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October 1999

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Date:
Tue, 5 Oct 1999 20:46:23 -0400
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few points:
(1) cross section of Au/Ni/Cu should be done with Ni over plating on top of
the Au prior to cross section mounting.  the cover layer can be any hard
metals, what ever you can easily put it on top of the Au without damage the
surface features.  The Ni would prevent the smear of Au layer.  The cross
section not only show the Au thickness, it also reveal the morphology of the
grain and grain boundary structure, columinar or equiaxial.  Most
importantly, it shows the interfacial structure of Au/Ni/Cu.  If the
interface is porous, the Cu will diffuse to the top surface and make poor
reliability products.  Plating parameters are important to achieve the flat
dense interface, uniform coverage and good quality Au/Ni layers. (I guess
all the plating experts will tell you more about the current density, pH,
tank design, edge flux distortions, organic additive entrapment, etc. much,
much more than I do).
(2) The reasons for XRF and cross section measurements have some discrepancy
are:
 (a) porosity of the plating compare to the calibration sample.  The
calibration sample (coupon) normally made by dense, uniform Au over Ni. The
actural measurement done on the PCB sometimes are not as uniform at the
interface and exist some porosity.. This is more problmatic when immersion
Au is used.  Since the immersion is based on atom (ion) exchange process,
the thickness of Au has a layer of mixed Ni/Au plus some organic additives.
It is rather difficult to define where exactly where the interface is when
cross sectioned.  Porous plating (e.g. columinar grain in electrolytic Au)
normally gave a thicker reading on cross section data than the XRF measurements.
(b) Correct me, if I am wrong,  based on my understanding, the immersion Au
is less dense than the "properly" electrolytic plating Au.  Some of the
discrepancy is due to the density difference between the calibration sample
and the immersion Au.  (The XRF is based on the absorption edge of the
charcteristic X-ray, the thickness calculation is based on the number of the
atoms/unit volume using calibration samples).
(c) wrong calibration sample (even the same stacking sequence,e.g. Au/Ni/Cu,
but different thickness) may raise the error.  (sometimes, we only have
limited calibration samples on hand... you have do some "fancy foot work" in
order to get the actual measurement... particularly, for the new stacking
sequence of material...)...If you can't trust your equipment, you might as
well forget the measurement.
....this is just my limited understanding of the issue (1 cent)...
hopefully, someone will give their insight (millions).
                                  jk



At 10:11 AM 10/5/99 -0700, you wrote:
>I'm not a PWB shop person, but I've supported many in the past.  I'm not
>sure the shops are set up for 0.5um resolution.  I'm not comfortable with
>those Beuer polish set ups for 1um resolution, but I don't know the product
>in detail.  I'd like to know if there is a need and I'll send in my
>suggestions (as one who has done microscopy 1nm-10um samples on a research
>basis for years).  But I'll save that lengthy verbage for later.
>
>I've found cross-sections can be very misleading because of metal smear at
>thicknesses of ~1um.  It is critical that the last polish be done parallel
>to the interface of gold/nickel with much care.  I'm not a fan of the XRF
>technique because it relies on mathematically modeling your metal system-
>perfectly flat interface, no thickness variations over sample area, and the
>interactions for fluorescence in each layer.
>
>I suggest you do some hard work up front, but then it will become routine
>and easy later: find a selective etch for gold or nickel (you'll probably
>have more luck with the latter) or one that highlights the interface (takes
>advantage of that mismatch of metal-metal interface...back to that wire
>bonding story, Inge!).  This also means, you can't just put your sample on
>the automatic polisher without paying good attention to the last step.  You
>may do it by hand using 600 grit (I think that is per square inch) followed
>by 1000 grit sand paper.  A true polish (<0.2 um in your case) may be
>needed, but I'd try to avoid it because it's a hassle to keep a clean plate
>for that.  And don't contaminate your polish plates with dirty parts
>holders.
>
>
>
>-----Original Message-----
>From: TOE /Torben Østeraa <[log in to unmask]>
>To: [log in to unmask] <[log in to unmask]>
>Date: Tuesday, October 05, 1999 1:05 AM
>Subject: Re: [TN] Measurements of Gold over Nickel
>
>
>Ingemar, and others:
>While we're on the subject:
>We also use 'stoneage' technique to inspect nickle/gold
>plating from sub-contractor. He's using some sort of
>non-destructive method to verify thickness, and we do not
>always agree on the result.
>Apart from the obvious problem of thichness variation over
>the panel, is there a predictable difference in thickness
>found using different techniques?
>
>Regards
>
>Torben Østeraa
>Printca AS
>Denmark
>
>
>>>-----Original Message-----
>>>From: Ingemar Hernefjord (EMW)
>>>[mailto:[log in to unmask]]
>>>Sent: Tuesday, October 05, 1999 8:42 AM
>>>To: [log in to unmask]
>>>Subject: Re: [TN] Measurements of Gold over Nickel
>>>
>>>
>>>Edward,
>>>we have found that there is nothing better than ordinary
>>>stoneaged crossectioning and study in a metalographic
>>>microscope. In mass production, samples are usually enough.
>>>Such examinations don't take time and they tell a lot more
>>>than betascope. And...you can store the images and have a
>>>look back up and then.
>>>Regards
>>>Ingemar Hernefjord
>>>Ericsson Microwave Systems
>>>
>>>
>>>I am looking after alternative method of measuring of
>>>electroplated Gold
>>>over Nickel for production .Gold thickness is in range from
>>>0.75 to 2.5
>>>microns.
>>>I have in my plant beta scope with Promethium source with lot
>>>of problems.I
>>>have also XRF,that can not be placed in production area.
>>>Any sugestions?
>>>
>>>Edward Szpruch
>>>Eltek , Manager of Process Engineering
>>>P.O.Box 159 ; 49101 Petah Tikva Israel
>>>Tel  ++972 3 9395050 , Fax  ++972 3 9309581
>>>e-mail   [log in to unmask]
>>>
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