Don't tell me someone either used wrong file for the solder mask (LPI), or opening of the pads were not sufficient (mis align), or chemical was weak, rinse was not sufficient or on time, etc,etc. Provide it is LPI finishing :( long shot.
--------------------------
Sent using BlackBerry
----- Original Message -----
From: David D. Hillman [mailto:[log in to unmask]]
Sent: Wednesday, March 12, 2014 06:09 PM
To: [log in to unmask] <[log in to unmask]>
Subject: Re: [TN] BGA non wetting pads
Hi Phil - ugg, not a fun problem! One suggestion to complete as part of
the initial homework efforts - using either an XRF system or the SEM,
confirm that the there is no gold on the pads. Gold diffuses into solder
at a rate of 100 uinches per second at typical soldering temperatures. If
there is no longer gold plating on the pads, that means you did have
molten solder in contact with the pad at some point of the reflow
exposure. Your lack of wetted solder could then be a solderability or
plating issue. If the gold is still present on the pad then you could
have a clogged aperture issue or a component warpage issue (i.e. molten
solder never came in contact). I always get a bit nervous with the visual
only inspection assessment as sometimes nickel or tin/nickel IMC can have
a gold appearance creating confusion.
Dave Hillman
Rockwell Collins
[log in to unmask]
From: Phil Bavaro <[log in to unmask]>
To: <[log in to unmask]>
Date: 03/12/2014 04:22 PM
Subject: [TN] BGA non wetting pads
Sent by: TechNet <[log in to unmask]>
Ok, I have a head scratcher.
We have a recently built boards which exhibited 3/4 failing at functional
test. The board finish is ENIG, solder mask is SMOBC, material is Hitachi
(formerly Rogers "Theta"), and we are using a ROHS BGAs reballed with
Sn63Pb37 spheres and water soluble paste. Oven profile was 220 C max,
preheat ramp, etc.
We focused on one BGA which had JTAG capability built into the circuit.
Comnponent would not respond to JTAG.
The xray inspection did not reveal any distinct anomalies. Most of my
group felt the balls had collapsed adequately but I remain the pessimist
in this situation. We did not have a board available to have the BGA
cross sectioned so I was concerned as I thought I say excessive gap
between the part and the PWB.
We tried reheating one board using the Rework system and bumped up the
reflow max to 230C. No help, but the voids in the xrays did grow (as you
would expect).
Here is where it gets strange....
We remove the part, and seven random BGA's pads are still completely gold
in color. Absolutely no solder wetting to these pads. A few appear to
have transparent contamination on the pads. I don't see what I have known
to be black pad (years ago) and some of these gold pads are bright gold.
I can only think of one scenario and that would be complete misprint on
seven pads but really doubt that as the cause.
I try to repeat this defect on a solder sample PWB with just paste printed
and every pad takes solder.
I am trying to send Steve my photos.
Any ideas?
Thanks,
Phil
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