I have a board that is showing dentritic growth between the cathode of an LED and a ground copper pour on maybe 5% of production. The spacing is
0.010" between the pour and the cathode pad. I am arguing that this spacing is reasonable and there is a board defect or contamination, while my process guys say the clearance is the problem. The dendrites allow several hundred microamps of current, enough to light the LED. We can see them under a Lynx magnifier.
I think I should be able to do even finer spacing...any comments?
You are correct. We've had dendrites grow between pads that had about 0.020" clearance. I've been through three separate rounds with these buggers. Contamination is the issue. The real bitch is that the dendrites take a little while to grow and only in the presence of an electric field, so it's tough to identify that your process has really been fixed.
In my opinion, your process guy is trying to pass the buck. 0.010" clearance is huge, these days.
Dendrites are nasty animals. I've measure shorts in the tens of ohms. Blow some air at the suspect area and the short disappears.
The best solution, that we've found, is to clean 100% after assembly -- even though we use no-clean flux. The real pain is what to do if some rework is required. Do you do a complete re-clean? We currently do not, but we have also stopped using liquid flux, with the hope that any flux (within the solder itself) gets deactivated due to the heat of the soldering iron. Then, we do not clean the reworked area (for fear of introducing contaminants). So far, so good.
Find the source of contamination before blaming any process. I have ben using no-clean processes for years without a failure on much closer tolerances.
Ideas to look for are things like perhaps the LED is hand soldered. Using an incompatible flux while hand soldering can "release" the normally trapped flux in the no-clean residue. Or, if after soldering it, they spot clean the area with a solvent, they same release may occur.
I would say start with "why" this area is seemingly different from the rest of the assembly.
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