DFM 101 DFM 101: Final Finishes—HASL

One of the biggest challenges facing PCB designers is not understanding the cost drivers in the PCB manufacturing process. This article is the latest in a series that will discuss these cost drivers (from the PCB manufacturer's perspective) and the design decisions that will impact product reliability.

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Final Finishes

Final finishes provide a surface for the component assembler to solder, wire bond, or conductively attach a component pad or lead to a pad, hole, or area of a PCB. The other use for a final finish is to provide a known contact resistance and life cycle for connectors, keys, or switches. The primary purpose of a final finish is to create electrical and thermal continuity with a surface of the PCB.

HASL (Hot Air Solder Leveling)

The next finish to discuss in this series is HASL. Choosing a surface finish means weighing the pros and cons associated with each, typically a combination of application, cost, and the properties of the finish. For example, Lead-free HASL is considered &#;RoHS&#; compliant (< 0.1% BW of finish, for Pb, Hg or Cd), while SnPb (tin/lead) HASL is not.

HASL is available in two versions: a tin/lead alloy and a lead-free option. HASL is a variable thickness metal alloy coating, typically 25 to µ in [0.65 to 50 µm] thick, designed to create a solderable surface and to maintain solderability over an extended period. Using specialized equipment, panels with exposed copper (pads and/or traces) are inserted into a vat of molten solder. A series of compressed hot-air knives then remove excess solder and &#;level&#; the surface before the solder solidifies and cools. HASL is the most prevalent and most solderable finish. Boards that have been HASL leveled will have a bright, silvery pad coloration. Solder thickness and uniformity on SMD pads is a function of pad size, pad orientation during processing, and HASL equipment. As pad size increases, thickness and uniformity decrease. This is caused by the high surface tension of solder and the airflow over pads that remove excess solder. Small pads (0.025" pitch SMD) will have a more uniform solder coating than large pads (0.050" x 0.050"). Large pads may be partially covered with a mound of solder while the rest of the pad is thin eutectic solder. Small pads may have the same condition depending on pad orientation during processing.

Pad orientation also contributes to solder thickness and uniformity. Fine-pitch SMD pads perpendicular to the direction of processing will have a uniform solder coating covering the entire pad. Fine-pitch SMD pads that are parallel to the direction of processing will have the leading half reduced to the thin eutectic solder with a mound of solder on the trailing half (eutectic solder is the optimum metal alloy to maximize solderability). Angled processing exposes both pad directions to similar air blasts resulting in more consistent pad thickness for similar pad sizes. 45-degree angle processing minimizes the pad orientation difference. HASL is not used for fine pitch components. This is because it increases fabrication and assembly rework for boards that have fine-pitch SMT devices with lead centers below 0.020" and presents coplanarity issues.

Pros and Cons of HASL

Pros

• Economical and widely available
• Excellent solderability
• Can be reworked at the fabricator
• Presents a very flat solder deposit

Cons

• Limited assembly reflow cycles, as the HASL process introduces a thermal shock before it gets to assembly

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• Not suited for fine pitch (due to uneven surface height) or high aspect through-holes
• Standard SnPb HASL is not RoHS compliant
• Lead-free HASL requires different assembly soldering parameters

Understanding the cost drivers in PCB fabrication and early engagement between the designer and the fabricator are crucial elements that lead to cost-effective design success. Following your fabricator&#;s DFM guidelines is the first place to start.

We all know that printed circuit board is an essential part of any electronic device. The electronic components are soldered on the PCB bare board to achieve the connection and functions of the electronic products. But do you know PCB finishes on the circuit board PCB? The PCB finishes play an important role in the soldering of the components on the PCBs. HASL, ENGI, immersion tin, immersion silver, and ENGPIG are the most widely used PCB surface finishes. In this article, we will talk about one of the most important types: HASL PCB finish.

What is HASL PCB Finish?

HASL is short for Hot Air Solder Leveling, it is a type of metallic surface finish which is coated on the copper pads on printed circuit boards to avoid oxidation of copper and provide better solder joints. The coating of the HASL process is quite simple where the boards are dipped inside a molten solder which has tin/lead alloys. The molten pot used to coat HASL consists of tin and lead solder. Solder on the surface is not evenly deposited at this stage. To make the surface even and smoother, boards are then passed through a hot air knife which is known to remove the excess solder coated on the copper surface. HASL PCB finish is low-cost and widely available.

Because of the excellent solderability, reliability and durability of HASL finish, it provides a stronger solder joint between components and the board&#;s surface thus protecting the circuit board. When the boards contain thru holes and larger SMT components, a HASL PCB finish is preferred.

In recent days, Lead-free HASL has been widely used due to the absence of toxic substances and also due to its being environmentally friendly. Unlike traditional HASL, this finish doesn&#;t contain any lead contents in its composition, which makes it compliant with ROHS restrictions too.

HASL is mainly divided into 2 types based on the manufacturing process:

• Lead-based HASL or traditional HASL.

• Lead-free HASL finish.

An In-Depth Overview of HASL PCB Finish

Surface finish refers to a thin layer of coating done on a copper surface to prevent copper from oxidation and to make the surface ready for soldering the components onto the board&#;s surface efficiently without any problem. HASL is one such metallic surface finish which is widely used in PCB industries because of its excellent properties. In this article, we will deep dive into understanding the advantages, disadvantages, limitations and the process involved in HASL finish during manufacturing.

HASL PCB Finish Process

HASL coating involves only a few steps as it is a very simple process as explained below:

• Preparing the PCB: Before applying the HASL surface finish, boards have to be cleaned thoroughly to remove any contaminants and oxides on the copper surface. This will help the surface of the board to be ready for the application of solder without any contamination.

• Solder Application: After cleaning, the boards are submerged in the molten solder (tin/lead). This covers all the exposed copper pads on the board.

• Hot Air Levelling: Excess solder on the boards is levelled by passing the boards over hot air knives. When blown to the board, These hot air knives not only remove the excess solder but also make the surface even with the uniform distribution of solder all over the board and also help to control the uniform thickness of the board.

• Cooling and Cleaning: Boards are cooled after passing through hot air knives to make the solder dry and finally, they are cleaned again to get rid of any chemical stains or contaminants on the surface caused during the manufacturing process.

Applications of HASL PCB

HASL PCB is used in several electronic equipment and a few of them are listed below:

• Consumer electronics

• Industrial equipment

• Automotive electronics

• Telecommunication devices

• Medical devices

Benefits of HASL PCB finish:

• HASL is a low-cost process which is suitable for mass productions

• It is easy to rework compared to other finishes.

• Solderability is excellent compared to other finishes

Drawbacks of HASL PCB finish:

• Uneven surface caused due to hot air knives makes the component soldering difficult.

• Not appropriate finish for fine pitch components due to larger solder mask dams.

• It is not suitable for wire bonding.

• Due to the presence of lead in the traditional HASL process, It is not environmentally friendly.

Introduction to Lead-free HASL PCB

Due to the restrictions of several toxic substances used in the PCB manufacturing industry which were having adverse effects on human health and the environment, the lead-free HASL process was introduced. Since it doesn&#;t contain lead, it is considered environmentally friendly which also complies with the ROHS restriction. It consists of an alloy which is free of lead such as tin-silver, tin-copper and tin-nickel. Due to the absence of a lead process, this finish is preferred by most manufacturers instead of a lead-based finish. The aim is to eliminate the use of lead, a toxic substance while maintaining the solderability and reliability of the PCB.

Advantages of Lead-free HASL PCB Finish

• It meets environmental regulations such as ROHS restrictions. Hence it is environmentally friendly.

• It provides a safer and healthier working environment as it doesn&#;t contain toxic substances.

• Provides Uniform coating thickness compared to standard lead-based process.

• Has Excellent Solderability.

Disadvantages of Lead-free HASL PCB Finish

• Formation of tin whiskers on the surface which would result in a short circuit.

• It has a higher melting point which leads to thermal stress & component failure.

• Require modifications to the regular manufacturing process to accommodate higher temperature processes and different solder alloys.

Limitations of HASL PCB Finish

• Uneven surface finish: HASL finish has variation in the surface due to the blowing of air and the solder levelling process. This might affect the performance of the board after soldering the components as the components may fail due to improper connection.

• Thermal stress: HASL coating involves several processes where molten solder after the coating is immediately cooled, which might result in thermal stress on the boards. Due to the thermal stress, some chances are boards getting delaminated and even causing warpage issues.

• Not preferred for components with very tight pitch: Fine pitch components require very little space between the pads which in turn the solder webbing will also be less. HASL finish causes solder bridging between pads which might even tend to short circuit due to the minimum spacing between the pads. Hence it is not preferred when the component pitch is very tight.

Conclusion

In summary, HASL PCB is widely used because of its cost-effectiveness, simple process and suitability for thru-hole component soldering. With the inclusion of a lead-free process, HASL PCB finish has become environmentally friendly and compliant with ROHS. Overall, understanding the manufacturing process, advantages, disadvantages and the different types of HASL PCB finish is required to ensure the quality, durability and performance of the boards in manufacturing.

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