Hi, Ioan,
I'm no EE, either - just a confirmed ME, but from the experience I've had,
via hole diameter is dependant on 3 things: 1) current-carrying needs, 2)
board thickness and 3) plating thickness.
1) If you know the power that the via hole has to carry, you can determine
the cross-sectional area of the copper plating you need in the hole.
2) Adequate plating inside a hole depends partly on the plating process
capability of your fab house and partly on the hole's aspect ratio
(diameter to depth). Ask your fab house what their minimum hole diameter
and aspect ratio is for achieving even plating thickness down the hole.
From the thickness of your board, you can then determine whether you can
use the minimum hole diameter as a baseline or diameter based on aspect
ratio as a baseline.
3) Once you also know from your fab house the minimum hole plating
thickness they can achieve, you have all the information you need to
determine your finished hole sizes. Knowing the plating thickness and the
X-sectional area of copper you need, you can simply calculate the length of
copper required, which, of course will be the same as finished
circumference of your finished hole. Divide it by Pi and you have the
diameter. Obviously if your calculations come up with a hole diameter
smaller than you fab house's minimum or, likewise, too large an aspect
ratio, increasing the hole diameter won't do any harm - it only adds a
margin of safety (which you should do yourself in any case).
There is no maximum plating thickness, but bear in mind that you often get
thicker plating on the board outer surfaces than you get in the holes. You
don't want it to get too thick, or you run into over-etching problems,
uneven coverage, etc., etc. You're safe enough with assuming a 1/2 oz (0.7
mils) min hole plating thickness for your calculations.
Conversely, for the future, you can work out that if the current carrying
requirements of a via are less than a certain value, you don't need to
calculate hole diameters, since your fab house's minimum size will cope
with it.
Peter
"Tempea, Ioan"
<itempea@POSIT To: [log in to unmask]
RON.QC.CA> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
Sent by: Aero/ST Group)
TechNet Subject: [TN] Via size
<[log in to unmask]
RG>
02/21/02 10:15
PM
Please respond
to "TechNet
E-Mail
Forum.";
Please respond
to "Tempea,
Ioan"
Hi technos,
first I want to say to the Moonman "got your lean thing, as well as all the
other documents you put there. Big thanks to you and Seth".
Now, my question to the forum is: what are the rules in determining the
diameter of vias? Has it anything to do with the current or the power? Can
a
power supply board, stepping down from 600V to 220V, have 0.018" vias?
I know it sounds trivial, but I'm no EE.
Thanks,
Ioan
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