Ken,

We have been using via in pad for over a decade and I sympathize with your
problems. VIP is great for the designer and a nightmare for the assembler.
HASL will give you some incremental improvement as it increases the
available solder. It can be significantly helpful with very small diameter
holes, but as the diameter increases the help from HASL decreases. Proper
drill size selection is critical for successful assembly. What size hole is
your via, what is the board thickness, and what is the smallest SM feature
size. Stencil aperture expansion is also a balancing act that must be
optimized. We do not recommend masking the bottom side and even the
suggestion of encroaching soldermask onto the bottomside annular ring is not
acceptable assuming you are utilizing these as test points. We have
successfully used eutectic solder in a pre-assembly stencil step to
eliminate defects on problem codes, but this was only used as a stop gap
measure. We would then specify smaller drill sizes to eliminate defects.
With the relaxation in Class II solder joint requirements for discrete
devises, making acceptable VIP solder joints is somewhat easier, but there
are a host of other (test, wave solder) issues associated with VIP designs.
Please feel free to contact me directly if you would like to further the
discussion.

Thanks,
Robert Furrow
Printed Wiring Board Engineer
Supply Chain Networks
Lucent Technologies
978-960-3224    [log in to unmask]


-----Original Message-----
From: Ken Patel [mailto:[log in to unmask]]
Sent: Monday, March 04, 2002 5:37 PM
To: [log in to unmask]
Subject: [TN] Via in Pad? Need Help!!!
Importance: High


Guys,
On our nickel gold board, solder joints are below IPC spec (insufficient
solder) where there are vias in pad. Solder is drained down the via
holes and deprived the joint of necessary solder.  I would like to know
the following.
(1) How about using the surface finish 63/37 coating HASL instead of
Ni-Gold? I think gold dissolves in solder so quickly that I rather use
HASL processed boards, any thoughts?
(2) Do you guys mask the via on bottom side to avoid solder draining out
on the other side?
(3) Does anybody try Hi Temp solder paste to fill the vias as our
assembly house ordered via stencils to fill the via using eutectic
(63/37) solder paste which didn't work?

Looking for
re,
Ken Patel

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