Hi Phil,

Are bga Pad connected in dog bones and the dog bone vias not partially filled by solder mask? 

If vias partially filled the enig plating can be affected all around these vias including on the next bga pads which can become not solderable.
It is one possibility.

Rgds
Grandrien

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Message du 12/03/14 22:22
De : "Phil Bavaro" 

A : [log in to unmask]
Copie à : 
Objet : [TN] BGA non wetting pads

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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