If you’ve ever had a circuit board professionally manufactured, you have likely been asked your choice between HASL and ENIG surface finishes. From solderability to cost, these two finishes can’t possibly be more different. While some engineers have a tried and true favorite, you may be left wondering the differences between the two variants.
Hot Air Solder Leveling (HASL or HAL) is a surface finish that involves the insertion of the copper into a vat of molten solder. A series of compressed hot-air knives then remove excess solder and levels the surface finish before the solder solidifies and cools. This creates a powerful coating that protects the copper from corrosion and makes for an easy assembly process. Thanks to these benefits, HASL is one of the most common contact plating technique in use today.
While HASL is an excellent technique there are certainly drawbacks. Electroless Nickel Immersion Gold (ENIG) is a gold plating technique commonly used by PCB manufacturers to allow for a wide range of benefits over other methods. It creates a significantly flatter surface, a vital necessity for large ball-grid array packages to adhere properly. It also allows for a significantly safer, cleaner, and more ecologically-friendly manufacturing process thanks to the complete omission of lead. Since there is no need for an air knife process, ENIG boards warp less – especially when very thin – and have slightly improved thermal durability.
While several variants of HASL exist, they generally fall into two different categories: lead and lead-free. Lead-free boards generally are plated in solder containing mostly tin with small amounts of silver and copper, while lead-based boards generally are coated in eutectic tin-lead solder with possible filler metals. Unfortunately, the large majority of HASL boards do indeed contain lead and very harsh flux chemicals, which can potentially cause a wide range of health and safety issues for individuals working with HASL in an improperly controlled environment.
On the other hand, ENIG tends to remain the same across most PCB manufacturers, although quality can vary. Low quality manufacturers can sometimes reduce cost by decreasing the amount of nickel or gold plating applied, leaving a barely-coated copper pad which can lead to unwanted diffusion or a form of corrosion called “black pad.” Well-manufactured ENIG plated boards will coat evenly with enough nickel and gold to ensure the preservation of the many benefits that ENIG has to offer.
HASL is generally preferred for hand-soldering due to the easy joint formed. If the soldering alloy is close to the HASL’s alloy, adhesion will come very easily and a very strong joint will form as the metals will want to interact on a molecular level. This strong joint also makes HASL a good finish for high-reliability applications.
However, HASL leaves an uneven surface despite the leveling process. ENIG, on the other hand, provides for a very flat surface making ENIG preferable for fine pitch and high pin count components especially ball-grid array (BGA) devices. This flatness allows for accurate placement, controlled amounts of solder, and even heating, thus reducing the risk of opens or shorts due to cold joints or excess solder. In fact, while HASL provides a stronger solder joint, some assemblers have come to prefer ENIG over HASL in all cases because of the flatness.
The Environmental Factor
As cohabitants of planet earth, we should all be mindful of our environment. As one of the most widely produced raw materials, nowhere is this more prevalent than in manufacturing of circuit boards. As noted earlier, solder leveling is an intensive process that involves the usage of several harsh, environmentally unfriendly chemicals such as lead and flux. This is why the European Union went through great measures to eliminate the usage of leaded HASL and solder with the RoHS program. The United States government has recently been joining the movement by devising tax incentives for businesses who utilize non-lead-based products.
Not to be overlooked, cyanide is also a prevalent health and environmental factor in the production and use of either finish. Bay Area Circuits was one of the first PCB manufacturers to transition to an entirely cyanide-free ENIG finish. This breakthrough allows for significantly lowered risk to assemblers and engineers, and end consumers are left with a safer product.
So, which should you choose?
While HASL can serve a low-cost option for quick and easy prototyping or high-durability solder joints, considering shrinking component sizes ENIG has become preferred in many applications. The choice comes down to whether the environmental benefits and solder reliability of ENIG outweigh the low cost, high strength, and ease of soldering afforded by HASL.
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