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Topic: 13mil vias for 2 layer PCB? (Read 1990 times) previous topic - next topic

13mil vias for 2 layer PCB?

The minimum hole for a 2 layer PCB is spec'ed as 12mil.  I'm curious if this can be used for a 13mil via?

I don't normally use such small vias, but this is for a TI PowerPAD device - basically an MSOP 0.65mm pitch with a grounded thermal center pad that needs to be soldered.  I want to connect it to a ground pour on the bottom.  TI recommends using 13mil or smaller vias to avoid wicking.  But I assume this would actually refer to the hole size?  If I want to use a 12mil hole what should set the via size to?

Re: 13mil vias for 2 layer PCB?

Reply #1
if you want an endsize of 12mil, the drill size should be 4mil larger (=16mil).

Re: 13mil vias for 2 layer PCB?

Reply #2
Ah, the problem is I'm using KiCAD and it won't allow a via size <= drill size.  Also, the drill size can't be left out or set to 0 - this will make it complain the drill size is smaller than the minimum size.  My assumption, which is all it is since I can't find even a single example online of how to add custom vias, is that the via "size" in KiCAD includes the pad and maybe soldermask.  (Which of course is pointless in my case since these vias are used for stitching.)

Re: 13mil vias for 2 layer PCB?

Reply #3
I'm 90% sure the kicad via size is the pad size, and the kicad via drill is the size of the drill to drill the hole. Solder mask is set by the 'solder mask expansion' option, although by default kicad exports all vias as tented (change in gerber output options!). This option will make ALL your vias exposed however.

So according to what eauth said, if you want a endsize of 12mil, then you need a drill size of 16 mil, and you can set your pad size bigger than that. Why do you need your pad size equal to/less than your drill size? I think eauth just meant that the size of a plated hole will be 4mil smaller than the drill used to drill it due to plating.

http://http://www.eurocircuits.com/drilled-holes-in-the-pcb-  (part 6)

This link above implies eurocircuits will take this into account automatically, but not sure what dirtypcbs will do.

Re: 13mil vias for 2 layer PCB?

Reply #4
Yep, thanks for the info guys - very helpful.  I just went with a drill size of 13mil and added (I think) 4mil for the pad and called it a day. So 17mil/13mil.  Will see how this works out, a little wicking isn't the end of the world as long as I get a reasonable thermal connection.  (Experiments with the SOIC part shows it's not a huge consideration anyway.)  13mil is a pretty dang small hole and even less if plated. :)  I was just confused since I couldn't make sense of the numbers.  Pad size, drill size, yeah that makes sense.

Re: 13mil vias for 2 layer PCB?

Reply #5
I need 12mil vias (or smaller, but as I know, it is not possible here). If I set 12mil vias size in Proteus 8, will I receive a board with 16mil holes, that are plated with conductor of 2 mil thickness, so the end diameter is 12 mil, or they will drill 12mil holes, than plate them, resulting 8mil vias?

Thank You in advance for your help!

Re: 13mil vias for 2 layer PCB?

Reply #6
The 12mil refers to the drill size, so it's the size of the actual drilled hole you get.  This hole is then plated unless it's a purely mechanical hole.  The plated inside diameter will be smaller than the drilled hole, of course.

Mine worked great, no problems with wicking.  Hot air pencil reflowed perfectly with a small dab of paste on the thermal pad.  (Boards were too small to bother with a stencil.)  I also have an EMI shield on this board and the only screwup was I pasted the pads for the shield then realized I had to reflow the shield as a second step, leading to excess solder, but on the whole for a first try with a board shield and thermal pad it went surprisingly well!

Re: 13mil vias for 2 layer PCB?

Reply #7
[quote author="janb"]The 12mil refers to the drill size, so it's the size of the actual drilled hole you get.  This hole is then plated unless it's a purely mechanical hole.  The plated inside diameter will be smaller than the drilled hole, of course.

Mine worked great, no problems with wicking.  Hot air pencil reflowed perfectly with a small dab of paste on the thermal pad.  (Boards were too small to bother with a stencil.)  I also have an EMI shield on this board and the only screwup was I pasted the pads for the shield then realized I had to reflow the shield as a second step, leading to excess solder, but on the whole for a first try with a board shield and thermal pad it went surprisingly well![/quote]

Thank you for your reply, this is very good news. That would be a serious problem for my design, if the drill size would be bigger than 12mil.