Several finishes or finishing processes exist for Printed Circuit Boards (PCBs). HASL is one of those processes but the difficulty is in choosing between the leaded and HASL lead free processes. If you have thought of the differences between these two and why one is better than the other, you have come to the right place.
This article explains first, what the HASL process is, as it pertains to circuit board finishing. We also elaborate on the potential benefits of choosing the HASL lead free procedure. Thirdly and most importantly, we compared the lead free HASL PCB with the other surface finishing processes.
Background to HASL Lead Free: The Need to “Finish” the PCB’s Surface
We hear about HASL (both the leaded and lead-free processes), ENIG and a host of other procedures due to one factor – the need to have the circuit boards’ surfaces finished properly. When we understand the underlying reason for this, we will gain better clarity as to why the HASL lead free process is relevant.
The term, PCB surface finish, refers to the protective measures in place to prevent the surfaces of Printed Circuit Boards (PCBs) from getting oxidated. Through these processes, the core PCB components, such as the copper, ground planes, exposed traces, holes and pads wouldn’t be overly exposed to oxidation.
An overt or excessive exposure to oxidation also means that the circuit boards’ copper will deteriorate, which, in turn, makes the PCB unusable. Using the surface finish helps to keep the copper and some of the other important components safe from oxidation.
What is the HASL Finish Process?
Let us look at the background of the HASL finishing process, before we delve into the meaning of the HASL lead free. According to Wikipedia, HASL stands for Hot Air Solder Leveling. It is a solder-based finishing process used to protect the copper and components of Printed Circuit Boards (PCBs).
The process of actualizing this function starts with the dipping of the circuit board into a bath of molten solder. The goal is to enable the coverage of the exposed copper with most parts of the solder mixture.
Once this has been done, the next step is to pass the PCB in-between hot air knives. This is done to remove the excessive solder on the surface of the PCB, after the main copper is covered.
What is HASL Lead Free Finish?
Now, there are two (2) major types of the Hot Air Solder Leveling (HASL) process. These are the Lead-Free and the Leaded. They both refer to the process of applying the HASL – whether lead will be needed or not.
It is common to go for the HASL lead free finish process, because of the many benefits it presents. One of such benefits is the compliance with RoHS, hence the description as lead free HASL RoHS.
Understanding the Lead Free HASL Composition
When the HASL lead free is said not to contain, it is just the surface meaning. The non-lead composition came about because of the following processes:
- Lead-free solder alloys are used for the solder coverage.
- This proves to be a better variant to the conventional Tin-lead eutectic alloys used by the Hot Air Solder Leveling (HASL) circuit board finish process.
The lead free HASL composition also includes a mix of 0.6% of copper and up to 99.6% of Tin. Also referred to as the SN100CL, the process also involves a usage with the PCBs made with the Surface Mount Technology (SMT) process.
How Does the HASL Lead Free Process Stackup?
The process of applying the HASL lead free takes a little detour from the conventional HASL process.
First, the circuit board is dipped into the tank or bath of molten solder. The second step is having the circuit board passed through the air knives to have the excess solder (molten) blow off or removed.
Unlike the traditional HASL process that usually ends there, the HASL lead free is applied only after the soldermask has been applied to the Printed Circuit Board (PCB).
Important Lead-Free HASL Composition Considerations
Sometimes, paying a closer attention to how something works or is designed to work is a means to revealing some of the potential issues. The HASL lead free is not entirely “fault-free” and we want to explain some of the reasons why that is so.
1. The Availability of Other PCB Surface Finishes
It is important to mention that there are several other types of surface finishes to use for your PCB. Due to the wide range of options out there in the market, consumers often prefer going for the main HASL process or any of the following:
- ENIG and
- OSP
2. Ideal for Specific PCBs
You may be unable to use the HASL lead free surface finish for most of the Printed Circuit Board (PCB) designs. A good example is with the rigid circuit boards, especially those with a thickness smaller than 0.5mm.
You may also experience difficulties with using the lead-free HASL finish with flexible circuit boards.
The similarities with these two boards are their non-conformation to the spraying force of the HASL lead free process. Spraying tin (the main component of the HASL lead free surface finish) could affect the rigid and the flexible PCBs in the following ways:
- The adverse effects often lead to spraying tin only at the middle of the circuit boards.
- If tin is to applied all over the board, it often results in bending the PCB’s surface.
- In severe cases, the PCB would break into parts. That is the case with rigid circuit boards with less than 0.5mm of thickness.
3. Temperature Considerations
HASL lead free surface finishes may be unable to function perfectly when exposed or used in higher temperature and humidity conditions.
The Benefits of Using HASL Lead Free PCB Surface Finish
If you are okay with those considerations, you may now want to know some of the chances you have with the process.
We present to you, some of the benefits of working with the HASL lead free surface finish process for circuit boards.
1. Large Processing Window
Considering that you may be working on a larger volume of PCB projects, it makes sense to choose a surface finish process that aligns with such volumes.
HASL lead free comes to the rescue, thanks to the larger processing window, which supports the application in larger, through-hole technology Printed Circuit Boards (PCBs).
2. Excellent Solderability
Applying the solder is simple, thanks to the streamlined process of the Hot Air Solder Leveling (HASL) process. For the best results, the surface finish should be applied after the soldermask.
3. Affordability
One reason PCB consumers look the way of HASL PCB finishes is because of the affordability. As an inexpensive process when compared to the likes of immersion tin and ENIG, it doesn’t cost much to apply this type of surface finish on your circuit board.
The other benefits of HASL lead free surface finish include being an option for the PCBs with RoHS protection and eco-friendliness. You can also expose the PCB coated with the HASL lead free surface finish, as a way of highlighting the delamination issues.
Downsides to HASL Lead Free Surface Finish for PCBs
Just as this surface finish presents a lot of opportunities, it also presents some challenges. Understanding some of the downsides will help you have a better perspective as to what works and what doesn’t.
1. Thermal Stress
More thermal stress is exerted in the process of applying the surface finish. This often leads to increased risks of defects or design errors in the circuit boards.
2. Fine Pitch Bridging
Bridging is noticeable on the fine pitch. This is obtainable because of the following factors:
- Low planarity on vertical levelers.
- Due to this, it is almost impossible to use the HASL lead free surface finish ideal for use with the fine pitch PCB parts. Examples of the affected components are the less than 20mil pitch for SMD and BGA.
3. Temperature Considerations
HASL lead free surface finish is not your go-to for circuit boards with lower temperature. Instead, it has been found compatible with PCBs with an operating temperature between 260˚C and 270˚C.
The other disadvantages to the HASL lead free surface finish include not being ideal for use with the High-Density Interconnect (HDI) circuit boards and having varied thickness between the large and small pads.
HASL Lead Free vs. HASL Lead PCB Surface Finishes
The simplest definition of the difference between the two is that one doesn’t use lead while the other does. However, there are a couple of other interesting differences between the HASL with lead and the HASL without lead surface finishes.
The following are some of the differentiating factors:
1. Supported Thickness
The thickness of the circuit boards makes the HASL with lead and the one without lead different. While the one with lead supports PCB thickness between 25.4µm to 50.8µm; the non-leaded variant supports thickness between 2.54µm and 25.4µm.
2. The Cost of the Surface Finish
When compared with the other PCB surface finishes, such as ENIG and HASL, the non-leaded HASL is median. It is less expensive than the ENIG surface finish and a bit expensive than the traditional HASL process.
3. The Properties
The properties or compositions make the two different too. While the HASL lead uses a combination of Tin and lead, the lead free HASL uses more of Tin and a little amount of copper.
4. Typical Applications
HASL lead surface finish is ideal with use with the circuit boards that require larger thickness. For example, it can be used with the simple/conventional Flame Retardant (FR4) PCBs, especially those with over 0.5mm of thickness.
The HASL lead free surface finish can be used with the circuit boards with a thickness less than 0.5mm. An example is the rigid Printed Circuit Board (PCB).
5. Safety Precautions
It doesn’t make sense to use a product that causes harm to either the body or the environment. Such is the case with the HASL lead PCB surface finish, which uses lead as one of the properties. This causes bodily harm and could trigger environmental concerns.
The reversal is the case with the lead free HASL surface finish, which doesn’t use lead as one of the solder alloys. That way, both the human beings using it and the environment where it is used are safer.
6. Melting Points
Just as HASL lead free uses higher temperatures, it also means a lot of time must pass before its solders melt. The reversal is the case with the HASL lead surface finishes, which has the solders melting faster.
7. Presentation
How the surface finishes are presented also differentiate the HASL lead free from the lead HASL. It is common for the HASL surface finish, made up of lead and tin to have a brighter interface, due to the brighter and shiny composition of the properties.
In contrast, the lead-free HASL surface finish often appears dull when applied to the surface of the PCB.
HASL Lead free vs ENIG
Comparing the HASL lead free and the leaded variant gives you a perspective of some of the differences that make one better than the other.
However, for a fuller perspective of how it works, you want to compare or put the HASL lead free sided-by-side with some of the other PCB surface finishes. Comparing it with the ENIG process gives us an insight.
First, let us define what ENIG means. The full name is Electroless Nickel Immersion Gold. Also called immersion gold, or chemical Ni/Au, it is a two-layer metallic coating of over 120-240 µin Ni and 2-8µin of Au.
The goal of immersion gold is to plait a nickel of alloy to the circuit’s pads before applying the gold coating with chemical reactions. Culminative, the process is very helpful in the application and solidification of the circuit board mounting components, such as Ball Grid Arrays (BGAs) and Surface Mount Devices (SMDs).
With the Electroless Nickel Immersion Gold (ENIG) PCB surface finish process, the following benefits can be derived:
- The plating of the nickel alloy is not limited to the PCB pads. It can also be applied to the PCB wires or edges to help bolster the thermal/heat dissipation capabilities of the board.
- The ENIG PCB surface finish process is RoHS-compliant. Hence, it is environmental-friendly.
- The Nickel serves as the surface, upon which the SMD and BGA components are soldered.
- On the other hand, the Gold serves dual purposes. The first is to provide the low-constant resistance, required for making the thin gold deposits. It also protects the ENIG’s Nickels during the storage process.
Now to the differences between the immersion gold vs free HASL. The following are the clear differences between the two:
- ENIG costs more than the HASL lead free surface finish process for PCBs.
- The thickness of the HASL lead free is higher (potentially reaching 25.4µm). This makes it higher than the 3µm thickness of the ENIG surface finish.
- While the usage of the HASL lead free is mostly limited to the common rigid circuit boards with less than 0.5mm of thickness, that of the ENIG can be used for almost all types of circuit boards.
- Lead-free HASL comprises of over 99% of tin and a little percentage of copper. On the contrary, the properties of the ENIG surface finish include gold and nickel.
- The finishing method for HASL lead free is spraying, while that of ENIG is via chemical reactions.
Choosing the Right Surface Finish for Your PCB
Due to the several options out there (and they all have unique features), picking the best circuit board surface finish out of the list might not be as easy as walking in a park.
Yet, certain parameters can help you niche down and choose the one that closely aligns with what your PCB project and design is all about.
Here are some expert tips on how to make this decision:
1. Affordability
In terms of comparing and contrasting between the HASL lead free and the HASL lead, we would say that the former costs more. So, if you are working on a tight budget or looking to save up on costs, going for the HASL lead surface finish is a better way to save money.
2. RoHS-Compliance
Given the health and bodily harm concerns associated with electronic device usage, it is worth it to crosscheck the depth of exposure that the circuit boards would be bringing to the users.
RoHS-compliance is now a standard for most PCB projects and most consumers are taking a heed in that direction too. To this end, if you are looking to make RoHS-compliant circuit boards, going for the HASL lead free is a better option.
It is so because of the larger concentration of tin, and a little amount of copper. Since lead is not used here, you can be sure that the negative health concerns it triggers wouldn’t be something to worry about.
However, one important factor to the success of this is if there are no fine pitch components on the PCB. Components like BGAs and SMDs are not ideal for this purpose.
3. Durability Requirements
Besides protecting the copper and other important components, the function of the surface finishes should also extend to improving the PCB’s durability. The more durability the board becomes due to the application, the longer it lasts.
4. Applications and Operating Capacities
The applications, i.e., where the boards coated with any of these surface finishes will be used is an important element for consideration. To this end, the applications or use cases must be factored in to aid in the proper selection.
5. Consider the Environment
Do not confuse the environment to be the same thing as the application or use case. The environment, as used here, refers to the type or level of exposure that the Printed Circuit Board (PCB) is likely to be exposed to after getting coated with the surface finish.
The environment often refers to the extent of the temperature – whether it is harsh or mild. For the best results, use the HASL lead free, due to the RoHS-compliance, which makes it both friendly to the users and the immediate environment.
6. Settling for ENIG instead of HASL Lead-Free Surface Finish
The three (3) major options before you for finishing the surface of the PCB are HASL, HASL lead free and ENIG. While the 3 have distinct properties, one of them might just be a better option than the rest are.
For one, you should go for the HASL lead free over the HASL because it is RoHS-compliant, and that means it is possible to use it in a wide range of PCB surface finishes. The fact that it offers excellent solderability is another selling point.
On the other hand, HASL makes a great deal if you are looking to save up on costs and work with PCBs requiring lesser temperatures.
Going for the Electroless Nickel Immersion Gold (ENIG) is the best option if you are unsure of whether both the HASL lead free and HASL can do the job. Besides, ENIG offers almost the same properties as the lead-free HASL, such as RoHS-compliance.
Conclusion: Talk to an Expert if You’re Unsure
Sometimes, it is better to speak to someone versed in a field, if you are unsure or undecided on which path to take. If you are stuck at choosing between the traditional HASL and the HASL lead free, and you are not sure if the ENIG surface finish process can do the work – consider consulting an expert in that field.
Talking about consultations, you need to speak to an experienced circuit board designer and there is no other that has the experience and know-hows than Hillman Curtis. Irrespective of what options you have on your table, we will professionally guide you through the ins and outs of each of those options to help you pick one.
Besides, you can count on our experienced team of engineers and our industry-leading machinery to deliver the best surface finishes possible for your Printed Circuit Boards (PCBs). We are responsive and are available to take your orders!
