[satishmatin (OP)]

in case of leakage in a submarine, a pressure of 5 kg is generated to obstruct incoming water in a project, and the electronics are designed to withstand the same 5 kg pressure (in case of leakage, ofcourse). but how do i seal the lcd display from such high pressure?

[graham.d]

I'm not sure what is meant by 5kg. Pressure is measured per unit area.

But, in any case, you raise a good point regarding LCD displays. I am not sure to what pressure they would work down to. The rest of the electronics would be OK, in the short term, to a much greater depth than humans could stand. Semiconductor devices may suffer reliability problems if pressure cycled many times in a humid atmosphere, but I doubt this would be a significant problem. If this was a large concern they could buy hermetically sealed packages, though this is increasingly unusual, even for the military.

Most commercial and military submarines maintain 1 atmosphere inside - I did not know that they had any mechanism to increase internal pressure to prevent water ingress though it seems a better alternative to drowning. Some craft, like diving bells, are always pressurised to the surrounding pressure of course.

[satishmatin (OP)]

5kg/sq.cm.............thats the pressure
its almost 5 times the atmospheric pressure
general pressure inside sub is 1atm.........true
but this is couter protection technique being used in this project........to save the lcd displays i have to see that my rugged lcd display withstands 5 kg/sq.cm.

i came across these hermetically sealed connectors....one good option
more suggestions required on the extent of damages that the high pressure can cause to lcd, mother board, inverter card, etc.

[graham.d]

I would not expect any damage due to pressure on many electronic components. Nearly all semiconductor devices are sample tested (autoclave tested) at 2 atmospheres at high temperature and 100% humidity for over 100 hours. It is not expected that the pressure itself would cause any problem; it is just to drive moisture into the packages to see if this results in any corrosion. The heat is to accelerate any effect. The hermetic sealing provided on connectors and other devices is to keep moisture out. Pressure itself is unlikely to be a problem on devices, boards or connectors.

You should speak with the LCD display manufacturers regarding the effect on these displays though. My feeling is that these will also be OK because the pressure will be even across the surface but I have no specific experience that can help here.

I guess you are generally speaking of rare emergency situations too. I may be more concerned if there was regular pressure cycling, especially in high humidity.

[satishmatin (OP)]

- hav been trying to find the max pressure semiconductors & transistors can withstand...........none of the datasheets mention abt pressure........all they say is they are tested at 2 kg pressure
- hermetically sealed connectors seem to be too expensive......can't afford individually at this stage of project
- my first simple doubt is if there's a leakage, it hardly takes 5 min for the air to enter my lcd enclosure........it might simply crack the lcd screen, boards, etc.
- ive asked few people from pressure testing dept. in a research institue........they've no idea about effect of pressure on electronics or semiconductor devices

[graham.d]

It really can't crack boards - at least I can't see any reason why it should. There is no reason to use hermetic components from what I understand of your design criteria. Manufacturers are unconcerned about pressure because it does not generally matter very much. Just check out the LCD displays, even if it means testing them yourself. I expect they will be OK too.

[techmind]

I used to work on LCD displays, but at the research end rather than product-based testing.

An LCD is made of two usually rather thin (e.g. 0.7mm thick) sheets of glass with about a 4um (0.004mm) gap between them, which is filled with the liquid crystal. There are normally some kind of mechanical balls or spacer structures to maintain the 4um spacing.

If you push an LCD in a point from the front then you risk breaking the glass just because you're stressing a thin piece of glass.


From what I know about the construction of the screens, I envisage...

If you raise the air pressure, then you'll simply be pushing the two panes uniformly against each other. Depending on how sparse or densely distributed the spacers are, you might still tend to flex the glass panels closer between the spacers - which might temporarily upset the colours, but wouldn't break the glass. What will happen is that the liquid will have a huge mechanical advantage to force against the edge seal and the liquid crystal may be squeezed out (then when the pressure returns to normal the LC will retreat by capillary action and only part of the screen will work).

If you were *designing* an LCD for operation at higher pressure I guess you would
* put the spacer structures much closer together, giving the glass much less opportunity to bend between them
* possibly use slightly thicker glass just to make it a bit stiffer (not seriously thick!)
* possibly have a wider (and therefore more pressure-tolerant) seal around the edge of the glass keeping the liquid crystal in.

Such displays would be a bit of a "special" and cost some more - but there's nothing fundamentally difficult there...

[satishmatin (OP)]

most importantly: thanks graham and techmind
i hid the fact that ive already got the display tested
             ..........and it passed, after sealing some 7 or 8 leak points at connectors (which i found by placing the dummy enclosure in water)
i started this topic to know the impact of pressure on lcd, boards etc.........thanks graham & techmind......ur info certainly encouraged me in thinking broadly