Hello Technet,

We have an issue of cracking of silicone conformal coating on tin coated pcb.
Please read below whole procedure of our process of wave soldering and 
reflow soldering.

Here is our process from beginning of assembly to end of temperature test:

1- SMT components:

Print  lead free No-Clean solder paste on the PCB.

Put SMT components on PCB.

Oven reflow process to solder components.

2- Trough holes components:

Install through holes components.

Solder the through holes components in lead free solder wave.

Liquid flux (No clean) is sprayed on bottom side of the PCB.

            The PCB goes into pre-heating section.

            The bottom side is dipped in SN liquid alloy at 260’C.

            The PCB is cooled down at room temperature.

3- Tests:

            Electrical/functionality tests are done on each unit.

4- Coating:

            Spray with gun (100% silicone not mixed with thinner).

            1st layer is applied on bottom side of the PCB.

            2nd layer is applied on bottom side of the PCB at least 10 minutes 
after the first layer.

            PCB’s are left to dry 1 hour.

            1st layer is applied on top side of the PCB.

            2nd layer is applied at least 10 minutes after first layer.

            PCB’s are left to dry 1 hour.

            The PCB’s are cured 1 hour in oven at 65’C.

            Inspection with UV light.

5- Temperature test.

            PCB’s are put in environmental chamber at -30’C.   
            After 1 hour, cracks are visible.

            PCB’s are left at -30’C about 15 hours.

            More cracks have appeared.  Air pockets and humidity are present  
under the coating.

            Coating can be rubbed off the PCB using small tool or pick.

 

I’ve included picture of cracked coating and a report of the tests we made.

Thanking you in advance for your help in this matter.

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