Protecting a sonar circuit board at 3000 ft.

[SVSeeker]

Hello
We have been racking our brains about one circuit board for a Lowrance sonar that we want to take to 3000 ft but which is too big for the hull of the ROV.
Below are several Ideas and we welcome insight or other alternatives. A complete narrative about the sonar is here: http://www.submarineboat.com/rov_sonar.htm

The Lowrance SonarHub is one single board just under 7 x 7 inches, so the board must be adapted for the 3 1/2" ID pipe hull, or the hull must be adapted for the board. Options include cutting the board in half and soldering wire connections between the two sides. Making a 1 ATM housing to protect the boards components from the pressure sounds easy until we do the math. 7 x 7 inches = 49 square inches at 1700 psi = 83,300 pounds. Over 41 tons of force.

1) Spherical Hull
A sphere would be the best shape but it would adversely impact the drag of the hull. Spheres are also require complicated machining processes.

2) Elliptical Hull Supported by Titanium Pins
Some circuit boards have locations on them that are free of IC's and conductors. These locations could be drilled and have titanium pins that pass through in order to brace one side of a slightly elliptical hull against the other. Grade 5 titanium can has a 138,000 psi of tensile strength. 138000 * (pi*(((1/8)/2)^2)) = 1,693 pounds. The inside of the aluminum hull would need to be machined to within 1/4" of the surface of the circuit board in order to support flexing of the pins. The titanium pins themselves would be 1/4" rods turned down to 1/8 as necessary to pass through the 3/32" circuit board. This would require a complicated machining process.

3) Syntactic Foam Potting
The compression strength of a typical FR-4 fiberglass circuit board is 60,200 psi - flatwise. Resin encapsulating 5 micron, 0.0002 inch glass spheres is commonly used as flotation for deep sea vehicles. It has an good compression strength of 8000 psi, but low tensile strength. It also has a much lower shrinkage than typical epoxy. IC's and capacitors on the circuit board would first be coated with a thin layer of air entrained silicon to form a compressible buffer around these components. The board would then be think coating of "mg Chemicals" 832TC-450ML thermally conductive potting resin which uses aluminum oxide for thermal conduction (.682 W/mK) but which is not electrically conductive. The next layer would be epoxy infused with copper powder to produce a layer with an approximate heat conduction ability of 30 W/mK. This 6 to 10 mm layer will also bond 14 awg solid copper wire to the board in order to conduct heat away from the board. The end of the wires would pass off the edges of the board and be bent to form bands of copper down the sides of the board. Once the copper layer is cured the exposed copper wires would be embedded in salt dough which can be dissolved in water after the final layer of casting is cured. The final outer shell will be cast HYTAC-C syntactic foam poured into a mold of the desired fin shape from 1 to 3 inches thick. After curing the salt dough would be dissolved to form water channels over the exposed copper. This method would also not be reusable should the board fail and need to be replaced but a second board could be potted so replacement is available. This process also simplifies the cable connections as each conductor can by isolated and stripped to bare copper where it passes through the innermost thermally conductive potting layer to form a watertight seal to the conductor. The fin could be installed through a flooded section of the hull to limit it's cross-section. It could also form a mounting structure for the transducers or a dive plane should the ROV be used without the assistance of a forward towed array that controls depressing the unit when under tow.

4) Aluminum Casting
A circuit board could be covered temporarily with plastic wrap and a female mold of each side of the board made of castable silicone. The silicone negative would then be used to make a wax positive of the board for investment casting in aluminum. The aluminum casting would then be machined for an o-ring to seal between the two halves and have glands fitted for the various connectors. This process like the syntactic form potting described above would rely on the compression strength of the circuit board that is free of components, but would avoid overheating the components. The compression strength of a typical FR-4 fiberglass circuit board is 60,200 psi - flatwise.

5) Aluminum Shell with External Frames and Syntactic Foam Frame Webbing
A two part elliptical aluminum hull with clearance for the board would be machined from one inche solid stock leaving a minimum of 1/2 inch thick material in the hull. External frames of 1/2 thick by 1 inch tall aluminum plate would be welded onto the exterior of the hull and 3/8 inch lightening holes would be drilled through the frames. An temporary exterior skin would be attached to the top of the frames and the cavity between the skin and the hull filled with syntactic foam. The lightening holes would allow the syntactic foam to attach to the frames. The compression strength of the foam would act like a webbing for the frames in order to resist buckling.

6) Cut the Circuit Board and Soldier it Back Together
It an interesting idea, but a long shot, but it is possible to cut circuit boards apart and join the pieces with flexible wire or ribbon conductors. However this board processes very low power analog signals from the transducer which complicates this process.
7) Cast Circuit Board Hull Housing
A complete section of the hull with winglets to house the circuit board can be cast from aluminum. The winglets would protrude from both sides of the 4" OD pipe section so as to minimize their surface area. One of the wing tips would be capped with a bolt retained hatch.

[martinw]

Hi,

I specialise in designing pressure tolerant electronic PCBs ....

http://www.wareham-engineering.com

Depending on what's on the PCB you may be able to make it pressure tolerant, put it in a thin walled oil filled box, compensate it and away you go ....

Just a thought

Martin

[SVSeeker]

Depending on what's on the PCB you may be able to make it pressure tolerant, put it in a thin walled oil filled box, compensate it and away you go ....

Martin

That certainly is an attractive solution. How do you determine if the various components can survive routine service at 1700 psi? These boards are $700 each so we don't want too many failed attempts. The board in question is the Lowrance SonarHub due to be available next month but I the R&D engineers are good about helping. Perhaps there is a list of components the are inclined to fail?

--Doug

[rossrov]

Hello Doug. Seriously, if there is something in it for the manufacturer then maybe they could re-route the board and make 2 separate boards that you could wire together. They might have a student working for them that needs a project...Simply cutting the standard board in half and wiring together would be a major headache at best and if it has mid layers then I'd definitely be giving it a miss.

Funny you mention the pins approach - I was thinking of a similar thing to house my Eagle fish-finder board, though in that case depth was no where near that great.

Oil/liquid would be great but as discussed component behavior under pressure/pressure reduction is unknown at present

CNC machining a housing from metal, or casting is what I'd be checking out, and as you say integrate that into the ROV design or vice-versa. Composite epoxy/carbon fibre if designed properly is something you could do in your shed and not need foundry or mill services.

Have you checked out what a standard transducer will stand up to WRT pressure or how are you addressing that? Also I'm guessing that you are using the electronically scanned systems and not for example Lowrance's Spotlight?

[martinw]

Depending on what's on the PCB you may be able to make it pressure tolerant, put it in a thin walled oil filled box, compensate it and away you go ....

Martin

That certainly is an attractive solution. How do you determine if the various components can survive routine service at 1700 psi? These boards are $700 each so we don't want too many failed attempts. The board in question is the Lowrance SonarHub due to be available next month but I the R&D engineers are good about helping. Perhaps there is a list of components the are inclined to fail?

--Doug

Hi Doug,

If you would like to send me a message from my website with an e-mail address, I may be able to help. If it's available in a month the components will have been chosen already. If you have any high resolution photos of the board I could take a look and see if it's even remotely possible

Martin