Francis,

So far all the responses you received deal with the assembly process as a
cause for solder balls. I would like to add that the bare board also could
be a source of solder balls:
1) If there are via holes partially plugged with solder (from the HASL
process) the solder would melt during the reflow process and form solder
balls protruding from the via holes on the bottom side. This would represent
a problem when screening solder paste on the bottom side because it will
prevent the gasketing of the stencil to the board.

2) If there are via holes partially plugged with solder (from the HASL
process) and the board has solder mask tenting on the via holes on one side.
During the reflow process the solder would be blown out of the via holes and
would be deposited as solder balls around the via holes.

These happen normally when the vias are small (below 0.013") and the board
is thick (above 0.080").


Sherif Refaat, Eng.
Technology Roadmap Consultants Inc.
Tel. (514) 624 9842



----- Original Message -----
From: "Francis Sun" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, December 11, 2002 11:00 AM
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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