Hi George, Just wanted to comment that this was a very well-reasoned response. Thanks. Best regards, Leo Director of Applications Engineering ASAT, Inc. 3755 Capital of Texas Highway, Suite 100 Austin, Texas 78704 ph 512-383-4593 fx 512-383-1590 [log in to unmask] www.asat.com The information contained in this electronic message is CUSTOMER/SUPPLIER PRIVILEGED AND CONFIDENTIAL INFORMATION intended only for the use of the individual or entity named above. If the reader of this message is not the intended recipient, you are hereby notified that any dissemination, distribution and copying of this communication is strictly prohibited. If you have received this communication in error, please immediately notify the sender by electronic mail. Thank you. -----Original Message----- From: TechNet [mailto:[log in to unmask]]On Behalf Of Wenger, George M. Sent: Tuesday, July 26, 2005 4:18 PM To: [log in to unmask] Subject: Re: [TN] PCB Finishes: Final or otherwise Bob, You'll going to get many different answers to your question. The type of surface finish one uses on PCBs would be chosen based on the design, assembly, service reliability, and possibly repair considerations. As you indicated the finishes being used on PCB's today are winding up being the use final finish on deployed products. For many years bare copper or rosin/resin coated copper was the primary surface finish for PCB's. However, because companies had large inventories and PCB's might sit on the shelf for several years before being assembled there was a need for a surface finish that protected the solderability for long periods of time (i.e., >> 6-12 months). SnPb HASL protected the solderability for much longer periods (some will time in > 5 years. The shelf life of the new "flat Pb-Free" surface finishes like IAg, ISn, ENIG, OSP, etc. is high dependent on the storage environment. If you are really concerned about the final finish on your PCB features during use then I would suggest that you stencil print and reflow solder paste on all exposed features during SM assembly. That way all the non-solder mask coated traces and features will have the same final finish. We use IAg (immersion silver) as a surface finish on our PCBs but we do not use it as a solderable surface finish! We use it as an indicator that the copper it was plated to is solderable. The solder pastes and fluxes we've historically used were developed to reduce copper oxide and unless our boards have gone through an aggressively harsh thermal process the fluxes do a reasonable job. As for protect during use, if our products were deployed in harsh environments that would attach and eat through copper features and traces there aren't many surface finishes that would be very helpful. Even if there were good protective PCB surface finishes we'd have to worry about the surface finish on each and every component on our assemblies. As much as conformal coating is hated by most people who have to use it, it is probably a necessity for harsh environmental exposure. Regards, George George M. Wenger, Reliability Engineer Andrew Corporation 40 Technology Drive Warren, NJ 07059 (908) 546-4531 or (732) 309-8964 (Cell) -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert L. Lazzara Sent: Tuesday, July 26, 2005 4:23 PM To: [log in to unmask] Subject: [TN] PCB Finishes: Final or otherwise Virtually all lead-free bare PCB finishes are intended as interim, pre- assembly finishes. Yet I also see a growing occurence of circuit features that are sent into the post-assembly world without protection of assembly solder, solder mask or conformal coating. Which begs the question: Are the lead-free PCB surface finishes safe in the field as final finishes? There's a design before me at this writing: It has nearly 100 test points. All immersion silver. They'll go through assembly as immersion silver, then they'll go out into the field as immersion silver. The assembler is very interested in knowing how long the silver can sit on his shelf and still promote soldering, but hasn't any interest in how long immersion silver will sit in the field and still protect copper. Immersion silver isn't being singled-out: Trade it with OSP or imersion tin, and I don't feel any better or worse. In-fact I have yet to find any producer of immersion silver, immersion tin or OSP that rates their products as final, in-service finishes. But it's happening... I'm not too concerned about lead-free HASL or even ENIG, but the immersion and OSP coatings seem like long term risks in circuit reliability. Anybody else thought about this? Is there a paper that can be recommended? Bob Lazzara --------------------------------------------------- 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 ----------------------------------------------------- ---------------------------------------------------------------------------- -------------------- This message is for the designated recipient only and may contain privileged, proprietary, or otherwise private information. 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