[SI-LIST] Re: [Possible SPAM] BGA Breakout.
- From: "Marc Humphreys" <mhumphreys@xxxxxxxxxxxxxxxxxxx>
- To: "Surita Chandani" <surita.chandani@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 21 Jan 2010 13:06:50 -0800
Surita,
Now lets get to solving your 2 between math problem. I assume at this point
you've read and gathered valuable insight from Lee's depiction of the
anatomy of a via structure and related feature terminology from his
webposting "Why Not Route Two Traces Between Pins on a 1 mm Pitch BGA?.
" Yes I've read that book. Your conclusions are all ... :-) Captain,
another missile. ...pause...Tell me.. " from the movie Hunt for Red
October.
Not lauching missiles here and not intending to start are war. Surita still
doesn't have his answer.
Lee, can't argue the conclusion just your presumed assumptions and unstated
facts on current fab DFM guidelines from top tier Fab. vendors. I think you
said as much in a follow up e-mail this morning.
So on we go...
The pad for the BGA ball along with its 35 mil antipad etc. are not the
numbers you need to worry yourself with for the solving the two between
breakout of a 1mm BGA. Is just the via padstack that comes into play here.
First off we have to assumes we're not trying to esacpe on the top layer.
Even single lines more than 2 row deep will problably stretch even the top
tier fab. houses DFM rules. as they typically want you to maintain a
minimum 5 mil pad to line and min 5 mil min outer layer traces)
So considering the inner layers of the brd only.
The via size for the brd has to maintain a maximum 10:1 or 12:1 ratio (
drill size to via length) Can go higher but no for the small drill sizes
neccessary for this practice.
From experience the numbers work fine if we start with a 10mil drill size.
which allow us to confidently build boards upto 120mil thick.
All number are in mils. after initial convertion of 1mm to 39.37mils.
So the pitch between BGA pads is 1mm = 39.37mils.
Assuming we keep the same grid for the vias were have 39.37 mils center to
center on the drill.
A 10 mil drill/ 20 mil pad and allowing a pad to line of 4 mils - typical
number for a top tier fab.
will leave you with a usable routing channel just over 11mils.
Allowing 4 mils between traces you can now run two 3.5 mil lines.
If high voltage isoalation is required these dimensions will not work.
Also to prevent breakout on the pads you should add or request your fab
vendor to add Teardrops or fillets to your pad connections.
Marc
----------------------------------------
From: "Surita Chandani" <surita.chandani@xxxxxxxxx>
Sent: Thursday, January 14, 2010 12:21 PM
To: si-list@xxxxxxxxxxxxx
Subject: [Possible SPAM] [SI-LIST] BGA Breakout.
Hello Gurus;
I am doing some preliminary calculations on the breakout of
a 1,400+ Ball , BGA. Roughly one half of them are signal
connections which would need traces running up to them. The vendor claims
you
can run two traces between Vias, my calculations are not adding up, I have
a
few questions.
1. Generally, do the Vias have a pad even on the signal
layer it is not connecting to?.
2. Do the Vias have a larger pad on the inner layers?
3. With a Ball pitch
of 1 mm (39 mils.) and an Antipad of 35 mils, there is hardly any room for
one
trace between Vias, what am I doing wrong?
Thanks,
Surita Chandani
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- » [SI-LIST] Re: [Possible SPAM] BGA Breakout. - Marc Humphreys
