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December 2014

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Subject:
From:
Wayne Thayer <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wayne Thayer <[log in to unmask]>
Date:
Thu, 11 Dec 2014 18:23:53 +0000
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Hi Jim-

You've got lots of room for increasing solder deposition by just enlarging
stencil apertures, but reflow-in-paste will never achieve the 100% wetting
(which I'm taking to mean "void free"). To do that you will need to use one
or more pre-forms. For example, prior to reflow, you may just be able to
place some rectangles of solder on those pads that look starved (these can
be packaged in T&R as any other component). That should take care of the
issue, but if you have to go to a preform underneath the part then you'll
have to figure out how to flux it. 

Another, cheesier technique, used on prototypes, is to pre-flow solder paste
on the component. That isn't fun if you're building more than a couple.

Wayne

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jim West
Sent: Thursday, December 11, 2014 12:12 PM
To: [log in to unmask]
Subject: [TN] Power switch IC inspection criteria?

All,

Regarding inspection and acceptable criteria for a Power Switch IC with a
heat slug underneath.  Per attached pictures (Thanks Steve Gregory!), we
will get solder conditions as shown where we are not obtaining a visual 100%
wetting on the side overhangs "ears". We know we have solder under the part
by verifying with removal of the part.

1 - We are not inspecting these with our AOI's due to the inconsistent
solder joint and wetting on the outside "ears"
2 - One of the pictures shows our stencil aperture design for this part and
it shows we are applying paste to the entire area of this power plane.
3 - Per IPC 8.3.14 (Components with Bottom Thermal Plane Terminations), we
must achieve 100% wetting to land in the contact area, or it is considered a
defect all three classes.
4 - The solder applied to the "ears" wicks underneath the part leaving
little solder left for the "ears".
5 - See attached photo showing board layout and pad design.  We
unfortunately do not have the option of redesigning this pad.


What criteria do we inspect by?  Is this acceptable?  I don't know how I can
control the solder so it will not wick to the underside of the part and
allow visual evidence on the outside "ears" of the part.

We would be required to 100% inspect each board and apply solder to the ears
to achieve passing criteria?

Hopefully some of you have seen this type of situation and have experience
on how to inspect and what is the criteria for inspection?

Thanks for your time!

Jim

http://stevezeva.homestead.com/IMG2.jpg

http://stevezeva.homestead.com/IMG3.jpg

http://stevezeva.homestead.com/IMG4.jpg

http://stevezeva.homestead.com/IMG6.jpg

http://stevezeva.homestead.com/Power_IC.jpg







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