Hamilton, Richard -4454 wrote:
>
> Hello to all,
>
> Well we are finally testing our SM capabilities, we are brand new at this.
> We have an issue at paste printing that I would like everyone's input on.
>
> When the squeege makes it's pass on the stencil and then lifts upward off
> the stencil, the majority of the paste remains with the squeege instead of
> on the stencil.
>
> What factors are there in the paste charactistics when applied?
>
> Thank you all in advance.
>
> Richard Hamilton
> Clemar Mfg. / Rain Bird
> [log in to unmask]
>
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Hello Richard,

Here is my "nickel's" worth.

Look at a few things...
1.  If the paste is refrigerated, be sure it is brought down to room
temperature before even opening the jar.  Condensation can significantly
affect the paste's charecteristics as well as cause oxidation of the
solder particles.
2.  Many of the pastes I have worked with require preparation in the
form of stirring if in a jar, kneading if in a packet.  Syringes come
with a built-in mixing "gizmo".  Solder pastes may experience some
separation during storage or shipment (e.g., open the jar and their is
flux up top) . Starting with a homogenous solder paste mixture is very
important. Ask your solder paste supplier for their recommended prep.
(One formulation I used required about a minute of continual stirring!)
And beware, you may get variation from operator to operator, which will
in turn, affect the paste's viscosity and it's printing characteristic.
3. Environment is certainly important.  Your room temp and humidity may
seem relatively stable, but the temperature in your screen printer may
be different.  You may want to check.  I have been told by paste
suppliers that 1 degree C of temperature change can affect paste
viscosity by about 10%.  Typically, the warmer it is the less viscous
the paste, but depepending on what flux formulation you are using,
sitting in a warm environment, may cause it to dry out.  Also, some flux
formulations are very sensitive to humidity, so although your
environment has been stable...is it within the specified limits of the
specification for that solder paste?

Just as a side note on temperature and mixing...
We use to have to test our incoming solder paste for acceptance.  I had
developed an internal procedure (based on IPC standards) that required
that the paste be in a temperature bath at a specified temp for 24 hours
before viscosity testing could be performed.  We used a Brookfield
viscometer with a spiral attachment and the resulting measurements were
averaged over 10 minutes (if I remember correctly).  If the measurements
taken in the first 2 minutes were significantly different than the
measurements taken toward the end (which happened a lot with one
particlular brand than with another), than the paste was not thoroughly
mixed and the measurements were not valid.  Before this procedure was
implemented, we had a difficult time getting the same readings as our
supplier who had measured and certified the paste before shipment.

4.  Speaking of shipment...how was the paste stored during and after
shipment?  It may be in the right conditions on your floor, but may have
been subjected to relatively extreme conditions while in transit or in
storage somewhere in your facility before it got out to the floor.
Typically, suppliers will ship the paste in a manner to ensure that it
gets to your facility in good shape.  But let's say the scenario was
that the rep dropped off the jar for your evaluation and it was sitting
in his car for the last 4 hours in 90+ degrees.  Just some things to
consider.
5.  How old is the paste?  Has its shelf life expired?  Has the jar been
open for some time and now has gone bad.  Have you been reusing it for
your evaluation?  Just asking.
6.  Someone mentioned in an earlier response to work with your
supplier.  I can't say enough about that.  Don't just talk to their
sales folk, get in touch with their technical guru for assistance.  Most
suppliers are more than happy to help out a new customer (potential
customer in this case).  Your success is their success.  The level of
support you receive now may be a good indication of what to expect from
them in the future...Another important factor to consider in the
selection of any material or equipment.

From the bit that you had mentioned in your posting, my first thoughts
are that the paste was not prepped and is still too thick.  FYI - as the
paste is worked through subsequent passes with the squeegee it begins to
thin, "shear thinning".  We use to regularly add some fresh paste during
a run, not only to replenish the supply but to try to maintain as
consistent a paste on the stencil as possible.

And just another thought...how do you plan to run production?  Are you
batch printing?  Will paste need to sit on a board for a while before
assembly and reflow?  The paste formulation you select needs to fit your
production application and environment.  If the normal temp and humidity
on your production floor is not within the recommended specifications of
the paste you are considering than you either need to look at another
formulation or you need to control the environment...which alternative
is most cost effective for you?  Does the solder paste need to be able
to "keep" for about a week before you use it all up?  Once a jar is
opened, some pastes start to degrade more quickly than others...Not
everyone runs a million boards and uses paste up right away.

Hope this helps.

Good luck and welcome to the world of SMT!

Enza

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