Hot air solder leveling
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HASL or HAL (for hot air (solder) leveling) is a type of finish used on printed circuit boards (PCBs).
The PCB is typically dipped into a bath of molten solder so that all exposed copper surfaces are covered by solder. Excess solder is removed by passing the PCB between hot air knives.[1]
HASL can be applied with or without lead (Pb), but only lead-free HASL is RoHS compliant.
Advantages of HASL
[edit]- Excellent wetting during component soldering.
- Avoids copper corrosion.
Disadvantages of HASL
[edit]- Low planarity on vertical levelers may make this surface finish unsuitable for use with fine pitch components. Improved planarity can be achieved using a horizontal leveler.
- High thermal stress during the process may introduce defects into PCB.
Detailed Explanation of the HASL Process Flow
[edit]Immersion Soldering and Hot Air Leveling (HAL)
After the solder covers the copper surface, the circuit board is quickly sent into the hot air leveling equipment. The key to this process is to use one or more pairs of high-pressure hot air nozzles to blow away excess solder, forming a flat, uniform coating on the pads.
The temperature of the hot air is typically around 250°C and has a certain pressure. The purpose is to shape the molten solder into a standard thickness without damaging the pads or mask. This process requires extremely high precision in equipment control to avoid the formation of solder bridges or solder stringing.
Cooling and Solidification
After hot air leveling is completed, the circuit board is rapidly cooled to solidify the solder quickly, forming dense and robust solder joints. This cooling step requires appropriate control. Too rapid cooling may cause solder cracking, while too slow cooling may result in uneven solder layers.
Post-cleaning:
In the HASL process, solder and flux residues remain on the board surface, especially when using flux containing rosin. To ensure subsequent assembly and reliability, the circuit board must be thoroughly cleaned using specialised cleaning equipment, and sometimes deionised water and cleaning agents are also required.
Quality Inspection:
After the PCB completes HASL, it undergoes visual inspection and thickness testing. A microscope is used to observe pad coverage, confirming defects such as solder bridges, bubbles, or voids, and XRF equipment is used to verify solder thickness compliance with IPC standards (e.g., IPC-6012 specifies HASL surface thickness ≥1µm).
Packaging and Storage
PCBs that pass inspection are packaged in vacuum bags with desiccants to prevent moisture and oxidation, then labelled and stored in the warehouse, awaiting shipment or entry into the PCBA assembly stage.
Technical Challenges and Difficulties of the HASL Process
Despite its maturity and cost advantages, the HASL process remains widely used in PCB manufacturing. However, in modern electronics manufacturing, particularly under the backdrop of high density, high precision, and increasingly stringent environmental requirements, the HASL process faces numerous technical challenges and difficulties.
Difficulty in Controlling Solder Thickness Hot Air
The thickness of the solder layer in the hot air leveling process directly affects subsequent assembly quality and circuit performance. Excessively thick solder can cause issues such as solder bridging and short circuits, especially when mounting fine-pitch components (e.g., QFP, BGA, etc.); conversely, excessively thin solder can lead to poor solder joints and poor conductivity. During the hot air leveling process, parameters such as air pressure, temperature, and solder immersion time must be precisely controlled; even minor deviations can result in quality issues.
Significant Thermal Stress on PCB Substrates
During the HASL process, PCBs undergo high-temperature tin immersion and hot air leveling, with temperatures often exceeding 250°C. For high-layer boards, multi-layer boards, or circuit boards made of special materials (such as high TG boards), thermal stress may cause issues like board warping, layer delamination, and reduced insulation performance. Especially in designs with fine line widths and fine pitch, thermal stress is more likely to cause manufacturing defects.
Not suitable for ultra-fine pitch and micro-pads
As the trend toward miniaturisation of electronic products intensifies, the application of micro-pitch chips and micro-pads is increasing. However, the HASL surface is relatively uneven, and the solder layer has certain thickness fluctuations. When soldering small pads and micro-components, this may result in excessive or insufficient solder, thereby reducing soldering reliability.
Environmental regulations and waste disposal
: Leaded HASL, although mature in process and cost-effective, faces increasingly stringent environmental regulations globally due to its lead content, such as the EU RoHS Directive. Lead-free HASL, while compliant with environmental requirements, involves a more complex process and higher costs. Additionally, waste generated by the HASL process (such as solder dross and flux residues) requires proper disposal, increasing environmental burden.
Flux Residue Issues:
The flux used in the HASL process may leave residues on the PCB surface. If not thoroughly cleaned, these residues may affect subsequent assembly, soldering, and electrical performance, potentially leading to product failures. In particular, some lead-free fluxes have higher viscosity, making them more difficult to clean.
See also
[edit]- Electroless Nickel Immersion Gold (ENIG)
- Immersion Silver (IAg)
- Organic Solderability Preservative (OSP)
- Reflow soldering
- Wave soldering
References
[edit]- ^ Surface finish of PCB. Surface finish of PCB.