Guy,  I'm assuming you are talking about rework.  (why else would you use a
micro stencil?).  The complaint with flux only seems to center around
loosing the additional solder volume.  However, with BGA's, you do not need
to flatten the pads; just make them level.  HASL boards work fine for BGA's,
so why not leave the BGA site "HASL"ed.  That way, the 2.5 mils of solder
people complain is missing by not using solder paste will still be there.
This leveling can be done with a flat bar iron.  (You have to touch the pads
with an iron anyway.  Just get rid of the braided copper wick.)  Since the
balls are eutectic, the collapse will take care of minor pad height
differences.  (Don't try this with high temp solder balls).

Personally, I have found that the resulting standoff on a BGA varies
significantly on any given BGA, especially PBGA.  So any difference the 2.5
mils of solder would contribute gets lost in the noise.

BTW, I did do a case study comparing "flux only" attached BGA's at rework
against "solder paste" attached BGA's and temp cycled the parts and found no
significant difference.

Kind Regards

Ryan Grant
Advanced Technology Engineer
MCMS
(208) 898-1145
[log in to unmask]


> -----Original Message-----
> From: Guy Ramsey [SMTP:[log in to unmask]]
> Sent: Friday, September 07, 2001 11:26 AM
> To:   [log in to unmask]
> Subject:      [TN] Paste verses tacky flux
>
> Anyone out there with data, case study, anecdotal evidence, strong
> opinions, relative to installation of PBGA components (with eutectic
> solder balls of course) using micro stencil and solder paste verses tacky
> flux.
>
> Guy Ramsey
> Senior Lab Technician / Instructor
>
>
> E-Mail: [log in to unmask] <mailto:[log in to unmask]>
> Ph: (610) 362-1200 x107
> Fax: (610) 362-1290
>
>

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