The surface finish is a crucial process in PCB manufacturing. Circuit boards work under conditions that expose them to contaminants in harsh environments. PCB corrosion is a major concern for PCB manufacturers. The sole purpose of surface finish is to prevent corrosion in PCB and enhance functionality. Therefore, all boards have copper finished surfaces.
Surface finish serves as a form of protection to circuit boards. The copper in printed circuit boards will oxidize and deteriorate if they are unprotected. PCB surface finishes create an interface between the circuit and the component. However, there are different types of PCB surface finishes. In this article, our main focus is on Hot Air Solder Leveling (HASL).
What is Hot Air Solder Leveling (HASL)?
Hot air solder leveling (HASL) is a reliable and efficient surface finish. This surface finish process involves submerging the PCB in molten solder. Also, the solder could be tin or lead. Therefore, solder will cover the exposed copper surfaces. HASL surface finish comprises the combination of tin and lead. The tin accounts for 63% while the lead accounts for 37%.
However, there are lead-free HASL circuit boards. For these boards, there is absence of lead. Hot air leveling technology involves a high pressure and high temperature environment. Therefore, it is quite difficult to control the quality. HASL offers a reliable solder joint and long shelf life. Also, this type of surface finish offers effective component soldering. However, the surface planarity may not be ideal for fine-pitch components due to HASL coating’s thickness.
PCB manufacturers can use the air knife to control the thickness of the coating. The hot air solder leveling process needs to be controlled to achieve success. Due to the benefits of this surface finish, it is widely used in aerospace and military applications.
HASL is very effective for larger SMT or through-hole components. Also, this surface finish is one of the most cost-effective finishes in the PCB industry. Therefore, it is an ideal option for general purpose printed circuit boards. The low cost and robust solution of HASL made it a popular choice for PCB surface finish. However, the introduction of more advanced surface mount technology exposed the drawbacks of HASL.
Also, HASL isn’t ideal for fine pitch SMT PCBA. This is because of its incompatibility between fine pitch components and uneven surfaces.
What is the HASL Process?
This PCB surface finish involves dipping the printed circuit board into a pot of molten solder after applying a solder mask. After this, the PCB manufacturer removes the excess solder through hot air leveler. This step involves using hot-air knives.
The thin solder layer offers protection against corrosion. Also, the finish enhances the soldering of components to the printed circuit board during PCBA. HASL can be lead-based or lead-free. Also, the lead-free HASL and lead-based HASL are identical in terms of appearance and usage. However, lead-free HASL comprises the combination of 99.3% tin and 0.6% copper.
This mixture features a higher melting point for lead-free HASL. Therefore, this increases the need for a slight change to the reflow soldering operation in PCBA solution. Also, the flux used in HASL is a special one. This PCB surface finish has to activate the exposed copper surface of the PCB to enhance the solder on the surface.
The solder applied to hot air leveling must have a uniform thickness. Also, the air knife helps to level the solder. However, the impact of the air knife on the solder leveling changes.
The Characteristics of Heat Flux Used in Hot Air Leveling
The HASL process involves the use of special heat flux. Also, this flux plays a crucial role in activating the exposed copper surface of the board. However, heat flux must have the following characteristics to perform well:
Non-toxic and water-soluble
Water-soluble flux has less board residue and is easy to clean. Also, this flux won’t form any ion pollution in the printed circuit board. This flux can meet environmental standards without any special treatment.
Thermal stability
With good thermal stability, the PCB won’t have green oil.
Good activity
Copper surface layer removal facilitates solder wettability on the copper surfaces. Also, it is important to consider the good activity and the minimum corrosion of copper. Also, you need to minimize the solubility of copper in the solder.
Certain viscosity
The flux needs a certain level of viscosity. Also, the flux needs certain viscosity in order to protect the laminate surface.
The Control and Selection of Hot Air Leveling Process Parameters
There are several parameters involved in hot air leveling process. These parameters determine the success of HASL.
Dip tin time
There is a strong relationship between dip time and solder coating quality. Also, the tin in the solder and the copper in the matrix create a metal compound layer. Generally, this process takes within 2 to 4 seconds. At this time, there is formation of intermetallic compounds. Also, longer time makes the solder thicker.
If the time is too long, the PCB material will have green oil blistering. It will be easier to produce a half-dip scenario if the time is too short.
Solder temperature
For PCBS and electronic components, the commonly used solder temperature for lead/tin is 1830C. There are low chances to produce intermetallic compounds when the temperature of the solder is 183oC-221oC.
Wind knife pressure
After the PCB has been welded to keep excess solder, the solder blocks the metal holes. Therefore, the air knife helps to blow off too much solder. It then turns on the metal hole to prevent reducing the aperture of the hole too much. The pressure of the wind knife is the brain behind this. Great pressure gives thinner solder coating. Therefore, the pressure of the wind knife is a crucial parameter of hot air leveling.
Wind knife spacing
The flow rate reduces when the hot air in the wind knife moves from the nozzle. Also, the diminishing degree is proportional to the air knife pitch’s square. Therefore, larger spacing results in smaller rate of air flow. The spacing of air knife is usually between 0.95 and 1.25CM. The air on the circuit board will product friction is the distance between the air knives is small. Therefore, the lower and upper air knife must have a spacing of almost 4mm.
PCB rising speed
The speed through the air knife is a crucial parameter in hot air leveling process. The solder thickness depends on the speed of the conveyor. The solder is thin by slowly blowing the air on the PCB. Otherwise, the solder will become too thick.
Air knife angle
The angle of wind knife will affect the thickness of solder coating. Also, the solder thickness on both sides of the board won’t be the same if the angle adjustment is not accurate. Therefore, this may result in molten solder splash.
Advantages and Disadvantages of HASL
HASL has some benefits and drawbacks.
Advantages
- It is reworkable
- Low cost
- Long shelf life
- It is widely available
- Meets military standards
Disadvantages
- Uneven surfaces
- It isn’t ideal for fine pitch
- Has solder bridging issues
- It could contain lead
- Not suitable for high density interconnect structures
- It is not ideal for aluminum wire bonding
Applications of HASL
Lead-free and leaded HASL surface finishes are widely used in applications like:
Hand soldering
HASL PCB surface finish is ideal for hand soldering processes. This is because the joints are easy to form. Therefore, this makes it widely used for plated through holes.
HASL surface finish offers automatic shielding for vias and test pads during electrical testing of a printed circuit board.
High-performance electronics
Hot air leveling is an ideal choice for high-reliability and high-performance applications like military and aerospace. This is because of its formation of strong joints.
Other PCB Surface Finishes
This PCB surface finish is a non-electrolytic chemical finish. PCB manufacturers immerse the circuit board into tank of silver ions. It is a reliable surface finish for wire bonding and dome contacts. Immersion silver is better than HASL in terms of its environmental friendliness. Also, it has a good shelf life. Immersion silver is a RoHS compliant surface finish.
The surface of immersion silver PCBs displays great weldability for soldering. Immersion silver uses displacement reaction. Therefore, this offers great electrical properties.
Immersion tin is one of the most reliable PCB surface finishes. This RoHS compliant surface finish is a suitable choice for fine pitch components and flat surface. Immersion tin process involves depositing a layer of tin on PCB copper layer. The PCB manufacturer applied a fine texture of thin layer to the printed circuit board surface.
The tin layer in this surface finish shields untreated copper from any form of oxidation. Also, immersion tin is ideal for soldering applications. Immersion tin has a shelf life of six months. It is planar or flat, making it ideal for fine pitch or BGA component.
Organic solderability preservative (OSP)
The OSP surface finish is specifically used for copper pads. Also, this surface finish bonds to copper and shield copper pad. OSP surface finish belongs to other lead free options. The OSP process involves applying a thin protective layer of material over the exposed copper surface. Also, this PCB surface finish achieves this by utilizing a conveyorized process.
Furthermore OSP is highly conductive and performs well in solderability wetting. As a result of its coplanarity, OSP is a better alternative to lead-free HASL. Also, OSP has a long shelf life and offers a smooth surface.
What is the Difference Between HASL and ENIG Surface Finish?
There is a huge difference between HASL and ENIG surface finish. While HASL uses tin and lead, ENIG uses nickel and immersion gold as its major coating material. Electroless Nickel Immersion Gold (ENIG) has greater advantages over HASL. Also, ENIG has flat surfaces.
HASL surface finish is available in two variants which are lead-free and lead. However, a good majority of HASL printed circuit boards feature lead. Lead is dangerous for the environment as it poses health and safety risks. Electroless nickel immersion gold is environmentally-friendly. However, this surface finish option is very expensive.
In ENIG, a gold layer shields the nickel during storage. Gold is expensive and as such ENIG is expensive. Also, electroless nickel immersion gold provides a flat surface when used. Also, it is ideal for plated through hole. The gold and nickel layers in ENIG offer great protection for PCB surfaces.
Hot air leveling leaves uneven surfaces. However, ENIG offers a very flat surface for a printed circuit board. Also, this flatness enables even heating and accurate placement. HASL is a low cost option for easy prototyping. ENIG is a great option for fine pitch and BGA components. Also, immersion gold offers a plated through hole solution and double layer metallic coating.
The HASL process involves dipping copper into molten solder. Also, this process requires the use of compressed hot-air knives to remove excess solder. On the other hand, immersion gold process is a complicated one that involves three layers. Also, these layers are copper, nickel, and gold. ENIG surface finishes protect exposed copper circuitry. Also, immersion gold plating is ideal for complex surface components.
Frequently Asked Questions
Is Lead-free HASL RoHS Compliant?
Lead-free HASL complies with standards of RoHS. This surface finish contains no lead. Rather than using tin and lead in, HASL uses copper and tin. A large percentage of this surface finish is tin. Also, this HASL features 99.3% tin and 0.6% copper.
Is immersion gold lead free?
Immersion gold is a lead free finish that offers a good shelf life. Also, this finish process doesn’t involve the use of lead. Therefore, it is environmentally safe.
Conclusion
PCB surface finishes help to protect exposed copper circuitry. Hence, it is important to understand how they work. If you need a low-cost surface finish with a less complicated process, HASL is a great option. Also, it is crucial to choose the appropriate surface finish for your PCB project.
Choosing the best surface finish can be challenging. RayMing has got you covered on your PCB surface finish problem. We offer reliable surface finish for your PCBs. Also, we advise customers on the best PCB surface finish for their project.
