Vias in SMD pads

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GuitarPhil
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Joined: 26 Feb 2015, 18:04

Vias in SMD pads

#1 Post by GuitarPhil » 14 Dec 2018, 12:11

I noticed an option in the Autorouter Setup dialog to allow 'Vias at SMD pads'.

This got my 2-layer PCB autorouted nicely but I was wondering if this is acceptable to all PCB fab houses or will it cause any issues?


Phil.

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KevinA
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Joined: 18 Dec 2015, 15:35

Re: Vias in SMD pads

#2 Post by KevinA » 14 Dec 2018, 17:23

Extra money from some (PCBCART), https://www.pcbway.com/pcb_prototype/PC ... n_Pad.html appears to enjoy doing Via in pad. Try 4mm trace/space and .2mm hole on 6 layer board plus via-in-pad/blind&buried vias, $7.30 a board now. Check the PCB houses Capabilities, if that information isn't there ask them and most of the 'quick' quote systems do not allow selection of vias pad/blind/buried.

d1wang
Posts: 31
Joined: 13 Nov 2018, 02:19

Re: Vias in SMD pads

#3 Post by d1wang » 16 Dec 2018, 20:09

Read this: https://electronics.stackexchange.com/q ... n-smd-pads

You'd need to do blind or filled vias, which is more expensive. It's cheaper to go with smaller vias if you run out of space (ask your fab house about the size)

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KevinA
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Re: Vias in SMD pads

#4 Post by KevinA » 17 Dec 2018, 00:55

Talk to your board house, if they say they can do via in pad then get a prototype built and try it, for a BGA256 project I was looking at it will be interesting getting a quote on via in pad. That would mean two leads between pads, three rows/columns of pads a layer, power and ground plus two inner signal, 16% P&G pins or 39 out of 256 pins. 1mm trace and space... Where they were three years ago to where they are today on the technology to manufacture PCB's has changed, I just purchased a linear encoder with 1 micron resolution, $17.00, who'd have thought it possible. Oh, and the response is 40 inch a second.

GuitarPhil
Posts: 4
Joined: 26 Feb 2015, 18:04

Re: Vias in SMD pads

#5 Post by GuitarPhil » 18 Dec 2018, 19:03

Thanks for the replies and links guys. My PCB is just a 2-layer job though there are quite a lot of caps and resistors (mostly 1206 size) to fit on a 106 x 55.2 mm PCB so it will go in a Hammond 1590B enclosure. I can route it without the vias in SMD pads if I place some of the components on the bottom of the PCB but that might create issues (cost?) if I ever went to have PCBA by some of the Chinese houses.

@Kevin: Agreed! Technology seems to be accelerating ever more rapidly and costs tumbling at the same time :-)

Edit: Just checked out PCBWay for a rough quote. Price for 10 boards jumps from $25 to $165 if I select the Via in PAD option!!! Think I'll have another go at routing without the vias in pads!!!

Phil.

d1wang
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Joined: 13 Nov 2018, 02:19

Re: Vias in SMD pads

#6 Post by d1wang » 18 Dec 2018, 23:25

Depends on your application. Generally it's good to have a big ground plane, which means having most traces on top side, which also means having most components on top side. Double-sided SMD is also more expensive.

Hammond 1590B is a fairly big enclosure. Why not just have a bigger board?

If you aren't going to solder the components yourself, why not go with 0603 and 0805 components?

Watch out for component height if you do place components on the bottom height.

GuitarPhil
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Joined: 26 Feb 2015, 18:04

Re: Vias in SMD pads

#7 Post by GuitarPhil » 19 Dec 2018, 12:19

d1wang wrote:
18 Dec 2018, 23:25
Depends on your application. Generally it's good to have a big ground plane, which means having most traces on top side, which also means having most components on top side. Double-sided SMD is also more expensive.

Hammond 1590B is a fairly big enclosure. Why not just have a bigger board?

If you aren't going to solder the components yourself, why not go with 0603 and 0805 components?

Watch out for component height if you do place components on the bottom height.
Thanks for the suggestions.

The PCB just fits the 1590B so the board is as big as I can make it - it's not that big an enclosure - maybe you are thinking of one of the other 1590 enclosures?

My plan is to solder the prototypes myself and that's why I went for the 1206 components as I hopefully can see them LOL

I''ve done some 3D modelling and have checked that the component heights on the bottom won't cause any problems but that's definitely something to be aware of.

Phil.

mbelectronicdesign
Posts: 23
Joined: 19 Oct 2014, 08:56

Re: Vias in SMD pads

#8 Post by mbelectronicdesign » 17 Jan 2019, 10:19

Phil,

Get yourself a good stereo microscope and buy kits for 0603 resistors and capacitors, they are easy to work with under the microscope. The PCBs get much smaller and cheaper. I use all SMT if possible and SOT23, SSOP, or SOIC devices (ones with 'legs').

For capped / plugged via in pad, how do you specify that for Diptrace? Do you just make a special via type?

General Question...
For 0.3 mm BGA pads at 0.5 mm spacing, is it possible to go the 'dogbone' + via route to reduce cost?

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KevinA
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Re: Vias in SMD pads

#9 Post by KevinA » 18 Jan 2019, 12:57

I was/am going to do a test; create a via style, place pads for a device, add via to each pad and apply style, set mask/paste to open (none) on the top and mask tented on the bottom with no paste. Save project, select one of the styled via.s and change the paste from none to apply paste, apply to all the same style, generate a Gerber top paste file and use it to create a paste stencil.
The process would be, apply VIA paste, bake it, apply standard paste, place parts, bake it. From what I've seen on filled, via-in-pad charges this might be a cost effective means for small projects, if it works.

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