David,
The original question about microvias is different from via in pad.
Both because their size is so small and because they only go through one layer
before they are blocked/plugged anyway. A via in pad is typically drilled
(right through the PCB) and are much larger size (more volume to fill with
solder). The microvia is laser obliterated between the outside layer and the
next layer in only (or possibly a few layers in). Then the hole is plated, so
there is only a small indent to be filled with solder during assembly
processing and typically the screening will fill the small void sufficiently.
Or at least that is my understanding without ever having done it.
Darcy,
Your questions would probably be best asked of a couple of target
fabricators. They should be able to give you max and min sizes, any other
generic details that would make up the design rules for such features. i.e.
minimum pad size internally on the receiving end. Outside pad design (donut
sizing details). My understanding is that you must have a donut pad on the
outside, the laser will only drill through laminate and not the copper,
otherwise it could drill through the internal pad as well.
Sincerely,
Brad Velander
Senior PCB Designer
Northern Airborne Technology
#14 - 1925 Kirschner Road,
Kelowna, BC, V1Y 4N7.
tel (250) 763-2329 ext. 225
fax (250) 762-3374
-----Original Message-----
From: David Kearney [mailto:[EMAIL PROTECTED]
Sent: Monday, September 26, 2005 2:50 PM
To: Protel EDA Discussion List
Subject: RE: [PEDA] Micro Vias
My board assembly shop said that vias in pads would need to be plugged.
I have done it with a HASL board many times before without problems but
most of those were hand assembled, it's easier to add more solder if it
wicks through the board. I've recently started using ENIG for all of my
boards and the holes are much cleaner than they were with HASL. I would
be more worried now than I was before.
The bottom line for me is that I no longer do via-in-pad. I would in a
pinch but the board fab will be higher.
Regards,
David Kearney
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
On Behalf Of Darcy Davis
Sent: Monday, September 26, 2005 2:33 PM
To: PEDA (E-mail)
Subject: [PEDA] Micro Vias
We're heading into new territory for us and so looking into using micro
vias. Currently we're using 10/26mil (drill/annular ring) vias, but
we're still finding that the via's are consuming too much placement
room. My understanding is that with micro vias, we can punch a hole
right through a pad and still be OK through reflow. Does anybody know
otherwise? Since micro vias will be laser drilled (I assume), they can
only go between two adjacent layers, right? Is there typically a
specific size of hole that must be used, or is there just a minimum hole
size requirement? Are there any other limitations to this technology
that I may not have thought of? All else being equal, does anyone know
what the cost differential would be for a board with micro vias vs. our
current 10/26mil vias?
Thanks again,
Darcy Davis
Design Engineer
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