Hi, Marie,
You have a bit of a "mule" here. I can comment with some confidence about
the via holes and conductive epoxy filling, and my general experience is
that holes 10 mils (0.25mm) are just OK to fill as long as the aspect ratio
(hole diameter : hole depth) is not more than 1:6. Many fab shops, though,
do not like aspect ratios greater than 4:1.
With a 2.3mm (92 mils) thick board, via hole diameter at 1:4 would have to
be 23 mils (0.575mm). At 1:6, the diameter would have to be 15.3 mils
(0.383mm). Adding the usual preferred "pad diameter over finished hole
diameter" size of 22 mils, you end up with pad diameters of 45 mils
(1.125mm) and 37.3mils (0.9325mm). I have 1.27 mm pitch BGA's with pad
diameter only 0.5 mm, so I think that the design idea of via-in-pad is a
non-starter.
With 1.27mm pitch BGA's, there is enough room to design in offset vias that
don't therefore need to be epoxy filled, except perhaps for test points.
I would also worry - a lot - about the copper thickness, especially at that
thickness of PCB. What is the working environment of this board to be?
Peter
ME Magnin <[log in to unmask]> 14/03/2003 03:45 PM
Sent by: TechNet <[log in to unmask]>
Please respond to me.magnin
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] 2.3mm thick PCB and BGA
Technetters,
I have received a query from a customer : he is designing a PCB for a BGA ,
as described below :
35 mm ´ 35 mm, cavity up, 352-pin plastic ball grid array (PBGA) package.
Package Outline 35 mm ´ 35 mm
Interconnects 352
Pitch 1.27mm
Solder Balls 62Sn/36Pb/2Ag
Solder Ball Diameter 0.75 mm
Maximum Module Height 1.65 mm
Co-Planarity Specification 0.15 mm
Maximum Force 6.0 lbs. total, uniformly distributed over package (8
grams/ball)
He needs a 2.3mm thick PCB, 10 layers, 4 or 6 of them with 70um copper.
He wants 0.25 mm via holes underneath the BGA, which must be also used as
test pads.
For this reason, he wants these holes filled with conductive material to
increase the contact surface.
He has contacted his usual PCB manufacturers who are not really confident
with such a design.
Who has any experience abour this topic ?
Which solution would you propose ?
Waitnig for your answer,
Kind regards
Marie Magnin
FIRSTEC SA
Rue du Grand Pré 70
CH 1211 Genève 2
Tél. : +41(0)22 918 36 85
Fax : +41(0)22 918 36 93
mailto:[log in to unmask]
http://www.firstec.ch
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