Hi, Carl
There are several possible causes for the solder balls. Something may
have changed in your process to have them start showing up suddenly. I
would like to know if it is happening on a single-sided SMT board or a
double-sided board.
But here are some suggestions:
1. Determine if the solderballs are showing up under all BGAs (and
perhaps other components as well), or only under a particular BGA part
number. If it is under all BGAs and extending into some non-BGA
components, then I would suspect the solder paste has been sitting out
too long prior to reflow. This will cause it to "skin over" and when it
goes through reflow the resultant trapped gases cause the spattering of
solder fines. Another cause related to the solder paste is too much
moisture in the paste from being sheared on the press in especially
humid conditions. Some brands and formulations of solder paste are known
to produce a lot of spattering and also voiding in high humidity (more
hygrophobic). You may email me directly offline and I will tell you
which do, and which ones do not.
If the extraneous solderballs are under only a single BGA part
number, then I would suspect a contamination problem with that
particular BGA. I have seen this before, and the problem disappeared
when I washed the BGAs and dried them with a 1/2 hour bake at 105 deg.
C. prior to assembly and reflow.
2. Determine if the problem is seen on all assemblies, or if it is on
some and not on others. If it is only on some assemblies but not others,
misregistration of the stencil may be the problem. Sometimes the paste
deposits are not fully on the pad but are partially printed on the board
itself, causing satellites of molten solder that are trapped by the
collapsing BGA before they are fully agglomerated into the solder ball
because they are not in contact with the metal pad.
3. If the bottom of the stencil is not cleaned thoroughly often enough,
residual paste deposits are left behind on the PWB. These extraneous
smears of paste will reflow and agglomerate independently, and then be
trapped when the BGA collapses during reflow. Try looking at the paste
deposits just prior to placement.
4. Don't be afraid to very carefully take the populated board after
placement but prior to reflow and put it in your X-ray machine (I'm
assuming you have one). Look at the BGA pattern in the X-ray and make
sure the paste is not smearing as a result of too much placement
pressure. See if you can correlate print defect data either from visible
inspection or from Cyber data or from an SVS or other paste inspection
system, or even from the X-ray. If the boards with lots of extraneous
solder balls correlates with a high defect count on the paste print
inspection, that would be an indication of improper printer or placer
setup.
5. Check the paste deposits to see if you have a slumping problem with
the paste. It may or may not be related to humidity. Some pastes slump
terribly if the boards are even slightly warm (warmer than 75 deg. F).
Other pastes slump terribly because of flux separation in the tube. As a
result, the first deposit of the paste has heavy flux and cannot hold a
good "brick" formation on the pad. Look at the tops of the tubes of
paste for the telltale brown appearance. You can usually see it right
through the Semco cartridges on the dispensing end.
5. Make sure the boards are clean and dry and do not have trapped
moisture in the vias. This can also cause the spattering. This is
another reason why many companies no longer wash the boards between
reflow of side one and side two. Some solder pastes (but not all!) are
formulated to be able to be reflowed twice prior to wash without any
problems. They usually tell you this on the data sheet if they are of
this type (but you cannot believe everything they claim, either).
I hope this helps. Let us know if you find the cause, we all need to
learn from each other's experience(s).
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Carl Beharry
Sent: Friday, July 07, 2006 2:58 PM
To: [log in to unmask]
Subject: [TN] Solder balls
Hello fellow Technetters,
Would like some advise from the experts out there.
We have always assembled pcb with minimal solder balls that usually gets
washed away in the cleaning process, hence not creating any major
problems.
However, recently we have been getting involve with more complex boards
that has alot of 1mm and .8mm low profile bga's.
The problem that we are noticing is that the solder balls that never was
a problem before are now getting stuck under the bga's, sometimes
creating shorts.
I am not sure if we can eliminate solder balls completely but would like
to know how the experts are resolving a problem of this sort.
Thanks in advance for your input.
Regards,
Carl Beharry
Ectronics, Inc.
855 Industrial Hwy
Cinnaminson, NJ 08077
Phone : (856) 829-7161
Fax : (856) 829-6950
Email : [log in to unmask]
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