Hi Jon,
If you can keep the stencil opening narrower than the pad width, you will
have less bridging and solder balls. You should not make the stencil width
too small, however, or you will not get good paste release. (Stencil
opening to stencil thickness should be equal to or more than 1.5:1, e.g.,
.009 opening is smallest usable for a .006 thick stencil.)
Another source of solder balls and, sometimes, bridging would be paste on
the bottom of your stencil. If you don't have a good seal between the
stencil and the pads, the paste will "bloom" on the bottom because of the
vacuum being pulled.
I believe you have received very good info on the intrusive reflow and the
MELF pad design (a "U" shaped pad). Some years ago I made a test board of
different shapes, sizes, and spacings of pads for MELFs. You can see real
quick, what works best. We wound up using a "U" shaped pad, but I don't
recall the dimentions today.
Good luck,
Charlie Barker
Sr. Manufacturing Engineer
Input/Output, Inc.
Stafford, Texas
[log in to unmask]
281-552-3328 Voice
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to Benedicto Cruz <[log in to unmask]>
To: [log in to unmask]
cc: (bcc: Charles Barker/US/I-O INC)
Subject: [TN] see items stated
Hi!
I would like to ask for an effective stencil aperture design for
fine-pitch ICs. We are encountering some solder bridging on these
ICs. Also, there are solder balls present beside these components.
Do you have any stencil design guidelines that you could share with
us to improve our Screen Print Process?
Also, Are you familiar with intrusive reflow? Can anyone share some
informaiton about this?
We also have a product that uses a MELF SMD resistor. The problem
that we are encountering is that this component rolls during solder
paste curing. So when it comes out, the component is already
misaligned. Do you have any suggestions to control this without using
any fixtures?
Thanks.
Regards.
Jon Cruz
Electronic Assemblies Inc.
Manila, Philippines
e-mail: [log in to unmask]
Tel #: 823-7317/7593885
Fax #: 7538629
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