Hot air solder leveling

Foreword

Hot air solder leveling, in the era of tin-lead solder, was the surface treatment method with the highest usage rate in the PCB industry in Taiwan because it had a certain level of competitiveness. With its well-developed technique, low production cost and rapid production process, it was widely used for over 20 years. However, with the consideration of environmental protection during recent years, the solder can’t be used anymore.

The melting point of lead-free solder is 30 °C higher than the conventional tin-lead solder on average; to adapt to the new high temperature process, adjustments to the equipment and techniques must be made. However, we believe that even in the future, lead-free solder will continuously be the most competitive surface treatment method.

Purpose of Hot Air Solder Leveling

The purpose is to immerse the semi-finished board applied with solder mask in the molten tin to let the hole wall and the bare copper bonding pad be coated by the solder, immediately pull it out of the tin tank (transported through roller), and then let two hot air knifes blow off the solder in the holes with high pressure hot winds while the hole wall and the bonding pad can still be coated with a solder layer that helps with soldering.

Principle of the Process

3-1 Vertical Hot Air Solder Leveling Process

Feeding → pre-treatment cleaning → immersing flux → spraying tin → post treatment cleaning → quality inspection → packaging

3-2 Horizontal Hot Air Solder Leveling Process

Feeding → pre-treatment cleaning → preheating → immersing flux → halfway inspection → spraying tin → air-floating cooling bed → post treatment cleaning → quality inspection → packaging

3-3 Pre-treatment Cleaning

1.Copper etching of 15–25 µin.

2.The main function is cleaning the copper surfaces, etching the thin layer of copper surface with sulfuric acid hydrogen peroxide to fully remove the organic pollutant on the surface, so that the clean copper surface can react with molten tin and successfully form the IMC.

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Vertical hot air solder leveling
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Horizontal hot air solder leveling

3-4 Preheating (Horizontal Hot Air Solder Leveling)

1.Preheating temperature of 100 ± 50 °C.

2.Board temperature increases to reduce the damage caused by the thermal stress due to instantaneous high temperature.

3.The activity and capability of the flux increases to remove the oxidized contaminants on the copper surface and increase solderability.

4.To prevent the board from absorbing the heat when it enters the solder pot and the temperature in the solder port decreases as a result to affect the gloss and smoothness of the surface. This will also affect the throwing power.

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3-5 Flux

1.Remove oxide from the copper surface.
2.Protect the copper surface from oxidation again at high temperatures.
3.Assist in delivery and distribution of the heat, making the heat in different areas distribute more evenly.
4.Increase the wettability of the metal and solder.
5.Prevent solder from attaching to the PCB surface.

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Vertical hot air solder leveling
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Horizontal hot air solder leveling

3-6 Vertical hot air solder leveling machine

The substrate is hung on a mechanical arm and enters the tin tank downward vertically along the rail (upper right picture). The substrate is then immersed in the tin bath for about 3–6 seconds (bottom right picture).

The pads and vias are filled with solder during the tin bath. After the substrate is pulled up using the mechanical arm and leaves the solder pot, the two hot air knifes blow off the solder from the vias with high pressure hot winds from the front and rear and smooth them to the required smoothness and thickness for hot air solder leveling.

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3-7 Horizontal hot air solder leveling machine

The roller moves the substrate forward horizontally and stays in the tin tank area with three rows of rollers for 2–4 seconds.

After the substrate leaves the tin tank area, the two hot air knifes blow the solder off the vias with high pressure hot winds from the top and bottom. The solder that was blown off is guided to flow back to the tin tank area to meet the requirements of smoothness and thickness.

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3-8 Air-floating Cooling Bed

The cooling bed is used to reduce the board surface temperature.

The blower in the air bed blows cool wind. When the substrate enters the cooling section from the tin tank area, the cool wind blows from bottom to top and makes the board floating to ensure that the board is cooled and the temperature is reduced properly. At this time the tin surface changes from a liquid state to a solid state.

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3-9 Post-treatment Cleaning

The purpose is to remove the residues after the high-temperature HASL process.

Mechanical rinsing is conducted with hot water at 60 ± 10 °C and by brushing to completely remove the residual fluxes, solder powder and grains from the board surface.

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Advantages and Disadvantages of Vertical HASL
principle 1

Advantages and Disadvantages of Horizontal HASL
principle 2

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