In the field of electronic circuit manufacturing, the production of PCB (Printed Circuit Board) is of paramount importance, and PCB ink is an indispensable key material. It not only affects the appearance quality of the PCB, but also directly affects the electrical performance and reliability of the circuit board. So, what aspects of the use of PCB ink do we need to pay special attention to? And how do we deal with common problems?
1. Precautions for using PCB ink
(1) Temperature control: adhering to the “comfort zone” of the ink
In actual production, the experience of many manufacturers shows that the temperature management of the ink is like walking a tightrope and must be precise. Generally speaking, the temperature of the ink should be maintained steadily within the range of 20-25°C, and the temperature fluctuation should be as small as possible. This is because the temperature is like an “emotion regulator” for the ink. Once it gets out of control, the viscosity of the ink will become unpredictable like a runaway horse, which will seriously affect the quality and final effect of screen printing.
Historically, the early PCB manufacturing process was relatively rough, and insufficient attention was paid to the temperature of the ink. For example, in some small electronic equipment processing plants, due to the poor production environment and the lack of effective temperature control measures, the ink becomes too thin during the high temperatures of summer, causing the ink to run during screen printing and the pattern to become blurred; while in the cold of winter, it becomes extremely viscous, making it difficult to apply the ink, which greatly reduces production efficiency and keeps the defective product rate high. With the development of the electronics industry, people have gradually realized the huge impact of temperature on the performance of inks, and have begun to pay attention to and regulate the storage and use temperature of inks.
Today, modern PCB production workshops are usually equipped with advanced constant temperature and humidity equipment to ensure that inks are always at the optimum temperature. From a professional point of view, the effect of temperature on the viscosity of inks stems from its effect on the intermolecular forces of inks. When the temperature rises, the thermal motion of the molecules increases, the distance between the ink molecules increases, the interaction force weakens, and the viscosity decreases. Conversely, when the temperature decreases, the interaction force between the molecules increases and the viscosity increases. Therefore, maintaining a stable temperature environment is the primary condition for ensuring screen printing quality, both during ink storage and use.
When the ink has been stored outdoors or has experienced large temperature changes, it should not be used hastily. Just as athletes need to warm up before a competition to adapt to the venue, the ink also needs to be “adapted” to the ambient temperature for a few days, or the ink in the ink bucket should be brought to the right temperature for use. This is because using “cold” ink is like driving a car on ice, which can easily cause screen printing problems and cause unnecessary trouble in production. Therefore, in order to ensure that the ink quality is consistent, storing it at room temperature is undoubtedly the best choice.
(2) Mixing and dilution: a double guarantee of uniformity
Stirring the ink before use is like rehearsing before the start of a great performance. It must be thorough and careful, whether done by hand or with the help of mechanical force. This is because during long-term storage, the internal components of the ink may separate, just like the different colors of a cocktail that has been left to stand for a long time. Mixing the ink allows the pigments, resins, solvents and other ingredients to be evenly mixed again, ensuring that they adhere evenly to the PCB during screen printing.
If air is inadvertently mixed into the ink during the mixing process, the ink needs to be allowed to “rest” for a period of time to allow the air to escape naturally. Otherwise, these air bubbles will adhere to the PCB during screen printing, creating defects similar to a “pockmarked face” that will affect the appearance and electrical performance of the circuit board.
When it comes to ink dilution, a rigorous operating procedure must be followed. First, the thinner and ink must be thoroughly mixed. This step is like the seasoning process in cooking, and requires patience and stirring to ensure an even flavor. After mixing, the viscosity must be tested to ensure that the diluted ink meets the requirements of the screen printing process. For example, in the production of some high-precision PCBs, even a slight deviation in viscosity may result in the width or thickness of the lines not meeting design standards, which in turn affects the performance of the entire circuit board.
In addition, the sealing of the ink barrel after use should not be overlooked. This is like preserving precious food. A good seal can prevent the ink from coming into contact with impurities such as outside air and moisture, which can cause it to deteriorate. At the same time, the ink used on the screen is like dirty laundry that has been worn. It should never be put back into the ink bucket and mixed with unused ink, otherwise it will contaminate the entire bucket of ink, just like a piece of rat droppings spoiling a pot of soup.
(3) Cleaning: the road to a clean screen
In the process of using PCB inks, cleaning the screen is as important as sharpening a sword. It is best to use cleaning agents that are compatible with each other to clean the screen thoroughly, without leaving the slightest ink residue. When choosing a cleaning agent, it is like choosing the right partner, and the best results can only be achieved when they are compatible with each other. For example, for some types of PCB inks, alcohol-based cleaning agents may be effective, while for others, ketone-based cleaning agents may be required.
When cleaning, using a clean solvent is like washing your face with clear spring water, which can ensure the cleaning effect. If the solvent itself contains impurities, then during the cleaning process, these impurities may be transferred to the screen, affecting the quality of the next screen printing. From a practical point of view, a PCB manufacturer once used a solvent containing impurities to clean the screen, causing the impurities left on the screen to mix with the ink during the subsequent screen printing process, resulting in many small granular defects on the PCB board and a significant decrease in product qualification rates.
(4) Dry environment: perfect ending with exhaust escort
The ink drying process is like the final sprint stage of a rocket launch, and it must be carried out in a device with a good exhaust system. This is because during the ink drying process, a large amount of volatile ingredients such as solvents will volatilize. Without a good exhaust system, these volatiles will accumulate in the drying device, just like a haze covering the city. This will not only affect the drying speed, but may also cause defects on the surface of the dried ink, such as bubbles and orange peel patterns.
Take a large PCB manufacturer as an example. Before upgrading its ink drying equipment, due to the imperfect exhaust system, problems such as uneven ink drying and bubbles on the surface often occurred during the drying process. After upgrading the exhaust system, increasing the exhaust air volume and adding a filter, the drying environment has been greatly improved, the quality of the dried ink has been significantly improved, and the yield rate of the product has also increased.
(5) Work site: a compliant environment for printing high-quality products
The screen printing work area is like the backdrop of a stage, and must meet the requirements of the process technology. A good work area should have stable conditions such as temperature, humidity, and lighting, and should also be clean and hygienic to avoid interference from dust and other impurities in the screen printing process. For example, in some aerospace and aviation electronic equipment manufacturing fields that have extremely high quality requirements for PCBs, the temperature and humidity control accuracy of the screen printing work area can reach ±1℃ and ±5% RH, and a dust-free purification workshop is used to ensure that each PCB board can meet nearly perfect quality standards.
2. Common problems with PCB inks, causes and solutions
(1) Uneven ink: often due to intertwined “messy threads
Uneven ink is a common problem in the use of PCB inks, and it can take on various forms, such as the ink not being able to be applied, not adhering evenly to the PCB board, but appearing in a dotted, striped or flaky distribution, or even white spots of ink. The reasons behind this are like a complex spider’s web, intertwined.
Insufficient ink mixing time is like insufficient heat when cooking a dish. The various ingredients are not fully integrated, resulting in uneven ink composition, which affects its fluidity and adhesion. For example, in some small processing plants, due to workers shortening the ink mixing time in order to meet production schedules, the ink does not flow smoothly during screen printing and does not adhere evenly.
Mixing the wrong ink is like using the wrong medicine. Mixing different types or proportions of ink components together will inevitably prevent the ink from achieving the desired performance. Residues of oil or water stains on the board are like stumbling blocks on a smooth road, which will prevent the ink from adhering closely to the board, resulting in poor adhesion. This may be due to incomplete cleaning of the PCB board during the pre-treatment process, or contamination during storage or transportation.
Ink impurities are like grains of sand mixed into pearls, which can destroy the uniformity of the ink. These impurities may come from the ink raw materials themselves, or they may be mixed in during production, storage, or use. A poorly made squeegee is like cutting silk with a blunt knife, unable to evenly apply the ink to the PCB board. If the screen is not cleaned properly, then old ink or impurities left on the screen will be mixed into the new screen printing process, just like old stains contaminating freshly washed clothes. The mixed ink is like expired food, its performance has changed and it cannot meet the normal screen printing requirements.
To solve these problems, we can investigate them one by one like a detective. First, check the pretreatment line to ensure the quality of the blow drying section, to ensure that the surface of the PCB board is dry and clean. Check whether each section of the pretreatment meets the process standards. Just like checking whether athletes comply with the rules of the game, only when each link meets the standards can the final screen printing quality be guaranteed. Confirm the ink mixing parameters to ensure that the ink mixing ratio is correct and the time is sufficient. Cleaning the screen, replacing squeegees and other tools is like providing soldiers with sharp weapons, ensuring that the screen printing operation is well equipped.
(2) Large copper surface voids: a “trap” with multiple hidden dangers
The problem of large copper surface voids is mainly manifested as the separation of the ink from the copper surface in the area fully covered with ink on the large copper surface. This problem is like a trap hidden in the PCB manufacturing process, and if not handled carefully, it can easily lead to product quality problems.
Poor pre-treatment is one of the main causes of large copper surface voids. For example, if the roughening and micro-etching processes are not properly controlled during the surface treatment of the PCB board, the roughness and activity of the copper surface will not meet the requirements, which will affect the adhesion between the ink and the copper surface. Just like a house with a weak foundation is prone to cracks, the adhesion between the ink and the copper surface will be affected. Impurities on the board surface, such as dust, grease, metal particles, etc., can also damage the bond between the ink and copper surface. These impurities are like destructive molecules that sneak between the ink and copper surface and weaken the connection between the two.
Dents in the copper surface are like potholes in the road, which can cause the ink to fail to adhere evenly in that area, resulting in voids. Poor ink mixing can make the internal structure of the ink unstable, and it is prone to separation when combined with the copper surface. Uneven ink thickness on the copper surface is like painting a house with uneven thickness, and problems are likely to occur in the thin places. Impact damage to the ink surface is like exquisite porcelain that has been damaged by external forces, and the integrity of the surface has been destroyed, which can lead to voids. Uneven oven temperature distribution and insufficient or excessive baking are like a cooking process with poor control of the heat, which can lead to incomplete or excessive curing of the ink, resulting in a decrease in the adhesion to the copper surface. Repeated tinning or excessive tin-spraying temperatures can cause thermal stress on the bond between the ink and copper surface, similar to the damage caused to a fragile building by strong winds.
In the face of the problem of voids on large copper surfaces, we need to comprehensively inspect the pretreatment line to determine whether each workstation can meet quality requirements, just as inspecting whether all parts of a production line are functioning normally. Confirm the baking temperature and oven temperature distribution curve to ensure that the ink cures under the right temperature conditions. Confirm the ink mixing parameters to ensure stable ink quality. Check the production process to reduce external impacts, just as protecting precious cultural relics from collisions, to avoid damage to the ink surface. Confirm the tin-spraying operation parameters and conditions to prevent the tin-spraying process from adversely affecting the bonding of the ink to the copper surface.
(3) Large copper surfaces or corners of circuit boards with full coverage of solder mask Solder mask peeling at corners: a “crisis”
Solder mask peeling at corners of large copper surfaces or circuit boards with full coverage of solder mask is also a problem that should not be ignored.
Solder mask printed too thinly is like painting the wall with too thin a layer of paint, which cannot provide sufficient protection and adhesion and is prone to peeling. Poor pre-treatment at the corners of the circuit board is like sloppy construction at the corners of a building, leaving hidden dangers. Insufficient baking will result in incomplete curing of the ink, just like an unripe fruit that is prone to falling off. Repeated soldering with excessive temperature, prolonged immersion in flux, or flux with excessive aggressiveness will all have a damaging effect on the bond between the ink and copper surface, just like the erosion of an object by various adverse environmental factors. Damage to the ink at the corners may be caused by external forces such as collision or friction at the corners during the processing or transportation of the PCB board.
To address this problem, we can adjust the thickness of the solder mask printing, just as thickening a weak city wall, to enhance the coverage and adhesion of the ink at the corners. At the same time, it is also necessary to check the process parameters of the pretreatment at the corners to ensure good treatment results; confirm the baking conditions, solder spraying parameters, and the use of flux to reduce the damage to the ink at the corners by external forces, so as to ensure the quality and reliability of the PCB board at the corners.
In summary, the use of PCB inks is a process that requires meticulous operation and strict control. From the storage of inks, preparation before use, to the precautions during screen printing, and then to the accurate judgment and effective solution of common problems, every link is closely linked, like a precise chain. Any problem in any link may affect the quality of the final PCB product. Therefore, PCB manufacturers and related technical personnel must have a deep understanding of the characteristics and usage requirements of PCB inks, continuously optimize the process, and strengthen quality control in order to remain invincible in the fierce competition of the electronics market. At the same time, with the continuous development of electronic technology, the performance of PCB inks will also continue to improve in the future, and their usage requirements and problem-solving methods may also change accordingly. This requires us to continuously pay attention to industry trends, keep learning and exploring, and adapt to new technological challenges.
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