Potting electronics /PCBs

Other Hardware General Electronics
1

Happy holidays from Washington DC!

I'm working on a small electronics project with my sons and could use some insight on a unexpected snag after encapsulating our PCB with a polyurethane resin. We created a small printed circuit board (PCB) and soldered it with a Micro ATTiny85 and Adafruit LoRa module as part of a small receiver. We then separately connected the PCB to a 3V coin battery and LED which lights up when receiving a LoRa signal. It works perfectly. The snag came in when we covered just the PCB with a polyurethane resin from Electrolube which claims to be perfect for electronic potting. The PCB and LED worked after the resin cured but we were surprised that after only 1 day the battery voltage dropped from 3.2V to 1.3V. Yes the battery was connected to the PCB but we certainly didn't think the battery life would drop so much by simply having it wired to the PCB. Do such coatings drain batteries that fast or should we use a different approach to weatherproof our PCB??

Thank you!
Travis

2

You neglected to post the link to the product in question, nor what the battery life was
before potting.

3

How long did the battery last when you tried it on an non coated\potted PCB ?

No information on the battery, PCB, schematic or code either ............................

4

Coin batteries are not noted for having a lot of Amp/hours capacity and you have some sort of RF module that probably uses a lot of current when transmitting - this is a common problem when people use a 9v battery on a project and wonder why the battery life is so short (yes I also did it once way back)

As noted above - did you measure battery life before potting?

5

Be assured the potting had no effect on the battery life problem. It just makes it REALLY HARD to find the source of problems with your project.

Paul

6

Potting should not effect PCB operation, and the UR5604 (if that is what you are using) claims to be flexible.

If it is the potting I would guess:

  1. was not fully mixed and has some uncured material in the assembly.

  2. the board was not cleaned well and dried well before potting.

Perhaps further curing time might help, especially if under elevated temperature (think incandescent work light pointed at assembly).

Question: What are you hoping to gain by potting? Will your device be in direct contact with water (i.e. rain etc)?

John

7

I did a little research and it seems polyurethane has a large dielectric constant (9 to 11 or so) and is
very lossy (power factor 5% or more), so its unlikely to be suitable for any RF circuitry, but that
doesn't really explain battery life issues.

Its quite likely it will completely detune the antenna matching circuit on the LoRa module, which conceivably
could cause large VSWR variation and damage the tranmitter output stage, which then could conceivably
lead to power drain.

8

@3Dtj

After reading MarkT's post it is not clear if you were potting the LoRa module or just the PCB. I'm not sure it matters regarding battery usage but would like to understand.

John