Well the basic laws of physics and chemistry haven't changed. The plethora
of papers at that time does identify the issues very well and explain what
is needed. The reason for that burst of effort was that Ag conductives - iso
or anisotropic - were seen as a possible alternative process in the light of
the looming lead ban. 
I was involved in such a project (at a somewhat low level technically, more
marketing orientated). Our conclusions were the same as most others. First
that at least three breakthroughs would be needed to have anything like a
drop in adhesive replacement for solder. Second that the use of adhesive
interconnects would continue to grow, but by designing for adhesives not by
designing an adhesive to be a solder. The lead ban would just tilt the
balance of choice slightly more away from solders. We do still solder of
course.
Overall the issues now are the same as then. They are better addressed in
newer products by a number of sequential incremental developments in process
and in in-service performance. Newer products can have snap cure schedules
which give comparable thermal exposure in processing, maybe less. They can
contain oxide reduction additives, and corrosion inhibitors and so on to
increase reliable conductive bond life and maintain mechanical integrity.
Conductivity is still relatively poor. Thermoplastics do overcome one of the
biggest draw backs which is reparability but not without performance costs.
So modern products can be closer to solders in service and may well be good
enough, but this is conditional on component surfaces being optimised for
bonding not soldering. Process parameters will also need to be adjusted for
the way the products behave. For example what you print is what you get, so
No self alignment and so on.
It may be that you could use a low temp soldering process, if not then of
course use adhesives, but do speak with suppliers to obtain optimum product
and process conditions. Keep in mind that each adhesive/process will be
application specific, so by and large you can't just switch to an other
suppliers functional equivalent as you can with solders, you will need to
revalidate.


Regards 
 
Mike Fenner 
Bonding Services & Products
M: +44 [0] 7810 526 317 
T: +44 [0] 1865 522 663
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eva J
Sent: Monday, April 29, 2013 1:59 PM
To: [log in to unmask]
Subject: Re: [TN] Conductive epoxy to install SMT parts

Conductive epoxy is being considered as an electrical and mechanical bond
since the component can not withstand reflow temperatures. The future
application will be an ENIG finish on PCB with gold finish on the component
termination. Late 80s and even 90s papers would be outdated as materials
have changed. So no one is aware of recent papers regarding conductive
epoxy?

Thanks
Eva


On Sun, Apr 28, 2013 at 7:41 PM, Mike Fenner <[log in to unmask]> wrote:

> It's not totally clear why you want to adhesive bond instead of solder.
Are
> you looking to adhesive bond special devices to otherwise assembled
boards?
> There maybe ways to solder using repair techniques for example.
> If you wish to use adhesives you will need to consider conditions of use,
> service life, and in production surfaces to be bonded as well as max
> process
> temp and time. These will be different from solders. Generally adhesive
> bonds are not as strong as solder, and in non-hermetically sealed packages
> service life can be shorter (but maybe sufficient in either case).
> As already said epoxies do not bond well to tin or tin based alloys. Gold
> is
> also a low energy surface and will give low bond strengths with adhesives.
> So bond strength could be an issue, this is in addition to any problems
> that
> might arise from the Ag:Sn interface. Some suppliers do claim to have
> addressed the long term conductivity issues.
> Do keep in mind that whereas adhesives can be classified into generic
> types,
> (epoxies, silicones, acrylates etc) they are all supplier unique within
> that
> classification. This means you will need to check or verify product
> performance claims with every supplier. [This is not like solder pastes
> where although flux chemistry may be supplier specific, a solder alloy
such
> as Sn63 is not.]
> You will need to observe the cure schedules in the supplier data sheets,
> treating them as minimum times/temps at the bond line or the product will
> not develop data sheet properties.
>
> Regards
>
> Mike
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
> Sent: Thursday, April 25, 2013 3:31 PM
> To: [log in to unmask]
> Subject: Re: [TN] Conductive epoxy to install SMT parts
>
> Eva,
>
> You should ask the formulator for this information.  Really attempt to
nail
> them down and get them to provide real, and detailed data - one real live
> part types.
>
> In general, it is quite difficult to get most adhesives to adhere to the
> diverse surfaces finishes presented in SMD work, at temperatures that the
> SMD assembler [and components] will tolerate.  Making it more flexible is
> only one factor to consider.
>
> Mechanical, thermal, or other environmental stresses typically quickly
show
> the weak links.
>
>
> Side note - if "cured" over the weekend, I suspect that it is more of a
> humidity cure RTV than an "epoxy" based system.  Do you smell acetic acid
> [vinegar] during cure?  = bad idea.  Or do you smell more of an alcohol?
=
> okay.  Silicones have a nasty way of tending to migrate to places you
don't
> really want them to go... switches, connectors,
>
> Can your production live with a 48-72 hr cure?
>
> I would be sure to ask the formulator whether the material has an epoxy
> backbone, or a silicone backbone so that I could get a better grasp of its
> mechanical properties.  I am assuming that if it is a silicone, under
> normal
> conditions, it will already be above its Tg, and therefore already in it
> 'high expansion' state.  I also make the generalization that it will
remain
> flexible until down in the -20 to 0°C range [subject to the specifics of
> the
> material]
>
> Steve C
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Eva J
> Sent: Thursday, April 25, 2013 10:01 AM
> To: [log in to unmask]
> Subject: [TN] Conductive epoxy to install SMT parts
>
> I am looking for guidance on conductive epoxy selection for components on
> PCBs. I heard that there is a formulation of conductive epoxy with
silicone
> as a filler that is more flexible. Do you know of any studies or
> evaluations
> available on conductive epoxy for electronics?
>
> We attached a 4 pin LGA component that has gold finish terminations to a
> HSAL finished PCB. We syringe dispensed the conductive epoxy to the land
> pattern. Optically machine placed part and allowed the conductive epoxy to
> cure over the weekend.
> During environmental stress test ( hot / cold cycling) one joint failed.
> All of the LGAs were removed and what we found is the component came off
> way
> to easily.
>
> I would like to continue using conductive epoxy to mount special
> application
> SMD's: however reliability is a concern. Any words of wisdom?
>
>
> ______________________________________________________________________
> This email has been scanned by the Symantec Email Security.cloud service.
> For more information please contact helpdesk at x2960 or [log in to unmask]
> ______________________________________________________________________
>
> ______________________________________________________________________
> This email has been scanned by the Symantec Email Security.cloud service.
> For more information please contact helpdesk at x2960 or [log in to unmask]
> ______________________________________________________________________
>
>
> ______________________________________________________________________
> This email has been scanned by the Symantec Email Security.cloud service.
> For more information please contact helpdesk at x2960 or [log in to unmask]
> ______________________________________________________________________
>

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________


______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________