Here's a reply by Mike Barmuta which I believe will answer your question. Joe: This galvanic reaction is known as bimetallic corrosion. It takes place when dissimilar metals are in contact with each other in the presence of an electrolyte. In your case the copper is acting as an anode and being deplated away in the microetch. Here are some things to consider that will help. 1. Minimize the difference in surface area between the Au and Cu features. Think of it as anode to cathode ratio. Make the anode(Cu) bigger than the cathode(Au). Don't run small Cu traces into large Au pads if possible. 2. Minimize the time/concentration/temp of the microetch. In your earlier posting you stated you where trying to remove 60u". That is a lot of Cu. What is it you are trying to prep the surface for? At one time in our OSP line we ran our peroxysulfuric micoetch at excessive immersion times based on vendor recommendations. We had similar etchout experiences, as you are seeing, on Cu/Au boards. We now run this process using 30 sec. Vs the original 4 mins. in the micoetch. This removes about 5u" of Cu based on the chemistry concentration and temp for our microetch bath. This has eliminated the severe etchout problem and has not affected board finish or solderabiluty quality. You may want to consider a similar approach. 3. Whenever possible cover exposed Cu and Cu/Au interface areas with soldermask. This will help prevent localized etchout. Regards Michael Barmuta Staff Engineer Fluke Corp. Everett Wa. 425-356-6076 -----Original Message----- From: Joe Aronson [mailto:[log in to unmask]] Sent: Wednesday, August 18, 1999 7:34 AM To: [log in to unmask] Subject: [TN] GALVANIC ETCHING MECHANISM ALL: Has anyone out there experimented with etches to see wether a more/less aggressive etch produces a change in the degree of galvanic etching? It sounds to me like the main limitation in the mechanism of galvanic etching is going to be the diffusion of the oxidizer to the more noble metal. Is this the case?? If there is a known rate determining step please let me know. I may e able to use it to fight this effect. Thanks. Joe Aronson Wet Process Engineer JMSPI ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or 847-509-9700 ext.5365 ##############################################################