1. Engineering Requirements for a Tin- Whisker-Risk-Controlling Conformal Coating to be used in a Missile
A generic presentation by Bill Rollins, representing Rollins Associates
Copyright 3/1/2016

2. Requirements of a Conformal Coating to be Military-grade
Military grade conformal coatings are designed to protect circuit boards from external environments, particularly condensation and conductive debris.
A Missile-grade / Whisker-grade conformal coating needs to be designed to do the same as military grade., … and
protect the circuit board from itself if it has pure tin surface finishes that inevitably will grow tin whiskers, ….and
meet the service life requirements (and some benefits) of a harsh missile environment.
Military-grade coatings will pass MIL-I-46058, and the similar commercial specification IPC-CC-830 testing , conducted at a Defense Logistics Agency (DLA) approved laboratory.
To qualify a coating to MIL-I-46058, 60 test coupons are coated for the required tests. There are fungal-resistance coupons, flexibility coupons, thermal-shock coupons, flammability coupons, Y-pattern electrical resistivity coupons,
Y-pattern high-voltage-breakdown coupons, Y-pattern-with-resistors coupons,
and B-25 coupons (which is an IPC standard test board).
Of particular interest for missile use, with often a 10 year service life, is the four-month 85C at 95%RH thermal-humidity reversion test. Urethane coatings can be susceptible to urethane hydrolysis reversion. This test determines if the coating has chemical stability with long exposure to water vapor at elevated temperatures. If not, it will lose physical properties and become sticky, dark-colored and have low-electrical-resistivity.
If all tests are passed, then the coating is added to the DLA’s Qualified Parts List (QPL) for conformal coatings .

3. Requirements of a Conformal Coating that can be used to control tin whisker risk
The conformal coating must provide complete and adequate coverage of all tin plated surfaces, including sharp edges because they too will grow tin whiskers.
Liquid coatings will thin out (often to nothing) over sharp edges found on component terminations.
The Physics of Fluids causes this: the surface tension over a sharp edge tends to go to infinity as the radius of curvature gets smaller.
The conformal coating must captivate all tin whiskers so that they are constrained under the coating and can not escape.
The coating must be able to captivate both long whiskers and large diameter whiskers, for the many years of the service life of the missile
Large diameter whiskers are the biggest challenge. The Euler buckling force for a whisker increases as the 4th power of the whisker diameter.
If the coating can not buckle a whisker, the whisker grow through it. The coating must captivate all whiskers, even whisker swarms: (see T. Woodrow photo below)

4. Requirements of a Conformal Coating that can be used in a missile
Protecting fragile electronic components from fracture - Many fragile surface-mount components can be damaged by conformal coatings during thermal cycling in missile Qualification testing, Environmental Stress Screening during manufacturing, and normal operational environments for deployed missiles.
Fractures are caused by cumulative fatigue from the cyclical stress the coating applies to the fragile component as the temperature varies.
A mismatch of the CTE of the coating and the CTE of the circuit board is the primary cause of stress
Low modulus coatings are very compliant and do not transfer the stress
Low temperatures can be a problem as it tends to cause compliant coatings to get stiff
A Missile grade / Whisker grade coating should not have caused any damage to a circuit board test vehicle containing coated glass-bodied MELF diodes after 1000 thermal cycles with a range of -50C to +100C.
Chemical degradation of polymer conformal coatings from ultra-violet light, oxygen, ozone, and vapor phase water - Since most missiles are environmentally sealed, and desiccated, the exposure of the polymer coating to these degradation mechanisms is very limited and they are not considered to be service life issues .

5. Requirements of a Conformal Coating that can be used in a missile - 2
Temperature ranges of the coating versus missile storage and operational temperature requirements This is a service life issue for any polymer. Maximum storage high temperature requirements for most missiles are 85C, and occasionally 100C. Maximum circuit board temperatures for continual operation would generally be 115 C, for a variety of reasons.
High temperatures cause Thermal-Oxidative Stress in coatings. Different polymer bonds involving different kinds of atoms in different locations, will begin to break at different temperatures, representing different "Activation energies" for those bond-breaking chemical reactions. Higher temperatures activate different chemical reactions, thus more and different degradation pathways. However, many thermal-degradation products produce broken-bond molecular fragments. Over time these can accumulate to the extent that their presence usually becomes catalytic in breaking more chemical bonds. At that point, the coating has essentially failed. Re-stated, a polymer coating can have a very long service life below certain temperature limits, and a much shorter one if subjected to temperatures above that limit. This “life expectancy” performance of the coating should be compared to the known operating environments of the missile .
Steve Smith may have some comments on the above discussion.
Of equal importance to conformal coatings having a suitably long service life is the fact that you can be sure that the tin whiskers will as well.