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...http://www.electrapolymers.com/Assemus%20-%20no%20solderballs.html



SOLDER BALLS: CHALLENGING THE INDUSTRY
Solder balling is a phenomenon that creates expensive difficulties for both
assemblers and fabricators. Yes, you can remove solder balls with various
cleaning solvents, brushes, pressure sprays, and the like, but as
environmental concerns increase, no-clean processes have become the norm.
Solder balls are just small particles of solder which have separated from
the solder pool which creates solder joints. As assemblers know all too
well, solder balls can cause bridging. With water soluble processes, solder
balls are not a problem - as they are generally removed during the cleaning
process. In no-clean processes, however, the formation of solder balls is a
problem - particularly in fine-pitch and BGA assemblies.
What causes solder balls? There is consensus, if not total agreement, that
factors can include moisture contaminated solder paste, a poorly controlled
reflow process and poorly formulated solder powder. The design of the PCB
and the overall processing can also affect solder ball production.
A first step for assemblers is to consider the reflow profile. If the
temperature ramp rate is not correct or if the temperature is too high, the
paste can splatter, causing the formation of solder balls. All too often
assemblers do not optimize the reflow profile to suit the type of paste
being used. Matching reflow profile and paste is an essential component of
eliminating solder balls.
Of course, the solder paste itself should not be exposed to high humidity:
the additional moisture splatters during reflow.
Proper cleaning of stencils can also help to eliminate solder balls - often
solder paste can gather on the bottom of the stencil. This excess paste will
ultimately increase solder ball formation.
Research also shows that solder paste containing a high amount of ultra fine
powder particles (sub 25 microns) can increase solder ball formation. The
fine particles are often carried away from the joint itself and left on the
PCB.



THE BIGGEST KEY: SOLDERMASK
By far the greatest single factor that most influences the formation of
solder balls is the type and composition of the soldermask. The wrong mask,
for instance, one that is too dark, perhaps one with a finish that is
conducive to solder ball formation or one whose formula is not adapted to
BGA technology, can spell disaster when coupled with no-clean processes
...
Early in the 1990s, research on soldermask and solder balls clearly
indicated that roughened soldermask, one that was, for example, brushed
before the soldering process was initiated, had fewer solder balls than an
untreated mask. The rough surface made it more difficult for solder balls to
stick to the PCB. However, such a system had its own set of difficulties. As
described by Sven E. Kramer in an article in Printed Circuit Fabrication
(Solder Balls, Vol. 18 No. 12, December, 1995) "the roughening process would
be costly, as it created yet another step in the processing of the PCB, and
the roughening process would likely damage the coating and shorten the
reliability of the finished assembly".


> -----Original Message-----
> From: [log in to unmask] [SMTP:[log in to unmask]]
> Sent: 19 de Janeiro de 2000 02:16
> To:   [log in to unmask]
> Subject:      [TN] Reflow soldering >>> Solder balls
>
> Hi,
>
> sorry for this basic basic question!
>
> Are there any resources for the solder balls problem? Where do the come
> from and how to prevent them?
>
> Is there any site for solder defects info ?
>
> regards
>
> Jens Behrens
>
>

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