Agree with Mike. (1) to formulate adhesive to bond one type of surface well is not easy, to bond two totally different surfaces, Au and HASL, is need a master. (2) HASL is active surfaces, adhesive such as epoxy might react with it to form a weak layer at adjacent to the surface. (3) Smooth surface of HASL lack of anchor sites that helps for physical locking of rough surface normally helps in adhesive bonding. (4) check late 80 and early 90s papers, many tried to use adhesive to replace solder. There are tons of work may not be on the google list. My 2 cent.
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----- Original Message -----
From: Mike Fenner [mailto:[log in to unmask]]
Sent: Sunday, April 28, 2013 07:41 PM
To: [log in to unmask] <[log in to unmask]>
Subject: Re: [TN] Conductive epoxy to install SMT parts
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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