Introduction
Hot Air Solder Leveling (HASL) is one of the most common surface finishes used in printed circuit board (PCB) manufacturing today. It involves coating the copper traces on a PCB with solder to protect them from oxidation and provide a solderable surface.
This comprehensive guide will cover everything you need to know about HASL PCB manufacturing. We’ll start by looking at what HASL is, then dive into the step-by-step HASL process. We’ll also compare HASL to other PCB surface finishes, discuss HASL specifications, examine the pros and cons of using HASL, and look at some best practices for working with HASL PCBs.
What is HASL PCB Plating?
HASL, also known as Hot Air Solder Levelling, is a widely used PCB surface finish where solder is applied to the copper traces of a printed circuit board to coat them with a thin layer of solder alloy. This solder coating serves two main purposes:
Oxidation protection – The bare copper traces on a PCB will oxidize and corrode over time when exposed to air. The solder coating protects the copper from oxidation.
Solderability – The solder coating also makes it easier to solder components onto the PCB by providing a pre-tinned surface. The components’ leads can easily wet and adhere to the solder coating.
The term “Hot Air Solder Leveling” refers to the process used to apply the solder coating evenly across the surface of the PCB. The board is dipped in a molten solder bath, then hot air knives are used to blow off the excess solder, leaving a thin, even coating of solder remaining on the board.
The solder alloy used for HASL is traditionally a tin-lead alloy, with a eutectic composition of 63% tin and 37% lead. However, lead-free HASL finishes using alloys like SAC305 (96.5% tin, 3% silver, 0.5% copper) have become common to meet RoHS requirements.
The HASL PCB Plating Process Step-By-Step
The HASL process involves multiple steps to prepare the PCB surface and deposit the solder coating evenly across the board. Here is an overview of the complete HASL process:
PCB Fabrication
The first step in the HASL plating process is manufacturing the bare printed circuit board itself. The PCB starts as a laminated sheet consisting of conductive copper layers separated by insulating dielectric substrate material.
The fabrication process transforms the raw PCB laminate into a functional board with the intended circuit layout. This is accomplished through multiple chemical, mechanical, and electrochemical processes including:
- Drilling – Holes are drilled through the laminate at locations requiring electrical interconnections between layers. High-speed CNC drill presses perform hundreds of holes per second.
- Copper Plating – The walls of drilled holes are plated with copper electroplating to form a conductive barrel to interconnect layers.
- Etching – Unwanted copper is removed from laminate surfaces by chemical etching, leaving only the desired conductive traces and pads.
- Masking – An epoxy solder mask layer is screen printed or photo-imaged over the copper traces as an insulator and to control solder spreading.
- Legend Printing – Labels, markings, and nomenclature are screen printed on the board surfaces.
- Electrical Test – Each board is electrically tested to verify conductivity between traces and identify any short or open circuits.
The order and details of fabrication steps depend on the PCB class, number of layers, circuit density, and other characteristics. But regardless of the exact fabrication process, it must be completed before HASL plating can be applied to coat the exposed copper traces. Thorough fabrication also ensures the baseboard is free of defects before plating.
Cutting and Beveling
After initial fabrication, the PCB panel consists of a large laminate sheet containing many individual PCBs arranged in an array. This panel must be divided into individual circuit boards or smaller panels.
First, the panel is sheared or routed to cut the boards apart along the borders and v-score lines. Precision cutting machines Using diamond cutters are used to avoid generating particles or contamination.
Next, the raw cut board edges must be beveled or chamfered at a 45-degree angle all along the edges. This is done using a special orbital grinding machine.
Beveling serves several important functions:
- It removes rough edges and copper burrs to prevent cutting or scratching of handlers.
- The angled edge improves the capillary flow of molten solder, ensuring the edges are coated.
- Bevels allow boards to gradually enter the solder wave rather than abruptly submerging. This prevents solder splash and turbulence.
- Chamfered edges minimize the entrapment of flux residues along the edges after soldering.
The boards may also have notches or radius cuts made along the edges to facilitate handling and separation after plating.
Careful cutting and beveling of the blank PCBs before plating ensures a smooth entry into the solder wave and uniform edge coverage of the solder deposit.
Cleaning
Before applying solder, the PCB surfaces must be thoroughly cleaned to remove any residual contamination from fabrication like dirt, grease, or chemical residues. Even minute amounts of contamination can interfere with proper wetting of the molten solder.
Aqueous cleaning is one method, using water mixed with detergents at temperatures up to 80°C to clean boards. Aqueous cleaners are inexpensive but slower acting on some contaminants.
Plasma etching is a dry process using ionized gas plasma discharge to aggressively remove organic residues. Plasma offers a fast, waterless cleaning method but at a higher equipment cost.
Chemical cleaning uses solvent baths, like modified alcohol, to dissolve away polar contaminants. Solvent cleaning is fast acting but the chemicals require proper handling.
After initial cleaning, boards may also undergo a short sulfuric acid dip lasting a few seconds to strip away any remaining surface oxides before soldering.
Thorough pre-soldering cleaning provides the pristine PCB surface required for successful HASL plating.
Fluxing
Once cleaned, the PCBs undergo flux application to prepare the copper surfaces for soldering. The flux serves several crucial functions:
- Removes any residual metal oxides not cleaned off the PCB pads and traces
- Applies an intermediate layer to improve wetting between the solder and copper
- Prevents re-oxidation of the copper before soldering
- Removes minor contamination from the PCB surfaces
There are a few methods of applying flux:
- Foam Fluxing – Boards pass through a curtain of flux foam leaving a thin film behind.
- Spray Fluxing – Automatic spray nozzles coat boards with a fine atomized flux spray.
- Wave Fluxing – Boards pass over a turbulent wave of flux, submerging and coating them.
- Dip Fluxing – Boards are dipped into tanks of flux for thorough coating.
Most HASL processes use no-clean flux, meaning residual flux left after soldering does not need to be cleaned off the boards. Rosin-based fluxes are common due to their good wetting power. Synthetic fluxes are also used for their wide process window.
The flux deposit should be uniform, leaving no dewretted bare copper spots. Flux is applied generously to account for a portion burning off in the solder pot and hot air knives. Proper fluxing leads to excellent solderability during HASL plating.
Preheating
Before immersion in the solder wave, the PCBs are preheated to an elevated process temperature, typically between 130-200°C. Preheating is done to avoid thermally shocking the boards when they enter the molten solder at 245-260°C.
Several types of ovens are used:
- Infrared Ovens – IR radiation rapidly heats board surfaces, but can cause uneven heating.
- Convection Ovens – Heated forced air provides gentle, uniform board heating.
- Vapor Phase – Boards are immersed in an inert vapor at a precise boiling temperature.
The flux coating is also dried and activated during preheating. But boards shouldn’t be overheated above 220°C to avoid damaging the flux or PCB.
Preheating time ranges from 2-5 minutes depending on factors like board size, copper area, and preheat temperature. Larger boards require longer preheat times for the center temperature to equalize with the edges through thermal conduction.
Automatic conveyors transport the boards through various preheating zones with independent control to develop the optimal thermal profile. Careful preheating is key to creating a HASL coating free of solder balls and other defects.
Solder Coating
The preheated PCBs are ready for immersion in the molten solder wave to coat the copper surfaces. The solder wave provides uniform and rapid coating of the boards with the solder alloy.
The wave soldering system consists of a stainless steel solder pot containing anywhere from 100 to 500 lbs of molten solder held at a temperature between 245-260°C for SnPb alloys.
Nitrogen is bubbled through the molten solder along with ultrasonic agitation to prevent dross formation and maintain optimal surface tension for coating.
The solder forms a standing laminar wave, 1-3 inches high, through which the PCBs pass on an automated conveyor. The dwell time in the wave is adjusted between 2-10 seconds depending on the solder thickness required.
As the boards enter the wave, the molten solder fully wets and coats the copper traces and pads on the board surfaces. The liquid solder displaces the flux layer and makes intimate contact with the copper.
The immersion soldering provides a uniform coating thickness across the boards within a tight tolerance. Precise control of solder pot temperature and conveyor speed/angle allows tailoring the solder thickness.
Hot Air Leveling
After solder coating, the boards enter the hot air leveling machine to precisely control the final solder thickness. Hot air knives evenly remove excess solder, leaving only a thin coating on the PCB.
The leveling machine contains powerful fans generating nitrogen heated to 300-400°C. The nitrogen flows through precision air knives or nozzles aligned over the board surfaces.
As the conveyor carries the solder-coated boards under the air knives, the hot turbulent nitrogen blasts the board surfaces. This blows off the excess molten solder, shearing it back down to the desired coating thickness.
The air knives are arranged in a series of alternating top and bottom nozzles. This allows sequentially leveling the top and bottom surfaces to achieve a uniform thickness.
The conveyor speed, nozzle clearance height, nitrogen temperature/pressure, and number of passes are all carefully controlled. This tailors the leveling to produce the specified HASL thickness across various board areas.
Any imperfections like beads, icicles, or thick edges are sheared away, leaving a flat, smooth, and homogenous solder finish. The leveled coating thickness is generally between 0.5-2.0 mils.
Cooling and Solidification
After the solder thickness has been precisely leveled, the boards must be cooled in a controlled manner to solidify and set the solder coating.
Rapid uncontrolled cooling can lead to non-uniform contraction of the solder layer. This produces stresses in the solder which may warp the PCB or crack the brittle solidified solder.
To prevent this, boards leaving the hot air leveler enter cooling tubes 3-5 meters in length. Cool nitrogen or ambient air flows through the tubes, gradually cooling the boards over 5-10 minutes.
The cooling profile starts at around 160°C and incrementally drops to 60°C at the exit. The cooling rate is maintained between -15°C to -25°C per minute.
This extended cooling period allows the molten solder time to evenly solidify and shrink across the board without distortion or cracking. It also enables some remaining flux residues to evaporate rather than becoming entrapped.
The result is a PCB with a smoothly solidified solder finish that is relaxed and stress-free. The solder coating is properly set and prepared for downstream processing.
Quality Inspection
The final step is an inspection of the solder coating quality. Parameters like solder thickness, coverage, and uniformity are evaluated using optical inspection. This ensures the coating meets specifications before the PCBs ship to customers.
How Thick is HASL Plating on PCBs?
The thickness of the HASL solder finish can range from around 0.2-2.5 mils (5-60 μm). Most boards with HASL will fall within the range of 0.5-2 mils (12.5 – 50 μm). Thicker HASL coatings above 2 mils were more common historically. But as PCB components have gotten smaller, thinner and more precise HASL thicknesses are now used.
Here are some typical HASL thickness specifications:
- IPC-610E Recommended: 0.8-1.6 mils (20-40 μm)
- IPC Class 2/3 Standard: 1.0 – 2.0 mils (25-50 μm)
- IPC Class 1 Advanced: 0.4 – 1.2 mils (10-30 μm)
The coating thickness can impact soldering performance. Thicker coatings above 2 mils can lead to the tombstoning of small surface mount parts. However, thinner coatings below 0.5 mils may not provide adequate oxidation protection and solderability over the PCB lifetime.
Comparing HASL to Other PCB Surface Finishes
While HASL is popular, it is not the only PCB surface finish option. Here is how HASL compares to some other common finishes:
OSP – Organic Solderability Preservatives like Entek coating provide excellent oxidation protection at a low cost. However, OSP has a shorter shelf life than HASL when exposed to the atmosphere.
Immersion Tin – Immersion tin is an affordable Pb-free finish. However, it is prone to whisker growth and has a shorter shelf life than HASL.
Immersion Silver – Immersion silver offers excellent solderability like HASL. It tarnishes faster but is more suitable for fine-pitch components.
ENIG – Electroless Nickel Immersion Gold provides superb oxidation resistance and shelf life. But ENIG is significantly more expensive than HASL.
Hard Gold – Hard gold electroplating is extremely durable but not suitable for soldering. It is used for selective plating of contacts.
So while HASL has some drawbacks compared to other finishes, it provides a very cost-effective combination of solderability, shelf life, thermal tolerance, and ease of application. This makes it a versatile, general-purpose PCB finish suitable for many applications.
Pros and Cons of Using HASL
Here are some of the key benefits as well as downsides and limitations of using a Hot Air Solder Leveling finish on printed circuit boards:
Benefits of HASL
- Low Cost – HASL is one of the least expensive PCB surface finishes, keeping PCB fabrication costs down.
- Solderability – The solder coating gives excellent solderability for mounting components. Achieves good wetting and adhesion.
- Reflow Tolerant – Withstands multiple passes through solder reflow ovens without issue.
- Shelf Life – Properly applied HASL finish has a typical shelf life of over 12 months when stored properly.
- Repairable – Damaged HASL coatings can be repaired by resoldering, unlike metal plating finishes.
- Process Control – Well-established and controlled HASL process yields high quality and repeatability.
Limitations of HASL
- Lead Content – Traditional tin-lead HASL contains hazardous lead. Lead-free HASL must be used for RoHS compliance.
- Non-Uniform Coating – Cooling variations can lead to non-uniform solder thickness across the PCB surface.
- Dull Cosmetics – HASL coatings have a dull matte appearance, unlike brighter metal finishes.
- Bridging Risk – Excess solder can lead to bridging between fine-pitch components. Requires tight process control.
- Thermal Shock – Rapid heating and cooling cycles can damage the solder coating and PCB materials.
- Copper Oxidation – Due to thin coating, copper oxidation can still occur over time, reducing solderability.
HASL Process Capability
For quality assurance, the HASL line process capability should be evaluated to ensure it can repeatedly produce coatings within specification. Two key metrics are:
- Cpk – Process capability index. Cpk above 1.33 generally indicates a capable process.
- Thickness Tolerance – Should have +/- 10% thickness tolerance relative to target thickness.
Statistical process control monitoring of parameters like solder thickness, coverage, and temperature is necessary. X-ray fluorescence (XRF) tools are commonly used to measure solder thickness at various board locations.
Furthermore, sample PCBs must be cross-sectioned and inspected under a metallographic microscope according to IPC standards like IPC-4552 to verify the coating meets internal quality specifications for thickness, voids, coverage, and grain structure.
Best Practices For HASL PCBs
To get the most out of your HASL finished boards, follow these fabrication and handling best practices:
- Specify your target HASL thickness based on component sizes and pitches. Thinner for finer features.
- Require solderability testing conformal to IPC J-STD-003 standards.
- Request cross-section photomicrographs from your PCB supplier to verify thickness and quality.
- Handle boards with care. HASL coatings are somewhat soft and prone to scratches during handling.
- Use adequate ESD protection, grounding, and moisture barriers when storing boards.
- Apply conformal coatings selectively after assembly to prevent coating wear or damage.
- Allow boards to warm to room temperature before unpacking to prevent moisture condensation.
Adhering to best practices ensures your boards have an even, high-quality HASL finish capable of protecting your boards and providing reliable soldering.
Conclusion
With its combination of low cost, ease of application, and suitable functionality, Hot Air Solder Leveling remains one of the dominant PCB surface finishes in use today. By following the optimal HASL process steps and quality procedures outlined here, PCB manufacturers can produce boards with uniform HASL coatings that protect copper traces and deliver reliable soldering performance.
HASL strikes an excellent balance between cost and functionality across a wide array of electronics applications. However, engineers should keep in mind the process and coating limitations of HASL when considering it alongside other finish options for more demanding applications with fine geometries or specialized requirements.
Frequently Asked Questions (FAQs)
What is the difference between lead HASL and lead-free HASL?
Traditional HASL uses a tin-lead solder alloy containing about 63% tin and 37% lead. Lead-free HASL substitutes lead for metals like silver, copper, bismuth, or antimony while maintaining a high tin content above 95%. This complies with RoHS regulations banning lead in electronics.
Is HASL suitable for fine-pitch PCB assemblies?
While HASL can bridge between smaller pitch components, it is suitable down to 0.5 mm pitch. Other finishes like immersion silver or OSP may be better for pitches under 0.5mm.
Can you HASL plate only parts of a PCB selectively?
Yes, selective HASL plating is possible by using solder masks, tapes, or resists to mask off areas that should not receive solder. This exposes only selected traces or pads to the solder wave.
What’s the shelf life of a HASL PCB?
Properly applied HASL with a relatively thin coating of 1 mil or less can achieve a 12-24 month shelf life when kept sealed in a dry nitrogen environment. Thicker HASL finishes over 2 mils have shorter shelf lives around 6-9 months.
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