Jack, what is the report number? ______________________________ Reply Separator _________________________________ Subject: Re: Unidentified subject! (SMT Repair) Author: [log in to unmask] at Internet Date: 11/4/96 2:51 PM >Date: 04 Nov 1996 09:08:21 -0600 >From: "Millsap, Pete (AZ77)" <[log in to unmask]> >To: Technet <[log in to unmask]> (Return requested) >Subject: Unidentified subject! > >The Navy did a surface mount repair study in June 1987. The conclusion of >this study is that the "long term reliability of a solder joint will be >jeopardized with multiple rework cycles. With copper dissolving away during >thermal processing, the pads will inherently lose their structural >compliancy. The copper goes into solution forming brittle compounds such as >Cu6Sn6. These brittle intermetallic layers may cause long-term solder joint >fatiguing problems." They state that on 2 oz copper 100% of the pad would >dissolve in 12 cycles. > > My question does anybody know of any recent studies that would support or >counter this study. Do other companies limit the number of heat cycle on a >pad? > >From the EMPF HelpLine in Indianapolis; prepared by Jack Crawford I was the project manager for the study you refer to while working for the Navy base in Keyport, WA. The specific conclusion on the copper solution and the itnermetallic was based on a dissolution formulas and other engineering documents, and not by test results. Subsequent to that study, a more intense evaluation was completed in 1992 that involved more actual testing. That was done by the Navy Surface Warfare Center, Crane, again coordinated by myself. We have copies of those test results in the EMPF Technical Library, and would be glad to share them. In summary, the test utilized a matrix of 3 board types (6 layer FR4, 6 layer ceramic copper thick film, and a 4 layer copper-invar-copper board) with all ceramic components of leadless, J-lead, and gull-lead configurations; 20 and 68 I/O 50 mil pitch (hey--it WAS 1991-92). The boards were a SEM "D" size, approx 4.6". Some of the large and small components were removed and replaced one time, some three times, some not at all. We used large and small components both at the edge and center of the boards. Some components were instrumented with internal thermocouples to monitor temperatures. We used nitrogen on five of the "state of the art" hot gas systems available at that time, and one IR. A sample of the boards were subjected to power cycling and magnified visual inspections. Additionally, some interconnects were microsectioned. Relative to the info you are asking about, we controlled the heat cycles (temp, duration) as tightly as we could. Heat cycles were: (1) HASL (1) Attach Components (2) single remove/replace and (6) triple remove/replace. We utilized residual solder to eliminate "partial, uncontrolled heat cycles" to wick or extract solder from the pads. We also wanted to see if the solder could or shouldn't be reused. The findings were that with controlled cycles at minimal temperatures, intermetallic growth was minimal, and there was no reason to suspect or predict solder joint fractures with as many as 8 reflows. Additionally, the pad thickness didn't seem to decrease at the rate predicted in the first study--perhaps the intermetallic actually acted as a barrier to further dissolution. The study is dated now, because of changes in flux and solder formulations, new heating control and application methodology, and different kinds of pad/lead finishes now available, but there is some core info that could very well be beneficial to process development. Anyone is free to discuss this further with me at the phone numbers/e-mail numbers in my signature file which follows this msg. Jack ***ALL NEW EMPF PHONE NUMBERS*** Jack Crawford HelpLine Manager Electronics Mfg. Productivity Facility ****NEW--317.655.3688--NEW**** *****FAX 317-655-3699 NEW **** 714 N Senate Ave, Suite 100 Indianapolis IN 46202-3112 VISIT OUR HOME PAGE AT: http://www.empf.org [log in to unmask] *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * *************************************************************************** *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * ***************************************************************************