Michael,
An aggressive wash should remove any loose solder balls. If you are applying conformal coating, then you can be pretty sure that anything left will be entrapped, and therefore not an issue (as long as it isn't violating minimal electrical clearance.
If you aren't using conformal coating, you'll have to scratch your head a little more (or perform some testing) to determine if you think the solder balls are attached enough so they won't "become dislodged in the normal service environment."
Ben
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Forrester, Michael (H USA)
Sent: Wednesday, November 30, 2011 10:39 PM
To: [log in to unmask]
Subject: EXTERNAL: [TN] Real World Acceptability of Solder Balls.
I have a PCA 6"X4" with a 144 pin LQFP on it. The lead spacing is 0.5
mm which leaves about 0.2 mm space in-between the pins. The board goes
through SMT reflow and a selective wave. We are seeing various degrees
of solder balls/spheres. The balls we see are basically two sizes
40-45um and 70-80um. The solder paste is a size mesh 3, which would
explain the smaller balls. At this time I am not sure where the larger
ones are coming from? We see around the LQFP any where from 2-5 balls
and literally hundreds of balls. The solder paste is a no-clean and
then the board is cleaned. The reason for using no-clean is because of
the process wait time from reflow to cleaning. The vendor does not want
to leave an aqueous flux on the board for an extended period of time.
My basic question is: Is there an IPC specification(or general rule of
thumb) on the acceptability of solder balls? I understand that the
presence of solder balls is a process indicator and that IPC-610 states:
Solder balls are not entrapped, encapsulated or attached or can become
dislodged in the normal service environment.
Does this really mean that if I find a single ball that is not
entrapped/attached, this is a reason for rejection? Is this practicable
or is there a spec./industry standard that says something like if you
have X balls smaller than X in an area X, this is acceptable? In the
real world, are companies holding the zero tolerance on solder balls?
Thank you in advance.
Best Regards,
Michael Forrester
Sr. Product Engineer
CSMTPE
Siemens Healthcare Diagnostics
101 Silvermine Rd.
Brookfield, CT 06804
PH: (203) 740-6452
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