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

Understanding the unique characteristics of different conformal coating types and how they interact with a wide range of products and materials is crucial for achieving optimal functionality, performance reliability, and product longevity. Designers and users of conformal coatings need to understand the distinct properties of each type and their compatibility with the components they protect, ensuring product durability in their intended environments throughout the design life cycle of each part.

Conformal Coating Guidelines

Various specifications for conformal coating have been developed to recognize and accommodate variations in operational and material conditions. End users applying conformal coatings may face challenges related to selecting or applying the coatings, leading to issues such as mismatches in coefficients of thermal expansion (CTEs), moisture entrapment, outgassing, corrosion, abrasion or the need for future rework or repairs. 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.

IPC Conformal Coating Standards

IPC, the Association Connecting Electronics Industries, is a renowned standards development organization based in Bannockburn, IL, USA, with a global presence. Accredited by the American National Standards Institute (ANSI), IPC’s mission is to standardize the assembly and production requirements of electronic components, covering interconnecting and packaging electronic circuits and assemblies. SCS has been an IPC member for nearly three decades.

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

PC-A-610, also known as Acceptability of Electronic Assemblies, outlines visual quality-acceptance criteria for producing electronic assemblies. It defines the necessary PCA/component features that surpass basic end-item performance standards. Requirements for coating thickness are also set by IPC-A-610, according to the coating material and the function of components being covered.

IPC-A-610 categorizes products based on their function, with Class 1 being the least stringent, followed by Class 2 and the most rigorous being Class 3.

• 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 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-830: Qualification 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                      

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/
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. The two most important IPC standards for conformal coating are:

• IPC A 610 Acceptability of Electronic Assemblies Standard – It is currently the most widely used IPC-standard for consumer and high reliability printed wiring assemblies. It contains valuable, but limited, criteria for conformal coating applications.
• IPC-CC-830B Qualification and Performance of Electrical Insulating Compound for Printed Wiring Assemblies – It is applicable to use of conformal coatings, qualifying the definition, use and conformance of all conformal coatings types. It has been designed and constructed with the intent of obtaining maximum confidence in the materials with minimum test redundancy.

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. SCS focuses on the importance of aligning coating types and processes according to the IPC standards for conformal coatings required to meet clients’ material and operational functions.