Phil:

When we run in to this kind of problem with protoype HW we use a simple
solderability test to determine if the BGA can be put back down to the
board. Our addage is "nothing sticks to solder like solder." In order to
get by clean with IPA carefully, scrape with dental tool and observe (this
may give you hints), tine with solder. If it accepts solder then move on to
the other pads. If not, XRF analysis followed by SEM will tell you why (the
last time this happened there was indeed a solder mask constituent on the
pad preventing the wetting).

Bob W/BEST


On Wed, Mar 12, 2014 at 4:21 PM, Phil Bavaro <[log in to unmask]>wrote:

> 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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-- 
Bob Wettermann
BEST Inc
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Cell: 847-767-5745


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