Most likely there is mask contamination on the pads. It is difficult to get all of the mask residue out of the tiny pockets in the mask where the BGA balls mount.
Easy to test: Try to wet by hand with solder and iron or solder paste and hot air. If the solder doesn't flow out smoothly, then the pads are contaminated.
Plating solution molecules are much smaller than the size of a "grid" of contamination which prevents wetting, so the ENIG still goes through. Hence, nice yellow pads which still don't wet.
To fix, hand scrape the pads with an Xacto and wet by hand. Laborious, but gets a prototype out while you argue with the vendor. I had this happen with a vendor I don't use anymore: Took several weeks before they grudgingly acquiesced to make new boards: But they made sure they let me know that I was nuts and they were just doing this to placate me!
That vendor hid behind the IPC guidelines on wettability testing, which apparently say to use a larger pad than most BGA's have. I always put a "fitness for use" clause in my pcb PO's, so I only need to show the boards don't work for me. That's OK contractually, but practically doesn't do me much good because I have to find a new vendor anyway. I find it kind of bizarre that most vendors don't have ANY solder paste available to see if the boards actually are "fit for use".
Wayne Thayer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Wednesday, March 12, 2014 5:22 PM
To: [log in to unmask]
Subject: [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
________________________________
This message and any attachments are solely for the use of the addressee and may contain L-3 proprietary information that may also be defined as USG export controlled technical data. If you are not the intended recipient, any disclosure, use or distribution of its content is prohibited. Please notify the sender by reply e-mail and immediately delete this message and any attachments.
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
|
