I don't know what frequencies you are working at... But, The 'prudent'
approach would be to proto the boards and have your supplier make 2 or 3
different 'flavors' of surface finish approaches for your assembly people to
populate and then test them for performance issues.
Immersion Gold works very well, Immersion silver also, so does OSP and white
Tin. ALL of them are sensitive to environmental and handling issues and some
of them are incompatible with certain processes or environmental
conditions... but all of them can be overcome by setting up procedures to
deal with the issues. None of them is a solution to all problems... but they
all have their special applications for solving issues with assembly and
manufacturing of the boards.
HASL is a good finish... it's just not real flat. If you have very fine
pitch parts, it may cause you some trouble with coplanarity of the leads to
be soldered on Quad flat packs or BGA's.
OSP works and is flat, has a shorter shelf life less than a year more like 6
months, and needs to be individually wrapped in sealed poly bags to protect
it on the shelf until assembly.
Gold works and is flat, solders well, and as long as the plating is not too
thick it doesn't present a problem... down sides are if the plating is too
thick it can embrittle the solder joint through gold contamination of the
solder and some issues with 'black pad' occur if the plating process isn't
kept clean. Good thing is it doesn't oxidize.
Silver solders well, is flat and needs protection as it tarnishes or
oxidizes but the oxide it makes is still conductive... in fact silver is the
best conductor, better than gold in most cases...
Tin also is very flat, and there is some promising work that has been done
with tin finishes, Florida Cirtech did some work with White Tin that seemed
very impressive.
Realize that you are only flashing gold or silver or Tin or OSP on the
surface and so little of it is present just to protect the copper surface
until soldering is done... After soldering is complete we really don't care
what happens to the plating, our solder joints are good.
Just a little food for thought... after all the choice you make will depend
on the type and complexity of the board you are manufacturing... and how it
is handled before soldering.
Good luck.
Bill Brooks - KG6VVP
PCB Design Engineer, C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
Datron World Communications, Inc.
_______________________________________
San Diego Chapter of the IPC Designers Council
Communications Officer, Web Manager
http://dcchapters.ipc.org/SanDiego/
http://pcbwizards.com
-----Original Message-----
From: Wenger, George M. [mailto:[log in to unmask]]
Sent: Wednesday, June 15, 2005 2:33 PM
To: [log in to unmask]
Subject: Re: [TN] RF board finishes
Martin,
The use of nickel or ENIG is not necessarily a "no- no" for RF circuits.
If your RF products have long strip lines than your designers may not
want to use PCB traces that are plated with nickel. When you start
using many small size discretes and fine pitch IC packages many people
have found that HASL is not a good choice. From a soldering standpoint
ENIG or immersion silver should work fine. Because of the brittle
solder joint failure that have been associated with ENIG we decided to
use immersion silver for all of our PCBs. It has been a good choice for
us and we've not had any reliability issue since 1997.
Regards,
George
George M. Wenger, Andrew Corporation
Reliability / FMA Engineer
Base Station & Subsystems Group
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Martin Butcher
Sent: Wednesday, June 15, 2005 5:15 PM
To: [log in to unmask]
Subject: [TN] RF board finishes
Hi All,
We are currently assembling an RF product (first time build) that
utilizes
several fine pitch devices, leadless fine pitch devices and a great many
0402's. The board finish is currently HASL and as the volume ramps up
there
is concern that defects related to the fine pitch and HASL will result
in
unacceptable costs.
We prefer to use ENIG as a surface finish for non-RF products that fit
this
Bill. I seem to recall reading something on this very Net that
mentioned
nickel or ENIG being a no no for RF ccts.
I am not an RF guy and would love to get some feedback as to the effects
of
various board finishes on RF products. Perhaps even recommendations as
to a
"good" finish that will give better yield than HASL.
Regards,
Martin Butcher,
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