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November 2011

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Subject:
From:
Robert Kondner <[log in to unmask]>
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Date:
Wed, 30 Nov 2011 23:41:24 -0500
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Michael,

A quick guess is you have smears of solder paste or a paste slump that
leaves a paste web. It might be related to a temp profile that sputters the
paste of drives off the flux before reflow. 

I assume you just started using a new paste from your description, you might
keep searching.

I don't know about the acceptability of small solder balls but they sure
don't look good.

I had an old paper on Solder Beading, see:
www.kondner.com/files/Solder_Beading.pdf  Not exactly what you are seeing,
the beads are typically larger than what paste smears leave. But, it might
help

Bob K.

-----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: [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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