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smallest via hole that can use a +/- 3 mils

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lsday View Drop Down
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    Posted: 17 Jun 2019 at 8:16am
Can anyone answer this question please? what is smallest via hole that can use  +/- 3 mils tolerance?
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Tom H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jun 2019 at 11:26am
0.20 mm (8 mils) However, there are high quality fabrication shops that say they can do 0.15 mm (6 mils). 

The big issue is the pad annular ring. 

The fab shop will add 3 - 4 mils to the finished hole size. 

Then you must consider drill drift and layer to layer tolerance of 3 mil. 

Then after the drill oversize and the layer to layer tolerance you still need to have 1 mil annular ring. 

IPC Class 3
Hole = 8 mil (0.20 mm)
Pad = 20 mil (0.50 mm)

IPC Class 2
Hole = 8 mil (0.20 mm)
Pad = 18 mil (0.45 mm)

The via hole tolerance would be +0, - the hole diameter

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