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Bad solder joints

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Bad solder joints | 29 May, 2009

We are getting what looks like cold solder joints on one of our boards. The boards have an aluminum substrate, and use OSP plating. The solder is lead free - 96.5 Sn, 3.0 Ag, .5 Cu. The flux is no clean. When we first had the problem, my solder profile was set for typical optimal - 90sec soak, 60 sec reflow, and peak temp. just under 255C. I adjusted the oven towards the recommended max limits of 2 minutes soak and 90 sec reflow. The screen shot is of a 110 sec soak and an 80 sec reflow. This must have been a step in the right direction because the joints weren't obviously bad, like in "no connect iso1_8" but still could be easily pulled loose like in "bad joint 4". We also tried soaking the parts in alcohol working on the theory that there was contamination - no difference. I also put a negative offset in the placement parameter of the part to make sure it was pushing the part into the paste with a 200ms dwell. Also, no other part on the board has this problem. And, multiple temperature profiles have shown any significant thermal differences between the bad IC and the rest of the board. Any insight will be greatly appreciated.


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Bad solder joints | 29 May, 2009

Have you tried different board orientation through the oven, or have the boards been put through the same way each time?

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Bad solder joints | 30 May, 2009

You're thinking the correctly that there's something screwy about that part, but it's not contamination. It's the alloy used as solderability protection. It would help if you could tell us the alloy.

Anyway, this alloy, when combined with your paste, has a higher melting point than you're expecting. Currently, you're at 217 plus 30*C for about 35 seconds. This is fine for the average situation and the paste you are using. In this case, the uncommon part requires further compensation. Try increasing that time to 45 seconds to adjust for the plating on that peculiar part.

For more on your "no connect issue" search the fine SMTnet Archives on=> head in pillow

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Bad solder joints | 1 June, 2009

What is your squeegee pressure and size. Also does the pad sit on a ground plane, as it looks like it is partially reflowed so surface has gone but not coallesced properly which would indicate it is more a problem with reflow above 217-219C OR obviously increase the Dwell to get the whole board up to temperature. I assume the profile given was on a populated PCB and not a bare PCB. If you have anymore Pics they would be good to see Cheers Greg

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Bad solder joints | 1 June, 2009

Sorry looking at your profile again and considering its Aluminium substrate I would opt for an increase to the preheat or introduce a soak to the profile to get the PCB up to reflow. Try soaking between 175 - 200C sufficient enough to heat the whole PCB evenly Cheers Greg

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Bad solder joints | 4 June, 2009

We wound up using a ramp to spike profile with 50 sec. between 150C and 217C and 90 sec. above 217C. The idea being the flux is still active when it hits reflow, and it stays in reflow long enough for the weird alloy in the parts lead to wet. All parameters are within specs, just near the edges.

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