Is there a standard for via sizes, or can you make them any size you want?
(I'm going to be using traditional PCB houses to manufacture my PCB's.)
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Quick answer: If you already know which PCB fab will make your boards, use their "preferred minimum hole size" for your vias. This should be posted in the capabilities section of their web site.
If you don't already know which PCB fab will make your board, 0.020 inch (0.5 mm) vias are pretty conservative -- a few years ago I found lots of PCB fab houses can handle 0.020 inch (0.5 mm) vias -- it was greater than or equal to their "preferred minimum hole size". I'm sure the "preferred minimum hole size" has dropped even smaller since then.
Long answer: In principle, you can specify vias of any size.
In practice, there is a balance between:
- If you make the vias too big, they require lots of space on your PCB (on every layer), making it more difficult to route and perhaps requiring a bigger, more expensive PCB.
- If you make the vias a little too small (less than the "preferred minimum hole size" of a particular manufacturer), then that particular manufacturer will claim it requires a "special process" which adds expense.
- If you make the vias much too small (less than the "absolute minimum hole size" of a particular manufacturer), then that manufacturer can't make it at all, and you'll be forced to find a different (probably more expensive) manufacturer or redesign the board.
The "metal barrel" of a via is practically free if you have any plated through holes. The PCB fab throws the PCB in the through-hole-plating bath until an appropriate thickness of metal grows in every hole.
I generally pick a via size that I know is greater than or equal to the "preferred minimum hole size" capability at many different PCB fab houses, so I'm not locked into a single manufacturer.
Alas, the "capabilities" page of some PCB fab houses is sometimes hard to find. Please help me find the "capabilities" page of fab houses and post the URL of that capabilities page to the wiki at http://opencircuits.com/PCB_Manufacturers ... which also briefly summarizes the "preferred minimum hole size" and "absolute minimum hole size" and a few other capabilities of each manufacturer. (A few of them have a limited number of "standard drill sizes" listed, which may be what you are looking for).
davidcary
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One manufacturer's list of drill sizes. They pick the drill they have that is nearly the size of the hole you asked for. http://www.pcbexpress.com/technical/faq.php#drill – joeforker Apr 26 '12 at 16:36
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Your PCB shop will have a list of standard drill diameters, which the drill tool can pick at any time. Other diameters will require manual intervention, and you'll pay extra for that.
I often use a 0.35 mm hole with a 0.7 mm diameter annular ring.
stevenvh
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"Manual intervention" would typically mean loading a new cartridge of drill bits instead of their standard selection of between 20 and 100+ bits depending on the drilling machine. – joeforker Apr 26 '12 at 16:39
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PCB houses will give you acceptable sizes for their process. They have a tolerance and a cost associated with different sizes, but unless you are making the next batch of Iphones you will probably just be given a cost per via, if even that.
Normally they just give me a cost/square inch and tell me my minimum size.
Kortuk
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Well, using a different via means a different drill size, right? Surely they can't have a drill size for everything, so do they have to make up drill bits? – Thomas O Oct 31 '10 at 23:34
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1They could use a 5 mill drill bit and move it in a circle to generate almost any size. I have also seen methods that use a laser. – Kortuk Oct 31 '10 at 23:36
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You are getting into something that falls into "too localized." This question is going to have an answer that varies by manufacturer. – Kortuk Oct 31 '10 at 23:41
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If you're smallest drill is 20 mil (roughly 17 mil vias after plating), most fab houses won't charge you extra. Some places will go as small as 12 mil after plating with no extra charge. But Kortuk is right-- each fab house has a list of 5-10 drills that are free, so there is no general answer to the question. – pingswept Nov 01 '10 at 03:14
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Yeah, the one I most commonly use does a min of 8mil and does not charge extra no matter the size I pick. – Kortuk Nov 01 '10 at 14:17
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@Thomas O - Actually, they often do. Usually most fab houses have drill sets from 0.001" to 0.250" in 0.001" increments. – Connor Wolf Nov 14 '10 at 07:00
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Generally, holes smaller than 0.010 increase the cost, because you break a lot more bits once you start getting **really** small. – Connor Wolf Nov 14 '10 at 07:01
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@FakeName, Makes sense, most of the fab I do we do not have large enough quantity for them to take effects like this into account. – Kortuk Nov 16 '10 at 16:13
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Don't forget the "minimum annular ring width", that's the amount of copper surrounding the hole. PCB suppliers will have a minimum annular ring width, which is defined as (diameter of the pad - diameter of the hole) / 2.
Leon Heller
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The "aspect ratio" defined as = board thickness / unplated drill dia is important. If it is too high, you may have via barrel cracking due to expansion when soldering. Typically a value of 6 is considered safe. E.g with 1.5mm (60 mil) board thickness, a drill size of 10 mils, which means after plating, about 8 mils plated hole dia.
Ajith
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If your drawing calls out +/- mil tolerances for PTH's and via's, your board house can select appropriate hole sizes that match their "standard" without upcharging for custom hole sizes.
For example, you might have something like:
+2/-0 PTH for < 50 mil +4/-2 PTH >= 50 mil.
+2/-2 for all via's.
Toybuilder
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