PCB surface finish is a metal or organic coating on PCB pads that protects the copper and boosts solderability. Lead-free HASL (hot air solder leveling) is the most common PCB surface finish, and if you don't specify which surface finish you want, we assume you want lead-free HASL for your boards.
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However, lead-free HASL is not allowed for flexible PCBs and rigid PCBs with a thickness smaller than 0.5mm. You have to replace it with ENIG (electroless nickel immersion gold) or OSP (organic solderability preservatives). This article reveals HASL and ENIG and compares them.
HASL means using tin/lead solder to spray the PCB pads and then using hot air to blow the PCB surface so that the pad surface is level. The thickness of HASL is 1mil to 2mil (25.4μm to 50.8μm, or 0.025mm to 0.05mm).
HASL is a cheap PCB surface finish. And its solderability is good.
However, HASL doesn't meet the lead-free requirements in most of the world, and in most cases, we use lead-free HASL.
The properties of lead-free HASL are pure tin. The thickness of lead-free HASL is 0.1mil to 1mil (2.54μm to 25.4μm, or 0.mm to 0.025mm).
Lead-free HASL is the default surface finish from the PCB manufacturer PCBONLINE. Compared with HASL, PCB pads with lead-free HASL look dim, and the soldering temperatures are higher.
HASL
Lead-free HASL
Properties
Tin and lead (37% of lead)
Tin (lead less than 0.5%)
PCB pad finish
Glossy
Dim
Solderability
Good
A little worse than HASL
Reflow soldering temperatures
210&#; to 245&#;
240&#; to 270&#;
Wave soldering temperatures
About 250&#;
About 260&#;
Though lead-free HASL is common, it can't be applied to rigid PCBs with a thickness smaller than 0.5mm and flexible PCBs, neither HASL. These boards can't bear the spraying force. If you spray tin on these boards, they bend, and only the middle of the boards is sprayed with tin. Sometimes, rigid PCBs with a thickness smaller than 0.5mm may even break if you apply HASL or lead-free HASL to them.
In such cases, you can choose the other PCB surface finish -- ENIG. Or you can also change lead-free HASL to OSP. But OSP is easy to oxide. If you don't solder the PCBs at once, OSP oxides. And the manufacturer has to seal and package the OSP PCBs at once.
ENIG, also known as immersion gold, or chemical Ni/Au, is one of the best PCB surface finishes for almost all PCBs. In any case, if HASL or lead-free HASL can be used, you can replace it with ENIG. But inversely, HASL or lead-free HASL can't replace ENIG.
ENIG means plating a nickel alloy coating on the PCB pads and then a gold coating with chemical reactions. Besides PCB pads, ENIG can also be applied to other areas, such as the PCB edges or wires for thermal dissipation purposes. The thickness of ENIG is 1μm, 2μm, and 3μm. (From the equation 1mil=25.4μm, you can understand how thin the PCB finish ENIG is compared to HASL.)
ENIG PCB pads have a gold color and look shiny. Besides, the solderability of ENIG is the best among all the PCB surface finishes. And because gold has a dense crystal structure and is hard oxide, the protection of ENIG to copper of the PCB pads is also excellent.
ENIG PCBs are high-standard PCBs and they are usually used for high-end applications. As we mentioned above, ENIG can take the place of HASL and lead-free HASL in any case if your budget allows, but HASL and lead-free HASL can't replace ENIG.
Below is a comparison between EENIG and HASL.
ENIG
HASL
Lead-free HASL
Meeting lead-free requirements or not
Yes
No
Yes
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Price
High
Low
Middle
Thickness
1μm, 2μm, and 3μm
25.4μm to 50.8μm
2.54μm to 25.4μm
Finish method
Chemical reactions
Spraying
Spraying
Properties
Nickel and gold
Tin and lead
Tin
Applications
Can be used for almost all PCBs
Simple FR4 PCBs with a thickness larger than 0.5mm
Common rigid PCBs with a thickness larger than 0.5mm
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This article gives an introduction to HASL and ENIG separately and compares them. ENIG is universal for PCBs, especially for high-reliability PCBs. HASL doesn't meet the lead-free requirements but lead-free HASL does. If you don't know whether HASL can be used for your PCB or not, ENIG must be okay. You can also contact PCBONLINE for suggestions.
© This article is an original work of the PCBONLINE team. Please indicate the author PCBONLINE if you reprint. If the article is reproduced without permission or indicating the author's source, PCBONLINE reserves the right to investigate the infringement.
LF Hot Air Solder Leveling or LF-HASL is identical to the standard HASL process. However, it does have an obvious difference, i.e., it does no use Tin and Lead Solder. Instead, you will use a Tin and Copper, Tin and Nickel, or Tin, Copper, and Nickel germanium.
This difference is what makes LF-HASL a cost-effective and RoHS compliant surface finish. However, similar to HASL, lead-free HASL is also not suitable for smaller components. For PCBs with smaller components, you can choose better surface finishes such as immersion coatings. They are a little more expensive but ideal for such purposes.
The lead-free HASL PCB surface finish became one of the most considered and frequently used surface finishes after immersion gold. It provided safer surface finish alternatives, but it never became the new standard and created a bit of hassle for the circuit board manufacturers.
It all came down to the complexity of applying LF-HASL finish, and some PCB manufacturers had to outsource this lead-free HASL solution.
The chemistry behind the composition of lead-free hot air solder leveling has evolved over the years, so have its applications. Both horizontal and vertical applications have issues identical to HASL, which are pooling and non-flat surface finish with a foggy appearance in certain areas of the printed circuit boards.
The pre-trial combination of lead-free HASL received bad reviews. The combination of using silver, tin, and copper alloy together originally produced poor results during processing levels. They left behind bumpy, uneven coats that appeared unattractive and dull, and demonstrated poor performance in PCB assembly.
However, removing the silver and changing the tin-copper along with a few tweaks to the manufacturing process showed improvements with smoother and better surface coatings than the initial results.
This promising development in the process and application increased the demand for the LF-HASL. Not only did it make LF-HASL a common practice but decreased the timeline related to customer product delivery.
Lead-free hot air solder leveling needs higher temperatures for its application. During the first pass, it leaves the surface dull and grainy. However, after the second pass, both the appearance and surfaces improve, displaying a flatter, shinier, smoother, and even coat on the circuit board.
That said- the exposure to excessive heat from dipping twice in the molten solution reduces the copper levels below the acceptable limit as per IPC standards. This shortcoming worked as a catalyst for another change to the process staining the lead-free hot air solder leveling surface finish.
After several changes to both chemistries and processes, LF-HASL is finally a stable PCB surface finish application. The new and improved process is finally capable of producing a consistent and quality surface finish using lead-free alloys. LF-HASL has drastically reduced the hassle experienced by the PCB manufacturers.
You may wonder, after all those tweaks and improvements, what stopped LF-HASL from becoming the standard finish? Lead-Free HASL is still probably the least utilized PCB surface finish in the industry.
The reason is the introduction of other innovative surface finishes such as ENIG (Electroless Nickel Immersion Gold), OSP (Organic Solderability Preservative). Even immersion silver and tin took over surface finish landscape by storm.
Although LF HASL has come a long way in terms of improved viability, it is still work in progress. It has to cover some groundwork to compete with other surface finishes. However, it is gaining popularity amongst consumers, prompting PCB manufacturers to consider using LF hot air solder leveling as a standard surface finish.
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