Lead-free solder does not have the same surface tension capability as leaded solder. The "self-centering" behavior of lead-free solders is therefore worse than leaded solders. You may not be doing anything differently yet getting a much different result on that basis.
Components can also jump around based on outgassing, and with higher lead-free soldering temperatures, the outgassing phenomenon only gets exacerbated. One way to mitigate this issue is to pre-bake boards and also insure that your moisture-sensitive components are properly stored.
Brian Smith
General Manager - Electronic Assembly Americas, DEK International
Mr. Smith has been supporting customers in the electronics assembly industry since 1994. His expertise is focused on solder paste printing and reducing soldering defects. He holds a BS in Chemical Engineering and an MBA in Marketing. He has authored several papers in trade magazines and at industry conferences. He is an SMTA Certified Process Engineer.
Component moving can occur due to the solvent pick up from the SMT environment or the solder paste formula having a higher content of solvent. A way to reduce this movement is to use a slightly higher preheat as to dry the solder paste,creating less part movements.
Peter Biocca
Senior Market Development Engineer, Kester
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.
The wetting action with Lead Free processes and components is typically slower than a leaded process, which doesn't allow the parts to self-center as well as a leaded process would. Even so, you shouldn't have a ton of component movement even with a Lead Free process. I'm going to assume that you're using the same pick and place machine, and the same oven, just going back and forth between profiles.
If you have air flow controls in your oven, I'd validate that the fans/air handling settings are the same in the lead free as they are in the leaded. If they fans are set for a higher speed for lead free, they could be moving your components around. I typically only change zone temperature and belt speed for the difference between leaded and lead free reflow profiles, fan speed is never adjusted.
If they are the same speed/settings then you need to look at a couple of other things. First validate that the components going into the reflow oven are aligned appropriately. With the requirement for faster cooling for Lead Free Processing, you may want to make sure your cooling section isn't too aggressive with its fan speed either. If you are using nitrogen for Lead Free, it may be possible that the amount of circulation is too violent or aggressive and could be moving components around.
It is also possible there is entrapped moisture in your PCB's that could be causing an issue. Moisture is more of an issue with Lead Free because it gets to a gaseous state much quicker than in a leaded process. If you are using different ovens for each process, check to be sure that the nitrogen curtains/air curtains (if you have them) at the end of the ovens aren't hitting the components on the PCB's when the solder is still in a reflowed state.
Good Luck!
T.J. Hughes
Manufacturing Engineer, Esterline Interface Technologies
Mr. Hughes has been in the electronics manufacturing field for 20 years. Operating the processes and as a manufacturing engineer for the last 14 years. He is also a CIT as well as an SMTA Certified Process Engineer.
I would look at the profile, especially the ramp rates. I suspect that you are heating too fast and causing the flux to outgas too quickly.
Additionally, some paste manufactures use different fluxes between lead-free and lead solder due to the liquidus temperature therefore the lead free flux might have a different temperature range that is critical for "light" parts.
It is best that you adhere to the individual solder paste manufacturer's profile specification.
Fred Dimock
Manager, Process Technology, BTU International
Mr. Dimock is the manager of Process Technology at BTU International. His extensive experience in thermal processing includes positions at Corning, GE, and Sylvania. He has authored numerous articles on lead free processing and process control, taught classes at SMTAI, and participated in the IPC Reflow Oven Process Control Standard committee.
It is difficult to know what is causing "components jumping around" without seeing the post-reflow boards. The Pb-free soldering profile should be checked to ensure it is roughly conforming to the solder paste manufacturer's thermal profile recommendations. If heated too vigorously before partially drying the paste, the paste could be boiling before the onset of solder wetting causing the parts to move.
Perhaps your Pb'd profile is more suitable and parts are wetting properly and remaining stationary. The fact that it is only happening with Pb-free and not Pb'd says that there is low probability of a bad conveyor system which may be disturbing the parts by vibration. Check your profile carefully with proper thermocouple attachment.
Gary Freedman
President, Colab Engineering
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.
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