Hi Steve-
Why are there "reinforcement" bonds on the stitches? Usually those are a bad sign, indicating very poor bondability.
Did you get the delaminated areas to turn blue using some kind of dye? Why does the leadframe material appear pink instead of silver?
I guess it is obvious that thermal expansion of the molding material is responsible for the wire breaks, but I'm sure you've researched the molding materials and determined they it is appropriate, so you must be seeing the end results of another defect, such as:
-If the reinforcement bonds were placed because the leadframes had poor bondability, it is possible that the same affect which caused the poor bondability caused poor bonding between the leadframe and the molding compound. (I once had some LED leadframes which were so hard and smooth that even the die attach adhesive wouldn't bond!) If the bond between the molding compound and leadframe is poor, then the leadframe wouldn't be able to help stabilize the molding compound, possibly allowing more movement than in a comparable unit without the bondability problem.
-If the molding compound were improperly cured, then the most common defect would be a reduced Tg. You could try baking the devices for several hours at an elevated temperature to see if that helps.
-Ball bonding wire usually has an elongation on failure of at least 3%, but wedge bonding gold wire has an elongation of 0.5-2%. Is it possible somebody ordered the wrong wire? I haven't tried to ball bond with the wedge bonding wire, but I suspect that you would be susceptible to cracks in the heat affected zone just above the ball. However, with nice long loops you might not notice. Again, those reinforcement bonds could be a red flag.
Wayne Thayer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
Sent: Sunday, June 13, 2010 12:22 PM
To: [log in to unmask]
Subject: Re: [TN] LED wiring bonding
Victor,
I have posted a few pictures on the IPC site -
http://ipc-technet.groupsite.com/galleries
I am probably way down at the bottom under "Mold Delamination from LED's and
Leadframes"
I had to crop them to hide the guilty, etc., but I believe they will still
get the point across
I've shown delamination of the mold compound from the lead frame and surface
of the LED as well as necking of the wire due to CTE issues.
Steve Creswick
http://www.linkedin.com/in/stevencreswick
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