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Going Beyond Your Solder Paste Work Life
Board Talk
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TranscriptPhil Welcome to Board Talk. We're Phil and Jim, the Assembly Brothers here to answer your SMT and related process questions. Jim An intelligent questioner has sent in a problem, proposed a solution and is asking us for validation. We encountered cold solder joints on all of the 0603 components on the bottom side of a small batch of board assemblies, yet the larger components had good solder joints. These components are all near via holes but this does not seem to be related to the cause. We replaced the solder paste on a new batch of assemblies and the problem was eliminated. Phil What changed? Jim The solder paste used for the problem batch had exceeded the 12 hour setup usage time when we applied the solder paste in our facilities. It was exceeded it by one hour. Was the problem likely do to paste exceeding our 12-hour work life? The amount of time that a paste stays on the stencil while you're printing is called work life. Obviously after a long enough time, any paste will dry out and degrade and need to be replaced. Apparently this material was rated for 12 hour work life and this small batch of boards was printed after 13 hours. All pastes have solvents in them as well as other materials. So over time they're exposed to the atmosphere, sitting out on the stencil; they dry out. Also chemical reactions take place, but mostly it's drying. So you print and the paste isn't what it's supposed to be. It's lost some of its chemistry. Whatever the mechanism, you see it on your smallest components because they have the smallest deposits on the board and they have the largest surface area to volume ratio. This is much more magnified with people doing 0201 and 01005 components where you have tiny deposits. What happens? You're going through your reflow. I'm assuming you're reflowing in air. You don't have as many solvents and materials in the paste. You exhaust them during the reflow process. They do their job cleaning the oxides on your 0603's and on the pads. But the chemistry of the paste is exhausted before you reach the reflow section and you get re-oxidation. Thus the impression that you're getting a cold solder joint. Worst case you can even get graping because you've reduced the fluxing capability and re-oxidized down to the paste level and the individual particles don't coalesce. Then you get a graping situation. Phil I think as a sidebar, one of the things is that expiration time, or in this case the work life, as Jim mentioned. You should know it and adhere to it. You can establish or re-establish the working life when you qualify your solder paste. But in lieu of that, pay very strict attention to the manufacturer's specification sheet on things like work life, pot life, shelf life, expiration dates. They tend not to be too conservative. Jim Establish your own realistic work life on your own conditions because it's obviously a function of temperature and humidity. Phil So whether you're using fresh solder paste or expired solder paste. Whatever you do - Jim Don't solder like my brother. Phil Don't solder like my brother. |
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