Normally, our coating thickness is around 3mils with Solithane 113 and Uralane 5750. Process control is critical , e.g. spray or manual coating processes.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee Hitchens
Sent: Monday, 28 October 2013 10:40 PM
To: [log in to unmask]
Subject: Re: [TN] Exceeding Maximum thickness of urethane conformal coating
HI Phil
Phil mentioned the cleanliness and this is one of the key factors when coatings wet out. If you get bad wetting or too much fluid getting scavenged around devices due to strange surface tension behaviour then you get extreme build up of coating around leads and devices which leads to cracking.
So, as Phil and Doug state get your coating on as laminar as possible and it shouldn't be a problem. How are you intending to or are apply the coating?
This can be a critical factor in the whole process?
Best Regards
Lee Hitchens
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Kinner
Sent: 28 October 2013 14:34
To: [log in to unmask]
Subject: Re: [TN] Exceeding Maximum thickness of urethane conformal coating
Hi Phil,
As Lee suggested, the main thing you should be looking for is coating coverage. At the higher end of the thickness range you mention, many of the commonly used Urethane materials will be susceptible to cracking. Multiple thin coats will always give better and more uniform coverage than 1 thick (or even 2 thickish) coat(s) and this is what you should be looking for.
When you factor in the possible extended curing times (especially oxidative cure systems), potential reliability issues (mentioned by Doug) and the additional cost of the materials, over and above what you really need, then I would say that 3 mils should be your maximum nominal thickness, and work hard on your process to get better coverage and uniformity at lower thicknesses.
Lee referred to the work done by Per-Erik Tegerstrom at the IVF. I think from memory, that this would have been over Tin/Lead solder and scrupulously cleaned... if this applies to you great. Most of the work that I have seen more recently have shown at best no-correlation and at worst, a significant correlation between greater coating thickness and significantly reduced thermal shock/cycling lifetimes. If you are coating over no-clean then that that is a whole new can of worms to consider, but the same basic premise, work hard on the process to achieve better coverage and uniformity at thinner coating builds.
Best Regards,
Phil.
Phil Kinner
Conformal Coating Consulting
Desk +44 1483 600293
Cell +44 7946 020614
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: 25 October 2013 20:45
To: [log in to unmask]
Subject: [TN] Exceeding Maximum thickness of urethane conformal coating
I was reviewing a process specification regarding Type UR conformal coating and it contained a sentence which struck a nerve.
This particular specification stated that the thickness shall be .003"+/-.002" and .010" maximum if two coatings are applied.
I always thought there was a good reason why the coating should not exceed .005" such as CTE mismatch which might cause cracking of the coating or perhaps worse, component damage in thermal cycling. But I haven't found any papers to that effect yet.
Naturally, any comments/experiences from this forum will be appreciated, especially if I am completely off based.
Phillip Bavaro
Senior Manufacturing Engineer, Manufacturing Engineering
L-3 Telemetry-West
9020 Balboa Ave. | San Diego, CA 92123
858-694-7874 | 858-740-8260 mobile
[log in to unmask]<mailto:[log in to unmask]> | L-3com.com/TW
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