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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