Alvin;

   There is no formula since the stencil opening depends on pad size at PCB,
as rule of thumb they recommend reduce 10% from pad width to give a good
gasketing. Stencil thickness generally is 6 mils, laser cut, trapezoidal
shape with eletropolish finish.You can use eletroformed stencil as well, but
NEVER use chemical etching type for fine pitch.
   The big problem to fine pitch apertures is the fact that when you reduces
stencil opening the area formed by the four sides of the stencil wall become
greater than the area of the pad and there is a limit that the solder paste
stay at stencil instead of release to the pad. Some people also recomend the
aperture be enough to accomodate 5 solder sphere in line, this way you can
define the solder type to be used. I have been using type 3 solder paste for
20 mils without problem.
  OSP finish is better to print due to planarity of the pad, some bad HASL
finish can cause bridging due to solder paste accumulation under stencil
during print cycle, if you have automatic stencil wiper just increase the
cleaning rate.
  Related with the process I always worked with No-clean process and has no
argument to compare with Clean process.

Thanx

Jorge Dourado de Santana
Maintenance / Process Engr
Microtec - Brazil


> -----Original Message-----
> From: Alvin Leong [SMTP:[log in to unmask]]
> Sent: 10 de Agosto de 1999 07:43
> To:   [log in to unmask]
> Subject:      [TN] Stencil aperture opening
>
> Hi Technetters,
>         I've a question of what is the formulae that used to calculate for
> the
> stencil aperture opening on the 20 mils pitch IC? This question is more on
> the amount of solder volume that sufficiently to meet the strength as well
> as the reliability. Is there any different of the clean vs no-clean
> process? HASL finishing vs Entek (OSP)?
> Hope to some words for reference.
>
> Thanks in advance,
> Alvin
>
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