Heat damage possible on boost reg?

T Chap

Ray I know you use this same regulator, I use this for driving the 4.3 Newhaven LCD. I don't get many failures but this one died in less than a year. I just got the part back from the client to repair it and I notice that I have gotten a layer of heat glue on the part. I use heat glue to put in the LCD and board in an enclosure as it is very easy to remove with heat if there is an issue, but I never paid attention to getting the glue on top of the regulator. This part gets very hot, so I am wondering if it was lack of heat escape that killed it? Any thoughts from anyone appreciated. I replaced the part and the LCD backlight now works.

Heater.

It's hard to make out anything from that fuzzy picture.

However from what you say it seems very likely that inhibiting thermal dissipation with glue, or whatever, might cause longevity issue

T Chap

Well the cell want let me get close enough to make a clear photo but the point is that it had a thin layer of heat glue over it and the glue looked quite discolored from the glue around it. The regulator runs hot, so I assume the glue added some extra fatigue to it.

Heater.

Sounds like it's time to check the data sheet for that device.

What is it's maximum operating temperature?

What is it's thermal dissipation?

What is it's efficiency and how much power must it dissipate in your application?

Peter Jakacki

TIP: don't get too close to take a photo, know your minimum focus distance which in the case of my phone needs about 5", those with smaller sensors usually can get closer. When the photo is sharp and focused you can always crop it down, but you can't do anything with a fuzzy shot.

I have the same or similar part here and hot glue will trap heat but remember that the pcb is also a heatsink especially if it has been designed properly, so "muck" on there as well as the part will impede heat dissipation.

Here's a shot of a boost reg on one of my pcbs (cropped down a lot but clear)

T Chap

Thanks guys for the input. I will be looking at some other dead boards this week to see if there is a pattern of glue over the regulator. I have a feeling this is the problem on several boards.

kwinn

Sure looks like an early failure due to overheating. Seen similar failures many times over the years. Even though a device may be below its rated max power dissipation and max operating temperature the warmer it gets the sooner it fails.

Tracy Allen

It looks carbonized next to the chip. If not heat damage to the chip itself, which is most likely, it might be unwanted "resistors" between the pins.

T Chap

More learning today. I got back 3 brand new modules of 6 I had just sent out. This is pretty rare to have such a high failure rate(cost me a lot to straighten this out). Of the 3 all of them there was heat glue on the boost reg like the one last week I was trying to solve. So apparently I randomly started putting glue on the PCB over the reg and was honestly never giving it much thought in thepast. The time to death of the regulator is random from days to weeks, also I notice that one of these will work for a short time before it shuts down.

Another problem in addition to the glue/heat issue, is apparently all 3 of these had some bridges removed from the 40 pin FPC and I always use this no clean flux that is water soluable. I use alcohol to clean the best I can but it is not easy to clean behind the pins and inside where the ribbon sits. I don't have any serious flux remover as my experience has been bad trying to use powerful cleaners.

Out of the 3, I decided to replace 2 of these LCD backlight regulators that died under heat(under load it is 18V) and see if that would solve the problem. On both boards, the identical results:

1. Replace Reg
2. Power up
3. In under 30 seconds Flameout on the FPC connector pins to PCB near the ground, Backlight LED Lowside(from regulator) and LED High side 18V. See photos.
4. Damage on the ribbon cable in the same area. Possibly heat from the pins.

I have used this same flux for years, often times it lives on pins forever and never an issue. I am assuming that the traces and pins are running hot and the flux is somehow solidifying into a conductor, not sure if this is possible but what else is flaming out?

Considering I have hundreds of these out there these were flukes and the lesson learned is if there is a bridge, never rework with flux.... if it cant be solved easily by wick then trash the board. Then the obvious of always be sure the PCB near the boost reg is clear of glue.

What is interesting in this, is that both regs were dead, and upon replacing both there was a flame out near the power pins for the LCD backlight. I have actually witnessed this same issue in the past but didn't try to resolve the issue as it was rare. But after losing a grand in the past week there needed to be some resolution


Photo1 glue on boost reg
Photo2 backlight power pins smoked on the left side of the FPC connector
Photo3 ribbon damage after flame out
Photo4 another ribbon damage after flame out

kwinn

Photo1 glue on boost reg:
Looks like the regulator is getting hot enough to turn the glue brown. Probably due to the glue trapping the heat and limiting air flow in that area. If you can remove the glue I would suggest measuring the resistance from the regulator power pins to a feedthru/pad/solder joint, or other point on the power supply path.
A bad solder joint can also result in high power dissipation on the chip pin which in turn can cause the chip to fail. Some glues are slightly conductive to start with and become more so with heat, which can cause a thermal runaway failure. The chip or joint gets warm, the glue carbonizes and conducts more current, and the chip or joint gets even hotter.

Photo2 backlight power pins smoked on the left side of the FPC connector:
Photo3 ribbon damage after flame out
Photo4 another ribbon damage after flame out

Could be a bad solder joint or ribbon connection, or it could be glue or another contaminant shorting two or more pins together. Check the resistance from the other end of the ribbon cable to the connector pin and the solder joint the pin is soldered to before disconnecting the ribbon cable at the board end.

Tracy Allen

How about this scenario? The original boost lost regulation due to overheating and the voltage spiked to much greater than 18 volts. Why the big flameout after the regulator replacement? Maybe the original one flashed a conductive path on the ribbon connector and set up a bigger flashover after the replacement. Those fine pitch pads underneath chips and connectors can be a real problem with water soluble fluxes.

It looks like the board in photo #4 has a heavy conformal coating, also a possible problem for heat.

T Chap

Sounds interesting on the conductive path, obviously for both failures to be identical with the flameout after replacing the regulator there was a path that occurred at some point post fabrication, otherwise the flameout would have happened on first power up.

As for board 4, that substance you see is where I heated the glue and scraped it off just to do the regulator replacement test.