With the widespread adoption of smartphones, the miniaturization of electronic products, and the EU’s push for lead-free processes, the surface treatment process of electroless nickel immersion gold (ENIG) has become simpler and more cost-effective compared to other surface treatments. Additionally, ENIG offers excellent repeatability, good flatness, suitability for fine-pitch components, long-term storage capabilities, and resistance to oxidation. Consequently, an increasing number of electronic products are opting for ENIG as their PCB surface treatment.
When users encounter issues such as parts dropping or poor solderability with ENIG surface treatment, the first concern that often arises is “black nickel,” also known as “black pad.” However, there seems to be a lack of understanding regarding what “black nickel” or “black pad” truly entails. This article aims to explore the concepts of ENIG’s “black nickel” and “black pad” from the perspective of working professionals in the field.
The phenomenon of “black nickel” in ENIG essentially involves two primary components: “phosphorus” and “nickel oxide.”
“Phosphorus” is derived from the electroless nickel plating layer. During the subsequent process of gold replacement and chemical nickel deposition, phosphorus remains inactive and accumulates between the gold layer and the nickel layer, resulting in a phosphorus-rich layer that ultimately leads to embrittlement, compromising solder strength.
“Nickel oxide” is mainly composed of a complex chemical formula represented as NixOy (where x and y are numerical values). The underlying cause of this issue is the excessive oxidation of the nickel surface during the immersion gold process, where metallic nickel is converted into nickel ions (a broad sense of “oxidation”). This excessive oxidation leads to irregular deposition of large gold atoms (with a radius of 144 pm), creating rough, loose, and porous crystalline structures. As a result, the gold layer fails to provide complete coverage over the underlying nickel layer, which remains exposed to air, continuing the oxidation process. This gradual formation of nickel oxide beneath the gold layer ultimately impedes soldering.
1. Most solders, such as SAC305, SAC3005, SnBi, and SnBiAg, primarily consist of tin (Sn). When a circuit board is heated in the reflow furnace, Sn reacts with the nickel (Ni) from ENIG to form the common intermetallic compound Ni3Sn4 (IMC). If the nickel layer is oxidized, forming an ideal IMC becomes challenging. Even if it forms inadequately, the IMC may appear intermittently and unevenly, reducing welding strength—similar to a brick wall where cement is poorly applied. This lack of cement between bricks mirrors the IMC’s role; gaps lead to fragility.
2. Circuit board surface treatments include “nickel-immersed palladium gold (ENEPIG),” which effectively mitigates “black nickel/black pad” issues. However, due to higher costs, it is predominantly used in high-end boards, CSPs, or BGAs.
3. Two potential issues with ENIG pads and their prevention: ENIG’s fundamental process. A key advantage of the ENIG surface treatment for PCBs is its straightforward manufacturing process, typically requiring only two chemical solutions—electroless nickel plating and acidic gold solution. While additional chemicals may be involved, the ENIG process generally begins with chemical nickel deposition on copper pads, controlling the nickel layer thickness through time and temperature management.
4. Following deposition, the freshly plated nickel is immersed in acidic gold solution, where a chemical displacement reaction occurs, depositing gold onto the pad’s surface and dissolving some nickel into the solution. This gold layer gradually covers the nickel until fully enveloped, halting the replacement reaction, and the process concludes with cleaning the pad’s surface. Typically, this gold layer measures about 0.05 µm (2 µ”) or thinner, making the ENIG process easy to manage and relatively cost-effective compared to electroplated nickel and gold.
5. The formation and effects of black nickel: The nickel layer’s quality relies heavily on the nickel plating solution formula and temperature control during chemical deposition, along with the acidic gold treatment process. Electroless nickel plating occurs through an autocatalytic reaction of hypophosphite and nickel salt on the pad’s surface, resulting in a coating containing a specific phosphorus (P) percentage. Research indicates that phosphorus content should ideally range from 7% to 10%.
6. If the plating solution formulation isn’t maintained or temperature control falters, phosphorus content may deviate from this norm. Low phosphorus levels lead to weaker coatings, while high levels significantly increase hardness, adversely affecting solderability and reliable joint formation. If phosphorus content is low and gold plating isn’t processed correctly, numerous cracks in the gold layer may develop, making it difficult to remove acid gold in subsequent cleaning, leading to accelerated nickel corrosion and the formation of black nickel—commonly referred to as black solder pads.
7. The formation and impact of phosphorus-rich layers: In ENIG surface-treated solder pads, the actual alloy that interacts with solder paste is the nickel, resulting in the typical intermetallic compound (IMC) Ni3Sn4. Phosphorus in the nickel plating does not alloy but occupies a certain proportion within the nickel layer, where excess phosphorus may concentrate at the alloy layer’s edges, creating a phosphorus-rich layer. If this layer is excessively thick, its strength diminishes, making it the likely failure point under external stress, thereby compromising reliability.
8. Preventing and controlling black nickel and phosphorus-rich layers: Although the emergence of black nickel and phosphorus-rich layers can be subtle and difficult to detect, understanding their causes allows for effective prevention methods. To combat black nickel formation, the manufacturing stage must ensure plating solution stability and process temperature control to maintain optimal nickel and phosphorus ratios. Additionally, acidic gold solutions require diligent maintenance and timely adjustments if they become overly corrosive.
9. For users:
1. The best approach is to utilize a scanning electron microscope (SEM) to microscopically inspect the solder pad’s surface, checking for gold plating cracks, and using EDS to analyze phosphorus ratios in the nickel layer.
2. Another method involves hand-soldering a typical pad and measuring the push-pull strength of the joint; abnormally low strength may indicate black nickel.
3. Lastly, conducting an acid gas corrosion test on ENIG samples can reveal surface powder or discoloration, indicating cracked gold coatings and potential black nickel.
10. Among these methods, the second is the most straightforward and rapid to implement. By employing these techniques, issues can be identified early, preventing extensive reliability problems in ENIG circuit boards and minimizing losses.
11. Regarding the phosphorus-rich layer, ensuring proper phosphorus and nickel ratios during nickel plating is crucial. The welding process must be carefully controlled, including time, temperature, and intermetallic thickness, ideally between 1-2 microns (µ). Excessively thick IMC formation inevitably leads to a thicker phosphorus-rich layer.
When users encounter issues such as parts dropping or poor solderability with ENIG surface treatment, the first concern that often arises is “black nickel,” also known as “black pad.” However, there seems to be a lack of understanding regarding what “black nickel” or “black pad” truly entails. This article aims to explore the concepts of ENIG’s “black nickel” and “black pad” from the perspective of working professionals in the field.
The phenomenon of “black nickel” in ENIG essentially involves two primary components: “phosphorus” and “nickel oxide.”
“Phosphorus” is derived from the electroless nickel plating layer. During the subsequent process of gold replacement and chemical nickel deposition, phosphorus remains inactive and accumulates between the gold layer and the nickel layer, resulting in a phosphorus-rich layer that ultimately leads to embrittlement, compromising solder strength.
“Nickel oxide” is mainly composed of a complex chemical formula represented as NixOy (where x and y are numerical values). The underlying cause of this issue is the excessive oxidation of the nickel surface during the immersion gold process, where metallic nickel is converted into nickel ions (a broad sense of “oxidation”). This excessive oxidation leads to irregular deposition of large gold atoms (with a radius of 144 pm), creating rough, loose, and porous crystalline structures. As a result, the gold layer fails to provide complete coverage over the underlying nickel layer, which remains exposed to air, continuing the oxidation process. This gradual formation of nickel oxide beneath the gold layer ultimately impedes soldering.
1. Most solders, such as SAC305, SAC3005, SnBi, and SnBiAg, primarily consist of tin (Sn). When a circuit board is heated in the reflow furnace, Sn reacts with the nickel (Ni) from ENIG to form the common intermetallic compound Ni3Sn4 (IMC). If the nickel layer is oxidized, forming an ideal IMC becomes challenging. Even if it forms inadequately, the IMC may appear intermittently and unevenly, reducing welding strength—similar to a brick wall where cement is poorly applied. This lack of cement between bricks mirrors the IMC’s role; gaps lead to fragility.
2. Circuit board surface treatments include “nickel-immersed palladium gold (ENEPIG),” which effectively mitigates “black nickel/black pad” issues. However, due to higher costs, it is predominantly used in high-end boards, CSPs, or BGAs.
3. Two potential issues with ENIG pads and their prevention: ENIG’s fundamental process. A key advantage of the ENIG surface treatment for PCBs is its straightforward manufacturing process, typically requiring only two chemical solutions—electroless nickel plating and acidic gold solution. While additional chemicals may be involved, the ENIG process generally begins with chemical nickel deposition on copper pads, controlling the nickel layer thickness through time and temperature management.
4. Following deposition, the freshly plated nickel is immersed in acidic gold solution, where a chemical displacement reaction occurs, depositing gold onto the pad’s surface and dissolving some nickel into the solution. This gold layer gradually covers the nickel until fully enveloped, halting the replacement reaction, and the process concludes with cleaning the pad’s surface. Typically, this gold layer measures about 0.05 µm (2 µ”) or thinner, making the ENIG process easy to manage and relatively cost-effective compared to electroplated nickel and gold.
5. The formation and effects of black nickel: The nickel layer’s quality relies heavily on the nickel plating solution formula and temperature control during chemical deposition, along with the acidic gold treatment process. Electroless nickel plating occurs through an autocatalytic reaction of hypophosphite and nickel salt on the pad’s surface, resulting in a coating containing a specific phosphorus (P) percentage. Research indicates that phosphorus content should ideally range from 7% to 10%.
6. If the plating solution formulation isn’t maintained or temperature control falters, phosphorus content may deviate from this norm. Low phosphorus levels lead to weaker coatings, while high levels significantly increase hardness, adversely affecting solderability and reliable joint formation. If phosphorus content is low and gold plating isn’t processed correctly, numerous cracks in the gold layer may develop, making it difficult to remove acid gold in subsequent cleaning, leading to accelerated nickel corrosion and the formation of black nickel—commonly referred to as black solder pads.
7. The formation and impact of phosphorus-rich layers: In ENIG surface-treated solder pads, the actual alloy that interacts with solder paste is the nickel, resulting in the typical intermetallic compound (IMC) Ni3Sn4. Phosphorus in the nickel plating does not alloy but occupies a certain proportion within the nickel layer, where excess phosphorus may concentrate at the alloy layer’s edges, creating a phosphorus-rich layer. If this layer is excessively thick, its strength diminishes, making it the likely failure point under external stress, thereby compromising reliability.
8. Preventing and controlling black nickel and phosphorus-rich layers: Although the emergence of black nickel and phosphorus-rich layers can be subtle and difficult to detect, understanding their causes allows for effective prevention methods. To combat black nickel formation, the manufacturing stage must ensure plating solution stability and process temperature control to maintain optimal nickel and phosphorus ratios. Additionally, acidic gold solutions require diligent maintenance and timely adjustments if they become overly corrosive.
9. For users:
1. The best approach is to utilize a scanning electron microscope (SEM) to microscopically inspect the solder pad’s surface, checking for gold plating cracks, and using EDS to analyze phosphorus ratios in the nickel layer.
2. Another method involves hand-soldering a typical pad and measuring the push-pull strength of the joint; abnormally low strength may indicate black nickel.
3. Lastly, conducting an acid gas corrosion test on ENIG samples can reveal surface powder or discoloration, indicating cracked gold coatings and potential black nickel.
10. Among these methods, the second is the most straightforward and rapid to implement. By employing these techniques, issues can be identified early, preventing extensive reliability problems in ENIG circuit boards and minimizing losses.
11. Regarding the phosphorus-rich layer, ensuring proper phosphorus and nickel ratios during nickel plating is crucial. The welding process must be carefully controlled, including time, temperature, and intermetallic thickness, ideally between 1-2 microns (µ). Excessively thick IMC formation inevitably leads to a thicker phosphorus-rich layer.
