Author Topic: What´s the purpose of the white gunk?  (Read 4106 times)

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Offline arildj78Topic starter

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What´s the purpose of the white gunk?
« on: December 02, 2022, 12:15:44 am »
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.
« Last Edit: December 02, 2022, 12:18:17 am by arildj78 »
 

Offline twospoons

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Re: What´s the purpose of the white gunk?
« Reply #1 on: December 02, 2022, 12:19:14 am »
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.
 

Offline gnuarm

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Re: What´s the purpose of the white gunk?
« Reply #2 on: December 02, 2022, 01:58:47 am »
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. 
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Online Bud

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Re: What´s the purpose of the white gunk?
« Reply #3 on: December 02, 2022, 02:48:13 am »
To dampen the inductors' singing ..?
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Offline Swainster

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Re: What´s the purpose of the white gunk?
« Reply #4 on: December 02, 2022, 03:07:29 am »
Hold the inductors in place during THT component assembly?
 

Offline Geoff-AU

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Re: What´s the purpose of the white gunk?
« Reply #5 on: December 02, 2022, 04:42:16 am »
Whoever worked on that board is clearly quite excited about inductors
 
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Offline Doctorandus_P

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Re: What´s the purpose of the white gunk?
« Reply #6 on: December 02, 2022, 04:56:43 am »
My best guess is they put it there to confuse    arildj78.
 

Online coppercone2

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Re: What´s the purpose of the white gunk?
« Reply #7 on: December 02, 2022, 05:46:10 am »
I would not worry about it
 

Offline Siwastaja

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Re: What´s the purpose of the white gunk?
« Reply #8 on: December 02, 2022, 11:25:06 am »
It's magical cargo 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).
 
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Offline Kasper

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Re: What´s the purpose of the white gunk?
« Reply #9 on: December 03, 2022, 07:46:45 am »
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.

I'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?

 

Offline arildj78Topic starter

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Re: What´s the purpose of the white gunk?
« Reply #10 on: December 05, 2022, 08:37:08 pm »
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.
« Last Edit: December 05, 2022, 08:44:22 pm by arildj78 »
 

Online wraper

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Re: What´s the purpose of the white gunk?
« Reply #11 on: December 05, 2022, 08:54:45 pm »
Actually it helps quite a bit. SMD power inductor terminals ar not as rigid as you may think.
 
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Offline gnuarm

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Re: What´s the purpose of the white gunk?
« Reply #12 on: December 06, 2022, 01:58:54 am »
Actually it helps quite a bit. SMD power inductor terminals ar not as rigid as you may think.

The 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.
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Offline arildj78Topic starter

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Re: What´s the purpose of the white gunk?
« Reply #13 on: December 06, 2022, 06:06:07 am »
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?
 

Online wraper

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Re: What´s the purpose of the white gunk?
« Reply #14 on: December 06, 2022, 09:06:41 am »
Actually it helps quite a bit. SMD power inductor terminals ar not as rigid as you may think.

The 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.
Particular type of inductor also has very small footprint. So PCB pads can be torn off.

« Last Edit: December 06, 2022, 09:08:17 am by wraper »
 

Online wraper

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Re: What´s the purpose of the white gunk?
« Reply #15 on: December 06, 2022, 09:13:49 am »


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?
Yes, also it helps from inductor flying off when device is dropped.
 

Offline arildj78Topic starter

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Re: What´s the purpose of the white gunk?
« Reply #16 on: December 07, 2022, 05:21:49 am »
Does gunk have a real name and a part number at DigiKey?
 

Online wraper

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Re: What´s the purpose of the white gunk?
« Reply #17 on: December 07, 2022, 12:51:45 pm »
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.
« Last Edit: December 07, 2022, 12:53:48 pm by wraper »
 

Offline Siwastaja

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Re: What´s the purpose of the white gunk?
« Reply #18 on: December 07, 2022, 04:25:11 pm »
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 heat
2) glue which is applied to the board by a CNC dispenser in small dots, before pick&place, which is cured during reflow.

Both are almost impossible to see from the finished board.
« Last Edit: December 07, 2022, 04:40:38 pm by Siwastaja »
 

Online SiliconWizard

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Re: What´s the purpose of the white gunk?
« Reply #19 on: December 07, 2022, 07:18:39 pm »
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.
 

Offline aduinstat

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Re: What´s the purpose of the white gunk?
« Reply #20 on: December 07, 2022, 11:30:48 pm »
Does gunk have a real name and a part number at DigiKey?

RTV162 is what all the assemblers I talked to use. It's kinda expensive though.
 

Offline T3sl4co1l

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Re: What´s the purpose of the white gunk?
« Reply #21 on: December 07, 2022, 11:40:26 pm »
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.


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 heat
2) glue which is applied to the board by a CNC dispenser in small dots, before pick&place, which is cured during reflow.

Both are almost impossible to see from the finished board.

I'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
« Last Edit: December 07, 2022, 11:43:46 pm by T3sl4co1l »
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Offline TimNJ

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Re: What´s the purpose of the white gunk?
« Reply #22 on: December 07, 2022, 11:58:38 pm »
We use tons of electronics RTV at work...it does work. 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.
 
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Offline aduinstat

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Re: What´s the purpose of the white gunk?
« Reply #23 on: December 08, 2022, 12:14:12 am »
We use tons of electronics RTV at work...it does work. 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.

RTV from Home Depot will release acetic acid as it cures and corrode your components.
 

Online wraper

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Re: What´s the purpose of the white gunk?
« Reply #24 on: December 08, 2022, 12:52:54 am »
We use tons of electronics RTV at work...it does work. 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.

RTV from Home Depot will release acetic acid as it cures and corrode your components.
It 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.
 


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