encapsulating diodes

[kelley]

Hi everyone,

Does anyone have any recommendations for a suitable material to encapsulate some diodes on a circuit board? These are not glass diodes by the way.

Thanks

kelley

[Suhas]

Hi Kelley,
So what are the diodes made of? Specific material? Also were you looking to mold of any other process would do? Honestly, I am not going to know the answer but I am curious.
Thanks,
Suhas

[tfrounfe]

Hello Kelley,

I'm interested to see if anyone has experienced this as well. I have insert molded circuit boards but they did not have any surface mounted components on them yet.

Trevor

[kelley]

We should have the info Monday on exactly what type of diodes they would like encapsulated.

kelley

[brentb]

Acetal (Delrin or Celcon has been used for encapsulation
Can you give more info on application and end-use environment, like in a gas tank or ambient air enclosure. If you want a specialty and /or heat dissapative resin, contact Kyocera. (they don't just make telephones)
KOM

brent

[kelley]

Acetal (Delrin or Celcon has been used for encapsulation
Can you give more info on application and end-use environment, like in a gas tank or ambient air enclosure.
brent

Hi Brent,

End use is automotive, mounted in cabin. That's really all I can say at the moment.

kelley

[ATI K ID]

PA 6/6 has good heat and chemical resistance. I make electric terminal connections with that.

[kelley]

Sorry for taking so long to update this project. We did settle on acetal for the encapsulation material and we've had some success encapsulation the one by two inch circuit board with diodes.

Now the bad news, our engineer just called me at home to tell me the samples we placed in thermal cycling (-40C to 100C) are cracking, basically he's saying the corners are cracked on most of the parts.

I suspect the plastic is shrinking more than the circuit board during the cold portion of the test, I'm not real sure where to make changes to the process to correct this. The biggest problem I'm having process wise is the minimal pack and hold pressures I'm having to use due to flashing that is occurring around the wires coming off the board.

I'm thinking I need to pack this part more to prevent this shrinking, but I'm not sure.

Any ideas?

Thanks

Kelley

[rickbatey]

POM needs to be packed for a long time, with high pressure, and typically a hot mold. This is due to the huge amounts of shrinkage that POM under goes. POM loses something like 25% of its volume while cooling. You also need the gate to be large enough to stay open for the continued shrinkage to be packed out.
If you have square corners and sharp features molded into the parts, you will have issues as POM is sensitive to molded in and features that increase stress.
Rick.

[kelley]

Thanks for the quick reply Rick. And you are correct, the tool right now does have square corners and sharp edges along the surface where the cracking is occurring.

The tool is going back to the mold shop tomorrow for some changes anyway, I will discuss opening the gate and adding radii to all the sharp corners.

Thanks again Rick


Kelley

[rickbatey]

Kelley,
If you haven't done so yet, sign up at DuPont for their free processing and design guidelines. They have great information and you will look like the Hero after studying to info presented.
A former plant I worked in ran a POM part, safety, for automotive. We had to remove the sub-gate and run a tab gate to get rid of the jetting. Next step was a degating station with air logic and pneumatic routers to run the cutters. Now I had great looking parts; I needed to get more parts out. Following the processing guide I was able to destroy the quoted cycle time without part failures or cosmetic issues.
Rick.

[kelley]

Rick,

I actually have that design guide. We did send the mold back and got rid of the sub gate and went to a tab gate. We ran some more parts and put them in thermal cycling and still had cracking. About -25f seems to be where the cracking begins.

We're sampling some filled materials right now and are having some success. I'll know a bit more in the morning.

Kelley

[kelley]

I just wanted to give an update for those who have been helping me with this project. We are successfully encapsulating our diodes and circuit board.

It did take a material change to get it done, but the end result has exceeded our expectations. Thermal cycling has not been and issue at all, we've not seen a hint of any cracking.

We've got an extremely robust process now, one that is forgiving of the slight cycle time variations that are inherent in hand loading parts into a mold, and since moving away from acetal we've been able to lower our mold temps to 100F from 200F, which is a big plus for the operators who are having to load the circuit boards in the mold.

Thanks again everyone!

kelley