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Subject:
From:
Sherif Refaat <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Sherif Refaat <[log in to unmask]>
Date:
Mon, 15 May 2006 10:01:55 -0400
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Richard,

What you mentioned about Immersion Silver is mostly correct. However, there
is the special handling required (not to contaminate the surface of the
circuitry) and the effect of the air quality on the solderability. I have a
customer that keeps complaining about the solderability deterioration and
Silver color change if the boards are left 48 hours between the assembly of
both sides SMT components and a selective wave process. I suspect that the
air quality at the assembly site is the cause of the problem. But it is,
still, a problem that you do not see on ENIG and OSP.


Technetter,

There was a webinar last week that talked briefly about DIG (Direct
Immersion Gold). It seems to me that this new finishing has all the good
things and none of the bad things of the other finishings. Is any one using
this finishing in the industry? Any one can share his/her experience?



Sherif Refaat

----- Original Message -----
From: "Stadem, Richard D." <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, May 15, 2006 8:12 AM
Subject: Re: [TN] Question about ENIG in SnPb process . . . .


Sefika,
The component plating has little to do with the surface finish of the pwb,
in most cases.
If you are going to use a regular leaded 63/37 reflow profile, then your
choices of surface finish other than ENIG are immersion silver, organic
solder protectant (OSP), immersion tin, and HASL. They all have advantages
and drawbacks.
Immersion silver provides a flat finish, does not break down during
processing, allows you to solder directly to copper after dissolving the
silver into the solder joint, has no known negative byproducts such as
brittleness, subsequent growth after soldering, no oxide development, etc.
It does have storage issues, but they do not really affect solderability,
and are easily mitigated with proper packaging (Silver Saver paper and
hermetic shrinkwrap seal).
OSP is another finish that allows you to solder directly to copper.
Sometimes it does not apply well, and there can be solderability issues. It
does not endure processing very well, and should not be used for an assembly
that is going to undergo multiple heat and wash cycles such as double-sided
reflow, wave solder, probable rework where the entire board is going to be
heated (BGA rework), and finally hand soldering. By the time you finish the
assembly, there is little OSP left after a minimum of 5 thermal excursions
with a minimum of 5 wash processes in between.
HASL provides a very solderable surface, but it is not flat and this can
lead to stencil gasketing and poor paste printing. It can also be difficult
to use for fine-pitch parts. Sometimes the board vendor has difficulty with
the HASL process and may need to do it more than once. You can then get
boards that are virtually impossible to solder because of dewetting issues.
I have been using immersion silver for several years on hundreds of
different assembly types, literally millions of pwbs, with no problems.
Zero. Zip. Not a single issue related to immersion silver finish, and from
several vendors. I have used it on military supercomputers, missiles,
undersea torpedo assemblies, cellphones, hearing aids and medically
intrusive products, telecommunications platforms and servers, automobile
under-hood instrumentation, flight control computers, navigational
equipment, underwater fishfinders, avionics systems including altimeters,
windspeed indicators, laser gyroscopes, MEMS gyroscopes, level indicators
for huge road graders, even ultrasonic wash machines. I have never seen a
failure in any of these due to immersion silver finish or application.
It is the finish I prefer above all others for flatness, solderability, and
cost. Most pwb vendors can provide it for less than HASL, ENIG, even OSP.
Immersion silver is a good finish to use for Pb-free processes. It does not
contribute further brittleness to the lead-free IMF. It has no plating
issues that can be exacerbated by processing heat cycles, such as ENIG
sometimes does.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Sefika ÖZKAL
Sent: Monday, May 15, 2006 1:19 AM
To: [log in to unmask]
Subject: Re: [TN] Question about ENIG in SnPb process . . . .

Dear Technetters,

Instead of ENIG what should we choose? For example, for high density (with
bgas) boards with some of the components lead-free plated?

Moreover what is the choice when the components assembled to the boards are
both lead-free alloy plated and lead alloy plated (mixed)?

Regards

Sefika

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Friday, May 12, 2006 11:52 PM
To: [log in to unmask]
Subject: Re: [TN] Question about ENIG in SnPb process . . . .

LOL Werner,
I totally understand where you are coming from. As a process engineer, you
should see how much hell I need to go through to verify that the root cause
of the solder reliability issues is not the process but the pwb finish, or
issues related to it. And identifying surface finish problems is not easy,
nor cheap. We usually need to go to an outside failure analysis house to get
verification. This is very costly. Thus I also have a somewhat biased
opinion towards ENIG using standard 63/37 solder. And as you pointed out,
Werner, the lead-free alloys combined with ENIG makes the situation
potentially much worse.
With ENIG, some of the plating issues are exacerbated by further thermal
cycles during normal processing. This is why it is difficult to prove "is it
the plating, or is it the process". There quite often are situations where
it is a little of both.
When I am working with military hardware (or really, any electronic
assemblies), the last thing I want to take a chance on is solder reliability
issues with the board finish that go undetected until hundreds or even
thousands of assemblies are built and in the field.
Because of ENIG's fickle nature, and because I do not like to solder to
nickel for all of the reasons that are well documented here in the forum
archives and hundreds of white papers, articles, and so forth, I simply try
to stay away from it whenever possible. Why would I ever need to use it,
except for in certain situations and space applications where it is not
qualified?

And Gerard, I want you to understand I respect your knowledge and opinions
fully, I simply choose to disagree with you on this.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Friday, May 12, 2006 1:15 PM
To: [log in to unmask]
Subject: Re: [TN] Question about ENIG in SnPb process . . . .

Thanks Lee, Richard & Daniel,for backing up my experience.

Gerard, of course, as a consultant clients come to you when they have
problems, not when everything works as advertised. So, purely statistically,
consultants get a one-sided picture.
As a consultant, one of your first obligations is to do no harm with our
advice [sort of like physicians]-having seen what I have, there is no way I
can advise a client in favor of ENIG. Of course it works most of the time,
but God help me i advice ENIG to a client and all hell breaks loose.

Werner

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