I have been soldering (prototype quantities) mini-Ball Grid Arrays to FR4
boards with 10-mil diameter solder pads.  Co-planarity is a major issue to
get these things soldered down.  I have had great success using the Precision
Pads Technology (PPT) solid solder deposit method to place uniform, co-planar
solder on my boards.  Two disadvantages?...you need a dry-film soldermask
and you need to ship your boards to Florida for processing (or get licensed
to do it yourself).  The process works GREAT!  You can even VP solder J-leaded
parts to the board without wicking all of your solder up the leads.  Just add
flux, place your parts, and reflow.

Of course I'm presuming that your HASL solder thickness spec. is rooted in
a co-planarity requirement for SMT attachment.

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Subject: FAB: Fine Pitch solder thickness w/HASL

    Our HASL solder thickness spec. is 0.3 to 1.5 mils.  Is it unreasonable 
to expect using a horizontal HASL process that 1.5 mils will be the actual 
max. on 100% of the pads on both sides of the board, even with fine pitch (< 
= 20 mils)?
    My guess is that the surface tension of the molten solder on the fine 
pitch pads, its low mass and the surrounding geography, make it difficult 
for the air knives to do consistant leveling.
    Some of our product mix here is Mil Spec work.  Converting the boards 
with fine pitch on both sides to Au/Ni may be more pain than it is worth.
    Any inputs on this subject will be greatly appreciated.

Norm Dill
MQC Specialist
[log in to unmask]
(716) 242-4210

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Gary P.
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                                  Gary D. Peterson
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