Re: How to protect circuit boards that must be exposed to weather?

On 2024-08-06 3:27 p.m., Glen Walpert wrote:

On Thu, 1 Aug 2024 17:50:29 -0700, John Robertson wrote:
 

I have another problem - display panels on an outdoor hammer game - the
style that you swing a hammer at a pedestal and it lights up a column of
LED panels with 8x8 graphics.
>
The one I am repairing was made in 2001 and it is breaking down. The
surface mounted serial drivers are loosing connection to the PCBs from
being thermally stressed over a couple of decades of outdoor use and I
want to make replacement panels (the original company has abandoned the
product) that will be more durable.
>
Currently on the rear (somewhat protected) side of the board is the
electronics, covered (or course) with a conformal coating. The outer
side is an 8 X 8 matrix of LEDs in groups of 8 for each pixel of the 64
pixels on the panel.
>
There are 40 - 8 x 8.75 inch panels going up around 30 feet...
>
Is there a better way to solder on SMD devices to help avoid thermal
solder failure? After connection corrosion, the biggest problem is
solder failure of the SMD pads to the PCBs.
>
Thanks!
>
John :-#)#

 If the column of lights is attached to the same base as the hammer target,
there could be significant solder fatigue from the vibrations traveling up
the light column.

No, the tower is on its own, no risk of shock at least!

 Switching to lead-free will improve fatigue resistance.  While there are
lead-free alloys specifically designed for good fatigue resistance, and
any solder mfgr app engineer could make a recommendation, the standard
SAC305 is a significant improvement over tin-lead according to numerous
test results published in the industry trade magazines like SMT
Technology.

Thanks for that information!

 Stiffening the boards with a thicker material or with stiffeners will
reduce thermal bowing from sunshine.  Leaving all unused area covered with
copper ground plane and white or light colored soldermask might help.

Copper ground plane is planned.

 Flexible mounting to isolate vibration might help, perhaps clamp between
silicone foam strips with rubber bushings around the screws or similar,
could also provide sealing of the back side.

It is not so much vibration as much as thermal changes over a day in the sun...times many days!

 Flexible conformal coating will prevent the coating from stressing solder,
as can happen with rigid materials due to differential thermal expansion.

Conformal coating is part of the problem for repairs. While it is flexible it doesn't peel off easily at all - which I guess is the intent - but this does make it harder to repair. Are there conformal coating that respond well to a solvent that isn't toxic?

 As Buzz mentioned, underfill can help if your assembler has the
capability.

I will look into that too.

 Regards,
Glen

Thanks for taking the time!
John :-#)#
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