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Solder Boards Not Holding Up to Shock Testing
Board Talk
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TranscriptPhil And welcome to Board Talk with Jim Hall and Phil Zarrow, The Assembly Brothers, pick and place. Today we are speaking to you from the Elegante Ballroom, high atop Mount Rialto. Today’s question has been submitted by ER. We are using a vapor phase oven which we have used successfully for a number of years with both leaded and lead-free solders. Recently we have experienced problems with a no-lead BGA that has low silver solder balls. The problem is that the solder boards do not hold up to shock testing. We have increased dwell to 85 seconds at 245 C and this helped but did not fully correct the problem. We increased the temperature to 253 degrees C and still the problem persists. We have changed solder paste to one that has a more active flux. Still no help. Any suggestions will be appreciated. How did they increase the peak temperature? Jim I designed and built vapor systems at the very beginning of my career in this industry. You are absolutely right, Phil. The fluid itself defines the temperature, it’s the boiling point. If you went from 245 to 253 you had to change fluids, which is not a trivial exercise. The low silver balls on the BGA, probably SAC105 or something like that and they are specifically put on there, versus SAC305 to do better in shock testing, particularly drop shock testing for handheld devices. So, this is really counter-intuitive here. Looking at the specs, I am thinking the problem with vapor phase is in the preheat. Vapor phase purely is really only the reflow zone. You still have to provide some alternative method of preheating the whole board close to the melting temperature of the solder you immerse it into the saturated liquid at the temperature of the liquid, the 245 or 253. Because if you don’t, if you take a room temperature board and immerse it directly into one of these vapors you will have severe warpage, you will tombstone every small chip on the board and you will have many other problems. The heating is just too fast for direct immersion. You have to preheat. I am thinking, particularly when they are talking about going to higher temperatures, that they didn’t preheat the boards enough so that they were getting BGA balls, shock testing, maybe they got some form of head in pillow or non-wet open that is showing up in shock testing. The logic I am thinking is that you didn’t preheat enough, you went into this very hot vapor, you got warpage, fast warpage while you were melting and wetting and it gave you some of these head on pillow type defects that are causing your lack of reliability in shock testing. Phil We are kind of shooting in the dark here because we are not privy to failure analysis. As Jim said, it may or may not be related to the thermal characteristics. It could be head in pillow. Are you getting wetting or dewetting? What is happening down there? It could be something totally unrelated like cleanliness, contamination. So many other issues that affect how well that connection is going to be made. A little more failure analysis information and we can see if they are heading down the wrong path with the thermal and where to go on that. Jim ER noted that they changed the solder paste to one that has a more active flux. Following up on what you said, contamination. If it is non-aenic contamination, most fluxes are not going to handle that. That may be a red herring there. You feel like you are putting more on to flux but is actually some contamination on the balls or the board that no flux is going to remove. Phil Don’t rule out your solder paste too. This sounds really simple, but make sure you are not using expired solder paste. Has it sat on the printer too long and interacted with the atmosphere and what its actual pot life is supposed to be? There are so many factors here for joint filling shock test. Jim I want to reinforce what you said Phil. This is a kind of failure. After all the things you have tried, you really need to get to the root cause. Sending some of these samples to an FA lab to really identify what the nature of the failure is, is probably warranted. Phil You have been listening to Board Talk with Jim Hall and Phil Zarrow, The Assembly Brothers. Please send in your questions, and of course we always look forward to the comments that we receive. Whatever you do, however you’re doing it, please don’t solder like my brother. Jim And don’t solder like my brother. |
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