Agree 100%. We see this phenomenon on 0402 resnets and some 0402 components (location specific) on thermally challenging lead-free boards (thick board, large BGAs, ground planes etc.,) The reflow profile, being tailored to ensure complete reflow of bigger BGA parts almost always leads to overheating of the smaller parts and greater than recommended soak times, resulting in little or no flux activity during reflow. Less flux contained in the small solder deposits for these smaller parts exacerbates the situation further.
But the important thing is (leaving aside the appearance part for a minute) that there is good wetting to the component and an acceptable solder joint per IPC 610. We have been building these lead-free boards for about 3 years now and no returns for solder defects related to this condition yet (telecom application). The process window however, is definitely shortened due to lower flux activity as now you need pristine parts to be able to get a good solder joint.
Amol S. Kane
Process Engineer
Harvard Custom Manufacturing, Inc.
941 Route 28
Owego NY 13827
Phone: 607-687-7669 Ext 349
Fax: 607-687-9733
[log in to unmask]
www.harvardgrp.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Burke
Sent: Wednesday, January 13, 2010 5:33 PM
To: [log in to unmask]
Subject: Re: [TN] Return of the "Snotballs"
Seen worse.........8-(
I have encountered this on a number of occasions and have come to a few
thoughts on the subject.
In my experience it is generally found on the smaller components (such as
0201) and often only on the ceramic parts.
The conclusion that I came to with this is that the flux itself is running
out of steam as the smaller components are experiencing overheating in the
ramp up phase of the reflow profile compared to the larger devices. They
have a smaller amount of solder paste and hence flux, and they are exposed
to the full effect of the reflow profile often in less densely populated
areas. Ceramic also has a very good thermal transfer co-efficient and heat
capacity compared to the other packaging materials such as BAG and other
plastic bodied parts.
The reason that I came to this conclusion is that often you will see this
effect on a small ceramic part but not at all on a QFP or a BGA.
Inevitably in the speed required to "fix" things I instructed our CEM to
back off on the ramp profile and the problem went away. And yes some
soldering materials are less "robust" in this respect.
Sad to say, but I have never seen this condition with leaded
solder...........8-)
John Burke
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of stephen gregory
Sent: Wednesday, January 13, 2010 12:24 PM
To: [log in to unmask]
Subject: [TN] Return of the "Snotballs"
Maybe some of you remember a post I made a couple of years ago when I was in
Tulsa and I had a reflow problem with a SAC solderpaste. It was then
I learned of a new term called "Snotballs". Here was the picture I posted
back then:
http://stevezeva.homestead.com/files/Snotballs.jpg
Well guess who decided to follow me out to Pennsylvania? Check out:
http://stevezeva.homestead.com/Type_4.JPG
Same vendor, same alloy.
All I can say is make sure you know where your powder is coming from.
Steve Gregory
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