Mr. Charles Windsor,
I love to, but I don't think I can converse in Swedish tongue.
And don't they have Swedish language police going around to
check everybody speaks Swedish like in Quebec, Canada?
Excuseeeee me for this nonsense. I'm getting my head put
together in early monday morning...
1. Your first question:
No, we have no problem at all with paste release. As per the
faxed stencil aperture spec, acute angles are 60' to 65'
degrees. I tried 45'degree acute angles with no paste
release problem. Fillet radius for these apertures are 130um,
0.005" or 150um, 0.006". Solderpaste release problem is
more of the function of stencil aperture aspect ratio,
seperation speed of board and stencil, and conditon of
solderpaste. I allowed a large process window to eliminate
this sort of problem by using the aspect ratio of 2:1(min of
1.5:1), a very slow seperation speed, 180um (0.007") stencil
for 1.27mm pitch and 130um (0.005") stencil for 0.65 and
0.5mm pitch, and electropolished aperture walls.
Trapezoidal stencil apertures will further enchance paste
release.
2. I presume you are still using polyurethane squeegees,
sometimes called as "rubber". I dropped using rubber
squeegees 4 or 5 years ago. Metal squeegees are the way
to go for printing solderpaste. If you must, use a hard rubber
saying a durometer of 95 or up and easy on the downward
pressure of squeegee.
regards
Matthew Park
>>> <[log in to unmask]> January
9, 1998 10:33 am >>>
Mr Park,
Feel free to call me Clinton please...
I recieved the pages this time thank you very much! When
we began
trialling different aperture shapes, we actually considered
trying a
similar shape to the one you have sent me. We were a bit
dubious about
using this shape for two reasons.
1 We felt a concave angle less than 90 degrees might cause
problems
with paste release. Do you know what fillet radius is used on
these
apertures?
2 If the PCB pad is not perfectly level or should paste be
trapped
under the stencil, we thought it could be possible for the
pointed part
of the stencil to "curl" or project up above the rest of the
stencil.
Thus when the squeegee passes over, it could tear the
stencil or
bend/kink the pointy part (excuse the techno speak!).
Have you had any experience with these problems or are
there any
guidelines to stop this from occurring?
Thanks for your help so far!
Clinton
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