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June 2010

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Subject:
From:
Joyce Koo <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Joyce Koo <[log in to unmask]>
Date:
Thu, 10 Jun 2010 15:12:58 -0400
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text/plain (2125 lines)
It is known failure (5-6 years ago) that improper selected optical encapsulant will pull out of the wirebonds due to mismatch CTE.  However, if you buy good stuff from brand name vendor, such as Nichia, Cree, Luxeon, etc, you should not have any problem (it should all be sort out years ago).  If you buy no-name brand, oh, well, a price to pay for their learning curve.  Being said that, there is another angle of the problem, if your thermal design is outside the vendor recommended range, you can pull wirebond out too.  Make sure you got sufficient heat sink with proper temperature differential (a block of metal with same temperature can not sink any heat).
If you really want to understand stuff, check FA 6 years ago... (most of them do not publish failure)... good luck.
           Jk (1.92 cents)

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Mahanna
Sent: Thursday, June 10, 2010 2:57 PM
To: [log in to unmask]
Subject: Re: [TN] LED wiring bonding

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]<mailto:[log in to unmask]>] On Behalf Of Victor Hernandez







Sent: Thursday, June 10, 2010 8:04 AM







To: [log in to unmask]<mailto:[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]<mailto:[log in to unmask]>] On Behalf Of Steven Creswick







Sent: Tuesday, May 11, 2010 5:32 AM







To: [log in to unmask]<mailto:[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]<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]<mailto:[log in to unmask]>] On Behalf Of Steven Creswick







Sent: Monday, May 10, 2010 6:49 PM







To: [log in to unmask]<mailto:[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]<mailto:[log in to unmask]>] On Behalf Of Chris Mahanna







Sent: Monday, May 10, 2010 5:49 PM







To: [log in to unmask]<mailto:[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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