Just spit balling here and you may have done this already.....but have you tested if the BEC is actually only putting out 7.4V?

Yeah I actually had to set it because they arrive default 5v. There is a programmer that comes with them and I set it to 7.4v. and I double checked with volt meter.

When you say "on occasion" how often would you say? Once every 10 power cycles? 100?

As it happens when power is applied, is the EZB operating yet when the servo blows? (We know you know your stuff, just trying to get a complete picture in my head.)

The servo that blows up, is it always the same make and model? If so, what is it?

You seem to have pretty solid parts with the BEC, I'm curious about the servo. Do you have more servos you can afford to blow up (I hate to say it that way). If you did, one experiment I would probably want to do is disconnect the servo from d12 (or the EZB all together) and then power cycle the BEC and servo multiple times to see if I could produce another blow out.

What voltage is the servo rated for? Is the servo digital or analog?

You could add a tanking capacitor on the input side (rated for 12V or higher) or the output side of the BEC (rated 7.4V or higher) to stabilize any voltage spikes and/or in-rush current. I'd say you'd likely want 4700uF or greater in value.

Thanks Jeramie, I'll give it a shot. Servos are hitec digital and 7.4 micro servo.

@justin, maybe 25 start ups. And when power applied . I'm running thru possible causes fixes but now thinking about a rebuilt of this particular build and using a standard servo instead of the micro.

Is this your servo HS-5087mh. It does seem like good quality servo if that is the same one.

The blue smoke is the head scratcher with your setup. I've smoked off brand el-CheapO servos, I've smoke servos putting excess load on them (trying to make them move too much) and I've smoked servos by giving them too much juice, like 12 volts to a 6 volt servo. You are not doing any of the sinful thing I normally do to smoke servos.

Your BEC limits the voltage the proper level of 7.4. Your digital hitec servo should be reliable.

Because you aren't commanding the servo to moving anything yet the servo should not be under load - or is it? Is it twitching at power up and sometimes it can't flex and maybe that causes the smoke?

Yeah that's the one. No unusual load or over pushing the boundaries of the servo limits. No buzzing or humming on power up. Unfortunately it's now happened more than one with two separate systems a week apart. So going to design in a standard size low profile servo. I have just enough room.

@fxrtst.

This may not be relevant but;

I worked with electric RC airplanes with large motors. A big problem was arking when the connection was made to the LiPo battery. The issue is that when there are large value capacitors at the input of the ESC (electronic speed control), a long distance (1 foot or more) between the batteries and the motor, and the very low internal resistance of the batteries; a large inductive kick voltage spike can be generated. Large enough that a "snap" can be heard and an ARC seen at the connector. I have heard that occasionally a flight controller was damaged due to the inductive kick.

There are several spark suppression techniques. They involve connecting the power through low value (~10 ohms) resister to charge the caps at a low current, over time, then connect the battery. There is a "anti spark" connector available. Anti spark connector

Hope this is of some help.