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January 2008

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Subject:
From:
Yuan-chia Joyce Koo <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Yuan-chia Joyce Koo <[log in to unmask]>
Date:
Fri, 18 Jan 2008 09:10:23 -0500
Content-Type:
text/plain
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text/plain (324 lines)
Is the ball are Sn plated and reflowed?  If it is, you have surface  
migration via residual plating solution... oxidation/corrosion  
product on surface due to improper rinsing.... (haven't seen that for  
a long time...).
           jk (wild goose....)
On Jan 18, 2008, at 8:36 AM, Bev Christian wrote:

> Sigh.  My replies below, again in BOLD.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M.
> Sent: Friday, January 18, 2008 7:25 AM
> To: [log in to unmask]
> Subject: Re: [TN] SV: [TN] SV: [TN] Solderability of Tin-Gold
> Intermetallic
>
> I usually like to respond with answers not questions but un
> Bev,
>
> I usually like to respond with answers not questions but until I
> understand an issue it is hard to respond with answers.  Your question
> is still vague to me.  Although you said that you are talking about
> trying to make joints and not talking about joints already made, you
> also said that what you are exploring is whether unusual amounts of  
> gold
> tin intermetallics on the surface of the solder balls pried off of
> boards that show head in pillow did not solder because of these
> intermetallics.  I view Head-in-a-Pillow solder joints as joints that
> have not wet for some reason.
> WELL I GUESS THIS IS A CASE OF SEMANTICS. I DON'T CONSIDER THAT A  
> JOINT
> WAS EVER MADE. If I HAD BEEN TALKING ABOUT A FRACTURE BECAUSE OF HIGH
> AMOUNTS OF GOLD in A SOLDER JOINT, ok, BUT THERE WAS NO METALLURIGICAL
> BOND EVER MADE IN THE CASES I AM DISCUSSING.
>
>   If that is the case then I assume the
> gold-tin intermetallics you are talking about are from the  
> materials in
> the BGA and not from the surface finish on your board.  I'm having
> difficulty believing that there is enough gold in under-bump
> metallization to cause a gold-tin intermetallic issues.
> TRUTHFULLY SO AM I, BUT WE ARE EXPLORING ALL AVENUES.
>
>   You indicated
> that you used XRF to measure the gold concentration on samples that  
> you
> pried off.  Ingemar anticipated that you meant the part of the ball  
> that
> was soldered to the board pads.
> NO. I MEANT THE BALL OF THE PART.
>
>   However, from the way you worded your
> inquiry I can't tell if you meant you looked at the board pad or  
> the BGA
> ball.
> THE LATTER.
>
>   I assumed, maybe incorrectly, that you looked at the surface of
> the ball after peeling the BGA off.
> CORRECT, AFTER PEELING OFF THE BGA. WE MEASURED 1.28 +/- 0.15% [Au]
>
>   If that is the case then I would
> assume that you should be able to take a non-soldered BGA and measure
> the gold in a BGA ball
> CORRECT AND GOT 0.86 +/- 0.8% [Au]
>
>  and then put the BGA though a reflow process
> without any solder paste and re-measure the ball after reflow to  
> see if
> the gold concentration changed.
> ON THE LIST OF THINGS TO DO.
>
>   I'm assuming that you're talking about
> small size uBGA's with ball diameters in the 10 to 13 mil range.
> THERE YOU AMERICANS GO AGAIN. I KEEP TELLING YOU THE METRIC SYSTEM IS
> NOT A COMMIE PLOT.  :) THE BALL DIAMETERS ARE 300 MICRONS.
>
>   Since
> the XRF penetration depth is typically between 1 and 2 mils you'll be
> analyzing the gold in the top surface of the ball.  I assume that  
> if you
> don't see a high gold concentration in the un-reflowed solder ball  
> that
> the gold-tin intermetallics you see after the reflow process  
> migrated to
> the surface of the ball during the cooling process.  If that is the  
> case
> they probably aren't the reason for the Head-in-a-Pillow soldering
> defects.
> THAT'S WHAT ONE WOULD THINK. HOWEVER IF THE DIFFERENT BGA NEXT DOOR
> SOLDERS PERFECTLY, YOU LOOK AT ALTERANTIVES.
>
> MAN, THE E-MAILS ARE COMING FASTER THAN I CAN ANSWER... AND I DO  
> HAVE A
> JOB - OTHER THAN TECHNETTING.  :)
>
> Regards,
> George
> George M. Wenger
> CommScope / Andrew Wireless Solutions
> Senior Principal FMA / Reliability Engineer
> 40 Technology Drive, Warren, NJ 07059
> (908) 546-4531 [Office]  (732) 309-8964 [Cell]
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
> Sent: Friday, January 18, 2008 4:45 AM
> To: [log in to unmask]
> Subject: Re: [TN] SV: [TN] SV: [TN] Solderability of Tin-Gold
> Intermetallic
>
> Finally, you disclosed what you really wanted to know. You pried off
> BGAs and found unusual amount of AuSn platetlets on the surface of the
> balls. This is my opinion: If the wetting on AuSn formations is  
> good or
> not is not the first thing you ought to think of. When you say the
> 'surface', I anticipate that you mean the part of the ball THAT WAS
> SOLDERED TO THE BOARD PADS. If THAT surface has lots of AuSn  
> formations,
> then I understand how the BGAs could be pried off = probably too much
> gold from the ENIG pads = something wrong in the soldering process. IF
> the solder joints were normal, you would have a real problem to get  
> the
> BGAs off the board, because healthy solder joints are enormously  
> strong.
>
> As to your question, whether the solder wets to AuSn intermetallics, I
> think all is depending on what atmosphere you had during the IMC
> formation. AuSns are, as you know, not chemical bonds, but alloys with
> its gold phases and its tin phases. Now, if the tin phases are not
> oxidized, then you ought to get good wetting, and vice versa. In
> practice, I think that there is a high risk of AuSn oxidation, that's
> why you do AuSn soldering of RF details in vacuum or inert atmosphere.
>
> Inge
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
> Sent: den 17 januari 2008 16:42
> To: [log in to unmask]
> Subject: Re: [TN] SV: [TN] SV: [TN] Solderability of Tin-Gold
> Intermetallic
>
>
> Bingo!  There is the crux of it. I am not talking about joints already
> made, I am talking about TRYING to MAKE joints.
>
> I hope I don't get in trouble for this.
>
> My original, vague question was because of - head in pillow. Inge
> guessed correctly.
>
> I don't mean to be rude, but we already have a very extensive fishbone
> for this, so please don't come back with the usual reasons.  They are
> being addressed, trust me.
>
> What I am exploring is whether unusual amounts of gold tin
> intermetallics on the surface of the solder balls pried off of boards
> that show head in pillow did not solder because of these materials.
>
> Bev
> RIM
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner  
> Engelmaier /*
> Sent: Thursday, January 17, 2008 10:36 AM
> To: [log in to unmask]
> Subject: Re: [TN] SV: [TN] SV: [TN] Solderability of Tin-Gold
> Intermetallic
>
> Hi All,
> We know that Cu6Sn5 is non-solderable after oxidation. However, I  
> do not
> know whether we really KNOW that it is solderable prior to that,  
> and we
> know even less about the solderability of AuSn4, Ag3Sn, or for that
> matter Sn3Ni4.
>
>
>
> Werner
>
>
>
> **************
> Start the year off right.  Easy ways to stay in shape.
>
> http://body.aol.com/fitness/winter-exercise?NCID=aolcmp00300000002489
>
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