Small air bubble under the IMU Module PWB


#1

Hi All
After potting my IMU Module into the epoxy in the acrylic housing and letting it harden overnight, I noticed two small air bubbles at the back and front of the Module between the Module and the bottom of the case. Both air bubbles are flat and totally isolated from the epoxy along the sides of the IMU Module.
The IMU Module works fine electrically. Will these minor air bubbles cause a mechanical issue at depth?
Regards,
TCIII AVD


#2

Do you have any pictures of the bubbles? What are the depths you are going to? I would suspect that you will be fine as the epoxy is pretty rigid.


#3

Hi Brian,
Thanks for the response, much appreciated.
The two bubbles are very flat, maybe .005 in thick and maybe .125 in wide. There is one at each end with roughly 75% of the bottom covered in epoxy. I plan to stay above 100 ft on any given dive.
Regards,
TCIII AVD


#4

As long as the circuit boards and all the chips all have an epoxy barrier between the chips and the water, you should be ok. If you experience a failure this would be a great data point for us and we would like to hear about it.


#5

Hi Brian,
Thanks for the update, much appreciated.
Fortunately the bubble at the wire leads end is not affecting the leads and they are covered with epoxy. The center and the edges of the module are covered and supported with epoxy.
I have wired up another IMU Module/Pressure Sensor and plan to exercise additional care when potting the module with the epoxy though I thought that I was doing a good job with the original module.
Any words of wisdom other than the original potting instructions will be greatly appreciated.
Regards,
TCIII AVD


#6

There are a few things that are helpful. These are written up in the general build instructions for the ROV, but here they are for your reference.

My final piece of advice is to pot in a warm environment to lower the viscosity and help allow air to escape.

Good luck!


#7

Hi Brian,
Thanks for the pointers, much appreciated.
What I think that I will do is to flow the 2 Ton Epoxy in a small mixing cup, stir again to get any leftover bubbles out, then scoop it into one of my 10 cc syringes that has an 18 gauge needle tip that will give me better flow control than the 2 Ton Epoxy mixing tube.
I live in Southern Florida so my workshop, which is in the garage, is always warm and ensures the lowest viscosity of the mixture that is possible.
Below is a picture of the bottom of the IMU/Pressure Sensor Module showing the two air bubbles:


Regards,
TCIII AVD


#8

If you lay down a layer of epoxy first it should help avoid this.


#9

Hi Brian,
Per the original instructions, I did put a layer of epoxy on the bottom of the Module case and worked the Module around in the epoxy before covering it on the top. That is why I was really surprised to see the bubbles between the Module and the bottom of the case.
Regards,
TCIII AVD


#10

I am surprised to see them there also if you put a layer down. Hopefully the second one turns out better.


#11

If your super concerned with it you could always carefully drill a hole through the acrylic and then flood with more epoxy


#12

@Teaguey,
An interesting solution, however there is no way for the trapped air to be removed as the epoxy is being forced into the bubble?
Regards,
TCIII AVD


#13

Well i meant if you drill into the heart of the air bubble and use a syringe to inject the epoxy it should force the air out.


#14

2 ton epoxy is great however I had one unit that was old- whatever and it flashed 5 second after ejection. Since the application is critical consider wasting a tip and testing to be sure is ok. Cheap insurance.