David, The short answer to your question "Is it possible to get an acceptable solder joint if a Pb-Free BGA is put onto a board using SnPb solder paste and normal SnPb profiles?" The longer answer depends on ones definition of "acceptable". If you had asked if mixed alloy solder joints are reliable my answer would have been that nobody right now can answer that question with any degree of certainty. I know of a few companies who have shipped product (either intentionally because they didn't have an option or unintentionally because they didn't find out until after they shipped it, that have area array components with SAC alloy solder balls that were soldered using SnPb solder paste and a typical SnPb reflow process and they are asking the same questions. There are probably many other companies who have also shipped products with mixed alloy solder joints and don't even know it yet. There are many consortia activities currently planning or doing mixed alloy reliability evaluations but my concern is that the data won't be available early enough to avoid having to ship mixed alloy solder joint product. I realize that there are many right now who have indicated that the reliability of mixed alloy solder joints isn't "good". However, I believe that the published information I've seen so far doesn't support their believes'. The data appears to suggest that reflow solder joint assembly quality and PCB surface finish have a big effect on the solder joint integrity/reliability. There are also others that are suggesting that mixed alloy solder joints are or can be reliable and the only data I've seen that tends to support that is from NPL. At the present time I've been asked to give my risk assessment about models that we've built with mixed alloy solder joints. The problem I'm having is the similar to the problems other companies are having. Companies want answers now not later and they don't have the resources to obtain the answers and they usually don't have the samples (i.e., products) to devote to a reliability evaluation. So those who have to do the risk assessment have to do it on limited data and the amount and quality of the data usually defines the "comfort" level in the risk assessment. In the next week or two I'm hoping to finish up finishing up an evaluation on a mixed alloy product. I don't want to call it a reliability evaluation because is doesn't follow IPC-9701 or for that matter any recognized reliability methodology. However, it is the only data I can obtain in the required time period to do a risk assessment and my attitude is any "Good" data is better than none. I must re-emphasize the "Good" data part because I've seen many cases where some "Questionable" data has actually been worst than none. Several months ago I was aware of a several models being built using a 16-Layer 0.100" thick FR370 PCB. Four out of nine area array components on these models had SAC alloy solder balls and all of the model boards were built using our standard SnPb solder paste reflow assembly soldering process. After the assembly process I was asked will the models be reliable. I could hardly stop laughing until I realized this question is being and will continue to be asked by many companies. As hard as I tried to obtain several model for temperature cycling the only assembled PCB board I received was one defective sample. This sample wasn't defective because of a failed solder joint. It was defective because a design engineer manually modified circuitry on the outer layers for design modification trails. Since this wasn't a daisy chained test vehicle and it was only a sample size of line as well as the fact that there was no time, doing a controlled ATC made no sense so we decided to do destructive physical analysis (DPA),i.e., cross sections, to determine the degree of alloy mixing and visual quality of the solder joints. The DPA revealed alloy mixing of 40 +/- 10 percent, minimum solder joint voiding, and "good" looking solder joints on the mixed alloys area array solder joints as well as the SnPb solder joints. Although I wasn't able to obtain an assembled PCB for temperature cycling one board was subjected to HALT testing and when it was finished with HALT testing it was used to develop a PEST (i.e., burn in) procedure. After HALT and PEST I was told I could have the board for DPA so that I could give a more informed answer to the reliability question. Unfortunately, this time period came when my work load was at it peak. Fortunately, this was just before Thanksgiving and our reliability organization had a temperature cycling chamber available so I was able to add an addition 240 temperature cycles to the assembly during the Holiday. So far during the DPA what I see is extremely encouraging. If I can find time I hope to put the result together into a paper and present the finding. Most certainly it won't be entitled "Everything You Ever Wanted or Needed to Know About Mixed Alloy Solder Joint Reliability" but it might be something like "Mixed Alloy Solder Joint Integrity/Reliability Sanity Check" My apology for the long answer but I'm on a six hour flight and the movie wasn't very interesting. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf of Guenter Grossmann Sent: Thursday, December 15, 2005 10:38 AM To: [log in to unmask] Subject: [TN] Antw: [TN] Mixing SnPb and Pb free BGA's on an assembly David Lead free BGAs can be soldered with SnPb solder in a SnPb soldering process. Even if you stay below the liquidus of SnAgCu the liquid solder will solve the ball forming a lower melting alloy at the interface. The process of the solution of the ball into the solder ends if - The alloy that formes due to the continuous solution of Cu and Ag into the SnPb has ist liquidus at the solder temperature - The temperature is lowered below the liquidus of the SnAgCuPb alloy This means that depending on temperature and time different fractions of the balls are are solver in the solder. we made some trials and used the temperature- time integral approach of James Vincent to characterise the influence of the production parameters on the solder structure. Originally I was worried that the interface where the original ball meets the solder will be the week point where degradation will take place. However, looking at the slides of Dave Hillman I see that the rupture takes place in the SnPb solder along the pad. This makes sense, since SnAgCu it mor creep resistant than SnPb and thus the strain occurring due to thermal cycling will be dominantly in the SnPb solder. I have no quantitative data in hand to say how much crack growth is accelerated due to this material combination. A problem which is evident is the fact that you have no collapsing balls (or not the entire balls) to compensate for coplanarity problems of the balls. Best regards Guenter EMPA Swiss Federal Laboratories for Materials Testing and Research Centre for Reliability Guenter Grossmann, Senior Engineer 8600 Duebendorf Switzerland Phone: xx41 1 823 4279 Fax : xx41 1 823 4054 mail: [log in to unmask] >>> [log in to unmask] 13.12.2005 15:30:24 >>> Is it possible to get an acceptable solder joint if a Pb free BGA is put onto a board using SnPb solder paste and normal SnPb profiles? The lead free solder balls are SnAgCu 96.5/3/0.5. Seems nigh impossible to get DDR2 packages with SnPb balls. I'm designing a high end DSP into an industrial instrument. The DSP is not available lead free, nor does the application call for lead free (thankfully). There is no option to use a different memory type with the DSP. Nor is there any possibility of using a different DSP or FPGA due to time/cost/sanity. Don't want to explore going lead free on this one (for obvious reasons), and certainly do not welcome the expense of reballing 0.8mm DDR2 chips. Best Regards David Greig ______________________________ GigaDyne Ltd Buchan House Carnegie Campus Dunfermline KY11 8PL United Kingdom t: +44 (0)1383 624 975 www.gigadyne.co.uk <http://www.gigadyne.co.uk/> ______________________________ --------------------------------------------------- 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 ----------------------------------------------------- --------------------------------------------------- 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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