Chris, Steve,
I happened to have an SEM picture of that "air gap" you described (had a case of faulty LED) so can send it over if it helps.
Regards,
Vladimir
Vladimir
SENTEC Testing Laboratory Inc.
11 Canadian Road, Unit 7.
Scarborough, ON M1R 5G1
Tel: (416) 899-1882
Fax: (905) 882-8812
www.sentec.ca
-----Original Message-----
From: Steven Creswick <[log in to unmask]>
Date: Mon, 10 May 2010 18:49:07
To: <[log in to unmask]>
Subject: Re: [TN] LED wiring bonding
Chris,
Ahh! Something I can help out with.
A great deal depends on the package structure, obviously wire bonding
process control, and whether it is Au or Al wire.
Wire bonding defects are rare, if the materials and process is under
control.
The caveat is that Pb-free reflow temps may cause extra expansion in the
molding compound as the material goes above it Tg during reflow/s.
Many/most clear mold compounds have Tg's in the <135-155°C range. Can't put
silica in there to help modify the CTE...
Moving mold compound during temp cycles can often break the bond wire near
mid-span. The wire will actually neck down to the shape of an hour-glass
funnel as it stretches. ... then breaks. Oops.
You can also get bond lifts, but if that is the case you may also see
delamination from the bonding surface. Hard to describe what it looks like
other than to say that you can see an air-gap there.. If you don't see the
delam, one just may have a poor bond.
You can also shear the ball/bond off the die, but many times one will place
a low modulus material on top of the chip to mechanically de-couple from the
higher modulus molding compound in an attempt to alleviate this.
First thing to do is x-ray to find where the discontinuity is. If that
fails, then move on to selectively polishing the device down so that you can
see inside clearly.
I have been known to take a really tiny 0.010-0.015" dia drill bit in a pin
vise to gain access to the interior for dye testing to check for
delamination [but that can be classified as destructive - although certainly
illustrative!]
Lastly, it may also be possible to shear the die off, but like you, I would
look at the wire first. A quick look at the bonds can often tell you if
they are over bonded, or something was moving during bonding. Once you get
it polished flat, you may be able to get a reasonable picture.
Steve Creswick
http://www.linkedin.com/in/stevencreswick
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Mahanna
Sent: Monday, May 10, 2010 5:49 PM
To: [log in to unmask]
Subject: [TN] LED wiring bonding
It's all LEDs these days!
We have an FA that shows poor wire bonds to GaAlAs/GaAs in microsection
after assembly. The failure is seen in the parts per hundred across
different lots and slightly different part numbers of LEDs.
The assumption would be that LED wiring bonding defects are very rare. Is
there something particular about these? Or is it the crimping and/or
Pb-free assembly that is pulling them apart?
The ones that seem more susceptible are GaAlAs Double Heteros.
Thanks.
Chris
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