Hi Mark -

Go back and look at the Bulwith paper again. The recommendation is 5 to 15 micro INCHES. It's the old problem of we Yanks vs the rest of the world in units. Converted to rest of the world units, Bulwith is recommending 0.13 - 0.38um which is well inside the NPL calculations. Many BGA manufacturers in their app notes recommend 0.15 +/- .05 um gold over nickel as the maximum to prevent gold embrittlement. They are being conservative to try and account for the variables of reflow soak time, temperature, paste thickness, and solder alloy variations. All of the above will affect Au diffusion, Pb migration, and the formation of the Au-Sn / Au-Sn-Ni intermetallics that cause embrittlement.

Regards - Harry


At 08:40 PM 11/24/02 -0500, you wrote:
Thanks Harry,
I've read both papers below and don't see how they agree. The NPL paper has 2 charts that max out at 3um gold pad thickness. Now I was told that the solder paste stencils that my company uses are 5 to 6 mils (127 to 152um) thick. If this is the case the maximum thickness of gold I should speck that won't cause embrittlement would be between 1.8 and 2.2um. I'm I missing the boat?
The Bulwith paper (which I was really impressed with ) stated that gold thickness should be between 5 to 15um. What was missing from that paper was the volume of solder (or thickness of solder paste) recommended to insure a good joint(<3.5wt % of gold by NPL paper). If I use 5um gold would I have to use 350um paste (a 13.78 mil stencil) to obtain the proper gold to solder ratio?
Brad
thanks for your input. I'll be hunting up those papers next.
Cheers
Mark
At 09:26 AM 11/22/2002 -0800, you wrote:
Hi Mark,

Here are two links to papers that discuss the subject of soldering to gold plated PCB's. The first is theoretical, and the second is practical (they agree with one another). The guidelines you are looking for are on the last page of the second paper.

http://www.npl.co.uk/ei/documents/gold.pdf

http://www.alphametals.com/products/solder_solutions/pdf/soldergld.pdf

Regards - Harry

At 06:56 AM 11/22/02 -0500, you wrote:


Hi Everyone,
Does anyone know the maximum recommended plating thickness you can use for gold with out causing gold embrittlement in the solder joint. My application is for BGAs. I attended a class and PCB east a few years ago and was introduced to a new term "Sacrificial Gold platting" . I interpreted it to mean the minimum thickness required to prevent oxidation and not cause embrittlement of the solder joint. So does anyone know what thickness I should speck for my boards?
Thanks
Mark
Mark E Witherite CID
469 Nilson Rd
Bellefonte PA 16823
[EMAIL PROTECTED]
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