Straightforward Explanation of MIL-I-46058C for Conformal Coatings

During your selection of conformal coating you ran in to a specification that you have seen before but not fully understood.  Conformal coating choices are vast, but with so many available, how does a user pick the correct one? Are there minimum standards that define what a conformal coating is supposed to do? Thankfully, yes there are. This column will focus on one such standard: MIL-I-46058C.

The official title for the specification is MIL-I-46058C, “Insulating Compound (For Coating Printed Circuit Assemblies).” The standard serves as a material standard, used to evaluate and document that a particular coating meets a list of specific performance attributes (more on those later). MIL-I-46058C was developed to define a uniform set of test methods and performance requirements for conformal coatings and gives users confidence that the material they select will perform.

MIL-I-46058C is managed by the Defense Logistics Agency (DLA).

DLA manages the standard and maintains the associated Qualified Products List (QPL). For a coating to be placed on (and stay on) the QPL, it must be tested annually by a DLA-certified laboratory. The data are reviewed annually by DLA to ensure that each coating proposed for inclusion on the QPL still meets the requirements of the standard. The latest version of the Qualified Products List is available from www.dscc.dla.mil.

MIL-I-46058C evaluates conformal coatings to an extensive list of properties. The tests are:

  1. Curing time and temperature: Coating must meet the requirements of the standard when cured according to the manufacturer’s instructions.
  2. Appearance: When cured per manufacturer’s instructions, coating should be smooth, homogenous, transparent, free of bubbles, pinholes, etc.
  3. Coating thickness: The recommended coating thickness shall be 1-3 mils for acrylic, urethane and epoxy coatings; 2-8 mils for silicone coatings; 0.5-0.7 mils for parylene coatings.
  4. Fungus resistance: The coating cannot support the growth of fungus.
  5. Insulation resistance: The average insulation resistance of all coated specimens shall be a minimum of 2.5 x 1012 ohms, with no specimens having a measured value below 1.5 x 1012 ohms.
  6. Dielectric withstanding voltage (DWV): Coated specimens subjected to 1500 VAC for 1 minute shall exhibit no disruptive electrical discharge (spark over, flashover or breakdown). The measured leakage current shall not exceed 10 microamperes.
  7. Q (Resonance): The resonance values for the coating, measured before and after immersion in DI water, must not change beyond specified limits.
  8. Thermal shock: Coating materials are subjected to 50 cycles of thermal shock. After thermal shock, coating must meet the Appearance and DWV requirements.
  9. Moisture resistance: The insulation resistance of the coating is measured under high temperature and humidity and must meet minimum specified values. After temperature / humidity exposure, the coating must meet the appearance, insulation resistance and DWV requirements.
  10. Flexibility: Coating is applied to a test substrate, cured per manufacturer’s instructions and bent 180° over a 0.0125” diameter mandrel. There shall be no evidence of cracking, crazing or adhesion loss of the coating.
  11. Hydrolytic stability: Coated specimens are subjected to four 28-day exposure of 85°C / 90% RH. After this exposure, the coating can show no evidence of softening, chalking, blistering, cracking, tackiness, adhesion loss or reversion to liquid state. The coating must also remain transparent enough to view nomenclature and color codes used to identify the components over which the coating is applied.
  12. Flame resistance: Coating shall be self-extinguishing and non-burning when subjected to a flame test.
  13. Shelf life: Coating must meet appearance, insulation resistance and DWV when tested after storage for six months at 25°C.

MIL-I-46058C was declared “inactive” in November 1998. This deactivation meant the standard was “inactive for new designs, except for replacement purposes.” This certainly does not mean MIL-I-46058C disappeared from the landscape. Today, MIL-I-46058C persists for coating users and specifiers due to its requirement for independent third party certification and remains the only published conformal coating standard with an associated QPL.

Caution!  As a user you have many choices of conformal coatings. Many materials claim to “meet the requirements’”of MIL-I-46058C. These are coatings have probably not been fully tested to the rigorous standard required to obtain and maintain MIL-I-46058C qualification. Treat these coatings with caution: “meets” does not mean “is” on the QPL. There is only one way to verify this and that’s through the DLA website.

The conformal coating selection process involves a check list of many variables. One of these variables is the need for MIL-I-46058C qualified coating. We hope that this article helps you understand not only how to find such a coating, but also what is behind this qualification.

Jeff Sargeant
www.humiseal.com

Simple Recipe to Avoid Conformal Coating Blush

Your conformal coating film is supposed to be clear and transparent, but it’s cloudy-white and dull … it’s blushing. This condition is an indication that your coating film is trying to tell you something, and it isn’t that it’s embarrassed.  This blog will help you better understand why your conformal coating film is blushing from acquired moisture contamination and help you eliminate this defect.

Coatings appear cloudy because they have inadvertently acquired moisture, either due to their hygroscopic nature or by artificial means (a cooling mechanism created by solvent evaporation), which then reacts negatively with the coating resin –  exhibiting itself as a milky, colloidal-type substance.  Many different types of conformal coating chemistries can be sensitive to blushing under certain circumstances.  These circumstances are usually related to specific ambient conditions, application, cure, condition of the assembly/substrate/associated components, storage environment and/or equipment under use.

blush-conformal-coating-example

These conditions include:

  • High humidity, ~70% or higher
  • Low (or cooling) temperatures, ~16oC or below
  • Assemblies and associated components exposed to ambient moisture during storage prior to coating, can significantly increases the chance of blushing.  To overcome this, baking is often recommended. Not baking your boards after cleaning is a prime source of moisture uptake into assemblies
  • Storage conditions (opened containers, inside pressure pot, RH resident in the charging gas of the application equipment) and interaction with moisture already on the assembly.

blush-conformal-coating-example-1

In conjunction with this variety of circumstances, certain solvent-borne chemistries can aggravate this condition due to the fast evaporation of internal solvents.  This creates a cooling mechanism as they volatize from the film.  This cooling mechanism will condense any moisture vapor that may be present in the immediate atmospheric area.

High solids, low volatile room temperature vulcanized, or UV conformal coatings (with secondary moisture cure functionality) may also show some degree of “blush.” They are more sensitive, possibly hygroscopic, to the presence of ambient moisture vapor as this is utilized to activate their primary or secondary curing mechanisms.

Problems that can be created due to moisture vapor intrusion/blushing:

  • Surface tackiness/incomplete cure
  • Below standard aesthetic quality
    • Coating discoloration
    • Poor gloss retention
  • Poor adhesion
  • Wrinkling during second coat application/over-coating.

Here are some simple steps to start with … DIY:

  • Maintain nominal ambient humidity in the production environment between 45% and 65%.
  • Maintain nominal ambient temperature in the production environment between 18oC and 27o
  • Increasing the flash time between coating application and cure may help, as moisture vapor in the wet film may have the potential to re-evaporate
  • Use different thinner to change evaporation rate.
  • Use desiccated air/nitrogen to push coating through applicator
  • Post-bake assemblies after aqueous wash (if applicable) to eliminate any entrapped (under or between components) moisture.
  • Ensure that all partial containers are properly sealed when stored. Nitrogen purge in partially used container helps to eliminate moisture.

We hope that this helped you understand what blushing is, how it occurs, and how to prevent/alleviate it from occurring.  Remember that moisture is the root cause of cloudy or blushing conformal coating.  With many potential sources of moisture contamination, following our simple recipe will lead you back to a crisp, clear, and transparent film, and zero defects.

Nick Naumovic

www.humiseal.com