PCB - Surface Treatments

I was interested to see how important surface treatment was when PCB's are exposed to the elements.

My approach is take a LED flasher circuit board which can be left undisturbed for months, apply a treatment, and leave it exposed to the elements. 

Initial exposure to the elements was mid-day on 22/11/2022. That night a fierce electrical storm hit and gave everything a good drenching. Several such weather events took place over the test period.

    Date            Status

    22/11/22    5 Boards with different treatment and one board without treatment.

                       Placed outside in the open, surface mount component side facing upwards. 

    20/12/22    All 6 boards still functioning 

    23/2/23       Results after 3 months of harsh summer weather and several very wet rainfall events
                       as follows:

 

 

The test fixture after 3 months.

Conclusion:

Not a definitive test by any means. However, it appears any kind of treatment you have to hand is going to be better than no treatment when a PCB is exposed to the elements.

For low cost battery operated sensors housed in any kind of container I'd probably not bother. The life of such devices is several years at best, not decades, and by the time the battery needs replacing I suspect the device will be technologically obsolete.

Do you have any experience you'd like to share? I look forward to hearing your comments below.

73's

10W Class AB with parallel transistors - Not yet, but 5W is OK

It appears a cheap, stable and effective 5W HF amp can be realised with parallel SOT89's and thermal bridges. 

 

A variety of transistors have been tested. Some could deliver 10W continuously for extended periods thanks to the thermal bridges and heatsink. However, none appeared capable of delivering 10W with low distortion.

 

Gain and power handling varied between transistor types. Some test boards could deliver well over 10W into the dummy load with no mishap. Harmonic distortion was present so a good low pass filter is essential. No issues with thermal runaway were noted.

 

I learned the hard way that a 1206 resistor had an insufficient power rating as an emitter ballast resistor when testing for extended periods. After replacing several sets of resistors I switched to a larger SMD resistor.

Having destroyed many sets of SOT-89 transistors I have concluded the SOT-89 package is not going to deliver 10W of RF continuously.  Using 10 parallel transistors the per package dissipation is around 2W (1W of Rf requires at least 2W of electrical energy). At times I saw 5 Amps of total collector current on the "guess" meter fitted to the power supply which was just too much for many of the devices. 

While several transistor types could deliver 5W with low distortion that wasn't my goal. So it's time to move on to a SOT223 package.

73's