How to make self-adhesive PCB standoffs actually stick to plastic box

Thread Starter

AlbertHall

Joined Jun 4, 2014
12,396
In the past I have used these standoffs to secure a board inside a box but they never seem to stick very well - typically, when removing the board, they all come off with board.

I have a new possible use for them but how do I get them to stick solidly?
Clean the plastic - water and soap or/and IPA?
Scuff the surface with sandpaper?
Or ??
 

Ya’akov

Joined Jan 27, 2019
9,265
The adhesive is "pressure sensitive", it actually cures from pressure. Clean the surface with a solvent like IPA or acetone (if compatible), make sure it is all evaporated, then place the the standoff and apply as much pressure as you can safely manage directly perpendicular to the surface.

It is literally the amount of pressure that strengthens the bond, and since they tend to stick pretty easily, people neglect this. It will make a very large difference if you don't.
 

jpanhalt

Joined Jan 18, 2008
11,087
The adhesive is "pressure sensitive", it actually cures from pressure.
Can you provide a citation for that definition? Sure, there adhesives that cure when pressure is applied to exclude air (Loctite is one well known brand) . 3M makes some very interesting pressure sensitive adhesives (PSA's) that are removed in special ways. From a chemical standpoint, there is no reason one could not make a pressure sensitive adhesive that "cures" from pressure alone. For example, the chemical reaction that is initiated in NCR paper (no carbon required) does not require oxygen to be initiated, but it does require oxygen to become visible.

I would be reluctant to define a PSA as an adhesive that cures with pressure; although, there may be some like that. In fact, 3M began as a company making what we call masking tape today, and the primary purpose of that invention was an adhesive that didn't cure with pressure or carrier evaporation so it could be removed easily days later. PSA's have to be designed to have selective adhesion unlike most other adhesives (e.g., epoxy, CA, rear view mirror adhesive that is chemically cured). As you know, PSA's are often sold in spooled rolls. They need to stick poorly to themselves while sticking avidly to other materials. So, one might say, add a liner. Same issue. You need to be able to remove the liner and frequently want the resulting adhesive to stick to other smooth items.
 

jpanhalt

Joined Jan 18, 2008
11,087
@AlbertHall ,

Clean and grease free are usually the prime necessities. Roughing the surface may actually decrease adhesion for PSA's. That depends on the adhesive. I use a "slick surface" tape from 3M that sticks quite well to smooth surfaces and not to rougher surfaces.
 

SamR

Joined Mar 19, 2019
5,081
Not sure what you are trying to mount... There are some double-sided tapes that are a bit thick (±1/16?) and sort of "puttylike" that I have used and found inside chassis'. It comes in a roll w/ a peel off for each side and cut to length needed. Or Velcro w/ adhesive backing for removable items where you can't use standoffs and screws?
 

Thread Starter

AlbertHall

Joined Jun 4, 2014
12,396
Not sure what you are trying to mount... There are some double-sided tapes that are a bit thick (±1/16?) and sort of "puttylike" that I have used and found inside chassis'. It comes in a roll w/ a peel off for each side and cut to length needed. Or Velcro w/ adhesive backing for removable items where you can't use standoffs and screws?
This is the kind of thing I am talking about: https://uk.rs-online.com/web/p/adhesive-backed-supports/0220787/
None of them seem to have any instructions about how best to attach them.
I haven't come across any mention of PSA for them before but it's definitely worth a try.
 

Ya’akov

Joined Jan 27, 2019
9,265
Can you provide a citation for that definition? lective adhesion unlike most other adhesives (e.g., epoxy, CA, rear view mirror adhesive that is chemically cured).
https://en.wikipedia.org/wiki/Pressure-sensitive_adhesive

"Curing" may not be strictly correct, but permanent (non-removable) PSAs change state after application to remain attached.

No oils or dirt and intimate contact, followed by pressure, is the way to make them stay.
 

jpanhalt

Joined Jan 18, 2008
11,087
It was the term "cure" that I fell was wrongly applied:
jpanhalt said:
I would be reluctant to define a PSA as an adhesive that cures with pressure
From that Wikipedia link,
Pressure-sensitive adhesives are designed with a balance between flow and resistance to flow. The bond forms because the adhesive is soft enough to flow, or wet, the adherend. The bond has strength because the adhesive is hard enough to resist flow when stress is applied to the bond. Once the adhesive and the adherend are in proximity, there are also molecular interactions such as van der Waals forces involved in the bond, which contribute significantly to the ultimate bond strength. PSAs exhibit viscoelastic (viscous and elastic) properties, both of which are used for proper bonding.
That does not describe "curing." Van der Waals interactions occur with all sorts of mixtures and surface interactions that are not considered "curing." Of course, I did not rule out that one could make an adhesive that cures by pressure and suspect there may be some. The underlined portion is an important consideration in many procedures, such as Brazilian hair removal. :) Also when you remove an unwanted label, it is usually best to pull slowly, rather than rapidly. In contrast, when using PSA tape to remove goo left by other tape, it is often better to pull fast.
 

SamR

Joined Mar 19, 2019
5,081
This is the kind of thing I am talking about
I know them well! They don't stay attached and they are not UV resistant (the black ones are). Don't have a good answer as they are the "better answer" to drilled and screwed down. They will stay attached for maybe a couple of years or so inside of a chassis in a climate controlled environment. Not so long outdoors even in a dry cabinet here in the south. I suspect heat is an issue with their failure. Hmmm... Maybe reattaching them (after cleaning off the residue) with the double sided tape I mentioned before. When I first discovered them I thought they were great, then I learned the reality of using them. Can't tell from the link, but the ones I used had a foam pad on the bottom which helped with vibration resistance I would think. Quick and easy installation, but no longevity.
 

djsfantasi

Joined Apr 11, 2010
9,188
Why do you want to use them in the first place? For permanently fastening a PCB in an enclosure, I use threaded hex spacers.
D7C7D351-F047-46D5-9D87-6E1FC0FA73AA.jpeg
and a screw with a countersunk head with a cone shaped bearing surface having the appropriate thread for the spacer. Those type of screws are flat to the enclosure surface and are extremely neat.
85415F04-3F1D-492B-B386-BC436DF834D3.jpeg
Then, I screw the PCB to the spacer with a round head screw.

I don’t tighten the screws until all are installed. Then I tighten the enclosure screws before the PCB screws to allow for slight misalignments.
 

djsfantasi

Joined Apr 11, 2010
9,188
Those would work if the enclosure didn't need to be water tight. Using a similar but plastic one they could be glued to the inside of the box. http://www.essentracomponents.com/e...off-hexagonal-metricimperial-threaded-plastic
Those are nylon. It’s difficult to find an adhesive that sticks to nylon. I’d still use the metal (aluminum) standoffs with epoxy.

I’d get a few nylon screws, lightly oil them and insert them in one end of the standoffs.
Then, I’d fill the other end with epoxy and place them in the enclosure. The epoxy will drips out of the hole and adhere to the enclosure. Some will remain on the threads, creating a mechanical lock with the epoxy. The oiled nylon screws won’t adhere to the epoxy.

dj
 
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