Wirebonding question

[laroche73]

Has anyone successfully used gold wire to wirebond to an immersion silver PCB surface finish? I'd like to know of anyones experience with this or any long-term issues with current immersion silver finishes (MacDermid "Sterling" for ex.)? The reason I'm asking is that a PCB I'm working on needs to be both wire-bondable & solderable. A common metal or alloy for the pads would be best, but these components are only available with certain pad coatings (few options, the parts are ~ 0201 size). Gold & aluminum pads and standard SMT components with a nickel/lead/tin finish will be used on the board. I've searched the web over, and immersion silver seems to be recommended for aluminum wire-bonding but not for gold. There's a bit of anecdotal evidence that gold WB/silver immersion finishes work well, but I haven't come across any definitive studies. I've done some experiments wire-bonding gold wire to silver immersion PCB pads, and the results (pull-test & visual quality of the joint) seem to equal gold-to-gold (ENIG) bonding, but long-term reliability is a concern.
A gold surface finish (ENIG or soft gold) is usually recommended for this kind of PCB, it would be a plus in production to use a lower cost, standard surface finish that is both wire-bondable and solderable. Immersion tin & OSPs are not suitable for wirebonding. Thanks for any help. - CAL

 

[Roff]

Claude, have you seen this article?

Ron

 

[laroche73]

wirebonding Au-Ag

Yes, thank you. I ran across it yesterday, and found it to be one of the more helpful documents in the limited information available on the net. A few key statements from the paper confirmed this is not a common procedure (yet).

"While this work provided data that shows evidence that this coating (silver immersion) will bond with aluminum wire, another course of study has been engaged to evaluate gold wire bond effectiveness. As of this paper's publication, no data is available to report and will be published in a later paper." The paper goes on to say "Silver has been used as a substitute for gold wire for ball bonding integrated circuits. It has shown to be effective providing strong bonds with both gold and aluminum (pads)." This describes a reverse usage of the bonding materials (Ag wire/Au pads), but is helpful in prodding me to explore the idea a little further. As noted in the first post, our simple experiments showed good results, but I'd like to see some long-term reliablity data, if any exists.

I'll try to contact a few technical gurus from these companies and see what they have to say, but it sounds like immersion silver isn't well tested for this application. Thanks again for the link - Claude