PCB Tech - PCB circuit board hot air leveling technology

PCB hot air leveling technology is currently a relatively mature technology, but because the PCB manufacturing 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 on PCB hot air leveling process control.

1. The selection and use of flux

The flux used in hot air leveling is a special flux. Its function in hot air leveling is to activate the exposed copper surface of the printed board, improve the wettability of the solder on the copper surface; ensure that the laminate surface is not overheated, and provide protection for the solder to prevent solder oxidation when it cools after leveling. At the same time, prevent the solder from sticking to the solder mask 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.

The special flux for hot air leveling must have the following characteristics:

1. It must be a water-soluble flux, biodegradable and non-toxic.

The water-soluble flux is easy to clean, has less residue on the board surface, and does not form ion pollution on the board surface; it is biodegradable and can be discharged without special treatment, meeting environmental protection requirements, and greatly reducing 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. In the selection, it is necessary to consider both the good activity and the minimum corrosion to copper. The purpose is to reduce the solubility of copper in the solder and reduce the damage to the equipment by the smoke.

3. Thermal stability

Prevent green oil and substrate from being impacted by high temperature.

4. It must have a certain viscosity.

Hot air leveling requires a certain viscosity for the flux, which determines the fluidity of the flux. In order to completely protect the surface of the solder and laminate, the flux must have a certain viscosity. The flux with low viscosity is easy to adhere to On the surface of the laminate (also known as hanging tin), it is easy to produce bridges in dense places such as ICs.

5. Appropriate acidity

The flux with too high acidity is easy to cause the edge of the solder mask to peel off before spraying the board, and the residue after spraying the board for a long time may cause the tin surface to black and oxidize. Generally, the PH value of flux is about 2.5-3.5.

During the trial, you can test and compare one by one according to the following performance:

1. Flatness, brightness, and whether it is plugged

2. Reactivity: select fine and dense chip circuit boards and test their tinning ability.

3. The circuit board is coated with flux to prevent for 30 minutes. After cleaning, use tape to test the peeling of the green oil.

4. After spraying the board, let it stand for 30 minutes to test whether the tin surface turns black.

5. Residues after cleaning

6. Whether the dense IC bit is connected.

7. Whether tin is hung on the back of the single panel (glass fiber board, etc.).

8. Smoke

9. Volatility, odor size, whether to add thinner

10. Whether there is foam during cleaning.

2. Control and selection of hot air leveling process parameters

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 following will discuss the effects of these process parameters on the quality of printed boards. Influence.

1. Dip tin time:

The tin immersion time has a greater relationship with the quality of the solder coating. During dip soldering, the base copper and tin in the solder form a layer of metallic compound JIMC, and at the same time a layer of solder coating is formed on the wire. The above process generally takes 2-4 seconds, during which a good intermetallic compound can be formed. The longer the time, the thicker the solder.

2. Tin bath temperature:

The solder commonly used for soldering temperature of printed boards and electronic components is lead 37/tin 63 alloy, which has a melting point of 183°C. When the solder temperature is 183 degree Celsius-221 degree Celsius, the ability to form intermetallic compounds with copper is very small. At 221°C, the solder enters the wetting zone, and the range is 221°C to 293°C. Considering that the board is easily damaged at high temperatures, the solder temperature should be lower. In theory, it is found that 232°C is the maximum soldering temperature, and in practice, about 250°C can be set as the best temperature.

3. Air knife pressure:

The printed board after immersion soldering keeps too much solder, and almost all the metallized holes are blocked by the solder. The function of the air knife is to blow off the excess solder and conduct the metallized holes without reducing the diameter of the metallized holes too much. The energy used for this purpose is provided by the air knife pressure and flow rate. The higher the pressure and the faster the flow rate, the thinner the solder coating thickness.

4. Air knife temperature:

The hot air flowing out of the air knife has little effect on the printed board, and has little effect on the air pressure. But increasing 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 effect on the appearance of the solder coating after leveling. When the air knife temperature is lower than 93°C, the coating surface becomes dark. As the air temperature increases, the dark coating tends to decrease. At 176°C, the dark appearance disappears completely.

5. Air knife spacing:

When the hot air in the air knife leaves the nozzle, the flow rate slows down, and the degree of slowdown is proportional to the square of the air knife spacing. Therefore, the larger the distance, the smaller the air velocity and the lower the leveling force. The spacing of air knives is generally 0.95-1.25 cm. The spacing of the wind knives should not be too small, and the palm will cause friction on the printed board. The distance between the upper and lower air knives is generally kept at about 4mm, too large and easy to cause solder spatter.

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 molten solder splash 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 plate type and the geometric distribution angle of the plate surface.

Third, the uniformity of the solder coating thickness

The thickness of the solder 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 thickness of the solder coating of the hot air leveling also changes. Generally, the printed wires parallel to the leveling direction have low resistance to air and large leveling force, so the coating is thinner. The printed wires perpendicular to the leveling direction have large resistance to air, and the resulting The leveling 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 with strong pressure and high temperature as soon as it is brought out from the high temperature tin furnace, it is very difficult to obtain a completely uniform and flat tin surface. But it can be as smooth as possible through parameter adjustment.

Although the thickness of the solder coating of the above-mentioned PCB hot air leveling has unevenness, it can meet the requirements of MIL-STD-275D.