Analysis of printed circuit board contamination, corrosion and protection
With the development of electronic technology, the pitch of device pins on the circuit board is getting smaller and smaller, the arrangement of devices is denser, and the electric field gradient is larger, which makes the circuit board more sensitive to corrosion. On the other hand, the expansion of the application environment of circuit boards and the continuous increase in product reliability life requirements have increased the risk of PCB corrosion.
Among them, the atmospheric environment is the external condition for PCB corrosion, and atmospheric pollutants play an important role in the process of product corrosion. Since the faults related to air pollutants usually appear after the electronic products have been used for a period of time, this means that once the faults caused by corrosion occur, products with the same service life in the same environment will enter a period of concentrated fault outbreaks.
At the same time, the impact of pollution on electronic products is irreversible, which will cause great difficulties in maintenance and even lead to the scrapping of products. Therefore, it is necessary to carry out the corresponding protection design of air pollutants at the beginning of product design.
In previous studies on PCB corrosion, there were few systematic introductions specifically aimed at the evaluation of weak points of protective coatings on a circuit board in engineering practice and on the corrosion protection of circuit boards.
On the basis of previous studies, this paper combines the actual problems of circuit board air pollutant protection, starting from the typical PCB corrosion and the weak points of protective coating coating, to discuss the specific protective measures for circuit board products to deal with air pollutants.
Classification of air pollutants
According to the description of ANSI/ISA-71.04, there are three forms of pollutants in the air that affect the operation of equipment: solid, liquid and gas. The substances that have a greater impact on the circuit board in each form are as follows:
Solid particles – dust
Dust usually contains water-soluble salts such as chloride ions, sulfate radicals, and nitrate radicals. In addition to directly causing poor contact of metal connectors or metal contacts inside the equipment, it will also promote the formation of a water film on the metal surface. The dissolution of water-soluble components in the water film will accelerate the occurrence of metal corrosion, resulting in a decrease in the insulation resistance of the circuit board. More severe galvanic corrosion may occur during board operation.
Liquid air pollutants – salt spray
In addition to liquids in a broad sense, the liquid air pollutants described here also include liquids carried by gases and aerosols of atomized droplets in the air. In the air of coastal areas, the content of salt spray is high, and the main component is NaCl. NaCl is relatively inactive chemically, but in the case of humidity and water, it will produce Cl-, which will react with Cu, Ni, Ag and other metals or alloys.
At the same time, NaCl, as a strong electrolyte, can condense on the attachment surface when the relative humidity is lower than the critical value, dissociate to generate Cl-, and dissolve in the liquid film or droplets on the surface of the circuit board. At a certain concentration of Cl-, electronic equipment begins to corrode locally, and with the appearance of new non-dense corrosion products, the protective layer on the surface of the equipment is further damaged, and the corrosion rate increases rapidly.
Gaseous air pollutants – SO2, H2S
Sulfur-containing compounds are one of the most important pollutants in the atmosphere. H2S and SO2 in the atmosphere mainly come from industrial processes such as mining, combustion of sulfur-containing fuels, metallurgy, and sulfuric acid manufacturing. H2S and SO2 are strongly variable components, and H2S can be decomposed into H2 and S under heating. The SO2 discharged into the air reacts with the O2 and water vapor in the humid air, and is combined to form H2SO4 under the action of catalysts such as dust.
Corrosion failure mechanism and morphology
Electrochemical migration (ECM) caused by corrosion is the main cause of corrosion failure of electronic products. There are two different forms of electrochemical migration: one is the migration of metal ions to the cathode, the reduction deposition forms dendrites, and grows to the anode; the other is the conductive anodic filaments (CAF) produced from the anode to the cathode. The electrochemical migration of metals will eventually cause the short-circuit leakage current of the circuit, which will cause the failure of the system.
Adsorbed liquid films on the surface of the material play an important role in the atmospheric corrosion mechanism that occurs on circuit boards. The corrosion is the most serious when the thickness of the liquid film is more than 1 μm, and the electrochemical reaction mainly occurs under the liquid film. The corrosion forms of common electronic equipment in the air can be roughly divided into the following categories:
Corrosion is concentrated in a small area of the surface of the metal material, and most of the rest of the surface is slightly or not corroded. Mainly due to the uneven distribution of the metal surface state (coating defects, chemical composition, etc.) and the corrosion medium, the electrochemical property is uneven, that is, different parts have different electrode potentials, thereby forming a potential difference and driving localized corrosion.
In the process of localized corrosion, the difference between the anode area and the cathode area is obvious, usually forming a configuration with a small anode and a large cathode, and the anode corrosion is serious.
A specific type of localized corrosion commonly seen in specific galvanic corrosion on gold-plated components. Due to the existence of micropores or other defects on the surface of the coating, the intermediate transition layer and even the base metal are exposed to the atmosphere, and Au and other metals form a galvanic pair with a large cathode and a small anode, and electrochemical corrosion occurs.
The appearance of corrosion products further leads to the increase of surface defects, which eventually leads to the destruction of the coating. Affected by microporous corrosion products on the contact surface, the corroded area will exhibit higher contact resistance and phase shift.
When adjacent conductors are closely spaced and a bias voltage is present, a stronger electric field will develop. If there is a liquid film in the conductor at this time, the conductor with a higher potential will be electrolyzed by the solution, and the formed ions will migrate to the other conductor, resulting in a rapid decline in the insulation performance between conductors, destroying the conductor, and eventually causing equipment failure.
Typical PCB corrosion and protection
A variety of materials are used on circuit boards, and the selection of materials has an important impact on the occurrence of corrosion reactions. Taking thick film resistor vulcanization, SMD LED two typical vulcanization failures and PCB copper corrosion encountered in engineering practice as examples, the influence of different device packaging structures and material selection on the PCB corrosion resistance is compared.
(1) Thick film chip resistor sulfide corrosion
The surface electrodes of thick film resistors contain silver elements, which are easily chemically reacted with sulfur when exposed to air. If the outer protective layer and the plating layer are not tightly bonded, the surface electrode will come into contact with sulfur in the air. When the air contains a large amount of sulfur compounds, silver reacts with sulfide to form silver sulfide.
Since silver sulfide is non-conductive and has a larger volume than silver, after compounding, the volume expands, causing the fault of the original silver layer, and the resistance value gradually increases, until the circuit breaks.
In order to prevent the vulcanization of the thick film resistor, a resistor with strong anti-sulfurization ability can be selected. Coat the surface electrode with a protective layer, and introduce an Ag-free and conductive vulcanization protective layer to protect the upper electrode and completely prevent the vulcanization path.
(2) Silicone encapsulated LED vulcanization corrosion failure
The porous structure of silica gel can adsorb sulfide in the air. If the PLCC surface perfusion light-emitting diode is packaged with silica gel, there will be a risk of sulfide.
Because silica gel has the characteristics of moisture permeability and oxygen permeability, sulfur ions in the air can easily penetrate the molecular gap of silica gel, enter the interior of the LED, and chemically react with the silver-plated layer of the bracket, resulting in blackening of the functional area of the bracket and a decrease in luminous flux, until dead lights appear.
If epoxy resin is used for encapsulation, it can effectively prevent the erosion of sulfide ions. The LED packaged with epoxy resin was selected, and no vulcanization phenomenon was found after one year of field use.
(3) Copper corrosion of printed circuit boards
Printed circuit boards use copper as the electrical transmission medium, and copper corrosion will not only affect the appearance of the product, but also easily lead to short circuit or open circuit problems in electrical connections.
In order to improve the corrosion resistance of copper clad PCB, common surface treatment methods include: hot air leveling tin spray, chemical nickel gold and chemical immersion silver. Relevant studies have shown that in a sulfur-containing atmosphere that is prone to condensation, hot air leveling and tin-spraying have the strongest corrosion resistance, followed by chemical nickel and gold.
Surface treatment does not fully ensure that the copper clad of the circuit board will not be corroded in harsh environments. As shown in the picture, copper clad corrosion occurs in the grounded copper clad area at the bottom of the chemical nickel gold circuit board, and even the via holes in the area covered by the conformal paint have obvious corrosion products blocking the via holes.
Corrosion occurs in the through holes of the circuit board after hot air leveling and tin spraying, and the position of the circuit board through holes is a high-incidence area for corrosion.
In addition to changing the surface treatment method and increasing the thickness of the coating, the process parameters during circuit board production and integration testing should also be adjusted, especially to avoid damage to the coating due to excessive probe pressure during ICT testing.
The device corrosion of printed circuit boards is usually induced from the edge of the pin or device, and goes through the stages of surface coating damage, interface corrosion expansion, metal corrosion expansion, and component inner cavity corrosion. Conformal paint is a specially formulated coating used to protect circuit boards from environmental erosion.
The type and coating thickness of the three anti-paint are important factors affecting the protective effect. The industry often measures the thickness of the coating material at the plane position according to GB/T 13452.2-2008, and there is a distinction between wet film thickness and dry film thickness.
According to the actual application, for a controlled environment, it is not necessary to apply three-proof coating or use a thin-layer coating process, and the coating thickness is at the lower limit of the range; for an uncontrolled environment or harsh environment, it is recommended to use a thick-layer coating process. Thickness is at the upper limit of the range.
In actual production, it is found that the dry film thickness at the pins can sometimes only reach 1/3 of the dry film thickness in the plane area. The reason is that the conformal paint has a certain fluidity.
After spraying, it is subjected to gravity and the capillary action between the pins. The thickness of the conformal paint at the pins of the device is relatively thin, which becomes the weak point of the conformal protection, easy to form corrosion. As shown in the picture, the missing conformal paint and corrosion of the pins appeared on the pins of the circuit board components that have been used for a period of time.
Under the premise of the same coating process of the conformal coating, the protective effect of the circuit board is quite different for the conformal coating with different physical parameters and coating thickness. Appropriately increasing the viscosity and thickness of the conformal paint material can effectively improve the protection effect of the device pins and device edges, ensure the integrity of the coating, and further improve the corrosion resistance of the circuit board device during work.
The structural sealing protection design is to isolate or reduce the influence of external corrosive media, and maintain the original performance of internal insulating parts and electronic devices.
Increasing the protection level may lead to problems such as heat dissipation, human-computer interaction, and cost. When introducing fans into the system, pay attention to the air duct design.
According to the environment in which the device is used, reasonably select the heat dissipation method of the product and the position of the fan. When the fan is placed at the air inlet, care should be taken to avoid the formation of eddy currents inside the equipment, and devices with high pin density should be avoided at the air inlet to reduce the occurrence of serious dust accumulation in local areas and avoid the accumulation of solid particle pollutants.
Aiming at the problem of air pollutant protection of circuit boards, a variety of analysis and verification methods and protective measures are proposed in terms of material selection, protective coating and structural protection design from the device level, single board level and equipment level respectively.
1) For corroded devices, metallographic microscope, SEM, EDS and other means can be used to determine the specific contamination source, and select a suitable packaged device according to the type of contamination source and the invasion path.
2) Device leads and edge locations are often weak points for coating application due to gravity and capillary action between leads. PCB corrosion with protective coatings is usually induced from the pins or the edge of the device, and the location of the device leads is the weak point of the protective coating. Increasing the viscosity and thickness of the coating material can effectively improve the corrosion resistance of the protective circuit board against pollutants.
3) Appropriately improving the IP protection level of structural design and reasonable air duct design can effectively reduce the intrusion of air pollutants.