If this is for a side-view mirror and you have 20-square inches, I would use 3M very high bond (VHB) adhesive tape. I used this to mount 12 monitors to the back side of a sheet of polycarbonate. While we were laying it out, we "tacked" a frameless (bezel-less) display on just the top edge (didn't press it place) just the weight of the monitor with the plan that we would reposition it to finalize alignment and then tip it up to bond bottom and side edges. It turned out to be a dry bad idea. We broke the display and spent a half day rubbing and scraping the rest of the foam tape off. We redid it correctly and the displays are still in position 3-years later (real-time process monitoring system for 36 work cells with histograms).

Also cool was that we ink-jet printed the polycarbonate sheet with UV curable ink to section off each display. That ink bonded to the polycarbonate so well that it didn't scratch through as we scraped the VHB tape. The tape is strong on contact and gets "fully cured after 72 hours". If you don't want it to come apart and you are bonding dissimilar materials, use tape.
https://www.amazon.com/VHB-Adhesive...locphy=1024046&hvtargid=pla-309391936041&th=1

 

I would use 3M very high bond (VHB) adhesive tape.

That stuff is very good both adhesive strength and is water proof. That is what most car trim is held on with today.

If this is a mirror they also make a DIY tape that is similar. https://www.michaels.com/scotch-mir...xA9vWYsO00qmvFOPL1ZM-0oADZ2sRurxoC4B0QAvD_BwE

 

That stuff is very good both adhesive strength and is water proof. That is what most car trim is held on with today.

If this is a mirror they also make a DIY tape that is similar. https://www.michaels.com/scotch-mir...xA9vWYsO00qmvFOPL1ZM-0oADZ2sRurxoC4B0QAvD_BwE

And most auto-stores carry a version they call "trim tape". The trim tape really does take 24hr to cure. One side is repositionable in the first minutes before pressure is applied.

https://www.pepboys.com/3m-scotch-molding-tape-1-2-x-5-/product/215365

 

And most auto-stores carry a version they call "trim tape". The trim tape really does take 24hr to cure. One side is repositionable in the first minutes before pressure is applied.

https://www.pepboys.com/3m-scotch-molding-tape-1-2-x-5-/product/215365

I am not sure why i did not get notifications that there were more posts here.
I tested the silicone and it works pretty good so i went with that. It's curing now i will leave it for a couple days or maybe a week. After 24 hours i will install a few very thin stainless steel wires that wrap around the plastic back and mirror, holding the mirror a bit better than just glue. The wires will not be visible from 4 feet away as the gauge is #36 AWG but that wire is fairly strong. I'll just have to be careful when clearing that mirror in the winter and if i clean it with spray in the summer. The wires will also be glue t the back of the plastic and a couple dabs on the front near the top and bottom. I figure that should be good enough and if the mirror falls off i'll drive the car off a cliff (ha ha). I'll have to buy another mirror glass that's all.
A very wise old chess master once told me, "You can't have everything". Although that was referring to the game of chess itself (every move has advantages and disadvantages) it also applies so well to life.

I found that the plastic back part was covered in a sticky pad. I had to clean it before i could redo it.
What surprised me though is once i got the stick pad off i found there were circuit traces on the front side of the plastic where the mirror glass mounts (gets glued to). There were also two connector prongs on the back side. So owuldnt you know it, it's a heated mirror. Didnt know this car had them ha ha.

The trace is once continuous zig zag with curved zags rather than sharp bends. They cover the entire surface spaced maybe 1/16 inch apart as they twist and turn to cover the surface.
The resistance measures 8.7 Ohms, cold.

 

A word of caution:
Some time ago I used silicon adhesive to secure and seal half a dozen twisted pairs of 22g wire where they passed through a window frame for a data acquisition system. I found that before the adhesive cured, it effected the properties of the insulation on the wires. It caused relatively low resistance (Kohms) between the conductors and I got some very strange results for several days.
Once it had cured, the insulation resistance of the wires went back to normal.
I hope you don't have a similar problem, with the adhesive being directly on the circuit board.

 

A word of caution:
Some time ago I used silicon adhesive to secure and seal half a dozen twisted pairs of 22g wire where they passed through a window frame for a data acquisition system. I found that before the adhesive cured, it effected the properties of the insulation on the wires. It caused relatively low resistance (Kohms) between the conductors and I got some very strange results for several days.
Once it had cured, the insulation resistance of the wires went back to normal.
I hope you don't have a similar problem, with the adhesive being directly on the circuit board.

Was it "electronics grade" silicone? The ordinary RTV silicones produce acetic acid while curing which is bad news for a lot of things. The "electronics grade" RTVs don't do that, and don't have the concomitant bad small.

 

Was it "electronics grade" silicone? The ordinary RTV silicones produce acetic acid while curing which is bad news for a lot of things. The "electronics grade" RTVs don't do that, and don't have the concomitant bad small.

No, I wasn't using electronic grade, just the general purpose stuff.

 

A word of caution:
Some time ago I used silicon adhesive to secure and seal half a dozen twisted pairs of 22g wire where they passed through a window frame for a data acquisition system. I found that before the adhesive cured, it effected the properties of the insulation on the wires. It caused relatively low resistance (Kohms) between the conductors and I got some very strange results for several days.
Once it had cured, the insulation resistance of the wires went back to normal.
I hope you don't have a similar problem, with the adhesive being directly on the circuit board.

Hi,

Yes that is a concern. I think the circuit board had a conformal coating the metal material that make up the traces were not exposed to the elements as you can see there was a clear coating. Not if the silicone messes with that i dont really know, i just have to wait i guess. It's also low voltage 12 to 14v so it may be ok anyway.
I have read the best conformal coating is silicone based, funny. I dont really know the acid properties of the silicone i used i just know that it works as a strong adhesive. It's called "Flex Shot" and although the name sounds kind of cliche it is strong i know that from several tests i did recently.
Also, i have to wonder, do i really need a heated mirror? I did fine for years without heated mirrors. If i dont hook up the leads to the trace it wont heat up but also no electrical problems then either.

At this point though i am stuck with it anyway as it has been applied and has cured now for about 14 hours. Tomorrow some time i will apply the fine wires and that's going to have to do it.
I wonder though why Volvo would make such a nice heated side mirror and not have a better way to hold the mirror glass in place. Some edge clips would have held it forever, combined with the glue, or even not with the glue, and made it easy maintenance.
What is strange here is that we see a mix of 'glue' and 'heat', which is a bad mix unless the glue can take the heat (like many modern glues and silicones can).
I know other people who have Volvos and they had same problems.

I'll post again later in the year once it falls off

Here is a pic of the traces. The glue goes on top of those traces and the mirror goes on top of the glue so you never see the traces again after the mirror is stuck on.
You can see the two studs where the connector terminals are attached in the upper left corner. The connector prongs are on the flip side and two wires connect to them that provide the power to the traces.
If you have the patience you can try to follow the traces around from one terminal to the other to see if they are really contiguous

 

Was it "electronics grade" silicone? The ordinary RTV silicones produce acetic acid while curing which is bad news for a lot of things. The "electronics grade" RTVs don't do that, and don't have the concomitant bad small.

Hi,

Now that you mention it i am not sure if this is electronics grade or not. It's called "Flex Shot". It is very strong once cured i found that out from several tests so maybe it will last.
The traces do have some kind of conformal coating though so that may be enough.
You can see the image in the post just before this one #28.
Interesting way they did it i guess.

 

Hi,

Yes that is a concern. I think the circuit board had a conformal coating the metal material that make up the traces were not exposed to the elements as you can see there was a clear coating. Not if the silicone messes with that i dont really know, i just have to wait i guess. It's also low voltage 12 to 14v so it may be ok anyway.
I have read the best conformal coating is silicone based, funny. I dont really know the acid properties of the silicone i used i just know that it works as a strong adhesive. It's called "Flex Shot" and although the name sounds kind of cliche it is strong i know that from several tests i did recently.
Also, i have to wonder, do i really need a heated mirror? I did fine for years without heated mirrors. If i dont hook up the leads to the trace it wont heat up but also no electrical problems then either.

At this point though i am stuck with it anyway as it has been applied and has cured now for about 14 hours. Tomorrow some time i will apply the fine wires and that's going to have to do it.
I wonder though why Volvo would make such a nice heated side mirror and not have a better way to hold the mirror glass in place. Some edge clips would have held it forever, combined with the glue, or even not with the glue, and made it easy maintenance.
What is strange here is that we see a mix of 'glue' and 'heat', which is a bad mix unless the glue can take the heat (like many modern glues and silicones can).
I know other people who have Volvos and they had same problems.

I'll post again later in the year once it falls off

Here is a pic of the traces. The glue goes on top of those traces and the mirror goes on top of the glue so you never see the traces again after the mirror is stuck on.
You can see the two studs where the connector terminals are attached in the upper left corner. The connector prongs are on the flip side and two wires connect to them that provide the power to the traces.
If you have the patience you can try to follow the traces around from one terminal to the other to see if they are really contiguous

View attachment 263192

Thanks for the maze, it I'll keep the grand kids busy for a while.

 

The swift Flex Shot is the acetic acid emitting kind of silicone. Ingredients include...
Triacetoxyethylsilane
Methylsilanetriyl triacetate
1,1-Difluoroethane (propellant)

The -acetoxy groups are displaced with water to make an acetic acid leaving group and Si-OH ( hydroxyl groups) or bridged SI-O-Si groups to cross-link and cure the past to a solid. Most will be emitted on initial surface cure but you may have more over a long period of time as the blend continues to cure towards the core of each dab of this material. The concentration drops off as the cured skin gets thicker and thicker.
As for your heater PCB, it looks like there are some scratches in the tinned traces so you may not have had much of a heater anyhow. 8.7ohms on a 12v supply is about 18 watts. I would have expected about 24 to 30w for a 15 sq-in defrosting surface - like most rear window defrosters which are typically 1.5 to 2W/sq inch).

 

The swift Flex Shot is the acetic acid emitting kind of silicone. Ingredients include...
Triacetoxyethylsilane
Methylsilanetriyl triacetate
1,1-Difluoroethane (propellant)

The -acetoxy groups are displaced with water to make an acetic acid leaving group and Si-OH ( hydroxyl groups) or bridged SI-O-Si groups to cross-link and cure the past to a solid. Most will be emitted on initial surface cure but you may have more over a long period of time as the blend continues to cure towards the core of each dab of this material. The concentration drops off as the cured skin gets thicker and thicker.
As for your heater PCB, it looks like there are some scratches in the tinned traces so you may not have had much of a heater anyhow. 8.7ohms on a 12v supply is about 18 watts. I would have expected about 24 to 30w for a 15 sq-in defrosting surface - like most rear window defrosters which are typically 1.5 to 2W/sq inch).

Hey thanks for the info Mr Salts very good to know.

So it is not really the right kind, i just have to hope the conformal coating holds up.
Yes interesting maze. I was thinking of following it around to try to figure out what kind of material they made the traces out of. Might be hard to do though that pattern looks weird too.

Well i didnt think 18 watts was too bad really but really as long as it stays on i am happy.
The scratches are not really present on the actual surface that is an unfortunate artifact of the camera and the way it captures some things. For example you can see a very light colored area on the right side up and down the whole height, but that's not really there either the color is very uniform.
There is two or three tiny tiny patches of glue left though i got tired of cleaning it i didnt have the right solvent on hand so i had to literally rub the glue off with my fingers.
.
It's kind of interesting too how they used glue right on top of the traces that get heated. The glue type has to be able to take the heat. The heating surface is about 24 square inches on this mirror so that's about 0.75 watts per square inch. 1 square inch with 1 watt of power can raise the temperature by about 60 degrees C with free air convection so 3/4 w/sqin will be lower yes. I wont need it again until next year though we are getting into the warmer weather now.

 

Hello again,

I realized that the worst that could happen if the silicone 'eats' the traces is that the mirror just wont be heated anymore. No big deal for me doesnt matter that much to me. I've had non heated mirrors for years never had a big problem with that
What i did like about this particular silicone is that it is strong so it should hold the mirror on for the life of the car.

Thanks to everyone for the ideas and suggestions.