Hi Bob,
we land far from the set down point, but it may be necessary to understand the said mechanism once and for all. You are probably right. What if we combine von Mise and Mohr? The first one (3D) gives that the stress level is much higher close to the ball-pad interface than in any other part of the ball. The later (2D) gives that the stress level in one and the same plane is maximum at the diagonal, as you say. If we pretend to plot the stress along the pad, let be in the P-rich band, then the force amplitude is of the elongative type with a positive sign and increases towards the periphery of the pad. When you round the edge of the pad and go vertically along the pad (thickness variations), then I suspect that the stress changes to compressive type with a negative sign. That model would explain why there is such a sharp rupture projection and would also explain why the solder is untouched along the circumperiphery . I'm not a math guy, so I would rather let our experts do the calculation. But noone is interested to pay for such a deep diving. So, for me it's enough to get thumb up or no to my theory.
My comrade in the game, the BGA expert I mentioned, had a theory about the good solder adhesion along the pad circumperiphery. He meant, that certain phenomenons in the electroless nickel process results in a cleaner and nearly P-free nickel along the edge. Comparing the two ideas, I cannot see why there should be such a local and good nickel, so I lean more to your idea, Bob, or the combination which I proposed.
I don't dare apply this on the failure analysis method until I get a go from someone with better insight than what I have.
Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: onsdag 3 mars 2010 22:45
To: [log in to unmask]
Subject: Re: [TN] Hackneyed?
Hi,
Correct me if I am wrong but the craters and edge effects are expected as most materials fails in shear when a sample is pulled apart. I also recall seeing small inverted craters at the edges of samples due to the nature of stresses at the outside edge of the sample. I would expect the failure mode to be very different along the outside edges of the sample being pulled.
At one time I did a control system for a material testing lab and we pulled apart a lot of 1/4 inch round aluminum samples. The breaks were classic examples of Morh's Circle showing maximum stress at a diagonal.
Bob Kondner
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Wednesday, March 03, 2010 2:37 PM
To: [log in to unmask]
Subject: Re: [TN] Hackneyed?
Hi Inge,
do not put words in my mouth.
I suggested that that crater-like ring is the only good solder connection with the center of the crater floor having a brittle interfacial separation was typical of BP.
Not those worm-like bands of whatever material-are these bands on top of the grain boundaries?
And no, BP is characterized by 'normal' Sn-Ni IMC development-the P-concentration does not, at least initially, inhibit IMC formation; it weakens the interfacial bond.
Werner
-----Original Message-----
From: Inge <[log in to unmask]>
To: [log in to unmask]
Sent: Wed, Mar 3, 2010 12:15 pm
Subject: Re: [TN] Hackneyed?
Hi all,
about 'black pads' . Werner suggested that this was typical BP. However, had a visit from a very experienced person who cast an eye on our problem.
After a few minutes of silence and looking at my images, he scratched his head and cleared his throat and said 'Well, this Engelmaier, know perfectly who he is and I respect him very much, but I'm not with him. IF.. if this is a black pad case, then you can't have such a well developed bands
(plural) of Tin/Nickel IMCs. The very central thing about black pads is that the P-rich surface effectively stops the growth of healthy tin/nickel products. In my opinion, you have rather normal such IMCs, with the typical variation in strength, and the rupture occured in a weak region of these layers'. When you pull or prey off the BGA, the thick solder hanging on outside of the periphery of the pad is stronger than the inner part of the pad, hence the separation will begin in the middle. '
His argument seems to fit better into the story. Furthermore, this new explanation seems to coincide with Bunni's theory on hyper Nickel boundaries. But before going on his line, I would like to hear your meaning. Can you have a very high P%age and still no BP?
Thanks for your valued help
Inge
--------------------------------------------------
From: "Hernefjord Ingemar" <[log in to unmask]>
Sent: den 2 March 2010 00:20
To: <[log in to unmask]>
Subject: Re: [TN] Hackneyed?
> HiWayne,
>
> thanks for your eye on this.--- Correct, no vias under balls. >
Non-soldermask defined. Not overbaked. VP soldered, shadowed profile from > Xilinx datasheet. Checked. Question is how the shipped boards will behave > in the future. Some reports have arrived about pressure sensitive boards. > Maybe a consequence of BP?
>
> Werner seems rather convinced that these are real BPs, and considering
> >
other's and my own investigations, I've come to the conclusion that this > is to be reported to our staff as being a issue.
>
> We have left the ENIG behind, use HASL'd board since a year. I'm going
> to do similar investigation on some samples from that production line
> for >
comparison. I really hope that I will see quite different solder joints > with elongation rupture'd Tin/Lead micro joints all over the pad instead > of these 'rotten' ones.
>
> I'll let you know what happens.
>
> /Inge
>
>
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne -
> IIW
> Sent: måndag 1 mars 2010 14:56
> To: [log in to unmask]
> Subject: Re: [TN] Hackneyed?
>
> Hi Inge-
>
> Just completed an emergency FA in another project, so I'm just tuning in.
> Others asked about a cross-section. Did I miss your reply? Looking
> at >
the grain structure and elemental analysis for the different zones of the > joint would probably help give us more insight.
>
> Also, I haven't seen any theories on the variation in properties
> across >
the joint (periphery looks great, but interior looks rotten). Has Werner > given a theory as to why that is?
>
> You haven't suggested dismissing the theory that microvias have
> anything >
to do with this phenomenon, yet I don't see evidence in your pix of any > microvias, so I presume the pix all came from pads which don't have > microvias. Is that correct?
>
> I agree with Werner that I've seen overbaked joints do this. In any
> case, such joints cause nightmares because they look nice (and are
> nice) on the outside.
>
> Wayne
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
> Sent: Saturday, February 27, 2010 11:01 AM
> To: [log in to unmask]
> Subject: Re: [TN] Hackneyed?
>
> Hi Inge,
> Yes, I have seen this before-not with those absolutely great pictures,
> but with very similar elemental distributions.
> In that case, the assembly saw nine (9!!!) soldering exposures.
> I looks to me, your assembly was either exposed to soldering
> temperatures too often and/or too long.
> Werner
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
> Sent: Wednesday, February 24, 2010 2:54 PM
> To: [log in to unmask]
> Subject: [TN] Hackneyed?
>
> Hi Folks,
>
> am asking you to sacrifice a minute on a topic that may make you
> sick, as much as it has been discussed. Have a look at the SEM images
> that
> (hopefully) will be beamed by our most honorable guy, Dr Gregory. By
> order:
>
> 1. Pic 38 BGA ball after the preying off the 560 ball BGA from the board.
> SEI mode.
> http://stevezeva.homestead.com/files/XILINX_Daniel_38.jpg
>
> 2. pic 39, same at x 5,000
> http://stevezeva.homestead.com/files/XILINX_Daniel_39.jpg
>
> 3. pic 40,samet at x 10,000
> http://stevezeva.homestead.com/files/XILINX_Daniel_40.jpg
>
> 4.pic, 41 same at x 20,000
> These four show the face of the pulled-off balls. The package/board
> was >
sectioned into halv by half inch pieces with a thin blade diamond saw and > then each BGA section was separated from the board by means of a steel.
> http://stevezeva.homestead.com/files/XILINX_Daniel_41.jpg
>
> After SEM 1-4 I made EDS, which can be seen below.
>
> 5. http://stevezeva.homestead.com/files/XILINX_Backlund_Ball_2.doc
> As you can see, there is nearly no Lead (!), a high w/% Nickel, a
> little >
Tin
>
> and lots of Phosphorous. Clearly, the separation has been in a
>
> Now, continue with having a look at the board pads after the preying-off.
>
> 6. pic 44, Board pad at low magnification and SEI mode.
> http://stevezeva.homestead.com/files/XILINX_Daniel_44.jpg
>
> 7.pic 46, same at x 5,000
> http://stevezeva.homestead.com/files/XILINX_Daniel_46.jpg
>
> 8.pic 48, same at x 10,000
> http://stevezeva.homestead.com/files/XILINX_Daniel_48.jpg
>
> 9. pic 50, same at x 20,000
> http://stevezeva.homestead.com/files/XILINX_Daniel_50.jpg
>
> 10. pic 51, same but with BS mode
> http://stevezeva.homestead.com/files/XILINX_Daniel_51.jpg
>
> 11. XILINX_backlund_pad_1.doc
> http://stevezeva.homestead.com/files/XILINX_Backlund_Pad_1.doc
>
> As you can see here, there is more Lead, a high w/% Nickell, much more
> Tin and still lots of Phosphorous.
>
> Discussion:
> Obviously, real and reliable solder joints are only in the >
circumperipheric
>
> of the whole (best understood with pic 10). How strong the adhesion
> was >
in the main part of the joint, we don't know. From the function (final) > electrical testing, we have experienced several cases with disturbed > signals. Many a times, we have had to remove the BGA and supersede with a > fresh one. The test engineer told me, that by slightly add pressure to the > BGA, he may get the signal through again. He thinks we have an issue with > the uvias, others think there is a problem with the BGAs, myself I'm > committed to my theory that this has to do with the rotten interface > between
>
> the Tin/Lead and the ENIG pads.
>
> Question:
> Is anyone familiar with the said? What is your opinion about the >
demonstrated solder joints? Can yoy get cracks and electrical misfunction, > despite that hundreds of balls are holding the BGA close to the board in a > 'iron grip' ?
>
> Wayne? Steve? Dave? George W? Vlad?
>
> Thanks in advance. You'll have to wait for Steve to fix the exhibition.
>
> /Inge
>
>
>
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