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June 1998

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Subject:
From:
"by Dr. Eden Chen XianSong" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 16 Jun 1998 09:15:13 +0800
Content-Type:
text/plain
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text/plain (157 lines)
     Dear Matthew,

     Thank you for your advice about the conductive epoxy.

     Why do we want transfer from Ag/Pd capacitor or solder/pure Tin
     capacitor? Because  (1) our supplier from Japan will stop supplying
     the Ag/Pd capacitor and it is very difficult to get Ag/Pd capacitor at
     different value. It is even more difficult to get Ag/Pd resistor; (2)
     Ag/Pd terminal is expensive. Our product is optoelectronic component,
     it is quite cheap about a few  dollar.

     We have very long history to attach Ag/Pd capacitor using conductive
     epoxy. it is without  any problem.

     The solder/Tin termination capacitor/resistor attached with some
     conductive experience resistance increase when exposure to 85/85 test.
     But we do find one epoxy can pass 1000 hrs 85/85 test with very small
     resistance change with encapsulant and without encapsulant.

     Your advice is very correct about stencil printing and dispensing.
     The printing quality is better than that using dispensing. The 4 mil
     stencil was used.

     When I am doing the process evaluation about the dispensing, I face a
     lot of problem such as tailing, insufficient coverage of epoxy and no
     fillet. After the unit was encapsulated, it go trough 3 times IR
     reflow, then the resistance increase has been found.


     Best Regards

     Eden Chen






______________________________ Reply Separator _________________________________
Subject: Re: [TN] Conductive Epoxy for Solder termination
Author:  Non-HP-MPARK ([log in to unmask]) at HP-Singapore,mimegw51
Date:    6/16/98 6:03 AM


Dear Dr. EC,

A successful implementation of electrically conductive epoxies
for SMT application is more than understanding those issues
you  stated.  And it is NOT definitely a replacement for tin/lead
solderpaste as some mfg of conductive epoxies claim.    It is not very
difficult to form fillet with the epoxy.  Simply  epoxies DO NOT form
fillets and wet.

I performed a few sample tests one/two years ago when
one conductive epoxy mfg claimed it prints like any other
solderpaste and it is a replacement for solderpaste.  I
concluded that some fundamental issues have to be addressed
to make electrically conductive epoxies useful in a SMT
manufacturing environment,

- impractical storage/shipping temp, -70'c to-40'c
- too suceptable to moisure and dry out too quickly
- do not adhere good to tin/lead surface. A test revealed it
adhered well to bare copper and solder mask  surfaces.  It is
proper to have a different metallic finish for SMT component
soldering terminals and board surface than tin/lead.
- redesigning  board layout for epoxy application such as pad
sizes, conductor space req's etc...
- redesigning component soldering terminals for epoxy
application.
- poor performance of printability of epoxy
- reworkability

To answer your questions, it is vital to provide only an exact
amount of epoxy for component terminal area.  A typical printed
thickness of epoxy should be at 0.1mm.  That is why I believe a
dispensing option is not suitable.   It won't provide a controlled
coverage of epoxy for soldering terminals.  There are too many
parameter variations to control such as epoxy consistency, dot
size, dot height, placement pressure etc...

regards
Matthew
NII-Norsat International Inc.


>>> "by Dr. Eden Chen XianSong"
<[log in to unmask]> June
14, 1998  8:35 pm >>>
     Dear Sir,

     Some silver-filled conductive epoxies in the market claim to
be solder
     replacement, but successful use of these material require a
very good
     understanding of the epoxies properties, the process and
CTE mismatch.

     The epoxy can be deposited to the pad by stencil printing or
     dispensing,

     Dispensing process has the tailing problem which can cause
short for
     0402 chip resistor/capacitor.  It is very difficult to form the fillet
     for epoxy, because the epoxy may have a gap between the
side of chip
     resistor/capacitor and dot peak of epoxy.  How much should
the
     placement force be applied and how big should the dot size
for 0402
     resistor/capacitor?

     I like to discuss the above-mentioned issue in the TECHNET

     Eden

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