Acoustic microscope results are in. We tested virgin parts, after solder, after wash, after ICT. All NDT results are clean.
Very good news for the CM, and Vlad- we owe him a beverage of choice from the wager he made.
Our working hypothesis is asymmetrical strain from casting resin shrinkage prior to any cutting or grind. Maybe related to non-release from the mould. We feel that this jives with the crack signature. When possible, we are going to try very slow epoxy curing (24 hours) with Teflon coated moulds only, and see if turns off the phenomena.
BTW Inge, I agree 100% with the finest epoxy, and diamond wheels. How much $ for a water jet?
Chris
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Thursday, February 18, 2010 4:09 AM
To: [log in to unmask]
Subject: Re: [TN] Die cracks
I'm late with comments. Just a short one. Agree with Trikeman. Preparation issue. I have exactly same photos to share with you. Cutting must be done with finest diamond blade, low speed, low weight and a mixture of water and YES. Do the cutting a bit from the chip, mold with hardest expoxy, and polish with 600 and finer. Use polish oil!! Will take time. Lots of time.
Faster solution: If you have a water jet, you will cut the board into pieces 10 times faster.
Inge
-----Original Message----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
Sent: fredag 12 februari 2010 18:57
To: [log in to unmask]
Subject: Re: [TN] Die cracks
Chris,
I looked at your photos. The first two links reference Die Crack 01, the last two reference Die Crack 02, so there are only two pics to look at.
Die Crack 01, I played with the exposure a bit and it appears that the crack runs to the right, to the edge of the die and stops. The corner of the device appears to be pretty badly chewed up, although the image is slightly out of focus at higher zoom. Would be nice to see the entire chip, to see what is going on over on the left side as well. Enhancing the contrast did not appear to show any delamination between the device and over-mold compound, therefore I am inclined to believe that it is NOT a popcorn situation.
Die crack 02 also did not appear to show and separation between the over-mold and the device. This crack appears to terminate on the right side of the chips surface before even reaching the edge of the device, but what does the left side look like?? The terminus of the crack on the right side appears to stop at the surface of the chip and not enter the over-mold.
Both of these would likely have shown up quite well with a low frequency thru transmission acoustic scan - thinking of my days on my good old Sonix acoustic scope...
I would like to see more info, but my knee-jerk reaction at this point [with caveat] is that this is a die packaging/handling related issue, not an issue related to your assembly/reflow. Need to see both ends of the cracks and some clear high-mag looking for delam in the structures.
FWIW
Steve C
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Mahanna
Sent: Friday, February 12, 2010 11:45 AM
To: [log in to unmask]
Subject: [TN] Die cracks
Hi TN
We are in desperate need of QUICK expertise and/or opinions wrt the die cracks that can be seen from the links below. Sorry Steve, didn't want to circumvent you but I've got to come up with an opinion by EOD. BGA/CSPs were encapsulated in slow cure epoxy prior to excision. We section BGAs everyday and rarely see die cracks.
I've got to decide whether we should recommend that the CM build up a few more for NDT (acoustics, CT..)
I know: max cool-down was probably 2.2-2.8C/ second; ICT was not strain engineered or tested; SAC305 balls, paste; parts passed ICT, ..but cracks don't look to reach active circuitry Don't know: laminate, peak reflow Can't say: pretty much anything else
Thanks!
Chris
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