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March 2012

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From:
Steven Creswick <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steven Creswick <[log in to unmask]>
Date:
Mon, 12 Mar 2012 13:05:36 -0400
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Steve,

Not to be a nay-sayer, but I echo much of what Richard is getting at.

Have used conductive and non-conductive adhesives within hermetic packages
for many years.  Still have items constructed this way orbiting in the
Saturnian system - Cassini.

Rework generally is easy, just heat the item up to the glass transition temp
[can be anywhere from 40-50°C for low-class adhesives to 125-185°C for the
better, more thermally stable materials].  Hot air is typically used for
localized heating with mechanical removal of the offensive component.
Standard equipment exists for this.  It is really quite easy.  Does involve
operator intensive mechanical clean-up or the area to remove old adhesive.
Far easier than the mess of solder and flux, etc.

Many, but not all, conductive adhesives form reliable and electrically
stable bonds that hold up quite well in a hermetic environment.  This is NOT
so, in the oxidizing, humid environment of a PWB assy.  Contact resistance
typically climbs quickly.  If your application is only driven by voltage,
instead of current, you may get away with it longer.  However, always be on
the alert for electromigration...

As someone said, they are significantly weaker than solder - and it can get
worse with age, thermal cycling, CTE mis-match, etc.

Adhesives do not allow for self-centering of components although accurate
dispensing of adhesive has been done for many, many years.  For example,
generic jelly-bean LED die are frequently on the order of 100µm square [4
mils].  Depending upon the particular device construction, sometimes side
fillets are not allowed [shorts out the junction on bottom junction
devices].  While dispensing 50-75µm dots is not trivial, it is done by many.
There are also some materials used for jetting of conductive inks, but they
typically have a great deal of solvents employed and never tested to be
stable in the microelectronics application.

There are many 0402 case size [and smaller] chip caps and resistors attached
using Ag-filled adhesives in high-rel Mil and Implantable medical
applications.  Needless to say, dispense volume and pattern is important,
but readily do-able.  Microelectronics applications also typically deal with
dispense and component placement accuracies of 12-25µm, so if one gets the
correct amount in the correct location, all is typically well.

In Microelectronics applications, one would never attempt to bond to a
solder-coated, component.  Suitable chip components typically have Pd-Ag, or
straight Ag termination metallizations.  Au or bare Si are other typical
component terminations.  Curing of the 'better/more thermally stable'
adhesives generically takes place in the 150-165° range.  I refer to it as
attempting to adhere to a moving surface when attempting this cure to a
solder coated surface - even more so when dealing with non-eutectic alloys.

Yes, the solder replacement adhesives have lower cure temps than 150-165°C.
Some are RT cure, UV, or in the 50°C and up range.  Just remember that the
glass transition temp [Tg] of the adhesive generally can only approach the
cure temp [with certain exceptions].  Therefore, it would be prudent to have
a very benign use temperature envelope as well.

I would not adverse to listening, reading, and evaluating samples if time
permits, but the real reason they are not being used in the PWB industry is
because they just are not as reliable...  IMHO


Steve Creswick
Sr Associate - Balanced Enterprise Solutions
http://www.linkedin.com/in/stevencreswick
                         616 834 1883







-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, March 12, 2012 12:14 PM
To: [log in to unmask]
Subject: Re: [TN] Assembling with conductive adhesive

You also need to understand the long-term reliability characteristics of
silver-filled epoxy, with your design. It does not usually last as long as
solder, especially not under severe thermal cycling.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Monday, March 12, 2012 10:52 AM
To: [log in to unmask]
Subject: Re: [TN] Assembling with conductive adhesive

(1) you need excellent pick and place machine (no self centering surface
tension to help you).  
(2) you need to pick a good vendor with high quality silver loading (higher
silver = good conductivity but poor adhesion).  Few of them can do it
properly.
(3) you need good interface bonding with close contact of silver flakes
during the life of the product.  Not all of paste can do that (changes
conductivity as time goes by)
(4) you need components termination that can be bonded to adhesive, not all
vendor supply that part or have that options.
(5) you need good dispensing machine or pad printing machine.  Not everybody
who get use to stencil would want to (high resistance implementation).
Unless you want to use ACF (capital equipment cost). 
(6) if you can overcome above 1-5, very nice to use the conductive adhesive.


Joyce Koo
Materials Researcher - Materials Interconnect Lab Research In Motion Limited
Office: (519) 888-7465 79945
Mobile: (226) 220-4760

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Monday, March 12, 2012 11:39 AM
To: [log in to unmask]
Subject: [TN] Assembling with conductive adhesive

Hey all,

 

Wish I hadn't lost that hour of sleep this weekend, I'll try to find it
tonight. :o)

 

Anyways, have any of you thought of trying to replace solder with a
conductive adhesive for PCB assembly? There's a few out there (Henkel,
Cookson, Ellsworth, etc.) that are touting them as solder replacements.

 

When I read the technical datasheets, I'm thinking to myself, why isn't this
stuff more widely used? There must be a reason. 

 

I do know it's pretty expensive almost all of them are silver filled, and I
have read that the mechanical strength and thermal conductivity is not as
good as solder, but what other reasons are out there not to use it?

 

Steve



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