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Reasons for poor coating of PCB circuit board
PCB Blog
Reasons for poor coating of PCB circuit board

Reasons for poor coating of PCB circuit board

2021-12-29
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Author:pcb

1. Pinhole of PCB circuit board. The pinhole is due to hydrogen adsorbed on the surface of the plated part and delayed release. The plating solution cannot moisten the surface of the plated part, so that the plating layer cannot be electrodeposited. With the increase of coating thickness around the hydrogen evolution point, a pinhole is formed at the hydrogen evolution point. It is characterized by a shiny round hole and sometimes an upward small tail. When there is no wetting agent in the plating solution and the current density is high, pinholes are easy to form.


2. Pockmarks. Pitting is due to the unclean plated surface, adsorption of solid substances, or suspension of solid substances in the plating solution. When they reach the workpiece surface under the action of electric field, they are adsorbed on it, affecting the electrodeposition. These solid substances are embedded in the electroplated coating of PCB multilayer board, Small bumps (pits) are formed. They are characterized by convexity, no brightness and no fixed shape. In short, they are caused by dirty workpiece and dirty plating solution.


3. Airflow stripes. The gas flow stripe reduces the cathode current efficiency due to excessive additives, too high cathode current density or too high complexing agent, resulting in a large amount of hydrogen evolution. If the bath flows slowly and the cathode moves slowly at that time, the arrangement of electrodeposition crystals is affected in the process of hydrogen rising against the workpiece surface, forming gas flow stripes from bottom to top.


4. Masking (bottom exposure). Masking is because the soft overflow at the pin on the workpiece surface has not been removed, so electrodeposition coating cannot be carried out here. The substrate can be seen after electroplating of PCB multilayer board, so it is called bottom exposure (because the soft overflow is translucent or transparent resin).


5. The coating is brittle. After the smdpcb multi-layer plate is plated and formed, it can be seen that there is cracking at the bend of the pin. When the nickel layer cracks between the nickel layer and the substrate, it is determined that the nickel layer is brittle. When there is a crack between the tin layer and the nickel layer, it is determined that the tin layer is brittle. Brittleness is mostly caused by excessive additives and brighteners, or too many inorganic and organic impurities in the plating solution.


6. Air bag. The formation of air bag is due to the shape of workpiece and gas accumulation conditions. Hydrogen accumulates in the "bag" and cannot be discharged to the liquid level of the plating solution. The presence of hydrogen prevents electrodeposition of the coating. Make the part where hydrogen accumulates free of coating. When electroplating PCB multilayer boards, air pockets can be avoided by paying attention to the hooking direction of the workpiece. As shown in the figure, when the workpiece PCB multilayer board is plated, when it is hooked perpendicular to the bottom of the plating bath, no air bag will be generated. When hooked parallel to the groove bottom, it is easy to produce gas bags.

PCB

7. A "tin flower" is opened in the center of the plastic sealed blackbody. There is a tin coating on the blackbody. This is because the upward parabola of the gold wire is too high when the electron tube is in the welding line. During plastic packaging, the gold wire is exposed on the blackbody surface, and the tin is plated on the gold wire, like a flower. It's not a plating solution problem.


8. "Climb tin". There is a tin layer at the junction (root) between the lead and the blackbody, which climbs up to the blackbody like a wall grass. The tin layer is a dendritic loose coating. This is because in the pre plating treatment, the SMD frame is brushed with a copper brush, and the worn copper powder embedded in the blackbody is not easy to be washed away, becoming a conductive "bridge". When electroplating PCB multilayer board, as long as the electrodeposited metal is put on the "bridge" , the dendrimer deposits crawl to connect with other copper powders, and the area of crawling tin is becoming larger and larger.


9. "Whisker tin" is at the junction of the lead and the blackbody, there is whisker like tin on both sides of the lead, and there is tin coke like tin pile at the junction of the front of the lead and the blackbody. This is because when SMD frame is plated with silver by masking method, the masking device is not tight, and silver is also plated where silver plating is not required. During plastic packaging, some silver layers are exposed outside the blackbody. During the pretreatment, the silver layer is pried up, and the tin plated on the silver is like whiskers or piles of tin. Overcoming the exposure of silver layer is one of the keys of silver mask plating technology.


10. Orange peel coating. When the substrate is very rough, or there is corrosion during the pretreatment, or when the ni42fe + Cu substrate is treated before plating, some copper layers have been removed, while the copper layers in some areas have not been removed, and the whole surface is not smooth. The above conditions may cause the orange peel state of the coating.


11. Cavity plating. There are irregular pits (different from pinholes) on the coating surface, which is a "ceiling face" coating. There are two situations that may form a "ceiling face" coating.

(1) Some units use the glass bead spraying method to remove the overflow. When the spraying air pressure is too high, the kinetic energy inertia of the glass bead will impact the plated surface into small pits. When the coating is too thin and the pits are not filled, it becomes a "ceiling face" coating.

(2) The metallography of the base material alloy is uneven, and there is selective corrosion during the pre plating treatment. (the more active metal is etched first to form pits). The "ceiling face" coating is formed when the pits are not filled after electroplating of PCB multilayer board.

For example, for ni42fe base material, if Ni and Fe are not fully mixed evenly in the metallurgical process, there may be uneven alloy metallography in some areas of the material surface after rolling. During pre plating treatment, because Fe is more active than Ni, selective etching is preferred to form pits. If the electroplating layer of PCB multilayer board cannot be leveled, it will become a "ceiling face" coating. Similarly, zinc brass also has such a phenomenon. If the copper zinc metallography is uneven, zinc is selectively corroded before copper during pre plating treatment, so that the substrate is concave, and the PCB multilayer board is concave after electroplating.


12. Loose dendritic coating. When the plating solution is dirty, the concentration of main metal ions is high, the complexing agent is low, the additives are low, the anode and cathode are too close, the current density is too high, and it is easy to form loose dendritic coating in the current area. The loose coating is like foam plastics, and the branches are uneven.


13. Double coating. The formation of double-layer coating mostly occurs when the operating temperature of the plating solution is relatively high. In the process of PCB multilayer board electroplating, the workpiece is put out of the plating tank and hung in again. In this process, if the workpiece is raised for a long time, the plating solution on the workpiece surface precipitates salt frost due to water evaporation and is attached to the workpiece. When the salt frost is not dissolved in time, the coating is plated on the salt frost surface to form a double-layer coating, like Huafu biscuit. A layer of salt frost is sandwiched between the two coatings.

To avoid double-layer coating, the workpiece can be shaken in the plating solution for a few seconds before continuous plating, and then electrified for continuous plating after the salt frost is dissolved.


14. The coating is blackened. The main reason for the blackening of the coating is the high metal and organic impurities in the plating solution, especially in the low current density area; In the case of insufficient additives, black coating will also appear in the middle of large plated area; If the temperature is too low and the ion activity is small, a gray black coating will be formed when the current is too high. For the treatment of metal impurities, corrugated plate can be used as cathode for electrolysis at 01-0.2a/dm2. Organic pollution can be treated with 3-5g / L activated carbon. Use granular and wash with pure water first.


15. Blunt peeling. Ni42fe alloy is easy to blunt. Activation before plating includes two chemical processes, one is oxidation process and the other is oxide dissolution process. If the oxidation process is not sufficient or the oxide is not dissolved in time, there is still oxide residue on the plated surface, and the coating will be peeling or rough.


16. Replacement peeling. If there are two different materials on the same workpiece. For example, the surface of copper substrate is nickel plated, and copper is exposed on the notch after cutting and forming. When the copper ion in the strong etching groove increases to a limit value, the replacement copper layer is easy to be generated on the nickel layer. With replacement copper, the tin layer will peel off after tin plating. In this case, only the strong corrosion solution can be updated frequently to avoid replacement peeling.


17. Oil pollution peeling. If the oil is not removed in the pre plating treatment, there is no coating in the oil polluted area of PCB multilayer board during electroplating. Even if there is coating coverage, it is false plating. The coating has no binding force with the substrate, which bulges one by one like rubella, and falls off when wiped.


18. Dark round spot coating. When the workpiece has a large plated area, such as the heat sink of the pipe. When there are many impurities in the plating solution or insufficient additives, a gray black dark round spot coating will be formed in the center of the heat sink, like a plaster. Because the center of a large area is a low current region, impurities are concentrated here. Or when the additive is insufficient, the depth ability of the plating solution decreases.


19. The gloss of the coating is uneven, and the thickness is obviously (visually) uneven. This is because the additives are just added, and the additives are not fully dispersed, resulting in inconsistent bath characteristics. After the additives are evenly dispersed, the fault will disappear naturally.


20. The plating solution is polluted by chemical fibers, and it can be seen that the coating is embedded with a trace of chemical fibers. This fault can be overcome by making PP cloth of anode bag with soldering iron ironing method.


21. Mold contamination in the plating solution (mostly in nickel plating bath, because ph4-5 environment is suitable for mold growth). It can be seen that there are many mold bacteria embedded in the electroplated layer of PCB multilayer board. In this case, disinfection and sterilization measures should be taken. In order to avoid mold contamination, attention must be paid to the implementation of cylinder opening procedure of the production line.


22. Moss pollutes water quality. The workpiece is rinsed in water containing bryophytes, which adhere to the workpiece and firmly adhere to the workpiece after drying, affecting the product quality. Every spring, we should pay attention to the possibility of moss pollution and establish a sense of prevention. If the moss contaminates the bath, the moss will be embedded in the coating.


23. The porosity of the coating is high. The high porosity of the coating affects the appearance of the coating, the protective characteristics of the coating, shortens the storage period, affects the weldability, and the brittleness of the coating is large. Most of the reasons are dirty plating solution, more metal impurities and more organic impurities. The method to identify the porosity of coating is to directly identify the characteristics of plating solution. Hang the polished degreased stainless steel sheet into the PCB multilayer board for electroplating for about 0.5-1h. If the coating completely encapsulates the stainless steel sheet, and the coating can be scraped with a knife from the edge, the whole coating can be torn off with good toughness to form a whole coated sheet. Aim the coated sheet squarely at the sunlight. If you can't see the pores, it proves that the characteristics of the plating solution are very good. If you can see the transparent electricity (pores) bit by bit, it proves that the characteristics of the plating solution are poor. If you can't tear off the coating from the stainless steel sheet and the coating tilts up like a fish scale, it proves that the characteristics of the plating solution are very poor and the plating solution needs to be treated greatly.


24. There are regular differences in coating thickness on the same hanger. This is because the projection of the Yin and Yang patterns is not accurate (the relative position of the anode and cathode is not suitable), and the power lines are unevenly distributed. There are regular differences in the plating thickness on the same hanger. This is because the elastic contact resistance of the hook where each workpiece is located is different. The plating thickness of the contact is good, and vice versa. It is a quality problem of the hanger. If there are two hangers in the same slot, one of which is thick and the other is thin, this is due to the aging degree of the two hangers Different, the newer hanger has low contact resistance and thick coating, and vice versa. If the projection of anode and cathode is correct, the aging degree of the two hangers is the same, but the coating thickness is thick on one side and thin on the other side, which changes regularly. This is due to corrosion or salt frost on the cathode on one side, resulting in poor electrical contact. In order to conduct electricity well on both sides of the plating bath and eliminate the defect of large voltage drop when one side is powered on, if the length of the plating bath is greater than 1m, both ends need to be powered on, and it should be cleaned regularly to maintain good electrical contact.


25. There are black spots on the workpiece surface of some PCB circuit boards. There may be two reasons for this:

(1) The hanger envelope is aged and cracked, and the acid-base salt exuded from the crack is sprayed by compressed gas and splashed on the workpiece, polluting the coating.

(2) The water level of rinsing water is too low, and the workpiece on the upper layer of the hanger cannot be rinsed. The workpiece that cannot be rinsed and the hanging gear cross pollute each other. Therefore, the rinsing liquid level must be higher than the workpiece on the top layer of the hanger.

(3) Droplet cross contamination.

(4) There is oil in the gas.

(5) Manual unloading operation pollution.


26. There are two possible conditions for the workpiece to change color (turn yellow) after plating and drying or change color after a short storage time:

(1) The concentration of neutralization solution is too thin and the temperature is too low to remove the film.

(2) The crystal of the coating is rough, which increases the difficulty of rinsing and removing the film.


27. There are tin nodules on the coating surface. This is because the anode mud pollutes the plating solution and the PP bag is broken. When the anode is dissolved, on the one hand, it is transferred into the plating solution in the form of ions, and some are rushed into the plating solution in the form of atoms and atomic groups to pollute the plating solution. When atomic clusters contact the workpiece, they are embedded in the coating to form tin nodules.

28. Blackbody color difference of PCB circuit board. That is, the black plastic body becomes gray black. This is because the frame stays in the alkaline solution for too long in the pretreatment or neutralization tank of PCB multilayer board electroplating, and the blackbody has been alkali corroded. The components of the blackbody include epoxy, leveling agent, curing agent, anti-aging agent, white filler, melanin and so on. When the blackbody is corroded by alkali, it will reveal the filler. White + black is a gray (heterochromatic) phenomenon.