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October 2007

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Mon, 15 Oct 2007 10:31:03 -0500
Content-Type:
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text/plain (152 lines)
This is good advice, however realize that in some cases a Type 4 paste
will print better than a Type 3. If you have very small stencil aperture
openings such as for a 0201 or even 0402 chip, or for flipchip
components for example, the Type 4 will print much better through the
tiny apertures than the Type 3. If you do not need a Type 4 paste, you
should be using Type 3 to avoid the non-agglomerated solder fines that
do occur. These small solder balls and solder fines occur for two
reasons. The first is because the solder has nowhere to go when printed
under relatively large flat areas, unlike a leaded component such as a
gullwing where the solder can flow up the lead. Chip caps and larger
chip resistors do have some solder flow up the endcap termination, but a
large amount of solder paste going into reflow under the part may not be
able to flow out fast enough, and the result is small solder balls "spit
out" from the pressure. This is why components with "belly pads" will
always have solder balls under the parts or surrounding the parts if the
paste is printed 1/1. An aperture reduction that gives a solder volume
of 35% to 50% or even less is needed to provide room for the molten
paste to flow. 

The other reason fines and small solder balls are seen after reflow is
because if you use a Type 4 paste your print process must be literally
perfect, with no solder paste being printed outside of the metal pad.
Even if the paste is compressed slightly due to the component placement
pressure, solder fines can be wedged into the opening between the edge
of the pad and the edge of the soldermask. These fines will agglomerate
together into small solder balls separate from the solder joint. 

To summarize, your solder paste printing can improve dramatically in
terms of perfect bricks of paste if you use a Type 4, but if your
printer's alignment, squeegee pressure, speed, and and other parameters
cannot be held to extremely tight tolerances, and you don't have good
control over the aperture sizes used for a given component, and the pad
library is not perfect, then stick with Type 3.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Monday, October 15, 2007 9:26 AM
To: [log in to unmask]
Subject: Re: [TN] Small solder balls

Morning Tom,

One thing might help is to go to the largest mesh size solder paste that
you can use, small mesh sized solder paste tend to have more fines and
oxidation that sometimes create solder balls that don't coalesce back
into the main fillet.

Kind regards,

Steve Gregory

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Friday, October 12, 2007 4:17 PM
To: [log in to unmask]
Subject: Re: [TN] Small solder balls

Hi Tom!

Got your picture posted. It's at:

http://stevezeva.homestead.com/files/1206_SolderBall.JPG

What kind of soldermask is on the board? LPI or dryfilm? Do you have the
mask relieved around the pads by a few mils?

I ask that because it almost looks like the solderball is caught right
at the edge of the mask sort of...

Steve Gregory

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gervascio, Thomas
Sent: Friday, October 12, 2007 1:38 PM
To: [log in to unmask]
Subject: [TN] Small solder balls

Has anyone ever established a baseline of the anticipated frequency of
solder balls under chip components? We are doing Space assembly work and
the specifications allow for no solder balls. We are using a ROL0 solder
paste and cleaning in an inline semi-aqueous cleaner with
saponification. What is being rejected are solder balls about a 1-2 mils
in diameter that are intermittently trapped under low profile chip caps
and resistors. I suspect it is an agglomeration of solder particles in
the paste occurring during reflow. I have tried all the usual things-
stencil aperture modifications: - homeplates; reverse homeplates(like a
bowtie design that reduced the printed pad by 25%); homeplates which are
offset 7 mils from the inside edge of the pad: tried reducing hot slump
of the paste (changed from 90sec soak between 150-170 sec (per vendor
recommendations) to ramp to spike; tried reducing placement force or
fudging part thickness.
 
What I am experience I believe is typical of the process but would like
some input from my peers.
 
Steve Gregory, can I post a photo on your website- stevezeva.com to show
what I am talking about?
 
Thanks
 
Tom Gervascio
 

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