At 01:48 PM 1/23/2002 -0500, [EMAIL PROTECTED] wrote: > > What is generally considered too close of spacing between a through > hole and > > an SMT pad? > > > >Anything which doesn't leave some soldermask in between the two, unless >you're doing some of the more exotic tricks which allow vias in pads.
If the vias are tented, one could put the hole very close, but I'd feel most comfortable if the via pad does not touch the SMT pad, since solder could conceivably wick under the solder mask tent. I've used a 5 mil clearance, via pad to SMT pad. The issue is especially important because of a desire, with high-speed design, to make the loop area as small as possible. Putting the vias inboard (i.e., more toward the center of the part) may improve high speed performance by reducing loop area. i.e. PAD VIA VIA PAD VIA VIA--------------gnd plane VIA-------------------power plane instead of VIA PAD PAD VIA VIA VIA----------gnd plane VIA-----------------------power plane (The vias would be offset so that they will fit in the space between the capacitor pads, or they will be to one side of the part.) From a noise perspective, we would put the vias in the pads, or fully inboard as in the first example shown above, and I'd think that placing them close to the center of the part and to each other as possible would be best. Blind vias will have little or no problem sucking solder, particularly if the power layers are just below the surface, so I'd think they could be placed in pads. Comments? [EMAIL PROTECTED] Abdulrahman Lomax Easthampton, Massachusetts USA * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *