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The Causes of Tiny Solder Balls After Reflow
Board Talk
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SummaryPhil And welcome to Board Talk with Phil Zarrow and Jim Hall, the Assembly Brothers who by day go as ITM Consulting. Today we are at Board Talk. Let’s see, what do we have for today’s question Jim? This is from GH. We have been seeing tiny solder balls across our boards after reflow. Tiny ones huh? Not big, huge ones, tiny ones. We bake the board, clean the boards, and stencil with alcohol, use brand new paste, have the oven manufacturer come in and go through our entire oven and have had the solder paste rep investigate the problem. Nothing yet has resolved the issue. It almost appears that the solder is exploding as the balls are found in areas where there is no paste or components nearby. Have you seen this before? Any suggestions on how to resolve the problem? I guess the first thing that comes to my mind is what does your reflow profile look like? Is it compatible with what the solder paste manufacturer is stating in their data sheet what they want to see, if you are in that window or not. Want to elaborate a little on that Jim? Jim I am looking at the specifics of the question, it appears that the solder is exploding as the balls are found in areas where there is no paste and or components nearby. In your preheat of your reflow profile you are going too fast. The solvents in the solder paste formulation are bubbling, evaporating rapidly, bubbling causing explosions that physical displaced tiny solder balls and in some cases significant distances from the solder deposits. I have designed ovens for 30 years, old and gray now. I feel comfortable opening one of the proverbial topics. A dirty little secret that nobody likes to talk about is the very beginning of the heating profile when the solder joints and the solder paste on the board first come into the first convection zone and start to heat. If you look at, measure the profile with a good profiler, and you zero in on the slope and you go down, down, down to the very beginning where it first enters you will always find at least a small section, couple of seconds or often more, where the slope exceeds that what is quoted on the solder paste spec. The reason for this is because you are coming in at room temperature and it is really hard, in most cases impossible, to keep stabilized the temperature of the first zone of a convection oven at a low enough temperature to prevent at least a few seconds of rapid heating. If that extends it causes a gas blow in the oven. Like your using nitrogen and you are pushing gas out of the end of the oven, that can exaggerate that effect, make it extend. So, I would drag out my profile and my slope tool and drag it down to the very beginning of the profile and see if you do have a small section, or maybe a longer section when you really look at it, where you are heating too fast which could cause this rapid boiling of the solvents in the solder paste formulation causing this explosion which are displacing the solder balls away from the paste deposits, areas where there is no paste of components close by. Phil Another thing I would also suggest, one of the tests that we use for qualifying a solder paste is also a good way of detecting what the solder paste itself is contributing to solder balls. Very, very simple straight-forward test. You use some stock or an old stencil, whatever thickness you are typically using. Basically, 250 thousandths diameter hole if you will, aperture and print that solder paste manually on a ceramic substate. If you don’t have any aluminum substrates kicking around, go down to the local hardware store and buy some ceramic tile. Make sure you profile that piece of tile or substrate for the proper profile, getting back to that again, that it wants to see. Then print the solder paste there and run it through the oven. Take a look anywhere between 10 and 30x magnification what you’re seeing that little puddle around the flux. You should see nice and clear. If you are seeing some of those tiny solder balls, you may have a problem with your paste. Jim Another thing that jumps out at me is clean the PCB instead of with alcohol. Alcohol is a great thing but it is not compatible with all formulations of solder paste and other materials. Make sure that you don’t have incompatibility that is leaving the end of your “cleaning” process with other residues, in particular your stencil or maybe even on the PCB that could be causing a corrosive type of reaction during heating, generating your solder balls. Phil This is Jim Hall and Phil Zarrow of ITM Consulting and Board Talk, the Assembly Brothers, thanking you for listening today. Looking forward to seeing what comments our readers or listeners, depending on your age, send in. Jim I want to continue to thank all who write in, whether you criticize us or not, I feel that the biggest advantage of Board Talk is the comments, in many cases industry experts who have very specialized knowledge in some of these topics that Phil and I don’t have. Phil Well again, thank you for listening on behalf of Jim and myself. Whatever you do, however you are soldering, whatever kind of solder balls you are getting, whatever you do, please don’t solder like my brother. Jim And don’t solder like my brother. |
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