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Printed Circuit Board Assembly & PCB Design Forum

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What is too much silver in a joint

Bob

#19320

What is too much silver in a joint | 28 March, 2002

I have been looking at cross sectioned joints. In some instances there are areas where there are large concentrations of silver.

Could anyone quantify what is too much and also explain how the intermetallics are formed.

Thanks.

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#19323

What is too much silver in a joint | 28 March, 2002

Intermetalic Layer (IL). A compound formed at the interface of two different metals, whose atoms have an extremely high natural attraction for each other, so high, that they do not bond to other elements by any other means. Also, intermetalic compounds have fixed stoichiometric ratios. Whereas, alloys can have varying ratios of metals.

So, yer forming an IMC between silver and ... ? What is the composition of material of your: * Pads * Solderability protection on the pads * Solderability protection on component leads * Component lead material * Solder * Other

How did you determine that the material that you see in the x-section is silver?

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Bob

#19339

What is too much silver in a joint | 31 March, 2002

Cheers Dave, to answer some of your questions. � The component lead is copper with a tin / lead coating. The pads are standard Copper / Nickel / Gold. � The solder is standard Tin / Lead / Silver (2%)

We have a lab that can determine the composition of all metals in the joint after a X-section, not sure of the exact way in which they determined the metals but it looks very colorful. We have several images, which show the lead, copper, tin, nickel and silver content.

Bob...

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#19345

What is too much silver in a joint | 1 April, 2002

Hey Bob,

Boy, you are lucky to have a laboratory, and now colorful pix!!!

The only silver IMC that you will see are: * Ag6Sn, much less likely at 'normal' soldering temperatures * Ag3Sn, much more likely

Allowable silver concentration: * Below 2% will not cause deterioration in solder connection. * At 4% IZOD impact strength begins to decline from ~80 in-lb to ~15 in-lb at 6% [like falling off a cliff, eh?]

So ... * How thick is your IMC? * What level of silver do you have?

Observations are: * Be concerned that component manufacturers, in their rush to be lead-free, have resorted to using thick, 100% silver lead finishes. [You said 'no-lead', you don't anything about silver. Gawd the @#%#!! Eurolanders are making everyone's products unreliable, even if we don't use no-lead!!!] * While you are correct to be concerned about the Ag3Sn IMC, your solder system will have IMC other than silver.

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