By Al Wright, PCB Field Applications Engineer
Epec Engineered Technologies
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Anyone involved within the printed circuit board (PCB) industry understand that PCBs have copper finishes on their surface. If they are left unprotected then the copper will oxidize and deteriorate, making the circuit board unusable. The surface finish forms a critical interface between the component and the PCB. The finish has two essential functions, to protect the exposed copper circuitry and to provide a solderable surface when assembling (soldering) the components to the printed circuit board.
Hot Air Solder Leveling (HASL) was once the tried and true method of deliver consistent assembly results. However, the ever-increasing circuit complexity and component density has stretched the capabilities of even horizontal solder levelling systems to their limits.
As component pitches became finer and a need for a thin coating became greater, HASL represented a process limitation for PCB manufacturers. As an alternative to HASL, alternative coatings have been around for several years now, both electrolytic and immersion processes.
Listed below are some more common surface finishes used in PCB manufacturing.
HASL is the predominant surface finish used in the industry. The process consists of immersing circuit boards in a molten pot of a tin/lead alloy and then removing the excess solder by using 'air knives', which blow hot air across the surface of the board.
One of the unintended benefits of the HASL process is that it will expose the PCB to temperatures up to 265°C which will identify any potential delamination issues well before any expensive components are attached to the board.
Printed Circuit Board with HASL / Lead Free HASL Surface Finish
According to IPC, the Association Connecting Electronics Industry, Immersion Tin (ISn) is a metallic 'nish deposited by a chemical displacement reaction that is applied directly over the basis metal of the circuit board, that is, copper. The ISn protects the underlying copper from oxidation over its intended shelf life.
Copper and tin however have a strong affinity for one another. The diffusion of one metal into the other will occur inevitably, directly impacting the shelf life of the deposit and the performance of the 'nish. The negative effects of tin whiskers growth are well described in industry related literature and topics of several published papers.
Printed Circuit Board with Immersion Tin Surface Finish
Immersion silver is a non-electrolytic chemical finish applied by immersing the copper PCB into a tank of silver ions. It is a good choice finish for circuit boards with EMI shieldingand is also used for dome contacts and wire bonding. The average surface thickness of the silver is 5-18 microinches.
With modern environmental concerns such as RoHS and WEE, immersion silver is environmentally better than both HASL and ENIG. It is popular also due to its lesser cost than ENIG.
Printed Circuit Board with Immersion Silver Surface Finish
OSP (Organic Solderability Preservative) or anti-tarnish preserves the copper surface from oxidation by applying a very thin protective layer of material over the exposed copper usually using a conveyorized process.
It uses a water-based organic compound that selectively bonds to copper and provides an organometallic layer that protects the copper prior to soldering. It's also extremely green environmentally in comparison with the other common lead-free finishes, which suffer from either being more toxic or substantially higher energy consumption.
Printed Circuit Board with OSP / Entek Surface Finish
ENIG is a two layer metallic coating of 2-8 μin Au over 120-240 μin Ni. The Nickel is the barrier to the copper and is the surface to which the components are actually soldered to. The gold protects the nickel during storage and also provides the low contact resistance required for the thin gold deposits. ENIG is now arguably the most used finish in the PCB industry due the growth and implementation of the RoHs regulation.
Printed Circuit Board with Electroless Nickel Immersion Gold (ENIG) Surface Finish
ENEPIG, a relative newcomer to the circuit board world of finishes, first came on the market in the late 90s. This three-layer metallic coating of nickel, palladium, and gold provides an option like no others: it is bondable. ENEPIG's first crack at a printed circuit board surface treatment fizzled with manufacturing due to its extreme high cost layer of palladium and low demand of use.
The need for a separate manufacturing line was not receptive for these same reasons. Recently, ENEPIG has made a comeback as the potential to meet reliability, packaging needs, and RoHS standards are a plus with this finish. It is perfect for high frequency applications where spacing is limited.
When compared to the other top four finishes, ENIG, Lead Free-HASL, immersion silver and OSP, ENEPIG outperforms all on the after-assembly corrosion level.
Printed Circuit Board with Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) Surface Finish
Hard Electrolytic Gold consists of a layer of gold plated over a barrier coat of nickel. Hard gold is extremely durable, and is most commonly applied to high-wear areas such as edge connector fingers and keypads.
Unlike ENIG, its thickness can vary by controlling the duration of the plating cycle, although the typical minimum values for fingers are 30 μin gold over 100 μin nickel for Class 1 and Class 2, 50 μin gold over 100 μin nickel for Class 3.
Hard gold is not generally applied to solderable areas, because of its high cost and its relatively poor solderability. The maximum thickness that IPC considers to be solderable is 17.8 μin, so if this type of gold must be used on surfaces to be soldered, the recommended nominal thickness should be about 5-10 μin.
Printed Circuit Board with Gold ' Hard Gold Surface Finish
It is important to select the appropriate surface finish for your project by considering the various options while factoring in performance requirements and material costs.
For an example, if you are looking for the lowest cost then Tin-Lead HASL might seem like a good choice, but it is not suitable for RoHS-compliant products. If your product does require RoHS, you might consider lead-free HASL. That is only if there are no fine pitch components, since LFHASL cannot be applied perfectly flat. If your design needs to be RoHS compliant and does use fine pitch components, then you'll need to select a flat, lead-free finish, such as Immersion Silver or ENIG. Bear in mind that doing so will necessitate the use of more costly high temperature laminate.
Unsure of what you will need? Consult with a PCB fabricator prior to you making a selection. This will ensure that the combination of the surface finish and material will result in a high-yielding, cost-effective design that will perform as expected.
For more information on circuit board surface finishes, check out our blog posts on why you should bake OSP circuit boards before use and proper circuit board handling with immersion finishes.
Our team of experienced engineers is here to help you design a custom PCB solution, utilizing the various surface finished from HASL, Immersion Tin, OSP/Entek, ENIG, ENEPIG, or Hard Gold.
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Download Your CopyIntroduction
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 FabricationThe 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:
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 BevelingAfter 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:
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.
CleaningBefore 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.
FluxingOnce cleaned, the PCBs undergo flux application to prepare the copper surfaces for soldering. The flux serves several crucial functions:
There are a few methods of applying flux:
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.
PreheatingBefore 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:
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.
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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 LevelingAfter 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 SolidificationAfter 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 InspectionThe 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:
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
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:
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- 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:
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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