In the procurement process of product development, the surface treatment of PCB on the bare board may seem like a trivial decision, but understanding the variations and complexities involved can help prevent issues.

The surface finish of a printed circuit board serves two main purposes. First, it helps protect the copper circuits from corrosion. Second, it creates a solderable surface for your PCBA components. Therefore, the following considerations need to be taken into account:

1. The components you are using

2. Expected output of the PCB

3. Requirements for environmental and general durability of the circuit board

4. Impact on the environment

5. Budget

This article will discuss five common types of PCB surface treatments and their pros and cons.

1. Hot Air Solder Leveling (HASL): The most commonly used PCB surface treatment is HASL or "Hot Air Solder Leveling." It is popular because it is always available and cost-effective. However, even though HASL is inexpensive, new technologies can offer higher surface smoothness.

In this process, the PCB is dipped into molten solder and then leveled using hot air knives, hence the name. HASL can work well if your circuit board uses through-hole or large SMT components. However, it may not be an ideal surface treatment for smaller SMT components like those smaller than 0805 or SOIC. The surface smoothness achieved through this process is not entirely flat and can cause issues with smaller components. The solder used in this process is typically tin-lead, which means it is not RoHS-compliant. If your project requires RoHS compliance or your company wants to reduce lead usage, you may need to specify lead-free HASL.

Advantages:

• Good solderability

• Cheap/low cost

• Allows for a larger processing window

• Widely used in the industry

Disadvantages:

• Thickness/shape differences between non-planar and large pads

• Not suitable for smaller and denser components (SMD and BGA with spacing less than 20 mils)

• Bridging in fine-pitch applications

• Not suitable for HDI products

2. Lead-Free Hot Air Solder Leveling (HASL): Lead-free HASL is similar to traditional HASL, but it does not use tin-lead solder. Instead, lead-free HASL uses tin-copper, tin-nickel, or tin-copper-nickel-germanium alloys. This makes lead-free HASL an economical and RoHS-compliant choice. However, similar to standard HASL, it is not suitable for smaller SMT components.

Advantages:

• Good solderability

• Cheap/low cost

• Allows for a larger processing window

• Widely used in the industry

• Many popular options available

Disadvantages:

• Thickness/shape differences between non-planar and large pads

• High processing temperature in the range of 260-270°C

• Not suitable for smaller and denser components (SMD and BGA with spacing less than 20 mils)

• Bridging in fine-pitch applications

3. Immersion Tin (ISn): In ISn PCB manufacturing, a chemical process is used. This process involves depositing a flat metal layer on the copper lines. Tin is one of the cost-effective immersion coating types. While it is a budget-friendly option, it does have some drawbacks.

The issue with tin is that it starts to discolor when deposited on copper. Therefore, if poor solder joints are to be avoided, components need to be soldered within 30 days. In large-scale production, this may not be a problem. If a large number of PCBs are consumed quickly, the loss of gloss can be avoided. However, if the production volume is low or the bare boards need to be retained in inventory, using coatings like silver plating may be wiser.

Advantages:

• Flat solderable area

• Suitable for small spacing/BGA/smaller components

• Reasonable price for lead-free surface treatment

• Suitable for press-fit surface treatment

• Good solderability even after multiple thermal cycles

Disadvantages:

• Sensitive to handling - gloves should be worn

• Tin whisker issues

• Solder resist erosion - solder resist dams should be ≥5mil

• Pre-baking reduces solderability

4. Immersion Silver (IAg): Unlike ISn, immersion silver does not react with copper. However, it loses its gloss when exposed to air. Therefore, all IAg PCBs should be stored in anti-tarnish packaging during storage and handling.

Once properly packaged, these PCBs can be reliably soldered within 6 to 12 months. However, once the PCBs are taken out of the packaging, reflow soldering needs to be done within a day. Gold plating can extend the shelf life.

Advantages:

• Flat surface

• Ideal for small-size, small-spacing, and BGA components

• Medium-priced lead-free surface treatment

• PCBs can be reworked

Disadvantages:

• Potential handling/tarnishing issues

• Special packaging to prevent discoloration

• Small window for achieving good solderability during assembly

5. Electroless Nickel Immersion Gold (ENIG): My personal favorite PCB surface treatment is ENIG. The process involves depositing a thin layer of gold over a chemical nickel or electrolytic nickel surface.

The gold plating is hard and durable. This makes it have a longer shelf life, and even good manufacturers can use it for several years. The materials, process, and reliability often make ENIG boards more expensive than other surface treatment boards. However, we have seen many boards starting to shift most of their processes to ENIG, even when the price difference is small compared to other options.

Advantages:

• Flat surface finish

• Suitable for small-size, small-spacing, and BGA components

• Reliable manufacturing process and procedures

• Suitable for wire bonding

Disadvantages:

• Sometimes higher finishing cost

• Black pad issues with BGA

• Signal loss (RF)

• Avoid using BGA defined by solder resist layer

Choosing the Right PCB Surface Treatment for Your Project: The surface finish of the PCB is a critical decision to be considered before manufacturing. It is important to consider factors such as component type, output requirements, durability, environmental impact, and cost, and discuss them with your team. By fully understanding your needs, you can select the appropriate surface treatment for your PCB. PCB fabrication factories explain PCBA's sensitive PCB surface treatments, which type to use, and the details of PCB surface treatment.