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July 2009

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Subject:
From:
Steven Creswick <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steven Creswick <[log in to unmask]>
Date:
Thu, 16 Jul 2009 05:33:09 -0400
Content-Type:
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text/plain (207 lines)
Eric,

As Inge has commented, conduction in the adhesive generally takes place as
the consequence of contact between adjacent flakes of silver.

The adhesives manufactures have various ingredients in the adhesives which
minimize oxidative effects on the powders before, during and after cure.

The contact resistance value which you are getting, does indeed sound quite
high.  Since you are 'doing everything correctly', you will have nothing to
lose in trying another manufacturers materials.  I strongly suspect that you
will find the problem goes away. 

When you hit the joint with a surge, you will break through the 'oxides' and
create a conduction path.  This path will remain intact until you subject
the joint to a heat or cold transition.  You will find that you can 're-set'
the joint many many times.

If your joint is a lap joint between a plastic enclosure and the PWB, you
may also have a few CTE issues that mechanically aggrevate the situation.

You are absolutely correct in noting that mechanical strength and electrical
conductivity go hand in hand.  There is a definite one to one correlation.
If you don't have one, you won't have the other.

Steve

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric CHRISTISON
Sent: Thursday, July 16, 2009 5:13 AM
To: [log in to unmask]
Subject: Re: [TN] Silver epoxy - related question

Thanks Inge,

Yes,I'm sure the joint is "good".

Mechanically it's reliable and resistance remains reasonably constant 
through environmental testing i.e. we might see an increase from ~3 to 
~3.5 ohms from T0, though 3 lead free reflows followed by temperature 
humidity cycling. An interesting result is that I can tell when a joint 
is starting to weaken mechanically just by measuring it's resistance.

The enclosure is plated plastic, stainless steel on top of copper and 
the resistance between two points on the plating is typically 0.30 Ohms. 
The pads on the substrate are Ni/Au plated and should be clean.

The data sheet tells me that volume resistivity of the glue we use is  
0.30 mOhm.cm. Yes we cure properly and apply the correct amount of force 
and all through development I've never seen much change in DC 
resistance. I've certainly never seen results in milliohms.

Hmmm, got me thinking now.

 



Inge wrote:
> 3 Ohms is not healthy silver epoxy joint. I've been using silver 
> filled adhesives for 20 years, and can say with certainty that 
> something is wrong when you get such a high resistance value. A normal 
> reading is 10 milliOhms or even lower. The resistive path consists of 
> thousands and thousands of silver flakes in contact with each other, 
> and they make a low  resistance network. Are you sure of doing correct 
> measuring? Have you followed the curing instructions? Were the sites 
> cleaned before application of the adhesive? Enough adhesive dispensed?
>
> Typical volume resistivity is some milliOhms/cm.
>
>
> ----- Original Message ----- From: "Eric CHRISTISON" 
> <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Thursday, July 16, 2009 10:14 AM
> Subject: [TN] Silver epoxy - related question
>
>
>> Interesting thread which prompts me to raise a related question.
>>
>> I'm using silver epoxy as part of an EMC/ESD shield. It connects a 
>> conductive enclosure to a PWB ground plane.
>>
>> Generally we get a DC resistance across the epoxy joint of about 3 
>> Ohms. However after we subject a device to an ESD spike we find the 
>> resistance drops by maybe an Ohm or so.
>>
>> Anyone got an idea what's happening?
>>
>> Thanks,
>>
>>
>>
>> John Burke wrote:
>>> Thanks to all for the replies on silver epoxy - they are as always much
>>> appreciated.
>>>
>>> Best regards
>>>
>>> John Burke
>>> (408) 515 4992
>>>
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>>
>> -- 
>> Eric Christison Msc
>> Mechanical Engineer
>> Consumer & Micro group
>> Imaging Division
>>
>> STMicroelectronics (R&D) Ltd
>> 33 Pinkhill
>> Edinburgh EH12 7BF
>> United Kingdom
>>
>> Tel: +44 (0)131 336 6165
>> Fax: + 44 (0)131 336 6001
>>
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-- 
Eric Christison Msc
Mechanical Engineer
Consumer & Micro group
Imaging Division

STMicroelectronics (R&D) Ltd
33 Pinkhill
Edinburgh EH12 7BF
United Kingdom

Tel:	+44 (0)131 336 6165
Fax:	+ 44 (0)131 336 6001

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