Hi Brad,
I did indeed.
Nothing much has changed with SAC solders in this respect. The IMCs will be still with Sn and the base metal. There is no optimum thickness.
Shear strength is not a useful metric for SJ quality--with SJs it is a  step-function, either you have proper wetting or you no not. What is important is not the shear strength, but the failure mode: it is either brittle interfacial [that means when you look at the 2 fracture surfaces they do NOT have the same metallurgy], or ripped out soldering pads [fractures in the SJs are indeed very rare] indicating good 'wetting' and a strong metallurigical bond. Your reflow profile is ditto an all-or-nothing proposition: either all SJs are properly 'wetted' or some of them [in the center of BGAs] saw insufficient heat. Do not fall for this non-technical advice of 60 to 90 seconds above 183C--this does not assure proper wetting. You need Liquidus+20C [Liquidus+35C when soldering to Ni] for at least 5 seconds for even the slowest heating SJ to assure 100% good quality SJs.
Werner Engelmaier

-----Original Message-----
From: [log in to unmask]
To: [log in to unmask]
Sent: Thu, 26 May 2005 16:05:37 -0500
Subject: IMC and Wetting


Werner,

In a previous Global SMT & Pkg column "Solder: Quality and Reliability
Issues" you indicated "proper wetting is the process of forming a
metallurgical bond between the base metal of the solder pad and at least
one constituent of the solder alloy, in most cases tin."  We would like to
apply this principle in determining wetting criteria for SnAgCu alloys.
Can you share any guidelines or point us to references for optimizing IMC
during assembly? (both thickness and constituents).  Do you consider shear
strength of as built PWA components to be a useful metric for comparing
variables (reflow profile, solder flux, etc)?

I realize you are very busy and I would appreciate any input you can share.

Thank you,
Brad Green, Engineer
Seagate Technology
Shakopee, MN
952-402-2451

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