Additionally I have seen some silicones and polymers suffer cure
inhibition/poor adhesion on high tin content alloys especially if there are
any tin compounds on the tin. These soft spots and low adhesion areas
look/behave similarly to poor cure overall but are localised.
Regards
Mike
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Tuesday, June 09, 2009 1:45 PM
To: [log in to unmask]
Subject: Re: [TN] cracking of coating
Your coating is not cured. Check instruction of the product about tack time
and full strength develop time. Contact your coating vendor application
engineer.
--------------------------
Sent using BlackBerry
----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Tue Jun 09 07:36:40 2009
Subject: [TN] cracking of coating
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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