Hot air leveling technology is a relatively mature technology at present, but because its process is in a dynamic environment of high temperature and high pressure, it is difficult to control and stabilize the quality. This article will introduce some experience in hot air leveling process control.
Hot air leveling solder coating HAL (commonly known as spray tin) is a post-processing process widely used in circuit board factories in recent years. It is actually a combination of dip soldering and hot air leveling on the printed board metal. The process of coating eutectic solder in the hole and on the printed wire. The process is to first dip the printed board with flux, then dip it in the molten solder, and then pass between the two air knives, and use the hot compressed air in the air knife to blow off the excess solder on the printed board , At the same time remove the excess solder in the metal hole, so as to get a bright, flat and uniform solder coating.
The most prominent advantage of solder coating with hot air leveling is that the composition of the coating remains unchanged, the edge of the printed circuit can be completely protected, and the thickness of the coating can be controlled by the air knife; the gap between the coating and the substrate copper Make intermetallic chemical bonds, good wettability, good weldability, and good corrosion resistance. As a post-process of printed boards, its advantages and disadvantages directly affect the appearance of printed boards, corrosion resistance and customer welding quality. How to control its process well is a concern of every circuit board factory. Let's talk about some experience in controlling the process control of the most widely used vertical hot air leveling.
1. Selection and adoption of flux
The flux used in hot air leveling is a special flux. Its role in hot air leveling is to activate the exposed copper surface on the printed board, improve the wettability of the solder on the copper surface; ensure that the surface of the laminate is not overheated, and provide protection for the solder when it is cooled after leveling to prevent solder oxidation. At the same time, it prevents the solder from sticking to the solder resist coating to prevent the solder from bridging between the pads; the waste flux has a cleaning effect on the surface of the solder, and the solder oxide is discharged together with the waste flux.
Special flux for hot air leveling must have the following characteristics:
(1), must be water-soluble flux, biodegradable, non-toxic.
The water-soluble flux is easy to clean, has less residue on the board surface, and will not form ion pollution on the board surface; it is biodegradable and can be discharged without special treatment, which meets environmental protection requirements and greatly reduces the harm to the human body.
(2), has good activity
Regarding the activity, that is, the property of removing the oxide layer on the copper surface to improve the wettability of the solder on the copper surface, an activator is usually added to the solder. When choosing, it is necessary to consider not only the good activity, but also the minimum corrosion of copper. The purpose is to reduce the solubility of copper in solder and reduce the damage to equipment caused by smoke.
The activity of the flux is mainly reflected in the tinning ability. Because the active substances used in each flux are different, their activity varies. Flux with high activity is good for tinning on dense pads, patches, etc.; otherwise, copper exposure is easy to appear on the board surface, and the activity of active materials is also reflected in the brightness and flatness of the tin surface.
(3), thermal stability
Prevent green oil and base material from being impacted by high temperature.
(4), there must be a certain viscosity.
Hot air leveling requires a certain viscosity of the flux, which determines the fluidity of the flux. In order to completely protect the solder and the surface of the laminate, the flux must have a certain viscosity, and the flux solder with low viscosity is easy to adhere to On the surface of the laminate (also known as hanging tin), and it is easy to generate bridges in dense places such as ICs.
(5), suitable acidity
Flux with too high acidity will easily cause the edge of the solder mask layer to peel off before spraying the board, and its residue after spraying the board for a long time will easily cause the tin surface to blacken and oxidize. Generally, the pH value of flux is around 2.5-3.5. Other performances are mainly reflected in the impact on operators and operating costs, such as unpleasant smell, high volatile substances, large smoke, unit coating area, etc. Manufacturers should choose based on experiments.
During the trial, the following performances can be tested and compared one by one:
(1), flatness, brightness, whether the hole is plugged
(2) Activity: Select a fine and dense patch circuit board to test its tinning ability.
(3) The circuit board is coated with flux to prevent it for 30 minutes, and after cleaning, use tape to test the peeling of the green oil.
(4) Leave the board for 30 minutes after spraying, and test whether the tin surface turns black.
(5), residue after cleaning
(6) Whether the dense IC bits are connected.
(7) Whether there is tin on the back of the single panel (glass fiber board, etc.).
(8), smoke
(9) Volatility, odor, whether it is necessary to add diluent
(10) Whether there is foam when cleaning.
2. Control and selection of hot air leveling process parameters
The hot air leveling process parameters include solder temperature, dip soldering time, air knife pressure, air knife temperature, air knife angle, air knife spacing and printed board rising speed, etc. The influence of these process parameters on the quality of printed boards will be discussed below .
(1), immersion tin time:
The tin immersion time has a great relationship with the quality of solder coating. During dip soldering, the base copper and the tin in the solder form a layer of metal compound IMC, and at the same time form a layer of solder coating on the wire. The above process generally takes 2-4 seconds, and a good intermetallic compound can be formed within this time. The longer the time, the thicker the solder. However, if the time is too long, the base layer material of the printed board will be delaminated and the green oil will foam. If the time is too short, it will easily cause half-immersion phenomenon, resulting in partial whitening of the tin surface, and it is also prone to rough tin surface.
(2), tin bath temperature:
The commonly used solder for the soldering temperature of printed boards and electronic components is lead 37/tin 63 alloy, and its melting point is 183°C. When the solder temperature is 183°C-221°C, the ability to form intermetallic compounds with copper is very small. At 221°C, the solder enters the wetting zone, which ranges from 221°C to 293°C. Considering that the plate is easily damaged at high temperature, the solder temperature should be selected lower. In theory, it is found that 232°C is the maximum soldering temperature, and in practice, it can be set as the optimum temperature around 250°C.
(3), air knife pressure:
Excessive solder remained on the printed board after solder dipping, and almost all metallized holes were blocked by solder. The function of the air knife is to blow off the excess solder and conduct the metallized hole without reducing the diameter of the metallized hole too much. The energy used to do this is provided by air knife pressure and flow rate. The higher the pressure and the faster the flow rate, the thinner the solder coating will be. Therefore, air knife pressure is one of the most important parameters for hot air leveling. Usually the air knife pressure is 0.3-0.5MPa.
The pressure before and after the air knife is generally controlled to be large at the front and small at the rear, and the pressure difference is 0.5MPa. According to the distribution of geometric figures on the board, the front and rear air knife pressure can be adjusted appropriately to ensure that the IC position is flat and the patch has no protrusions. For the specific value, refer to the factory manual of the tin spraying machine of the factory.
(4), air knife temperature:
The hot air flowing from the air knife has little effect on the printed board and has little effect on the air pressure. But raising the temperature inside the air knife helps the air expand. Therefore, when the pressure is constant, increasing the air temperature can provide a larger air volume and a faster flow rate, so as to generate a larger leveling force. The temperature of the air knife has a certain influence on the appearance of the solder coating after leveling. When the temperature of the air knife is lower than 93°C, the surface of the coating is dark, and as the air temperature increases, the dark coating tends to lighten. At 176°C, the dull appearance disappeared completely. Therefore, the lowest temperature of the air knife is not lower than 176°C. Usually, in order to obtain a good flatness of the tin surface, the temperature of the air knife can be controlled between 300°C and 400°C.
(5), air knife spacing:
As the hot air in the air knife leaves the nozzle, the velocity of the flow slows down proportional to the square of the distance between the air knives. Therefore, the larger the spacing, the lower the air velocity and the lower the leveling force. The distance between air knives is generally 0.95-1.25CM. The distance between the air knives should not be too small, otherwise the air will cause friction on the printed board, which will be detrimental to the board surface. The distance between the upper and lower air knives is generally kept at about 4mm, if it is too large, solder spatter may occur.
(6), air knife angle:
The angle of the air knife blowing the board affects the thickness of the solder coating. If the angle is not adjusted properly, the thickness of the solder on both sides of the printed board will be different, and it may also cause splashing of molten solder and noise. The angle of most front and rear air knives is adjusted to a downward slope of 4 degrees, which is slightly adjusted according to the specific board shape and the geometric distribution angle of the board surface.
(7), PCB rising speed:
Another variable related to hot air leveling is the speed at which it passes between the air knives, i.e. the speed at which the conveyor rises, which affects the thickness of the solder. The slower the speed, the more air is blown onto the PCB, so the solder is thinner. On the contrary, the solder is too thick and even blocks the hole.
(8), preheating temperature and time:
The purpose of preheating is to increase the activity of the flux and reduce thermal shock. The general preheating temperature is 343°C. When preheating for 15 seconds, the surface temperature of the printed board can reach about 80°C. Some hot air leveling does not have a preheating process.
3. Uniformity of solder coating thickness
The solder thickness applied by hot air leveling is basically uniform. However, as the geometric factors of the printed wires change, the leveling effect of the air knife on the solder also changes, so the solder coating thickness of the hot air leveling also changes. Generally, printed conductors parallel to the leveling direction have less resistance to air and greater leveling force, so the coating is thinner, and printed conductors perpendicular to the leveling direction have greater resistance to air, resulting in a flattened surface. The flat effect is small, so the coating is thicker, and the solder coating in the metallized hole is also uneven. Since the solder is immediately placed in a dynamic environment of high pressure and high temperature as soon as it is lifted from the high-temperature tin furnace, it is very difficult to obtain a completely uniform and flat tin surface. However, it can be as smooth as possible through parameter adjustment.
(1), choose good active flux and solder
Flux is the main factor for the smoothness of the tin surface. Flux with good activity can get a relatively smooth, bright and complete tin surface.
The solder should choose lead-tin alloy with high purity, and regularly carry out copper bleaching treatment to ensure that its copper content is below 0.03%. Refer to the workload and test results for details.
(2), equipment adjustment
The air knife is the direct factor to adjust the flatness of the tin surface. The angle of the air knife, the pressure of the front and rear air knife and the pressure difference change, the temperature of the air knife, the distance between the air knife (vertical distance, horizontal distance) and the lifting speed will cause a great impact on the board surface. influences. For different board types, the parameter values are not the same. Some advanced tin spraying machines are equipped with microcomputers, and the parameters of various board types are stored in the computer for automatic adjustment.
The air knife and the inside of the guide rail are cleaned regularly, and the residue in the air knife gap is cleaned every two hours. When the production volume is large, the cleaning density will increase.
(3), pre-processing
The micro-etching treatment also has a great influence on the flatness of the tin surface. If the depth of micro-etching is too low, it is difficult for copper and tin to form copper-tin compounds on the surface, resulting in local roughness of the tin surface; poor stabilizer in the micro-etching solution will cause the copper etching speed to be too fast and uneven, and will also cause the tin surface to be uneven. It is recommended to use the APS system.
For some plate types, sometimes it is necessary to pre-treat the baking plate, which will also have a certain impact on the leveling of the tin.
(4) Pre-process control
Because hot air leveling is the last treatment, many previous processes will have a certain impact on it, such as dirty development will cause poor tinning, etc. Strengthening the control of the previous process can greatly reduce the problems in hot air leveling.
Although there is unevenness in the thickness of the above-mentioned solder coating for hot air leveling, they can all meet the requirements of MIL-STD-275D.
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