Dhawan,

So, you try to skip all research headaches and jump right into the
muddy water.  Check Enthone and kester web sites.    They have
many excellent technical articles written on the subject of OSP's
- www.kester.com
- www.enthone-omi.com

Also check July 1997 & August 1998 editions of Printed Circuit
Fabrication for OSP and electroless immersion gold/nickel
articles.  If you don't have them, name your fax, I will try my best to
fax them on Monday.

If you plan to use no-clean in near future, I recommend
immersioin gold/nickel finish coating.  This is a drop-in
replacement for HASL boards.  You don't have to fiddle
around with the existing process.  Wettibilty of gold is far superior
than HASL and solder joint fillet formation is far better.
One major solder appearance change from HASL is all solder
joints are dull-looking.  Solder joint embrittlement due to too
much gold content (usaully 3/5% more) are unfounded.
Immersion gold/nickel provides consistent gold thickness of
1um.

One problem you might encounter (depending upon who your
board vendor is for immersion gold)  is gold fall-out around SMT
pads.  This could cause shorts especially for QFP pads.  Gold
fall-out (fine gold dusts) is usually caused by too much plating.
The problem can easily fixed by fine-tuning gold plating process
by the board vendor.

OSP coated board provides the same or better flatness than
gold.  But the manufacturing process requires some fiddlings
and the handling of boards are more sensitive.  It provides a
superior  solder joint shininess and less intermetalic and cold
joints.  This is because there are no oxidized and contaminated
solder from HASL.

Some of issues you might tackle when using OSP coated
boards:

Watch out how many heat cycles a product go  thru, and boards
should be assembled and completed within a limited time.
Don't leave them out on the floor too long when they have to go
thru next assembly and heat cycle.  I guess I am talking about a
few days..  Non or de-wetting could be a problem if not
processed within a given time.

Cleaning of mis-printed board could cause non-wetting problem
as well.  This tends to remove a fraction of OSP coating.  0.35um
thickness is indeed thin.

Solder filling of thru-hole in the wave is another issue.  It will
never get filled fully the way HASL coating.  Usually solder wicks
up to about 50% or 75% of hole.  This is acceptable under IPC
600  class 2 and 3.

You also have to deal with exposed copper area.  I don't think it
will be a quality issue.

Training of assembly people for the proper handling of OSP is
also important.

More active flux (Water-soluble) works better with OSP, and there
are no-clean flux formulated for OSP.  I tested one no clean flux
type but the result isn't better than the one I am using.

I guess I am telling OSP is not a drop-in replacement for HASL

regards
Matthew Park
SMT Manufacturing Engineer
NII-Norsat International Inc.
Surrey, BC.

sorry I can not help you with surface flatness.  I don't think there
is any guideline for that.  But the pad surface level  should be
within a twice thickness of  leaded pin for QFPs.

>>> "Dhawan, Ashok" <[log in to unmask]> August
7, 1998  1:48 pm >>>
For fine pitch devices 15.7 mils and small chips 0603 ,do we
have any
IPC standard stating that what kind of surface leveliing /planarity
required for placement of FPDs and 0603/0402 chips.Even
accurate
placement can go offline once the surface is not flat. the situation
is
worse when we are trying to place 0603 components on glue
dots.


Also, there are no. of options for HAl alternatives . We are
comparing
gold immersion and OSPs.Our circuit boards are having fine
pitch devices
extendable to 12 mils QFPs and 50/60 mils BGAs and chips
down to 0402
package size.We also intend to implement No-Clean process in
near
future.  We have limited experience with Au immersion/Ni
electroless
and OSPs.
What are  views of experts who had some point of time decided
course of
action. What you think we go for?We want to go for process
where we do
not have to modify /readjust our current process based on water
soluble
soldering. Pro and Cons?

Not to mention that I consider technet as my vital tool for problem
solving. I will appreciate comments on this asap as I am running
out of
time and our process meeting is scheduled on Monady morning.

thanks

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