Workingbear previously wrote an article about how insufficient gold thickness in the ENIG surface treatment of a PCB led to the oxidation of Electroless Nickel, weakening solder joints and causing components detachment during plug and unplug. However, many sources explain that gold’s primary role in PCB surface finishes is to prevent nickel oxidation by acting as a barrier. Gold (Au) doesn’t strengthen  solder joints and may form brittle intermetallic compounds (IMCs) like AuSn and AuSn₂, leading to gold embrittlement.

Recommended reading: Two Major Potential Problems with ENIG PCB Pads (Black Nickel and Phosphorus-rich Layers) and Preventive Measures

If you’re familiar with ENIG soldering, you’ll know that the actual solder joint on an ENIG PCB should be entirely formed on the nickel layer, creating a Ni3Sn4 IMC to ensure solder strength. So, while gold does form IMCs with tin during soldering, these tin-gold IMCs must stay away from the tin-nickel interface and dissolve into the overall solder.

Recommended reading: What is an IMC (Intermetallic Compound)? How Does IMC Relate to PCB Solder Joint Strength?

To help everyone better understand the role of “gold” in ENIG soldering, the following article and images refer to the “Microsection and Standards Manual” published by TPCA (Taiwan Printed Circuit Association).

In a PCB with ENIG (Electroless Nickel Immersion Gold) surface finished, the gold layer quickly dissolves into the molten solder during high-temperature soldering, forming AuSn, AuSn₂, or AuSn₄ intermetallic compounds (IMCs) and rapidly separating from the Electroless Nickel (EN) layer, diffusing into the solder. Therefore, the solder joint in ENIG should entirely grow on the nickel layer. The gold layer’s primary function is to protect the nickel layer from direct contact with air, preventing rust and oxidation. If the gold layer is too thick, it won’t improve solder quality; instead, if the gold content exceeds 3% of the solder joint weight (Au has a density of 19.3), the gold will fail to diffuse smoothly into the solder, remaining at the soldering interface and forming weak IMCs, leading to gold embrittlement issues.

The following four images, magnified 4,500 times under SEM, illustrate what happens when insufficient heat is applied during reflow soldering. As gold (Au) begins to separate from the nickel layer and merge into the solder, there is a high chance that under external force, the component may crack along the line of AuSn, AuSn2, or AuSn4 IMCs.

The following four cross-section images show that when soldering heat is insufficient or not maintained, although a thin Ni₃Sn₄ IMC layer forms and bonds well, AuSn and AuSn₂ may not have fully dissolved into the solder. Instead, they stay near the IMC interface, making it prone to cracking under external stress. This is one of the main reasons why ENIG solder joints are typically not as strong as copper-based solder joints (Cu₆Sn₅).

The next two images show that when sufficient and sustained heat is applied during soldering, the gold layer completely dissolves into the solder. Small particles of AuSn can be seen floating and dispersing throughout the solder, leaving behind a Ni-Sn IMC that provides good solder joint strength. You can imagine that if the gold layer is too thick, too much AuSn IMC would form, preventing it from fully dissolving into the solder, ultimately weakening the joint.

According to IPC-4552 standards for ENIG, the gold immersion layer must be at least 0.05µm thick (which converts to 1.97µ”). However, this 0.05µm thickness must be controlled within 4 sigma of the minimum thickness. The typical range for the gold layer thickness is 0.075µm to 0.125µm (2.955µ” to 4.925µ”), so we usually recommend a gold thickness of 2µ” to 5µ”. The electroless nickel layer, meanwhile, should be between 3µm and 6µm (118µ” to 236µ”).

IPC-6012 (Rigid) requires a minimum gold thickness of 1.97µ” and a nickel thickness of at least 118µ”. IPC-6012 (Flex, Rigid-Flex) requires a minimum gold thickness of 1.58µ” to 3.94µ” and a nickel thickness of 118µ” to 236µ” (Rigid) or 50µ” to 236µ” (Flex).

For soldering purposes, thinner gold layers are preferable to avoid residual gold at the interface, which could lead to gold embrittlement and reverse corrosion. Since gold is an inconsequential element in the soldering process, a thinner layer is better. However, if the gold layer is too thin to fully cover the nickel layer, the PCB may oxidize after long-term storage, leading to solder rejection. Therefore, the main purpose of the gold layer is to prevent oxidation of the PCB surface.

In another article, “Does the Gold Thickness in ENIG PCBs Affect Component Detachment?” Workingbear mentioned that in recent years, due to rising gold prices, our company lowered the minimum gold thickness to 1.2µ” to reduce costs. However, this halved the shelf life of the product, and we had to carefully measure the gold thickness across large areas since gold tends to be thinner on larger pads. This change is best suited for PCBs that aren’t used for buttons or contact points. From 2012 to 2022, after about 10 years of implementing this standard, we experienced only occasional quality issues, like oxidation on some PCBA units delivered to customers. These issues were mostly due to human error rather than significant quality failures, so we continue to use the 1.2µ” specification.

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