Why some parts on PCB are glued together? - Page 1
Forum section: electronics / projects

author: Folxs
author_rank: Contributor
timestamp: November 28, 2016, 04:46:55 pm
content: Hi, I'm wondering why some parts on PCB are glued togeter with white silicone-like glue."Why is it so?"

author: cowana
author_rank: Frequent Contributor
timestamp: November 28, 2016, 04:53:45 pm
content: It's to stop vibration - without it, those larger parts would rattle about and potentially break off at the leads due to stress.

author: helius
author_rank: Super Contributor
timestamp: November 28, 2016, 04:55:25 pm
content: There are three common reasons: protection from high-voltage arcing, vibration resistance, and thermal coupling (as with compensating PTCs/NTCs).

author: wolf32d
author_rank: Contributor
timestamp: November 28, 2016, 04:57:25 pm
content: That rubbery glue provides mechanical damping (absorbs vibrations) and is cheaper than chips (and cheaper than proper mechanical restraints such as brackets, clamps, bolts).Some ultra-cheap chemistries tend to fail miserably after a few hours.

author: Folxs
author_rank: Contributor
timestamp: November 28, 2016, 10:05:12 pm
content: In this case it is power supply from a LCD monitor. So I think, that vibration isn't critical. Someone say that is for preventing high voltage arcing - that could be. And also thermal coupling - big diodes on right, shaunt resistor on the middle, varistors on the left.Thank You for answers!

author: hans
author_rank: Super Contributor
timestamp: November 28, 2016, 10:14:10 pm
content: Vibration is critical considering the cheap hard foam packaging TV's and LCD's are shipped in (they offer little damping). During transport (either postal or car) it needs to take a decent beat from road vibrations and UPS delivery guys throwing boxes over fences.In particular, you don't want rattling 400VDC leads in a product. That could be a nasty fault.

author: tautech
author_rank: Super Contributor
timestamp: November 28, 2016, 10:34:17 pm
content: Quote from: Folxs on November 28, 2016, 10:05:12 pmLook at the size of the leads on those.....any mechanical stress can be transferred to the pads on the PCB IF the through holes are a little oversize. They're tied together to minimise this risk.Normal practice these days.Even when used to excess.Thermal bonding use is less common but still seen.Some products used harden into a porous hard foam state and are hydroscopic and which are responsible for many HV explosions and product failures. The industry has learnt its lesson about those and they are now rarely seen.

author: mariush
author_rank: Super Contributor
timestamp: November 29, 2016, 01:08:46 am
content: Another reason to glue them down is due to the way the components are soldered... the pcb is often part of a bigger pcb which goes on rollers to a wave soldering machine ... the whole bottom of the board is sprayed in advance with liquid flux and then a wave of solder touches the bottom of the board soldering everything. The wave however cab be powerful enough to push up the leads a bit (for lightweight components) and also the rollers on the band moving the pcb to the wave soldering machine can shake the bigger parts enough to get one or both leads out of the holes.