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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