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August 2006

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Thu, 17 Aug 2006 22:22:20 -0400
Content-Type:
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Hi Richard,
You are absolutely correct—there is always an Ni/P layer between the Ni 
layer and the Ni-IMC layer. The amount of this Ni/P layer is determined 
by the original P-concentration within the plated Ni as well as the 
amount on Ni being dissolved in the Sn leaving the P behind. The 'Black 
Pad' condition is not an all-or-noting issue, but a gradual 
one—thehigher the concentration the weaker this layer.

Werner

-----Original Message-----
From: [log in to unmask]
To: [log in to unmask]; [log in to unmask]
Sent: Thu, 17 Aug 2006 3:56 PM
Subject: RE: [TN] LEADFREE BGA ISSUE

   Thank you for your reply, Werner.
If the dissolution rate is not affected by the varying amounts of Pb in
the alloy, or at least not significantly, then I suspect that the
inability of Sn to dissolve in Ni at higher temperatures seen with
Pb-free processing is more likely related to the presence of the varying
levels of phosphorus. These are directly affected by temperature.
While a small amount of P is needed during plating and is deposited with
the electroless nickel to avoid acidic corrosion, too much of a layer of
P under the gold after the subsequent immersion gold plating process
creates a barrier to soldering to the nickel underneath. During
soldering the gold is dissolved into the solder and this leaves the
phosphorus barrier exposed which of course does not solder very well.
The pad is left de-wetted. The greater the heat excursion, the greater
the p-barrier and also the greater the nickel oxide barrier. While it is
not necessarily a "Black Pad" in that sense of the word, it is still a
barrier that gets worse with an increase in temperature.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Thursday, August 17, 2006 1:18 PM
To: [log in to unmask]
Subject: Re: [TN] LEADFREE BGA ISSUE

Hi Richard,
I do not know to what extent the Pb-content would affect the dissolution
rates, but it stands to reason that they would be lower. I have not seen
any evidence that the dissultion rates in SAC solders should be less;
qute the contrary, all evidence shows higher dissolution rates as
evidenced by Cu dissolutions at PTHs and the dissolution of soldering
irons and wave solder machines.
Now, the evidence shows that SAC does not wet as well as SnPb, but I do
not think-it sure does not seem so-that has anything do to with the
dissolution rates per se.
The dullness of the surface of the Ni layer under the Ni-IMC layer most
liekly has to do with the presence of Ni/P enen though is has not led to
a 'Black Pad' problem.

Werner

-----Original Message-----
From: [log in to unmask]
To: [log in to unmask]; [log in to unmask]; [log in to unmask]
Sent: Thu, 17 Aug 2006 1:55 PM
Subject: RE: [TN] LEADFREE BGA ISSUE

    Don't always assume that just because the finish was ENIG, that it
was the root cause. It may have contributed to it, but usually there is
no one root cause when this happens.
Secondly, even when good wetting does take place on ENIG-finished pads,
upon removal the pads often appear dull. This is just the nature of the
nickel finish. It does not always indicate a solderability problem with
the finish.

The problem may have been a reflow profile that was not hot enough, with
TALT being insufficient to form a good IMF to the nickel base. It is
slightly more difficult to achieve this with standard 63/37 solder on
nickel than on copper, and appears to be even more difficult with SAC
alloy on nickel than on copper. While the temperature is significantly
hotter, the SAC alloy does not seem to form a good IMF with nickel as
readily as the 63/37 solder does.
Werner, the information on the dissolution rate of nickel into a tin
alloy from Klein-Wassink's charts are based on pure tin, not a tin/lead
alloy. What is the effect on nickel's dissolution rate when the amount
of lead in the solder alloy is varied, say from 37% to 0%?

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Colin McVean
Sent: Thursday, August 17, 2006 9:34 AM
To: [log in to unmask]
Subject: Re: [TN] LEADFREE BGA ISSUE

<Certain pads were very dull and when solder is applied on it, it seened
there was dewetting issue>

Sounds like an issue with the solderability of the ENIG rather than the
substrate. Check with your PCB fab supplier for their comments.

<By the way, such kind of issue happened only to one customer...>

It only takes one instance of poor process control on ENIG to see faults
occurring. Looks like this is what has happened in this case.

Rgds
Colin

Colin McVean M.Inst.C.T.
Production Manager
Artetch Circuits Limited
Main: 01903 725365
DD:    01903 712926
Email: [log in to unmask]
www.artetch.co.uk
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Poh Kong Hui
Sent: 17 August 2006 15:24
To: [log in to unmask]
Subject: Re: [TN] LEADFREE BGA ISSUE

Hi Technetters,
Could someone advise me the reasons to following issue.

1.      There are total 25 boards for the evaluation purpose.
2.      Round 1, we assemble d11 units SAC BGA onto board of ENIG. The
result
was 10 passed functioonal, 1 piece was not working. There was no signal
& power to the BGA unit.

3.      Round 2,  4 boards were being assembled, 3 passed, 1 was not
working

4.      Round 3, 10 boards were biring assembled, 1 was working, 9 not
working.

The above-stated boards undergo the same temperature profile.

At the beginning, we were suspecting the board of ENIG. When 1 BGA was
being removed from the failed board, and intend to perform the
reballing, we realise the not all BGA substrate pads were able to
solder. Certain pads were very dull and when solder is applied on it, it
seened there was dewetting issue.

My question is, if the failure of the BGA that was due to BGA's
substrate or otherwise.

By the way, such kind of issue happened only to one customer...
Thanks

KH Poh

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