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From: John Robertson <jrr@flippers.com>
Newsgroups: sci.electronics.design
Subject: Re: How to protect circuit boards that must be exposed to weather?
Date: Thu, 8 Aug 2024 14:49:34 -0700
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On 2024-08-07 6:14 p.m., Glen Walpert wrote:
> On Wed, 7 Aug 2024 14:34:14 -0700, John Robertson wrote:
> 
>> 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!
> 
> Board stiffeners to reduce warping, compliant mounting to allow thermal
> expansion without restraint, and cooling with a fan blowing up the column
> should all help some with that.

Drat, I didn't think of that as a problem. These panels are currently 
screwed (4 screws on each of two sides) to an aluminum column...it makes 
sense that the column expands with temperature changes and if the boards 
are secured tightly (8 screws/panel) then this could be part of the 
issue. Flexible mounting needs looking into!

> 
>>> 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?
> 
> The non-toxic part is the issue there.  Miller-Stephenson sells a
> conformal coat stripper which will remove all coatings except epoxy which
> they claim is environmentally friendly but I haven't used it or read the
> MSDS:
> 
> https://miller-stephenson.com/chemicals/conformal-coating/
> 
> I have used their MS-460H - soft silicone, solder thru (messy), easily
> scraped off. easy poke-through with a sharp probe, as well as the similar
> Chemtronics Konform SR which strips with xylene or acetone:
> 
> https://www.chemtronics.com/conformal-coating-selection-guide

I'll take a look into that.

> 
>>> As Buzz mentioned, underfill can help if your assembler has the
>>> capability.
>>
>> I will look into that too.
> 
> Underfill is generally epoxy, glues the components to the board, great for
> shock and vibration but terrible for rework.

I think with new panels that securing the chips to the board with 
underfill might well be a good idea.

>   
>>> Regards,
>>> Glen
>>
>> Thanks for taking the time!
>>
>> John :-#)#
> 

Thanks!

John :-#)#

-- 
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