Heat causing automotive LCD display to dim. Can't find a solution.

[RickB93]

Hi all,

Not sure if this is the right place but hoping someone can help.

I had a automotive LCD manufacturer produce some new LCD displays for an older vehicle (used to be manufactured between 1999 and 2005) as the original LCD displays had a design flaw in the way two ribbon cables were bonded, where delamination would occur causing lines to appear on the display.

Unfortunately there is no build data or designs available for the LCD displays, however the automotive LCD manufacturer was able to replicate the displays using the original ones, with a new improved soldered connection between the two ribbon cables. All works well however we have discovered that the LCD display dims when the instrument cluster's circuit board (to which the LCD display is mounted to) gets warm. We've traced the issue to a SRAM chip on the board for the LCD display (part number ZMD U62H64SA). This chip seems to be extremely sensitive to heat. It doesn't require much heat for it to get warm enough to cause the dimming problem with the display. For example, having the dash heater vents on hot for about 15 minutes generates enough heat behind the dashboard to cause the display to dim, or the interior of the vehicle getting hot on a hot day (when temperatures are around 25oc or more) causes it too. Even applying heat to this chip with a hair dryer set to the lowest heat setting quickly causes the problem to occur. The original factory LCD displays were also affected by this issue, however it required slightly more heat to cause the problem. It's not the display itself, as that has been tested by heating just the display to 60oc and it still operates absolutely fine. Apply even half that amount of heat to the SRAM chip and you'll see the display dim to the point it's unreadable.

The manufacturer has been unable to find any effective way to keep the SRAM chip cool, or understand why this is happening. The problem is the chip isn't getting particularly hot, but is so sensitive to heat that the normal ambient temperature is causing issues. The manufacturer has tried adding a heat sink and a cooling fan, but the issue is this just sucks the warm air around the instrument cluster in towards the SRAM chip so doesn't help at all.

The first pic attached shows how the LCD displays when the SRAM chip is at a cooler temperature. The second pic attached shows how the display looks when it dims, caused by applying heat directly to the SRAM chip. The third pic attached shows the SRAM chip on the board.

I don't have a electronics background, so finding a solution to this myself is well beyond my capabilities. I'm just a member of the enthusiasts club for this model vehicle and have been trying to find a effective replacement display made to solve the issue of failing factory displays. If anyone has any suggestions or potential solutions that would be really appreciated. We've been trying to solve this for around a year now.

Many thanks
Rick

 

[kellys_eye]

I'm surprised that an SRAM chip can affect a display that way and suspect this isn't the case anyway. It looks 'typical' of a poor CONTRAST issue. Many LCD's either have a PIN to apply an external voltage to 'set' the contrast. Other displays manage this internally on an automatic basis. Got any spec/details for the display itself?

IF (and it's a big 'if') the SRAM is the cause then try to obtain a MIL Spec version (much broader safe-operating temperature range) and see if that helps.

Personally I'd be checking with the display manufacturers as this seems, to me, to be the area of concern.

 

[kellys_eye]

Update - just checked the datasheet for the SRAM

https://pdf1.alldatasheet.com/datasheet-pdf/view/105048/ZMD/U62H64SA35.html

and noted its automotive application and wide operating temperature range which certainly clarifies my suspiscion that the LCD itself is the issue here and NOT the SRAM itself. That said, the device does come in two versions, one good for 85degC and one for 125degC so maybe try the higher range version?

If all else fails then fit a small TEC (peltier) arrangement.

Monitoring the data to/from the SRAM whilst observing the display might reveal something although only a failure of the actual logic levels would really make a difference. Have you checked the TIMING to/from the chip to see if it changes with temperature (often does)?

 

[RickB93]

Thanks for your reply. As I said above, it is not the display. We have already tested the display by applying heat directly to it and it operates perfectly fine. From memory the manufacturer tested the display heated up to 100oc and it displayed perfectly fine. The manufacturer and I have both tested the instrument cluster's board by applying heat to 1 individual chip at a time in isolation with a soldering iron or hot air tool, and have found no chip causes the dimming issue apart from the SRAM chip. It doesn't make sense to us either why this happens, as I believe the SRAM chip just stores the trip information data for this display, but we've done a lot of testing over the last 9 months and it all points to the SRAM chip. See this video here demonstrating what happens when heat is applied to the SRAM chip:

We tried installing brand new replacement U62H64SA chip (125oc rated) and it made no difference. The 125oc rated version is what's also fitted as factory standard.

If I apply heat to the rest of the board and isolate the SRAM chip to that it doesn't get hot, the display continues to operate fine.

I tried a couple peltier modules from china via Aliexpress but they were not effective. The cold side would start off ice cold but would quickly go warm due to the heat from the hot side. That was just during bench testing, without any testing in warm conditions.

 

[kellys_eye]

As I said above, it is not the display.

If it's not the display then what is it about the data going TO the display that makes the displayed data 'wrong' What parameter of the display changes to cause the 'fading'? Can you provide a datasheet for the display?

There may be a timing (heat) issue related to the way the displayed data changes and if the data in the SRAM doesn't meet the required parameters of the display you get the wrong result. Yes, this means the SRAM is the CAUSE but the display - and the way it deals with the data - is the answer as to WHY. I'd like to see the spec for the display - it might only be a matter of changing the clock speed.

 

[kellys_eye]

Ah ha..... that's a sh1t load of heat you apply to the chip before we see a change in the display and I reckon that it's not the heat on the SRAM that's the cause but the heat moving AWAY from it to some of the components adjacent TO it. This is why I want to see the datasheet for the display as the components seen on the left-hand strip connections may be related to the display 'contrast' and if the heat from the SRAM is affecting THOSE parts then you have the reason for the change.

Whilst heating the SRAM, try COOLING those components beside the display connection strip at the same time.

 

[RickB93]

You are correct!
So been looking again at other chips surrounding the SRAM to ensure no other resistors etc are actually causing the problem rather than the SRAM. The yellow chips to the left of the SRAM I've tested before, and just to be sure I tested them again this morning by applying heat to them with a soldering iron and they do not affect the display what so ever.

However.... what I had not noticed is that there is a very small chip on the opposite side of the board, almost directly behind the SRAM chip. See 1st and 2nd pics attached. This is what is actually causing the problem, not the SRAM chip. If I apply heat to this chip with the soldering iron the text on the display immediately goes blank.

So what has been happening during our tests is, when we heat the SRAM chip, the heat is transferring to the board and also heating this small chip located nearby on the opposite side of the board. And when we're cooling the SRAM chip and seeing the display quickly restore, it is also cooling the board and again also the chip on the opposite side of the board. So it made it appear that the SRAM chip is causing the problem when in fact it's this small chip on the opposite side.

If I remove the chip completely the contrast of the display is badly affected. See 3rd pic attached (ignore the vertical black lines, this is the problem the old original displays have due to delamination of the ribbon cable joint which I mentioned earlier in my previous post - this is why I've had the new displays made).

As a test, what I've done is removed the chip from the board, and then installed 3 wires from the board to the chip so that the board can be heat tested with the chip away from it. Now if I apply heat to the SRAM chip, it has no affect what so ever to the display (I tested by applying heat to the SRAM chip for 4 minutes with the soldering iron with no problems). If I apply heat to this chip with the soldering iron it immediately causes the text to fade and disappear from the display.

See my video here:

I cannot believe I didn't spot this small chip and test it earlier!

It looks like this chip is a switching diode? The marking on it is A7. So now my question is, what can be done to prevent this chip from causing my problem?

 

[kellys_eye]

So now my question is, what can be done to prevent this chip from causing my problem?

Per my previous advice, a larger copper 'ground' to dissipate the heat would work. There looks to be plenty of space to achieve this. Alternatively you could find a different device that has better heat dissipation properties - either way I reckon you'll have to rework the pcb layout.

Haven't looked for the device type but suspect it's a diode (maybe zener?) which are commonly available in higher dissipation vaues (250mW, 400mW, 1.3W etc) so the next 'size' up could do it???

https://datasheet.datasheetarchive.com/originals/library/Datasheet-093/DSA0067309.pdf

 

[Harald Kapp]

Glueing a metal tab on top of the chip to better sissipate heat may also help.

 

[H2814D]

Isolate and insulate that chip from the entire board with wires and see if the issue remains. The IC is likely heating up the board beneath that chip. If the chip isn't still touching the board there, the problem may be solved.

 

[kellys_eye]

So now my question is, what can be done to prevent this chip from causing my problem?

Since it's not the SRAM but 'heat' that's causing the issue then you need to insulate the A7 device from where the heat is coming from. This might be possible using just a 'blob' of thermal grease on top of the device - this would dissipate the heat evenly over the area of the thermal compound. It might even be possible using plain old silicone compound.

My preferred solution would be to try another device that has better thermal characteristics as mentioned above. The device is a 'simple' diode so higher dissipation devices may be available. Let us know how you get on.

Can you reveal what the component does in relation to the LCD? Is it (as I suspect) part of the signal path related to setting the display contrast? (sometimes fitted as an 'automatic' i.e. temperature compensated adjustment rather than a fixed-but-variable using a preset pot as is common arrangement).