Hi Ben - ah, now I see where you are going. Unfortunately, some of our industry "tribal knowledge" is need of a refreshing. The old rule of thumb was that if you were able to disperse the gold uniformly across the solder joint, you were safe and the issue of gold embrittlement was a closed topic. When we began using ENIG pwb surface finishes in much greater quantities, we discovered that the gold would migrate back to the solder/nickel solder joint interface and if the conditions were favorable, you could form a brittle gold/tin IMC that would degrade the solder joint (HP first reported this back in the early 90s). A bunch of good research (there have been a number of good articles in the Journal of Electronic Materials over the last 3 years) has demonstrated that the reason for the gold to migrate back to the solder/nickel interface is that a gold/nickel IMC phase is thermodynamically favored over a gold/tin IMC phase (sometimes material science isn't our friend). So, if you are soldering to a copper surface (aka immersion silver or immersion tin) the old tribal knowledge rules hold true. However, if you are soldering to a nickel surface (aka ENIG) then you need to understand how much gold you are distributing across the solder joint and impact of the use environment time/temperature interactions. The smaller the solder joint, the greater the risk of issues (i.e. CSPs versus BGAs). The Binghamton paper that Richard recommended is a good paper to assist in your assessment. Good Luck. Dave "Gumpert, Ben" <[log in to unmask]> 10/27/2008 06:27 AM To [log in to unmask], TechNet E-Mail Forum <[log in to unmask]> cc Subject RE: [TN] Au embrittlement on Copper vs Nickel Dave, I guess I wasn't clear. I'm not concerned with the Sn/Cu and Sn/Ni IMCs that initially form at the interfaces of the joint. I'm concerned with the ternary IMCs containing AU (such as Au0.5Ni0.5Sn4) that can form over time/temp along the joint interface. reference: http://physics.binghamton.edu/pub/cottsectc.pdf This paper indicates that there can be long term effects to the solder joint, even if there is less than 3-4 wt% Au. Ben From: [log in to unmask] [mailto:[log in to unmask]] Sent: Friday, October 24, 2008 3:12 PM To: TechNet E-Mail Forum; Gumpert, Ben Subject: Re: [TN] Au embrittlement on Copper vs Nickel Hi Ben - the major difference between the Au/Sn intermetallic (IMC), the Sn/Cu IMC and the Sn/Ni IMC is the structure of the IMC itself. All IMCs are brittle materials in comparison to solder joints. Both the Sn/Cu and Sn/Ni IMCs form as coherent structured layers at the interface between the pwb plated surface and the solder joint. The Au/Sn IMC is a needle-like or block-like structure that is located in the solder joint matrix or forms at the pwb plated surface/solder joint interface then intrudes into the solder joint matrix. That explanation is very very simplified (and yes, there are some other forms of the Sn/Cu and Sn/Ni I am leaving out) but it gives you a relative idea on how the IMC structures differ. These IMC structure differences result in very different cracking modes. Also, you must form either the Sn/Cu or Sn/Ni IMC to have a proper solder joint. The Au/Sn IMC forms only when you get approximately 3-4 wt% Au in the solder joint matrix and is not necessary for good solder joint formation. Hope this helps. Dave "Gumpert, Ben" <[log in to unmask]> Sent by: TechNet <[log in to unmask]> 10/24/2008 01:03 PM Please respond to TechNet E-Mail Forum <[log in to unmask]>; Please respond to "Gumpert, Ben" <[log in to unmask]> To [log in to unmask] cc Subject Re: [TN] Au embrittlement on Copper vs Nickel Dave, I'll see if I can track down a copy of the AWS Soldering Handbook, but the IPC-STD-001 HDBK doesn't go into the detail that I'm looking for. Are you differentiating between the brittleness caused by AuSn4 in the bulk solder and the brittle intermetallic layer that tends to form with aging of the joint? I can understand the bulk solder brittleness being the same, but are you saying that the effect of Au on the strength of the intermetallic layer is not significantly different? Ben ________________________________ From: [log in to unmask] [mailto:[log in to unmask]] Sent: Friday, October 24, 2008 11:29 AM To: TechNet E-Mail Forum; Gumpert, Ben Cc: [log in to unmask] Subject: Re: [TN] Au embrittlement on Copper vs Nickel Hi Ben - gold embrittlement is a function of the amount of gold in the solder joint and not dependent on the pwb pad being copper or nickel. The IPC-STD-001 Handbook has a very good and short tutorial on gold embrittlement in section 5.4.1.1 on page 63/64 which would be useful. Also, there is good coverage of the topic in the AWS Soldering Handbook, section 2.2 Contamination, pages 182-183. FYI - the AWS Soldering Handbook, ISBN 0-87171-618-6, edited by Dr. Paul Vianco, Sandia National Labs, is one of those references that everyone must have on their desks (IMHO). It covers a huge range of solder and soldering process topics in a well written format. Dave Hillman Rockwell Collins [log in to unmask] "Gumpert, Ben" <[log in to unmask]> Sent by: TechNet <[log in to unmask]> 10/24/2008 09:58 AM Please respond to TechNet E-Mail Forum <[log in to unmask]>; Please respond to "Gumpert, Ben" <[log in to unmask]> To [log in to unmask] cc Subject [TN] Au embrittlement on Copper vs Nickel Technetters, Anyone care to share some comments on Au embrittlement? Specifically, what is the relative difference between joints on copper vs nickel? (For a SnPb joint) Does Cu inhibit AuSn4 from forming in the bulk solder? Does this have any effect on Au in the intermetallic layer? Do the intermetallic compounds containing Au of each joint result in a similar brittleness? Do the joints have the same propensity for growth of intermetallic compounds containing Au? Any thoughts are welcome. Ben --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------