Cables - silicone seal in PCB

[fedimakni]

Hello,
I have pcb that it's currently produced i made a mistake that the length of the cable is a little bit short by few millimeters. I couldn't find longer cable because they are out of stock and i need to deal with what i have (pcb already assembled - cables already purchased...) Sometimes when i use the pcb the cable could be pulled out, so i would like to seal it, in order to prevent that issue.

I would like to ask if using silicone could be a good solution?
Any cons about silicone?
Any other solution can fix this issue?
Thank you in advance.

[Berni]

Silicone is a good choice, but MUST be the correct kind.

Never use the regular silicone used for sealing your bathroom shower. The stuff produces nasty acidic compounds that will corrode your electronics. You have to buy silicone specifically designed for electronics use (sometimes called RTV) as it has a different formulation. The stuff doesn't smell nasty at all, even the little odor it has is not all that unpleasant. It produces harmless alcohol compounds in its curing process so it won't damage any electronics. The downside however is that the curing time is a lot longer than regular silicone. It typically needs like 24 hours to cure at room temperature.

If slow cure is a problem then you can get special types that cure at higher temperatures, those can be cured in under 1 hour in an oven. Or alternatively you can use a 2 component silicone that will harden in about 10 to 60 minutes from the time of mixing (depending on type)

[fedimakni]

Thank you very much for the information,
Could you please suggest a brand so i can be more confident?
I also would like to know could that silicone be applied to small places because i don't want it to be around all over the place? So can we control it?
last how can we remove it after we apply the silicone?
Thank you again for the information

[jpanhalt]

What you need to search for is called "neutral curing silicone."  Here's a discussion: https://www.chemical-concepts.com/blog/2018/12/whats-difference-acetoxy-vs-neutral-cure-silicones/

There are many brands and part numbers.  Or, just search on your electronic supplier's website for a more limited list.

EDIT: Silicone will be flexible and is a good sealant, but it may not be the best adhesive.  Check Henkel/Loctite for alternative adhesives.

[Ian.M]

Silicone sealants are *NOTORIOUS* for not sticking when you want them to (unless all surfaces are chemically clean), yet being very difficult to remove well enough for *ANYTHING* else (including a fresh application of the same sealant) to adhere successfully.

If I were you, I'd try Kapton tape brdging between the connector body and the flat flex before resorting to any adhesive, let alone silicone sealant.

[Berni]

As long as the silicone package says it's for use on electronics then it should be fine.

When it comes to FPC cables they tend to be stuck down with kapton tape, but i assume the cable is too short to do that. More fine application of silicone can always be done using a small syringe or just simply a large toothpick to dip into the silicone and dab it onto where needed.

An alternative for gluing down these can also be two component epoxy. If you don't take something super strong they can often be softened by heat to get it off again. However the issue with a lot of them is that they love to wet surfaces, so it pulls itself down into connectors and makes a mess inside. You want one that is fairly thick and "pastey" rather than thin and easy flowing.

[Gyro]

I'd go with Ian.M's suggestion of tape. You're actively trying to stop the cable moving and silicone is reasonably stretchy. Hot melt or 'nail varnish' type adhesive might hold better, but could still let go or crack.

You can get some very strong adhesive tapes, fibreglass etc. which are used to restrain ribbons in laptops. Something like that would probably resist even strong tugs. They're also quick to apply, with no drying time.

[Siwastaja]

Silicone is soft, it does not provide the necessary mechnical rigidity here. It does work when filling large gaps between components, for example gluing down electrolytic capacitors.

Use tape instead. Doesn't need to be Kapton. Polyester tape or even just standard thin transparent packaging tape would do the trick. Test a few tapes, the idea is that a force pulling the tape shouldn't let it go. You can't achieve this target (keep the FFC in place within fractions of millimeters) with a silicone sealant which is soft, even if it adhered perfectly.

[tooki]

Silicone is a good choice, but MUST be the correct kind.

Never use the regular silicone used for sealing your bathroom shower. The stuff produces nasty acidic compounds that will corrode your electronics.

The “nasty acidic compounds” is actually a single acidic compound which isn’t particularly nasty, for most situations: acetic acid, the acid in vinegar. It will corrode copper, so indeed it’s no good for electronics, but don’t make it sound like it’s outgassing toxic chemicals or something!

[Terry Bites]

I've always been happy with DOWSIL™ 744 for all PCB mounting issues.
And Dow 3140 for coatings
It worth the extra cost.

[rcbuck]

Per Gyro's suggestion, try Red Green's "the handyman's secret weapon" (duct tape).

[Siwastaja]

Duct tape is no good, the glue is sticky but needs large surface areas and is best when wrapped around itself. In a tiny connector, it won't hold up at all. The tape needs to be the type where adhesion is not based on thick layer of sticky glue, but removal of trapped air between the two surfaces, where the surfaces need to be flat. Correct type of tape is similar to office tape or packaging tape, or Kapton, with thin glue that does not hold in uneven surface (where duct tape might be better), but gives great longitudinal pull strength with smooth surfaces.

[Ian.M]

I'd trust Kapton tape to stick and stay stuck on clean smooth surfaces (except low surface energy plastics, which require special pretreatment). Its also reasonably strong and tear resistant.

The other office/household tapes suggested may well work initially, (especially glassfibre reinforced packing tape) but you risk their adhesive drying out and crumbling or turning to goo over a period of several years.

[TimNJ]

Depends what's available to you, in your country, but I've never had problems with the cheap GE Silicone II (2) from the hardware store. It does not use acetic acid. I think the main concern with acetic acid is corrosion to copper or other conductors, as far as I know.

GE Silicone I (1) cures with acetic acid, and is cured in about a day. GE Silicone II (2) cures with ammonia, and cures in about a week.

Maybe the Dow/Corning stuff is good but the price is wild. I could never truly justify the price.

[Gyro]

I'd trust Kapton tape to stick and stay stuck on clean smooth surfaces (except low surface energy plastics, which require special pretreatment). Its also reasonably strong and tear resistant.

The other office/household tapes suggested may well work initially, (especially glassfibre reinforced packing tape) but you risk their adhesive drying out and crumbling or turning to goo over a period of several years.

It depends on where you source the Kapton tape mind you, some seem to have very poor adhesive these days.

I wasn't suggesting household tapes, if you've ever pulled a laptop apart, you will find various tapes, including woven fibreglass. One that really comes to mind is the type used to retain the FPCs on the edge connectors of LCD panels, very tenacious but will stick for a second time, and with no residue. I wouldn't advocate Duct tape - nasty creeping white adhesive.

I'm sure something from (genuine) 3M would be suitable.

EDIT: 3M 361 might be a good candidate - Woven glass, high temperature, Silicone adhesive, clean removal.... https://www.3m.co.uk/3M/en_GB/p/d/b40068300/ (only available down to 19mm)

[Siwastaja]

If you use Kapton, get genuine 3M stuff.

In manufacturing, thin light-yellow opaque tape is used to secure ffcs. I believe this is polyester tape with good adhesive. Cheaper than Kapton because you don't need the temperature resistance of polyimide, but OTOH that doesn't hurt.