Hello Al,

The ongoing usage of alternate PCB HASL finish and its relationship of the
BGA is like the first dive into water.  It is risky and you proceed
cautiously gathering as much information as possible.  After a while you look
back and wonder why did I ever go through the bother of wading in.

The number one reason for "non solder" finish is flatness.  This is important
to PBGA's and I believe moreso for the less than 20 mil pitch part having
over 200 leads.  I much prefer BGA's.
The surface finish of longest heritage are the OSP and ENIG.  Different forms
of OSP have been around for over twenty years lends itself well in "ongoing"
volume mode where repetition is readily predictable.  The OSP finish is
quickly depleted during cleaning reflow profile temps and repeating the same.
 Once the finish is lessened or removed time becomes critical.  Poor
management of time usage and you will have problems.  It is damaged by
handling; hold bare boards by edges only.
Electroless Nickel Immersion Gold (ENIG) has been around for well over ten
years.  The "Electroless Nickel" portion of the equation is where all the
interest lays.  The I-Gold is self limiting (as with all Immersions) and
thickness is realistically non inspectable at 2-8 micro inches (much closer
to 2 than 8).  The thickness of nickel is the bone of debate.  I like to
categorize my ENIG in three technology groups: 1) RF, 2)Digital FIFO, and 3)
Digital nonFIFO.  The IPC ENIG specification being published calls for
120-150 thick Ni (from recall),  and I place that with Cat 3.  A robust
"bullet proof" finish the Cat 3 ENIG will shelf life forever (over a year
anyway).  The issue I have with it is it requires longer duration and higher
temp times especially with BGAs.  This reflow profile is easier to handle
with the self professed "Cat 2."  The third configuration of RF is relative
to Nickels resistive nature.  A thicker build up of Ni will dramatically
increase paracitics of trace transmission.  For those traveling in the Cat 1
and Cat 2 classes I like 45 mircoinches thickness.  The Ni quality and timing
of I-Gold is critical, you must find a supplier that has a solid history of
delivering quality ENIG, I suggest based on the three Categories.  (Be warned
I am the only one that uses the 3 Cat terminology).
Immersion Silver is rapidly growing in usage.  It is a wonderful finish
(hence I am a board geek) and as close a plug in replacement to solder as you
will find.  Being a self limiting process again the thickness callout is 2-8
mircoinches (it falling near the 3-4 range).  Silver is "deposited" directly
on copper and there are no Nickel issues.  It is flat and a slight organic
anti-oxide coating that requires gloved, by the sides only handling.
Immersion Tin is basically the same as Silver, although today I get much more
request for Silver than Tin.  Tin is great, needs to be handled the same way;
with respectful caution and tends to be a little thicker in the 2-8uIN it is
near the 4-5 end.

My opinion is OSP, ENIG, Silver and Tin are good viable finishes that can be
tailored to your assembly floor.  It tends to be a matching ingredient for
pastes, cleaners, fluxes, etc.  The good news is there isn't a "bad" one.  A
string of commonality is relative to test.  The HASL makes for one nice ICT
pin landing pad where the flat hard oxided/tarnished (sometimes raggy
appearance) surfaces are more challenging.  Some advocate inclusion of the
ICT test points in the solder stencil.  It sounds like a good idea, although
I have no first hand experience doing it.

Well, that's it, my rendition of surface finish and BGAs all in one email....


Boston Brad
781 858 0783

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