What´s the purpose of the white gunk? - Page 1
Forum section: electronics / projects

author: arildj78
author_rank: Regular Contributor
timestamp: December 02, 2022, 12:15:44 am
content: What is the purpose of the white gunk on the solder terminals of both the inductors on this dual channel DC-DC converter?The rest of the board has conformal coating, but for some reason theses inductors have been given special attention.

author: twospoons
author_rank: Frequent Contributor
timestamp: December 02, 2022, 12:19:14 am
content: Looks like glue intended to stop the inductors falling off under vibration.They are heavy compared to their solder pad area, and as a result more vulnerable to joint failure under vibration.

author: gnuarm
author_rank: Super Contributor
timestamp: December 02, 2022, 01:58:47 am
content: The white, soft stuff is not going to do diddly for a part that is soldered onto the board. The soft stuff would allow the inductors to move without becoming detached, perhaps, but only after the much more rigid solder has long since broken.

author: Bud
author_rank: Super Contributor
timestamp: December 02, 2022, 02:48:13 am
content: To dampen the inductors' singing ..?

author: Swainster
author_rank: Frequent Contributor
timestamp: December 02, 2022, 03:07:29 am
content: Hold the inductors in place during THT component assembly?

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: December 02, 2022, 04:42:16 am
content: Whoever worked on that board is clearly quite excited about inductors

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 02, 2022, 04:56:43 am
content: My best guess is they put it there to confuse arildj78.

author: coppercone2
author_rank: Super Contributor
timestamp: December 02, 2022, 05:46:10 am
content: I would not worry about it

author: Siwastaja
author_rank: Super Contributor
timestamp: December 02, 2022, 11:25:06 am
content: It's magicalcargo cult gunk. Somebody thought adding it next to the inductors (barely touching them) magically stops the inductors from "falling off". The inductors however are of SMD type where the mechanical connection between the pads and the body is rigid. Also solder is rigid. As a result, any soft glue around it won't take any mechanical load or shock.The same approach is more successfully used to glue leaded components (inductors, electrolytic caps) together or to the PCB. This works when the component otherwise flaps in the breeze because it has flexible leads (the legs).

author: Kasper
author_rank: Frequent Contributor
timestamp: December 03, 2022, 07:46:45 am
content: Quote from: twospoons on December 02, 2022, 12:19:14 amI'm guessing this is right. I used to apply RTV that looked like this to smt electrolytic caps, SD cards and clips for RAM, for avionics equipment.Not sure if it was for joint failure or pad/pcb adhesion failure. I ocassionally had to remove some of those caps and it was relatively easy to remove the pad from the PCB, compared to anything else I've worked with.What product did this come out of?

author: arildj78
author_rank: Regular Contributor
timestamp: December 05, 2022, 08:37:08 pm
content: It is from a Milwaukee M18 (18V) battery to 12V/USB adapter.https://www.milwaukeetool.eu/en-eu/heated-jackets-system-attachments/The surfaceof the gunk is similar to silicone, but it can be removed a bit easier and comes off in chunks when scraped away.

author: wraper
author_rank: Unknown
timestamp: December 05, 2022, 08:54:45 pm
content: Actually it helps quite a bit. SMD power inductor terminals ar not as rigid as you may think.

author: gnuarm
author_rank: Super Contributor
timestamp: December 06, 2022, 01:58:54 am
content: Quote from: wraper on December 05, 2022, 08:54:45 pmThe solder connection will be very solid and rigid. However terminal structure may not be so rigid. Often they are metal tabs bent under the part, and could have some flex, I suppose.I expect this is one of those lessons learned by experience.

author: arildj78
author_rank: Regular Contributor
timestamp: December 06, 2022, 06:06:07 am
content: So to sum it up;The gunk provides extra mechanical stability for heavy parts to avoid fatigue in the solder joints and legs/terminals of the part?

author: wraper
author_rank: Unknown
timestamp: December 06, 2022, 09:06:41 am
content: Quote from: gnuarm on December 06, 2022, 01:58:54 amParticular type of inductor also has very small footprint. So PCB pads can be torn off.

author: wraper
author_rank: Unknown
timestamp: December 06, 2022, 09:13:49 am
content: Quote from: arildj78 on December 06, 2022, 06:06:07 amYes, also it helps from inductor flying off when device is dropped.

author: arildj78
author_rank: Regular Contributor
timestamp: December 07, 2022, 05:21:49 am
content: Does gunk have a real name and a part number at DigiKey?

author: wraper
author_rank: Unknown
timestamp: December 07, 2022, 12:51:45 pm
content: It's either neutral RTV silicone or two component silicone. Electronics grade neutral RTV silicone is quite expensive and usually comes in large tubes. I previously bought stuff from ACC silicones (now CHT), like AS1723 which I couldn't spend before more than half of the tube gone bad. But later shifted to 70X types (701-708) from Aliexpress, which can be purchased in small tubes.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 07, 2022, 04:25:11 pm
content: I'm still sceptical of the effectiveness of the silicone so far off the sides. Most of it is just touching the leadframe, which is rigidly soldered, and not the inductor body. You usually see silicone being used in larger components where leads can bend significantly in relation to the component body, and the silicone is then applied between the board and component body (not the leads, like in this picture).When SMD parts need added mechanical support, one usually sees one of the two:1) underfill for parts like BGA packages, this is thin polymer fluid which is applied after assembly right next to the component and flows by capillary action into the small gap, totally filling the voids between the component and PCB, and then is cured using heat2) glue which is applied to the board by a CNC dispenser in small dots,beforepick&place, which is cured during reflow.Both are almost impossible to see from the finished board.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 07, 2022, 07:18:39 pm
content: Yes it's silicone-based and, yes I too find the dispensing of some silicone just over the inductors' pads a bit odd. I would have either dispensed it all around the inductors, or none at all.

author: aduinstat
author_rank: Contributor
timestamp: December 07, 2022, 11:30:48 pm
content: Quote from: arildj78 on December 07, 2022, 05:21:49 amRTV162 is what all the assemblers I talked to use. It's kinda expensive though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2022, 11:40:26 pm
content: So in summary:- Strain relief for the bulky component.- But the material is most likely too soft to do anything.I'd say it's an erroneous choice by the assembler or designer. Epoxy, superglue or other more rigid substance would be fine though.Quote from: Siwastaja on December 07, 2022, 04:25:11 pmI've seen a hybrid approach: red gunk squirted liberally around the corners of the (BGA) chip, of firm consistency -- possibly component glue itself, just put on much thicker than the tiny dab you'd use underneath small chip components. (I don't think the same chip was underfilled, but I'm not sure; I didn't try to remove it or anything.)Tim

author: TimNJ
author_rank: Super Contributor
timestamp: December 07, 2022, 11:58:38 pm
content: We use tons of electronics RTV at work...itdoeswork. e.g. Plenty of examples of shock & vibration tests which failed (transformers, inductors shearing their pins, transformers getting knocked right off the board (non-plated thru-holes worse offenders). Add RTV and largely is fixed.Probably the key thing is using the right RTV. The stuff we use cures to almost a rubber tire hardness...well, maybe a little softer than that, but close.I don't think using silicone RTV sealant from Home Depot is going to do very well. It's far too soft.

author: aduinstat
author_rank: Contributor
timestamp: December 08, 2022, 12:14:12 am
content: Quote from: TimNJ on December 07, 2022, 11:58:38 pmRTV from Home Depot will release acetic acid as it cures and corrode your components.

author: wraper
author_rank: Unknown
timestamp: December 08, 2022, 12:52:54 am
content: Quote from: aduinstat on December 08, 2022, 12:14:12 amIt will not if you buy neutral cure stuff. However it's still would not be the best choice. Dielectric properties not rated, likely some anti-mold additives added. Likely too soft.

author: TimNJ
author_rank: Super Contributor
timestamp: December 08, 2022, 01:21:17 am
content: If you are inclined to use RTV from Home Depot for your electronics, try GE Silicone II, not I. But they both kind of suck though, way too soft.

author: alanambrose
author_rank: Frequent Contributor
timestamp: December 08, 2022, 05:57:02 pm
content: I think that it acts like a damper for high vibration circumstances. Probably the pad/solder combination by itself will work harden and crack after a while with the vibration energy. So this stuff absorbs some of the max amplitude vibration.Alan

author: arildj78
author_rank: Regular Contributor
timestamp: December 08, 2022, 08:42:57 pm
content: Is there a video to be made here @EEVblog?

author: gnuarm
author_rank: Super Contributor
timestamp: December 09, 2022, 08:30:34 am
content: Quote from: alanambrose on December 08, 2022, 05:57:02 pmThe gunk can't absorb any of the vibration, because the part isn't actually vibrating until the solder cracks... unless the leads allow movement. Then the problem is the leads breaking, which is my guess as to what is being protected, rather than the solder joints.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 09, 2022, 07:33:52 pm
content: Notice the contrast of examples: for the SMT part, leads are short and stiff, and a rigid resin is required to further secure it.For a THT part, the leads have some length, so the part can flex against the board. It might also have enough pins that it's rather well tied into the board, so the board itself flexes more (a potential hazard for nearby ceramic chip caps, say!). Consider a big ETD39 laying flat with dozens of pins, and now compare to the case of something floppy, like a THT electrolytic capacitor. The former might not need anything, while the latter might benefit most from a softer resin, as the peak deflection could still be within the elastic range (little/no lead fatigue), whereas a very rigid compound might.. tear the capacitor's skin away, or something?Maybe that's not the best contrasting example, I've certainly seen caps glued to boards with fairly stiff glue, and at that, mainly because the layer was thin. The skin / jacket is usually pretty tough by itself. But you can imagine components that can't be pulled just any which way, that might be damaged if glued improperly and subjected to mechanical shock.Conversely, soft resin on a rigid component, just doesn't do much. It's adding a large inductor in parallel with an already small one, it does nothing. (Note that electrical analogies are no accident, there is a perfect equivalence between inductance and mass, resistance and, erm, dashpot, and capacitance and spring. Mechanical systems are less often reasonable to treat as lumped element equivalents, because of the fairly low speed of sound and many degrees of freedom, but it can still be illustrative of some cases.)Tim