Parylene Coating Blog by Diamond-MT

Conformal Coating Specifications:  Guidelines for Design, Selection and Application of Conformal Coatings

Posted by Sean Horn on Fri, Sep 02, 2016 @ 07:30 AM

Understanding the characteristics of various conformal coating types, and their interactions with the extreme range of products and materials to which they are applied, ensures optimal function, performance reliability and product-life.   Designers and users of conformal coatings should be aware of the properties of various types of conformal coatings and their interactions with the parts/materials they cover, to protect the products in their respective end-use environments for the expected design-life of each component.  

In this context, different specifications for conformal coating have been devised to identify and account for differences in operational and material conditions.  End-users providing conformal covering are confronted by problems with coating selection or application that can generate difficulties including mismatched coefficients of thermal expansion (CTEs), trapped moisture, outgassing, corrosion, abrasion or future rework/repair.  For example, someone who manufactures printed circuit assemblies (PCAs) for standard commercial usage, will be best served using the IPC-610 standard for their conformal coating.  However, NASA standards are far more applicable to aerospace projects. IPCMemberLogo.jpg

IPC, the Association Connecting Electronics Industries                    

            Headquartered in Bannockburn, IL (USA), with offices throughout the United States and the world, IPC is a recognized and respected standards-development organization accredited by the American National Standards Institute (ANSI).  Its objective is standardizing assembly and production requirements of electronic components, including those for interconnecting and packaging electronic circuits and assemblies.   Diamond MT has been a member of the IPC since January 2012.

IPC Classifications Pertinent to Conformal Coating

Particularly important is IPC’s Class 3 Standard for Coating Requirements; its guidelines ensure conformally coated surfaces maintain exemplary performance under challenging operational conditions.  

IPC-A-610:  Additionally titled “Acceptability of Electronic Assemblies,” this standard provides visual quality-acceptability requirements for manufacture of electronic assemblies, delineating appropriate PCA/component characteristics that exceed minimal end-item performance criteria. 

Requirements for coating thickness are also set by IPC-A-610, according to the coating material and the function of components being covered.

Table 1

Conformal Coating Application Thicknesses

Coating type                                                                          Application thickness, mm. (in.) 

Acrylic (AR), Epoxy (ER), Urethane (UR)                         0.03-0.13 mm. (0.00118-0.00512 in.)

Silicone (SR)                                                                       0.05-0.21 mm. (0. 00199-0.00827 in.)

Parylene (XY)                                                                      0.01-0.05 mm. (0.000394-0.00197 in.)

            IPC-A-610 also classifies products according to their function.  Class 1 is the least stringent of these categories, proceeding through Class 2, with Class 3 being most rigorous.  More explicitly

  • CLASS 1 applies to the General Electronic Products’ category, including products whose primary requirement is efficient function of the completed assembly.
  • CLASS 2 encompasses Dedicated Service Electronic Products. This category emphasizes the product’s ongoing performance and extended life, under operating conditions where (1) uninterrupted service is preferred but not critical, and (2) the end-use environment would not stimulate failures.  
  • CLASS 3 is for High Performance/Harsh Environment Electronic Products – those that necessitate performance-on-demand and absence of equipment downtime in end-use environments that may be uncommonly harsh. Although CLASS 3 is sometimes known as the ‘aerospace class’ of IPC-A-610, this is not strictly true.  While CLASS 3/IPC-A-610 has many aerospace functions, it also applies to any end-use environment where the equipment must function on-demand, such as life-support, fire-control. or similar critical systems.

It is essential in most cases that aircraft flight controls require CLASS 3 certification.  An aircraft’s fly-by-wire controls must function without fail every time, while being subjected to temperature/vibration extremes at high altitude.  However, CLASS 1/2 assemblies may be more than adequate for such aircraft-internal functions as its entertainment system or communication among flight stewards.  CLASS 3 is mandated in aerospace, military or other end-use environments where the highest quality performance reliance/capacity is required; CLASS 3 is also costly, and is not recommended where other classifications will serve.

IPC-7711/7721: Rework and Repair – This standard delineates acceptable procedures for conformal coating and component removal and replacement; modification/repair of laminate material, conductors, solder, and plated through-holes are also stipulated. 

IPC CLASS 3:  This document provides the Standard for Coating Requirements of the Class 3 High Performance Electronic Products described for IPC-A-610, those components and systems requiring performance-on-demand, frequently for extended durations of operation, including aerospace, life-support, and military applications, as described above. 

IPC J-STD-001 End Item Standards:  Like the MIL-S-45743 it replaced, IPC-J-STD-001 establishes procedures and requirements for achieving optimal levels of performance quality and reliability, for uninterrupted operation in the harshest of end-use environments.  These are minimal end-product acceptable requirements for soldered electrical and electronic assemblies; evaluation methods, testing-frequency, and process control requirements are also described.

IPC-CC-830Qualification and conformance requirements for conformal coatings are the subject of IPC-CC-830.  Devised as a replacement for MIL-I-46058, a military-grade standard, in 1998, it delineates criteria for optimal confidence for coating materials, with the added benefit of minimum test redundancy

Table 2

IPC-CC-830 Conformal Coating Guidelines for Substrate Type and Quantity

Test                                                              Test Vehicle                            Average Success Rate

1) Dielectric Withstanding Voltage         IPC-B-25A test boards                                           (DWV)                                                 (with the D-pattern wired)                                  90 – 100%

2) Moisture and Insulation Resistance    IPC-B-25A test boards                                              (MIR)                                                               (with the D-pattern wired)                                      80%    

3) Thermal Shock                    IPC-B-25A test boards (with the D-pattern wired)         90 – 100%

4) Temperature/Humidity Aging        “Y” shape test assembly 9 (with resistors,                                    (Hydrolytic Stability)                  (1 color coded, 1 with numbers/alphabets, soldered)  90 – 100%

IPC-CC-830 standards encompass three categories, testing:  

  • 1) Qualification of the processes/materials for the coating purpose at hand, with testing required on each product batch;
  • 2) Retention of the qualification over a determined performance duration, with testing required every two years; and
  • 3) Overall quality conformance of the coating under these conditions, testing required every year.

Military-grade systems and ruggedized products benefit from adhering to IPC-CC-830 standards.  

Summary        

            IPC Standards present achievable, easily recognized benchmarks for the assembly and production requirements of electronic circuits, components and equipment.  For conformal coating, the two most important are:

Those using conformal coatings need to recognize the properties of various coating-types to select the kind most applicable to the product and its purposes, for extended design-life.  Diamond MT focuses on the importance of aligning coating types and processes according to the IPC Standard specifications required to meet clients’ material and operational functions.

To learn more about conformal coating specifications and which ones make sense for your project, download our whitepaper:

Conformal Coating Specifications

Tags: conformal coating, conformal coatings, conformal coating specifications