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December 2003

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Subject:
From:
Bev Christian <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 9 Dec 2003 17:52:05 -0500
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Doug,
OK that's clearer.  Thanks.  With regards to metal filled epoxy, I was under the impression that they did not have a long lifetime.  I know Nortel never used them in central office switches.  I would guess that the work was done by the Ottawa labs, I sure didn't.  And you're building what kind of wind up toy there in Cedar Rapids?  :)  Hmm, I seem to recall that Ken Gilleo did a one day course on these things (metal filled epoxies) at SMTAI and/or APEX.  Why not ask him?
Bev

-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: December 9, 2003 4:09 PM
To: Bev Christian; [log in to unmask]
Subject: RE: [TN] Adhesive strength vs. overhang






Depends... just kidding!
**And I bet it just tickles you pink to use my favorite expression back at
me!

"Our specifications are based on solder joint connections and the solder
specifications are not in question."  What does this mean?
What solder joint requirements?

**What, you can't read my mind?  Shocking.  OK, here is an example.  If I
pick up a copy of IPC-A-610 (Rev C), and I go to section 12.2.2.1 Chip
Components - Rectangular or Square End Components, Side Overhang, the
target is no overhang.  It becomes a defect for class 3 when overhang
exceeds 25%.  Looking at the requirements for end overhang, none is
allowed.  These requirements come as the result of many solder reliability
studies, and are based on the dynamics of a metallic solder joint.

Now suppose we replace the solder material with a silver filled epoxy
adhesive.  Different material, different dynamics.  Should these same
solder joint requirements of 25% side overhang and 0% end overhang be
applied to the conductive adhesive joint?

I was looking for any studies that have delved into this issue.  Could I
allow more side overhang or some end overhang because I am now using a
different material with different mechanical bonding strengths and
different creep characteristics?  Or, is it not as reliable as solder
(Hillman, shup up)and we should require less overhang?

Jack Crawford, does the IPC have a specification like A-610 which addresses
the issues of electrical connection using conductive adhesives?


Is epoxy rigidity good or bad? Is it brittle?  Is it stronger?
<grin> It depends.  I think you get better mechanical strength from the
epoxy, but at the cost of reworkability.


Doug Pauls
Rockwell Collins

Lets see how many out of office replies I get on THIS one.....


                                                                                                                                       
                      "Bev Christian"                                                                                                  
                      <[log in to unmask]        To:       "TechNet E-Mail Forum." <[log in to unmask]>, <[log in to unmask]>      
                      et>                      cc:                                                                                     
                                               Subject:  RE: [TN] Adhesive strength vs. overhang                                       
                      12/09/2003 02:23                                                                                                 
                      PM                                                                                                               
                                                                                                                                       
                                                                                                                                       




Doug,
Depends... just kidding!
"Our specifications are based on solder joint connections and the solder
specifications are not in question."  What does this mean?
What solder joint requirements?
Is epoxy rigidity good or bad? Is it brittle?  Is it stronger?

No, I do not know of any public report of the type you are talking about.
I would expect some of the big boys have their own internal ones.
Bev Christian
Research in Motion


-----Original Message-----
From: Doug Pauls [mailto:[log in to unmask]]
Sent: December 9, 2003 3:14 PM
To: [log in to unmask]
Subject: [TN] Adhesive strength vs overhang


Good afternoon all,

Internally, we are having discussions on the amount of allowable overhang
of passive SMT components (R and C).  Our specifications are based on
solder joint connections and the solder specifications are not in question.
We have an application where a silver filled adhesive is used to make the
connection between SMT metalization and mounting pad.  The solder joint
requirements were applied to these adhesive joints and I am wondering as to
the validity of this.  The adhesive is epoxy based and, in my mind, more
rigid than solder.  I think we are unnecessarily rejecting parts that are
acceptable.

So, before I go off and do a complex designed experiment, I thought I would
check with the Technet crowd first.  Do you know of any studies that show
the adhesive force (e.g. pull strength) vs. the degree of pad overhang?

Doug Pauls
Rockwell Collins

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