Wayne, others -
The object of the photos was to illustrate delamination of mold compound
from the surface of the device - which is what Chris was asking about in the
beginning, I thought...
Valid points have been made, but the object was not looking for a ball bonds
to fail, none of the die were sheared off, these were test parts subjected
to grueling test regimes [exception was the axial], safety bonds were there
by design - not because of any bondability problems, colors may be off due
to camera, lighting, etc., for confidentiality reasons I only illustrated a
portion of the total assembly. The purpose was to provide Chris with a
representative photo of what it looks like to have mold compound delaminate
from the surface of the die [and yes, the mold compound can easily shear the
die off].
Yes, the wire had to stretch along its length to ultimately fail as is did.
Oh yes, these had some very unusual wire looping and EXTREMELY long wire
spans for LEDs.
Remedies include alternate molding parameters, optimized post-mold cure
parameters, alternate materials, de-coupling materials, platings, mechanical
anchor features, etc. etc...
The blue color is a dye - I find that a blue Sharpie magic marker works very
nicely. For some reason, the blue flowed better than Black or Red. The
Sharpie ultra fine tip does not work as well as the larger tip.
Regarding the axial LED delamination. After they cleaned up the process, it
was much better.
Enjoy
Steve Creswick
http://www.linkedin.com/in/stevencreswick
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne - IIW
Sent: Monday, June 14, 2010 12:51 PM
To: [log in to unmask]
Subject: Re: [TN] LED wiring bonding
The pix make it seem like you are looking for the ball bond weld to fail.
That should be impossible. Some of your pix look like there may be a
smeared-over crack/cleavage at the wire in the heat affected zone. That can
absolutely happen: Molding material moves, causes wire to shear, molding
material moves back to original position, shear closes up, and you get
continuity if you're lucky. Then cross sectioning smears the gold across
the gap, and you're left scratching your head!
Wayne Thayer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Mahanna
Sent: Monday, June 14, 2010 11:33 AM
To: [log in to unmask]
Subject: Re: [TN] LED wiring bonding
Just uploaded some of pictures from the project that started this thread.
Kevin, I think your warning from Nichia applies to this job. I have not
seen such warnings from the manufacturer of these.
http://ipc-technet.groupsite.com/galleries Ir Emitters
Chris
Au. No necking or breaking was noted. We believe that there may be some
correlation to the increased CTE of "optically clear" encapsulation vs. not
so clear or tinted. But the underwriting of my project is gone, for now.
Chris
From: Werner Engelmaier [mailto:[log in to unmask]]
Sent: Thursday, June 10, 2010 2:34 PM
To: [log in to unmask]; Chris Mahanna
Subject: Re: [TN] LED wiring bonding
Hi,
Are we sure it is the bond and not the wire itself that is breaking? We eed
to know whether we are dealing with Au or AlSi wire.
Been working on a wire break case.
Werner
-----Original Message-----
From: Chris Mahanna <[log in to unmask]>
To: [log in to unmask]
Sent: Thu, Jun 10, 2010 1:13 pm
Subject: Re: [TN] LED wiring bonding
I forwarded some optical pictures to Steve.
Steve, you're welcome to post them.
Chris
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor Hernandez
Sent: Thursday, June 10, 2010 8:04 AM
To: [log in to unmask]
Subject: Re: [TN] LED wiring bonding
Fellow TechNetters:
Was anyone able to provide a photo(s) of this LED bonding issue.
Victor,
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
Sent: Tuesday, May 11, 2010 5:32 AM
To: [log in to unmask]
Subject: Re: [TN] LED wiring bonding
Chris,
I get the shivers when people say they have a bond lift, because my instinct
tells me the process was not where it should have been, but there is still
room for optimism.
If properly bonded, and subsequently torn or sheared apart, I expect to see
a good deal of viable intermetallic beneath the bond. If there is little
sign of intermetallic formation, there was little bonding taking place -
read as not a very good bond in the first place.
By analogy, if one is crafting with wood and gluing panels together with a
good glue, then subsequently breaks the panel - the break should occur
somewhere other than at the glue line. The glue should be stronger than the
wood. As it is with wire bonding. The bond should be stronger than the
wire itself.
Regarding the blue dye. Not sure of the specific mold compound being used
[you could at least ask to see the data sheet ...], but it has been my
experience that the amount of dye used is actually extremely little. I
doubt that it would have significant effect on CTE, but you could be on to
something. Normally one tosses in a bunch of fumed silica, etc to help
control CTE. This doesn't do too well for maintaining light transmission,
however. I would also check to see if the clear and blue were actually
cured the same way, etc. That may have more effect on the result than the
pigmentation alone.
If you have intermittent bonds due to delamination, etc. sometimes you can
simply press on the device and get it to work again. You might also try a
bit of gentle heat/cold - say refrig to room, etc. Am not a fan of the
freeze spray stuff.
Wire necking will come about after many temp cycles - aka hundreds depending
on temp limits. It had better not occur after 1-4 reflow cycles!
I may also have a pic of delam if Vlad can not locate his.
Best wishes,
Steve Creswick
http://www.linkedin.com/in/stevencreswick
-----Original Message-----
From: Chris Mahanna [mailto:[log in to unmask]]
Sent: Monday, May 10, 2010 10:33 PM
To: TechNet E-Mail Forum; Steven Creswick
Subject: RE: [TN] LED wiring bonding
Steve,
Awesome. Now that you've walked me through it, I think our customer has a
mixed bag- some bond lifts and delam, all damaged by excessive soldering
excursions. I guess maybe part of my question was whether delam was a
latent defect from poor bonding.? Haven't found significantly necked down
Au wires yet.
Do you think they can get significantly better CTE, Tg out of blue lenses?
On our job, the blue had half the failure rate of the clear, even though
they are rated the same for soldering. BTW they are both really Ir
emitters; clear lenses with the better spectrum.
Thanks for the help. I will certainly try to get permission to post the
pictures. I'd like to see Vladimir's SEM image. Is there further FA to be
gleaned from SEM?
Thanks Werner!
Wayne, yes I believe that was the official blame game from the component
manufacturer. What do they look like when you burn them up? I'd expect to
see burnt plastic too.??
Chris
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
Sent: Monday, May 10, 2010 6:49 PM
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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