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June 1998

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Subject:
From:
"Stephen R. Gregory" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 10 Jun 1998 15:08:39 EDT
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In a message dated 6/10/98 11:19:30 AM Pacific Daylight Time,
[log in to unmask] writes:

--> To all,
-->
-->  My R packs have a mind of there own,  at pick and place they are aligned
-->  then during reflow they decided to move. What causes this and how can I
-->  prevent this?
-->
-->  Thank you in advance
-->  Angie

Hi Angie!

     I'm assuming you're talking about chip resistor networks, because I know
there can be a problem with them if you have a certain style termination.
There's quite a few people not aware that there ARE two different styled
terminations; Concave and Convex, or as I prefer to call them; "Innies" and
"Outies"...ya' know, like belly buttons (GRIN).

     To describe them a little further, is that Concave styled terminations
are also known as castellated terminations, or what you see on LCCC's
(Leadless Ceramic Chip Carriers), there will be a square shaped metallization
with a half circle shape on the face of each resistor on the network. The
Convex terminations have the metallization plated all around the end of each
resistor looking more like the standard termination on individual resistors.

     The best ones to use are the "Innies" or concave terminations. They don't
have the skewing or misalignment problems that the convex terminated networks
have.

     The "innies", because of the termination design, utilize the surface
tension and wetting forces occuring when the solder is liquidous to a much
better degree than do the "outties" and actually self-center during reflow. In
fact, I have a report that was given to me by a Phillips component rep, of a
study done by engineers in the Phillips product application group to quantify
that there really is a difference between the two styles. It has some really
nice pictures of the two styles before reflow, and after reflow. They've both
were placed misaligned to the same degree, and the pictures show that the
"innies" self-aligned while the "outties" did not.

     Unfortunately, I only have a hard copy of this report, and it's in color
so it doesn't copy clearly in a black and white copier, otherwise I could FAX
you a copy. I don't have a scanner, but I do have a digital camera, so I'll
try and take a clear picture of the one page of the report that shows the
before and after pictures clearly and email them to you if you want...

     But, I bet you that's your problem...I went through the same headaches
when I first started placing them. We we're building SIMM cache' modules that
had 16-per of these little jewels on a board.

                                -Steve Gregory-

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