It is a big item, indeed, John. Sometimes Kirkendall voiding is confused with planar microvoids that occur from using improper copper types. I saw some of that with an assembly using something called tough-pitch copper. When they switched, the problem disappeared. I also know that improper reflow profiles can cause the planar voiding seen with the use of immersion silver. It really is seen with all finishes, but the root cause is too fast of a cooldown. But the biggest contributor to microvoiding is the type of solder paste and the condition of the pwb. I know of one solder paste that is extremely prone to microvoids (as well as regular voids). -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of John Burke Sent: Tuesday, May 16, 2006 11:57 AM To: [log in to unmask] Subject: Re: [TN] Question about ENIG and other surface finishing None that I don't understand..............8-) Surface finish topography is a big item. McDermid did a paper at APEX last year which I believe covers most of the issues. John -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of John Parsons Sent: Tuesday, May 16, 2006 9:46 AM To: [log in to unmask] Subject: Re: [TN] Question about ENIG and other surface finishing Any concerns yet amongst this group regarding planar microvoids and immersion silver? http://www.smta.org/files/oregon_chapter_presentation0905.pdf John Parsons -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D. Sent: Tuesday, May 16, 2006 9:17 AM To: [log in to unmask] Subject: Re: [TN] Question about ENIG and other surface finishing Yes, it depends... To use ENIG instead of immersion silver finish as a means of mitigating storage issues is a bad reason to switch, unless there are other compelling reasons, such as having a storage environment with a high chlorine content (sea salts) or other contaminants in the air that can potentially react with the immersion silver finish on the pwb's. Potentially, the ENIG is going to cause you a lot more problems than any issues with storage and tarnishing of immersion silver boards ever will. If you choose to solder to ENIG, you are soldering to nickel, not gold as many think. The gold is absorbed into the solder and you form your solder joints to the nickel underneath. However, soldering to nickel is much more difficult than soldering to the copper traces and pads on an immersion silver board, where the silver is absorbed into the solder joint and you form your solder joint directly with copper, which can be dissolved into the intermetallic formation with the tin/lead solder alloy (or just tin in the case of a Pb-free process) much easier than with nickel. The solder joint formed with the nickel is inherently much more brittle than one formed with copper, and is not as strong. That is if the ENIG is plated properly. If it is not done properly, which it often (still) isn't, then you will really regret ever switching. Packaging of immersion silver boards should be done with silver saver paper between each board, and the entire brick is shrink-wrapped in a special low-outgassing wrap that seals the air out of the package, along with a dessi-pak. In a previous life, I had immersion silver solder coupon boards that sat under my desk for 5 years exposed to the air. We needed some working scrap assemblies for test set-up. Even though the boards were somewhat discolored (slightly tarnished) they soldered right up, and have been in use for three years as calibration units without any failures. But there is very little sea salt in Minnesota air, and most factories control their air quality very well. If you do choose to change your finish, qualify the process before you jump into changing them over. I would suggest performing an evaluation using a small quantity of immersion silver boards and a small quantity of ENIG, and then running durability studies that include thermal cycling and perhaps some type of shock or stress testing. Werner can provide more information on the appropriate types of testing than I can. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Roger Stoops Sent: Tuesday, May 16, 2006 9:34 AM To: [log in to unmask] Subject: Re: [TN] Question about ENIG and other surface finishing I just received a question from someone inside my organization asking about why not use ENIG instead of Iag because of storage issues... Doing a quick Internet search took me to a paper at http://www.mse.uiuc.edu/Faculty/Shang/Preprints/1997-2006/JMR00-pilin.pd f#search='tin%20nickel%20intermetallic%20strength' discussing "a comparative fatigue study of solder/electroless-nickel and solder/copper interfaces." It's dated from 2000 and does not include any study using Pb-free solders. Final thought is that Sn-Ni SJs do not behave well under high stress environments after aging. Found a few others, Daan's website was also very helpful, but time does not permit further discussion. We have used ENIG, Iag, HASL, Pb-free HAL, and it seems our choices (as far as I have anything to say about it) will likely be Iag and Pb-free HAL, due to our operating environment and quality requirements. So, should one use ENIG instead of other SFs? In the now immortal words of (can't remember who, sorry), "It Depends"... Roger -----Original Message----- From: Stadem, Richard D. [mailto:[log in to unmask]] Sent: Monday, May 15, 2006 4:50 PM Subject: Re: Question about ENIG and other surface finishing Good points, George. Trouble is, in order for the temperature under a large BGA to reach 220C for any duration, the rest of the component leads on the assembly will be close to what? 235C? Not a good process for some components. The problem with the logic that the ENIG will work better at higher reflow temperatures seen with Pb-free processes is this: The plating issues that cause the nickel oxide formation during the immersion gold step (related to the phosphorus content, amongst other variables) are exacerbated by subsequent temperature excursions. If the plating is properly done, then yes, perhaps the IMF formed to nickel will be more easily accomplished. However, if NOT done properly, the higher temperatures will make the Black Pad and other nickel plating process-related problems much, much worse. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of George Milad Sent: Monday, May 15, 2006 2:15 PM To: [log in to unmask] Subject: Re: [TN] Question about ENIG and other surface finishing I believe one of the key issues in assembling ENIG parts is that the inter-metallic formed is a Ni/Sn. Ni/Sn inter-metallic requires higher peak assembly temperature and a longer dwell at peak temperature. The right re-flow temperature profile is a must. The recommendation of ENIG assembly is 220 C for 20 seconds. Some of the failures with ENIG may be attributed to colder assembly temperatures. The transition to Pb-Free should favor ENIG as the assembly temperature for Pb-Free is going to be higher and the Ni/Sn inter-metallic will always be formed successfully. Best regards George Milad [log in to unmask] National Accounts Manager for Technology Uyemura International Corporation Technical Center 240 Town LIne Rd Southington CT 06489 (516) 901 3874 (mobile) (860) 793-4011 (office) (860) 793-4020 (fax) --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e 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 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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 1.8e 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 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