Three proofing paint coating process | sealing determines success or failure
Time:2023-10-16
The practice of using three proof paint to protect printed circuits can be traced back to the 1960s, when the US Department of Defense developed the MIL-I-46058 specification. The general industry standard IPC-CC-830 has been driving the development of the three proof paint industry in recent years. Three proof paint can be used to protect printed circuits from current leakage, short circuits (arc, corona), corrosion, solder fatigue, mechanical stress (such as impact and vibration) It has a protective function against dust and dirt
Therefore, this paragraph's introduction to the history of the three proofing paint basically summarizes the role of using good coatings. Now that we understand the role of the three proofing paint, let's talk about my favorite part - how contract manufacturers constantly make various mistakes on the three proofing paint issue. Please remember, I work in an analysis laboratory.
Four main types of materials
There are several types of coating materials, and when deciding which type of material is most suitable for your product, it usually depends on the final usage environment of the circuit. Coating materials generally choose acrylic resin, silicone resin, polyurethane resin, and polyxylene. Other materials may also be used, but most of the materials we most commonly encounter belong to these four major types.
Among these four types of materials, the material with the fewest problems in the coating process is polyxylene, which is because it uses a gas phase coating process. This process requires the substrate to be very clean in order for the coated material to correctly adhere to the substrate. When this coating process is completed correctly, residues that may cause current leakage or electrochemical migration are usually removed. Most importantly, compared to other coating materials, poly (p-xylene) forms a nearly completely sealed coating.
The drawbacks of paraxylene are its cost and the time it takes to apply. We have not seen any examples of using poly (p-xylene) in mass production, such as consumer electronics products. However, high-end products with high reliability requirements such as medical equipment and aerospace applications will use this material. The other three materials are the focus of this month's column, as there is no need to pre clean the coated surface before applying these materials, which is the starting point for us to consider coating issues.
Our laboratory often receives reports from some people that their three proofing paint has failed, and they find that the coating cannot prevent the growth of "dendritic crystals" on their products, which surprised them very much. Sometimes, three proofing paint is considered a magical application that can prevent all electrical leaks, but this is not the case. Previously, I had discussed in detail the risk of increased levels of ion residues in any component of the manufacturing process, and this risk is not different when it comes to three proofing paints. The coating can effectively block most dust and debris, but if the atmospheric humidity is high enough, coupled with the presence of active and hygroscopic ion residues, the ion residues will eventually penetrate the coating and may cause electrical leakage.
Application and curing
There are many methods to apply the three proofing paint, including manual coating with brush, wet coating, automatic spraying with air pressure spraying equipment, and spray coating with nozzle. The effectiveness of all manual coating methods usually depends on the operator's ability and level of experience. The method of manual coating may also include considering appropriate masking areas not to be coated during design, and using polyimide tape to mask areas that do not require coating is the most common practice. This may prohibit the use of wet coating methods on some components, where areas that do not require coating are difficult to cover. The automatic spraying system is the most reproducible method for applying the three proofing paint. This is the most common method we see in the three proofing paint industry, usually used for large-scale coating processes. If some problems occur, the spray system is the easiest to diagnose and optimize.
Nowadays, it is a common practice to apply three proof paint on the residue of cleaning free flux, and there are some different opinions on this coating process. Generally speaking, we rarely see problems with the coating of circuit boards that have been properly cleaned before coating, such as circuit boards coated with poly (p-xylene). We have noticed that various coating adhesion problems can occur when the flux residue is not completely treated and a sturdy outer surface layer is not formed that allows the coating material to adhere. If the residual flux is not properly treated, the soft outer surface layer of the circuit board will mix with the coating, and the coating will not fully cure. The adhesion of the coating on components treated with cleaning free flux is still an issue, even after the cleaning process. Many times, most of the flux residue is removed during the cleaning process, but it leaves behind unimolecular layer residues that are not easily visible. Residues that you cannot see may cause many problems, just as there are problems with residues that you may see. Therefore, it is necessary to limit the specifications of the cleaning process to ensure that all residues are removed.
Then, there are two curing methods: heat exposure and ultraviolet exposure. Heat exposure curing requires more time than UV curing, and may take at least 10 minutes to dry the adhesive between the coating and the outer surface layer before proceeding with subsequent assembly processes. The coating continues to cure during these processes. UV curing is much faster, but there is a risk that the coating in shaded areas will never cure. This situation is most common on high-density components. If the UV cured coating migrates below the component, it will never fully cure and will remain moist throughout the product's lifespan. There will be dust and debris in the coating in this state that it wants to block. If the fragments are metal fragments, the risk of failure for your product may be higher than for products without any coating.
Due to its luminescent properties that other types of coatings do not have, the advantage of UV coatings is that they are easier to inspect. Inspectors can accurately determine the position of the coating on the component and determine whether the UV curing process is sufficient to reach all coating areas. UV curing coatings also make it easier to determine the consistency of coating thickness. Generally speaking, the brighter the coating, the thicker the coating. When there is residue present, the dehumidification effect of the coating is common, which often occurs on the sharp edges of the wire and the component body. This may make some areas in the component more susceptible to the impact of the operating environment.
The level of adhesion can be determined using IPC TM-650 2.4.1.6. This method uses a 10 × 10 grid, 1 millimeter in the grid × A 1mm square is etched onto the coating, followed by applying adhesive tape and pulling it away with a stable movement at an angle of 180 °. After pulling off the tape, check the grid to see how much coating inside the grid has been taken away by the tape. The area of the grid area where the coating is retained is judged based on six levels from 0 to 5. 0 indicates that the coating area taken away by the tape exceeds 65% of the grid area, and 5 indicates that the tape has not taken away any coating. This level determination is mainly completed on test samples during process evaluation, rather than on actual products. We suggest conducting such testing on actual products, if possible, by combining the coating process with the material you choose, which can help you better understand the expected coating effect.
Now, let's return to the theme: 'Sealing determines success or failure'. After selecting the coating material, it usually determines the outcome of the coating process. If maintenance is required, it is difficult to find easily remedial measures. Of course, you can use various chemical methods or dry ablation processes to peel off the coating, but these peeling methods are not only time-consuming, but also have inherent risks and the possibility of failure. The important lesson learned from this column is that coatings do not always prevent the occurrence of faults; The equally important lesson is that when considering the cleanliness level of the components that you must apply the three proofing paint on, you should do the same as when deciding not to apply the three proofing paint on the components. If your component is dirty, you may need to spend some time cleaning it, but in the end, sealing determines success or failure.
