In some cases you can't have solder mask between the pins.

For IC lead dimensions to pad dimensions, it's best to follow the manufacturer's recommendation for that package. Sometimes it's not on the datasheet and you have to hunt it down in other documents. My second choice after that would be to use another manufacturer's recommendations for the same package, unless you have something like Altium's IPC footprint wizard which asks for various dimensions and other information and uses IPC recommendations to create pad dimensions.

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IC Pad dimensions are the physical dimensions for the pads. A QFN, for example, might have pins that are 0.2mm wide and 0.4mm long (I just pulled those numbers out of nowhere). PCB Pad dimensions are the land patterns, or the "footprints". They must be larger than the maximum dimensions shown by the IC pad dims, as they must be able to fit the pins AND have room for solder to adhere to them. For example, PCB pad dimensions for the above example might be 0.4mm wide and 0.6mm long (again, this isn't ideal--look up IPC standards to get actual values). Soldermask dimensions show where the soldermask needs to stop so as not to cover pads (you don't want to cover the pads with soldermask, otherwise the solder can't stick). The soldermask expansion (distance around the pads that you want to leave free of soldermask) can be set in your design rules (hotkeys D-R).

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  • \$\begingroup\$ RE: "Soldermask dimensions are generally only used to outline the physical package" Did you mean silkscreen rather than soldermask? Soldermask dimensions need to be large enough to not overlap the pads even with mis-registration between copper and soldermask, but not too large to expose other copper features. \$\endgroup\$
    – The Photon
    Jun 23, 2015 at 16:26
  • \$\begingroup\$ Haha, woops! You're right, that was supposed to be silkscreen. I'll edit my answer. \$\endgroup\$
    – DerStrom8
    Jun 23, 2015 at 16:28
  • \$\begingroup\$ My answer has been edited. Sorry for the mistake (it's been a long day =P ) \$\endgroup\$
    – DerStrom8
    Jun 23, 2015 at 16:30

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