PCB Blog - HASL hot air solder leveling process for PCB

HASL = Hot Air Solder Level technology is a relatively mature technology at present, but its quality is difficult to control and stabilize because its process is in a dynamic environment of high temperature and high pressure. This paper will introduce some experience in Hot Air Solder Level (HASL) process control.

HASL solder coating Hal is a post-processing process widely used in circuit board factories in recent years. It is actually a process of coating eutectic solder in metalized holes of PCBs and printed wires by combining immersion welding and HASL. The process is to dip the solder flux on the PCB first, then dip it in the molten solder, then pass between the two air knives, blow off the excess solder on the PCB with the hot compressed air in the air knife, and eliminate the excess solder in the metal hole at the same time, so as to obtain a bright, flat and uniform solder coating.

The most outstanding advantage of solder coating with HASL is that the coating composition remains unchanged all the time, the edge of PCB can be completely protected, and the coating thickness can be controlled by air knife. The coating and the base copper are bonded to each other, with good wettability, weldability and corrosion resistance. As the post process of PCB, its quality directly affects the appearance, corrosion resistance and customer welding quality of PCB.How to control the process is a matter of concern to PCB manufacturers. Next, let's talk about some experience of controlling its process control on the most widely used vertical HASL.

Selection and application of flux

The flux used in Hot Air Solder Level  is a special flux. Its function in HASL is to activate the exposed copper surface on the PCB and improve the wettability of solder on the copper surface. Ensure that the surface of the laminate is not overheated, provide protection for the solder during cooling after leveling, prevent the solder from oxidation, and prevent the solder from sticking to the solder resist coating to prevent the solder from bridging between the pads. The waste flux can clean the solder surface, and the solder oxide is discharged with the waste flux.

The special flux for Hot Air Solder Level must have the following characteristics

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

Water soluble flux is easy to clean, with less residue on the PCB board surface, and will not form ion pollution on the PCB board surface. Biodegradation can be discharged without special treatment, which meets the requirements of environmental protection and greatly reduces the harm to human body.

2. It has good activity

As for activity, that is, the characteristic of removing the oxide layer on the copper surface and improving the wettability of solder on the copper surface, activator is usually added to solder. In the selection, both good activity and minimum corrosion to copper shall be considered, in order to reduce the solubility of copper in solder and reduce the damage of smoke to equipment.

The activity of flux is mainly reflected in the tin loading ability. Because the active substances used by various fluxes are different, their activities are different. High activity flux, good tin on dense pads, patches, etc; On the contrary, copper exposure is easy to occur on the board surface, and the activity of active substances is also reflected in the brightness and flatness of tin surface.

3. Thermal stability

Protect green oil and substrate from high temperature impact.

4. Must have a certain viscosity

HASL requires a certain viscosity of 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 the surface of the laminate, and is easy to bridge in dense places such as IC.

5. Suitable acidity

The solder flux with high acidity is easy to peel off the edge of the solder resist layer before spraying the PCB, and its residue after spraying the plate for a long time is easy to cause the tin surface to blacken and oxidize. The pH value of general flux is 2.5-3.5 About 5.

Other performances are mainly reflected in the impact on operators and operating costs, such as bad smell, high volatile substances, large smoke, unit coating area, etc. manufacturers should select them on the basis of experiments.

During trial, the following performance can be tested and compared one by one

1.Flatness, brightness, whether the hole is plugged

2.Activity: select fine dense SMD PCB and test its tin loading ability.

3.The PCB shall be coated with flux for 30 minutes. After cleaning, the green oil peeling shall be tested with adhesive tape.

4.Place the spray plate for 30 minutes and test whether its tin surface turns black.

5.Residue after cleaning

6.Whether the dense IC bit is connected.

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

8.Smog

9.Volatility, odor size, whether diluent needs to be added

10.whether there is foam when cleaning.

Control and selection of Hot Air Solder Level process parameters

HASL process parameters include solder temperature, immersion time, air knife pressure, air knife temperature, air knife angle, air knife spacing and rising speed of PCB. The influence of these process parameters on PCB quality will be discussed below.

1. Tin dipping time

The tin dipping time has a great relationship with the quality of solder coating. During immersion welding, the base copper and tin in the solder form a layer of metal compound into IMC, and a layer of solder coating is formed on the conductor. The above process generally takes 2-4 seconds, during which good intermetallic compounds can be formed. The longer the time, the thicker the solder. However, if the time is too long, the base material of PCB will be layered and green oil will bubble. If the time is too short, it is easy to produce semi immersion, resulting in local tin surface whitening and tin surface roughness.

2. Bath temperature

The solder commonly used for the welding temperature of PCBs and electronic components is lead 37 / tin 63 alloy, and its melting point is 183 degree Celsius. When the solder temperature is 183 degree Celsius - 221 degree Celsius, the ability to form intermetallic compounds with copper is very small. At 221 degree Celsius, the solder enters the wetting zone, and the range is 221 degree Celsius - 293 degree Celsius. Considering that the plate is easy to be damaged at high temperature, the solder temperature should be lower. It is found that 232 degree Celsius is the most suitable solder temperature in theory, and about 250 degree Celsius can be set as the best temperature in practice.

3. Air knife pressure

There is too much solder on the PCB after immersion welding, and almost all metallization holes are blocked by solder. The function of the wind knife is to blow off the excess solder and conduct the metallization hole without reducing the diameter of the metallization hole too much. The energy used to achieve this is provided by the wind knife pressure and flow rate. The higher the pressure, the faster the flow rate, and the thinner the solder coating thickness. Therefore, blade pressure is one of the most important parameters of Hot Air Solder Level (HASL). Generally, the air knife pressure is 0.3-0.5mpa

The pressure before and after the air knife is generally controlled as large in front and small in rear, and the pressure difference is 0.05Mpa. According to the distribution of geometric figures on the board surface, the front and rear air knife pressure can be appropriately adjusted to ensure that the IC position is flat and the patch has no protrusion. Refer to the factory manual of tin spraying machine of the factory for specific values.

4. Air knife temperature

The hot air from the air knife has little effect on the PCB and air pressure. However, increasing the temperature in the air knife helps to expand the air. Therefore, when the pressure is constant, increasing the air temperature can provide a larger air volume and faster flow rate, so as to produce 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 air knife temperature is lower than 93 degree Celsius, the coating surface darkens. With the increase of air temperature, the darkened coating tends to reduce. At 176 degree Celsius, the dark appearance disappeared completely. Therefore, the minimum temperature of the air knife shall not be lower than 176 degree Celsius. Generally, in order to obtain good tin surface flatness, the temperature of the air knife can be controlled between 300 degree Celsius - 400 degree Celsius.

5. Blade spacing

When the hot air in the blade leaves the nozzle, the flow rate slows down, and the degree of slowing down is directly proportional to the square of the blade spacing. Therefore, the larger the spacing, the smaller the air velocity and the lower the leveling force. The spacing of air knives is generally 0.95-1.25cm. The spacing of air knives should not be too small, otherwise the air will produce friction on the PCB, which will be unfavorable to the board surface. The distance between the upper and lower blades is generally kept at about 4mm, which is too large and prone to solder splash.

6. Blade angle

The angle of the air knife blowing the board affects the solder coating thickness. If the angle is not adjusted properly, the solder thickness on both sides of the PCB will be different, and molten solder spatter and noise may also be caused. The angle of most front and rear air knives is adjusted to 4 degrees downward, which is slightly adjusted according to the specific plate type and the geometric distribution angle of the plate surface.

7. Rising speed of PCB

Another variable related to Hot Air Solder Level (HASL) is the speed of passing between blades, that is, the rising speed of the conveyor, which affects the thickness of solder. The speed is slow, and there is more air blowing on the PCB, so the solder is thin. On the contrary, the solder is too thick and even blocks the hole.

8. Preheating temperature and time

The purpose of preheating is to improve the activity of flux and reduce thermal shock. The general preheating temperature is 343 degree Celsius. When preheating for 15 seconds, the surface temperature of PCB can reach about 80 degree Celsius. Some Hot Air Solder Level (HASL) do not have a preheating process.

Uniformity of solder coating thickness

The solder thickness applied to Hot Air Solder Level is basically uniform. However, with the change of geometric factors of printed wires, the leveling effect of air knife on solder also changes, so the solder coating thickness of hasl also changes. Generally, the printed wire parallel to the leveling direction has small resistance to air and large leveling force, so the coating is thinner. The printed wire perpendicular to the leveling direction has large resistance to air and small leveling effect, so the coating is thicker, and the solder coating in the metallized hole is also uneven. Since the solder is in a dynamic environment of strong pressure and high temperature as soon as it is raised from the high-temperature tin furnace, it is very difficult to obtain a completely uniform and flat tin surface. However, it can be as flat as possible through parameter adjustment.

1.Choose active flux and solder

Flux is the main factor of tin surface flatness. A relatively flat, bright and complete tin surface can be obtained with a good active flux.

Lead tin alloy with high purity shall be selected for solder, and copper floating treatment shall be carried out regularly to ensure that its copper content is less than 0.03%. Refer to workload and test results for details.

2.Equipment adjustment

The air knife is the direct factor to adjust the flatness of tin surface. The air knife angle, the change of front and rear air knife pressure and pressure difference, air knife temperature, air knife spacing (vertical distance, horizontal distance) and lifting speed will have a great impact on the plate surface. For different plate types, the parameter values are different. Some technologically advanced tin spraying machines are equipped with microcomputers to store the parameters of various plate types in the computer for automatic adjustment.

The air knife and guide rail shall be cleaned regularly. The clearance residue of the air knife shall be cleaned every two hours. When the production is large, the cleaning density shall increase.

3. Pretreatment

Micro etching treatment also has a great influence on the flatness of tin surface. If the micro etching depth is too low, it is difficult for copper and tin to form copper tin compounds on the surface, resulting in local tin surface roughness; Poor stabilizer in micro etching solution will lead to excessive and uneven copper etching speed and uneven tin surface. APS system is generally recommended.

For some plate types, baking plate pretreatment is sometimes required, which will also have a certain impact on tin leveling.

4. Pre process control

Because Hot Air Solder Level (HASL) is the last treatment, many previous processes will have a certain impact on it, such as poor tin loading due to unclean development. Strengthening the control of previous processes can greatly reduce the problems in Hot Air Solder Level (HASL).

Although the solder coating thickness of the above HASL is uneven, it can meet the requirements of mil-std-275d.