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May 2002

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Subject:
From:
"Houston, Terri" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 23 May 2002 09:24:33 -0600
Content-Type:
text/plain
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text/plain (66 lines)
Dennis  - thanks for sending me your attachment separately.

The ground-plane flooding on layer 1 that your EE wants will result in BGA
solder ball pads that are defined by the soldermask opening.

More than 10 years ago, Motorola and other companies went thru quite a bit
of thermal-cycling testing and found that this design (dubbed "captured
pads") causes solder joint fatigue and failure faster than non-captured
pads.

Basically, captured pads create a hi-stress-concentration area in the solder
joint that is more susceptible to creep and failure due to the differences
in thermal expansion between the PC board and the BGA package (which is
dominated by the die bonded to it in the 'shadow' of the die).

To address your concern on assembly, I'm not aware of any studies (and would
not think) that a flooded plane would cause assembly problems.

Good luck - EEs can be hard to deal with.

Terri
(an EE)

-----Original Message-----
From: Denis Lefebvre [mailto:[log in to unmask]]
Sent: Thursday, May 23, 2002 10:10 AM
To: [log in to unmask]
Subject: [TN] BGA Solderability


Technet;
As a designer, I diligently try to understand and account for fabrication
and assembly issues in my designs. It can be frustrating when working with
EE's that are not well versed in manufacturing as they request things that
you know are not good practice from a DFM viewpoint.
I have a case now where an EE is asking for a copper fill on BGA ground
pins. Since this is not easy to describe in a way that every one can
understand, I am including a small JPG to illustrate. It is a 356 pin BGA,
1mm pitch, 0.5mm solder lands.

I am convinced that this will not fare well when they try to solder this
part, yet the engineer insists on doing it this way. What argument can I
make, with supporting data, to persuade him? Simply telling him that I know
it is poor practice isn't cutting it! [Adjusting tie..."I tell ya, we get no
respect!"]


Denis Lefebvre, C.I.D.
Senior PCB Designer
408-542-3832
Finisar Corporation
1308 Moffett Park Drive
Sunnyvale, CA 94086
www.finisar.com <http://www.finisar.com/>

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