Holy smoke! If no-one else is gonna answer this one, I guess I'd better....

Assuming perfect release from the stencil, a 90% by weight metal solder paste
will give you 50% or so by volume (varying with the flux-type) of metal
INDEPENDENT of the particle size, as long as the aperture dimensions are an
order of magnitude or so larger than the diameter of the solder powder. However,
as the apertures shrink relative to the solder powder, at some point the powder
particles will become too big to fit through. Your 20mil pitch situation fits
somewhere between these two extremes.

If you read the J-STD-006 (and its amendment 1), the size 4 specification is
just a kind of slightly tighter-spec'ed version of size 3. The very largest size
'3' powder particle is only 53µm (microns) or 2.1mils, compared to the largest
for size 4, which is 45µm or 1.8mils across: not much difference at all.

Beware: in reality, some vendors' interpretations of 'size 4' can be very broad
(i.e. a lot of fines), which may lead to fish-egg solderballing and short
stencil-life. The short stencil-life is attributable to increased tackiness of
the paste, which retains the paste in the apertures, and consequently may give
you a lower volume deposit than previously seen with size 3!

If you're home-plating because of capillary balls under passives, always
remember to reduce the aperture volume at each end of the component by the
volume of the capillary ball.

Cheers,

Andy





Ryan Jennens <[log in to unmask]> on 05/14/99 02:11:51 PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond to
      Ryan Jennens <[log in to unmask]>

To:   [log in to unmask]
cc:    (bcc: Andy Mackie)
Subject:  [TN] FW: Fine Powder Solder Paste







  -----Original Message-----
  From: Ryan Jennens [mailto:[log in to unmask]]
  Sent: Thursday, May 13, 1999 11:51 AM
  To: TechNet
  Subject: Fine Powder Solder Paste


  What stencil design guidelines differ when one decides to go with
  a -400/+500 particle size solder paste? The paste companies tout that the
  finer particle paste offers print improvements below 20 mil pitch. But I
  believe one would have to reduce the aperture sizes all around because
more  metal would be deposited in each one. Is this accurate? How much reduction
  is needed. Is home-plating the apertures even more important? I could not
  find this topic in the archives. We are considering switching for our
  finer-pitch boards, as their pitch decreases, if it really offers an
  advantage. However, we have been printing 20-mil for a long time
  with -325/+500 paste for quite a while.

  -Ryan Jennens

  TelGen Corporation