Eva,

I was also researching conductive epoxies and found a paper here:
http://www.masterbond.com/whitepaper/electrically-conductive-adhesives
I had to register to access it. I don't endorse this company in any way, but maybe this paper will help you in some way. 

Good luck!

Howard A. Watson


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eva J
Sent: Tuesday, April 30, 2013 4:20 PM
To: [log in to unmask]
Subject: Re: [TN] Conductive epoxy to install SMT parts

Thank you very much

Indeed it is a difficult situation with limited solutions for our specific application.

Onward TechNet Soldiers!
Eva J


On Tue, Apr 30, 2013 at 5:07 PM, Mike Fenner <[log in to unmask]> wrote:

> 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?
> >
> >
> > ____________________________________________________________________
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