Protective Coating – field failures and protection concepts

This Whitepaper-Collection contains a selection of papers discussing field failures caused by climactic stresses and the selection of a protective concept.

A special focus is set on the topics electrochemical migration (ECM) and protective coating.

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According to GfKORR Guidelines

PROTECTIVE COATINGS for ELECTRONIC ASSEMBLIES

SELECTION | APPLICATION | REQUIREMENTS | RECOMMENDATIONS

With our regular posts on Linkedin we inform you about the most important topics about protective coating of electronic assemblies. 

The useful tips and information are in accordance with the guideline „APPLICATION AND UTILIZATION OF PROTECTIVE COATINGS FOR ELECTRONIC ASSEMBLIES – Selection, fields of application, requirements and application recommendations“ provided by the GfKORR working party “Corrosion protection in electronics and microcircuitry”.

On this website we provide further details to you.

INFLUCENCE OF THE ASSEMBLY ON THE PROTECTIVE COATING

Not only the coating material itself has an influence on the protective coating. Also the assembly is influencing the coating result.

BASE MATERIAL

The base materials used to manufacture the printed circuit board play a significant role in determining the electrical, mechanical and thermal properties and, last but not least, the expected costs of the assembly to be manufactured. The selection of the right base material is therefore essential.

COMPONENT AN CIRCUIT BOARD LAYOUT

On the circuit board, the distances between voltage and current and the insulators between them determine the sensitivity. The design and requirement of areas which need to be coated are based on these conditions.

SOLDER RESIST

Since the solder mask remains on the PCB, it must also fulfill its insulating and protective function during the entire lifetime of an electrical assembly. Therefore, solder resist masks have to meet the same high requirements as base materials and components.

SOLDERING MATERIALS AND SOLDERING PROCESS

A soldering process with not proper set parameters can leave large and uneven quantities of flux on the PCB. Moreover, a not proper flow of the liquid tin on the PCB will also leave more residues, which have a direct influence on the electrochemical stability and adhesion of protective coatings.

DRYING PARAMETERS

A protective coating can only fulfill the requirements and achieve full chemical and physical properties after full cure. For this reason, it is important to check the drying and curing parameters for each type of protective coating and assembly, which needs to be coated.

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KEEPING AREAS CLEAR AND EXPOSING AREAS

Some parts of electrical assemblies must remain uncoated, e.g. SMD connectors, THT connectors or
structures that are damaged by coating. In special cases, it might be required to remove the coating.

INFLUENCE SOLDERING MATERIALS & PROCESS

A soldering process without proper set parameters can leave uneven quantities of flux on the PCBA. Moreover, residues from the soldering process can be critical in terms of electrochemical stability and adhesion. Solder material and process have to fit together to reach reliable solder results.

SOLDER PASTE

A solder paste consists mainly of a flux and a solder powder. After re-melting, residues of the flux remain at or around the soldering joint. The chemical composition and physical properties of these residues determine their further treatment before a protective coating is applied onto the pcba.
Flux contains (depending on flux types) resin, solvents, activators and additives.

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SOLDERING PROCESS

In reflow conditions various chemicals may start outgassing. These gases will condense as deposits in colder areas of the reflow equipment. These residues are applied evenly to the assembly during the reflow process.

RESIDUES

∎ „No-Clean“ residue
∎ Water-Soluble residue
∎ Synthetic resin residue
∎ Residues of reactive epoxy resind solder pastes

SUBSTRATE AND PRETREATMENT

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Prior to applying the protective coating on the pcb, the surface needs to fulfill a certain cleanliness level. Several questions like „How to measure the cleanliness of the pcb surface?“, „How clean is clean enough for my application?“, „Do I have to clean?“ and „How to implement a suitable cleaning process?“ have to be answered.

Scroll down to learn more about requirements & decisions for the cleaning of assemblies, purity requirements and the measurement, detection and analysis of ionic impurities.

REQUIREMENTS & DECISION FOR THE CLEANING OF ASSEMBLIES PRIOR TO CONFORMAL COATING

Since coating the assemblies is generally the last step in the manufacturing process, errors in this manufacturing step may have a drastic effect on production costs and lead to appalling field failures. Cleaning before protective coating allows optimum adhesion of protective coatings and prevents subsequent crack formation and delamination.
In particular, crosslinking toxins, such as those produced during soldering, are safely removed during cleaning. In addition to standardized testing of the ion equivalent, it is also important to check for crosslinking toxins in particular for the verification of the purity of components to be coated.

After more than ten years of field experience with no-clean processes, an increasing number of field failures can be observed, especially in coated or encapsulated assemblies. Furthermore, the increased use of power-connecting, controlling and securing assemblies in the market has drastically increased the requirements for these high-voltage assemblies with regard to dielectric strength and aging resistance.

The combination of a cleaning application with effective surface cleanliness tests yields an economical, cost-effective solution to coating processes and substantially enhances process reliability and operational reliability of the coated assemblies.

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PURITY REQUIREMENTS

Traditionally, the result of the cleaning processes is optically evaluated according to IPC-A-610D1. However, mould release agents, grease layers or many fluxes leave behind visually invisible thin residue films depending on the process. Even unsuitable cleaning processes can leave behind invisible adsorption films. These residues impair the possible protective coating and the adhesion of protective coating compounds.

This therefore requires the definition of function-oriented evaluation criteria to characterize the result for cleaning prior to protective coating. In addition to the improved evaluation of process reliability, which can be documented in accordance with DIN ISO 9001:2008 2015, such evaluation criteria also enable more efficient cost optimization of the cleaning process, since quantifiable safety and imperfections of the cleaning process are now visible.

Learn more about the contaminants and their effect in our graphic.

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Protective Coating – field failures and protection concepts