PCBA Conformal Coating Process Corrosion Failure Analysis

Conformal coating, refers to the addition of a layer of protective coating to protect products and improve product reliability, three-proof paint plays a role in isolating electronic components and circuit boards from harsh environments. Conformal coating with excellent performance can greatly improve the service life of electronic products. Narrowly defined three-proof usually refers to anti-moisture and heat, anti-corrosion (including salt spray, acid and alkali corrosive liquids, corrosive gases, preventing electrochemical migration), anti-mold, in fact, three-proof also includes a variety of environmental stress protection, such as anti-vibration, anti-dust, anti-radiation, anti-static, anti-rodent, etc., in order to ensure that the PCBA will not be improperly protected and failures to prolong the service life of the product.

As the function of electronic products continue to integrate and intelligent direction, the size of electronic components and electronic assemblies also continue to shrink, which puts forward higher requirements for PCBA and three-proof processing technology. In recent years, improper selection of conformal coatings has caused component corrosion, corrosion and electrochemical migration failure due to poor processing control, which has brought double losses of time and money to enterprises.

Types of conformal coating

The types of conformal coatings commonly used in the industry are: AR-acrylic resin; ER-epoxy resin; SR-silicone resin; UR-polyurethane; XY-poly-para-xylene; fluorocarbon resin FC, etc.. Except for the chemical weather deposition process of poly-p-xylene, conformal coatings are liquid coated and attached to the product surface.

Failure Mechanism of Conformal Coating

When foreign matter (objects, liquids) is present on the surface to be attached, the foreign matter is sealed at the bottom after conformal coating is cured. The sealed foreign matter includes flux residues, board washing water (such as alcohol or isopropyl alcohol, etc.) and various solid residues. When the bottom of the conformal coating is sealed with part of the liquid, the conformal coating can not be effectively attached to the surface of the product, the bottom of the formation of non-insulating channels; product power work process, the potential difference between the action of electrochemical migration, resulting in the bottom of the conformal coating short-circuit burn board. The residual flux at the bottom of the conformal coating plays a role in assisting the formation of free metal electrons by etching the solder joints, while the residual liquid plays the role of the pathway for metal ions to swim. Whether residual liquid or solid, will affect the results of conformal coating adhesion, resulting in the existence of micro gaps in the bottom of the three-proof paint, providing a prerequisite for the formation of electrochemical migration. As conformal coating can not completely prevent the product from absorbing moisture, the product absorbs moisture in a humid environment, the metal ionizes under the action of potential difference or dissociates into a free state under the action of corrosive residues, and there is a channel under the film of conformal coating, which constitutes a sufficient condition for electrochemical migration. The corrosion and migration caused by foreign matter at the bottom of conformal coating is often accompanied by insufficient adhesion of the paint film, the skin film floating away from the phenomenon of shedding, which is manifested as unclean paint film and insufficient adhesion of the paint film in the daily inspection.

How to prevent corrosion migration phenomenon under conformal coating

1. The conformal coating should be coated completely, and there should be no missing coating area.
   

2. three-proof paint composition selection needs to be appropriate: for the use of environmental temperature differences in the product, three-proof paint needs to be able to withstand drastic temperature changes without cracking; for the use of the product in the process of stress products, three-proof paint needs to have considerable toughness without cracking; three-proof paint curing conditions need to be matched to ensure that not due to the curing of the cracking; the three-proof paint needs to be able to withstand the environment of the corrosive substances erosion. 

3. Before coating, the product should be thoroughly cleaned and baked to ensure that the product is dry and clean, to avoid liquid and solid sealing under the conformal coating.

4. the three conformal coating can only be used after curing, to avoid the three conformal coating curing process of device work heat resulting in skin film damage.

5. Avoid high power heat generating devices, fragile devices, optical sensing devices, etc. being coated with conformal coating.