Hi Francis,
Solder balls and solder beads are related, but distinct problems.  Beads
larger spheres formed during reflow, commonly with low stand-off chips.
Balls are small, widely scattered solder particles found within the flux
pool.
Some causes for beading:
        -paste printed on mask
        -poor aperture design
        -large overlap between pad & component
        -excessive paste volume due to stencil thickness
        -switching from rubber to metal squeegee (no more scooping)
        -high component placement pressure
        -high flux activation temp.
        -flux and moisture outgassing
        -fast ramp rate
        -excessive solder powder oxides or contaminates
        -low metal load
        -fine powder (type 4 or 5)
        -paste slump (hot or cold)
Some causes for balling:
        -print smear (usually caused by poor gasketing)
        -poor aperture design
        -large number of fines in powder
        -excessive humidity
        -insufficient flux activity
        -excessive solder powder oxides or contaminates
        -solder mask and paste interaction
        -flux and moisture outgassing
        -fast ramp rate
        -component outgassing
        -low metal load
        -paste slump (hot or cold)

Solutions:  modify stencil apertures & thickness. Investigate different pad
geometries.  Evaluate component and board metallizations for solderability
and contaminates.  Adjust the reflow profile (slow stead ramp).

Chris Christian
Regional  Manager - Canada & Great Lakes Area
Indium Corporation of America
Ph:  +1 519 725-4185
Fx:  +1 519 725-3925
Mobile:  +1 315 534-3207


-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Francis Sun
Sent: Wednesday, December 11, 2002 11:00 AM
To: [log in to unmask]
Subject: [TN] Solder ball causes


Hello,

I would like to compile a list of what may cause solder balls to form after
reflow for our internal consumption.
Can anyone add to my limited knowledge on some of the causes that can
create solder balls?
My knowledge is limited and some may not be correct. If any are not correct
please point them out.
If there is a comprehensive list available in the public domain, it would
be great if someone can point me in the general direction of where it is
located.


Some of the causes that I sort of know of:   !! BE WARNED !! some or all
may not be correct. :

Moisture from substrate and components
Dry solder paste
Wet solder paste?
Stencil aperture(s) too big
Wrong pad geometries
Poor mixing of solder paste
Solder did not reach room temperatures?
Solder paste passed shelf life?
Environment to humidity?
insufficient heat
Dwell time under liqudius is too long?
Dwell time under liqudious is too short?
Time interval too long between solder application and reflow?

Any help is appreciated.


 Happy Holidays to all

Francis Sun
MARK IV Industries
(905) 624-3025 ext. 1235
Fax: (905) 624-4572

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