During the PCB manufacturing process, the darkening of the gold plating layer is a relatively common cosmetic defect. This not only affects the visual appearance of the product but may also indirectly impact its electrical conductivity and resistance to oxidation, resulting in unnecessary production losses.
Based on practical production experience, this issue is not caused by a single factor; it is primarily closely related to the condition of the nickel plating bath solution, the control of nickel layer thickness, and the operating status of the gold plating tank. Consequently, targeted solutions must be implemented. The details are explained below.
Main Causes of Blackening in PCB Gold Plating
1.Poor condition of the nickel plating bath
In the PCB gold plating process, the nickel layer serves as the substrate for the gold layer; its quality directly determines the appearance and performance of the gold layer. Consequently, the condition of the nickel plating bath is one of the key factors causing blackening of the gold layer, and it is also one of the most easily overlooked control points in production.
In actual production, if the nickel bath solution is not maintained in accordance with regulations over a prolonged period and carbon removal is not carried out on schedule, organic and metallic impurities will accumulate continuously within the solution, thereby affecting the crystalline structure of the nickel plating layer. In such cases, the deposited nickel layer will exhibit flaky crystallisation, resulting in increased hardness and brittleness.
This manifests as blackening on the subsequent gold plating layer; in severe cases, it may even cause the nickel layer itself to blacken, thereby affecting the appearance of the gold layer. This type of gold layer blackening caused by improper maintenance of the nickel bath is relatively common in mass production and is a major reason for the recurrence of the problem.
2.Nickel plating thickness failing to meet standards
When encountering issues with blackening of the gold layer, many production staff tend to focus directly on the gold plating tank itself, whilst overlooking the core influencing factor of nickel layer thickness. In fact, gold plating layers on PCBs are typically designed to be relatively thin, and many of the surface appearance issues that arise are fundamentally caused by the poor condition of the underlying nickel layer. Among these, an insufficient nickel layer thickness is one of the primary causes of blackening or whitening of the gold layer.
If the nickel layer thickness fails to meet process requirements, it cannot provide a uniform and stable substrate for the gold layer. Consequently, the gold layer may exhibit uneven coverage or localised light transmission, resulting in a visually blackened appearance. In accordance with industry standards, the thickness of the nickel plating layer on PCBs should be controlled at around 5 μm to provide adequate support for the gold layer and effectively prevent the gold layer from turning black.
3.Abnormal operation of the gold plating tank
Compared to the nickel plating tank, the gold plating tank is more stable and has a lower probability of contamination. Provided that the solution is properly filtered and replenished on a regular basis, serious problems generally do not arise.
However, if key control parameters in the gold bath deviate from normal, this can still lead to blackening of the gold layer. Specific areas requiring close inspection include:
The dosage of gold bath additives; insufficient additives can cause uneven gold deposition, whilst excessive amounts may lead to an imbalance in the bath composition, both of which affect the appearance of the gold layer;
The pH of the bath; a pH deviating from the standard range disrupts the deposition environment for the gold layer, leading to abnormal crystallisation and blackening;
The condition of the conductive salt: insufficient conductive salt affects current distribution during electroplating, leading to uneven deposition of the gold layer and, consequently, surface defects.
Solutions for Blackening of Gold Plating on PCBs
1.Strictly control the thickness of the nickel plating layer
To address blackening of the gold layer caused by an excessively thin nickel layer, factory engineering staff must prioritise checking the thickness of the nickel layer. During production, the nickel layer thickness must be strictly controlled at approximately 5 μm in accordance with process requirements. Regular inspections of the nickel layer thickness after plating should be carried out to prevent insufficient or uneven thickness. At the same time, the nickel plating process parameters must be optimised to ensure uniform deposition and dense crystallisation of the nickel layer, thereby providing a stable substrate for the gold layer and reducing the likelihood of blackening at source.
2.Standardising the Maintenance of Nickel Plating Baths
The key to resolving issues caused by the nickel plating bath lies in effective daily maintenance and carbon treatment. Production workshops must establish a comprehensive maintenance regime for nickel plating bath solutions, conducting regular tests to analyse impurity levels and component ratios, and carrying out carbon treatment in a timely manner.
Carbon treatment effectively removes organic and metallic impurities from the solution, restoring its activity and ensuring the cleanliness of the plating solution. This prevents issues such as flaky crystallisation and abnormal hardness in the nickel layer, thereby indirectly addressing the risk of blackening in the gold plating layer. It is important to note that carbon treatment must be carried out strictly in accordance with operating procedures. If operators are unfamiliar with the carbon treatment process, they must undergo professional training in advance to avoid solution failure due to improper operation, which could lead to more serious production issues.
3.Optimising Gold Bath Operational Control
To address gold layer darkening caused by abnormalities in the gold bath, daily management must be prioritised, focusing on the following points:
Reasonably control the dosage of gold bath additives, replenishing them regularly based on solution consumption to avoid shortages or excesses, thereby ensuring the stability of the solution composition;
Regularly test the pH of the solution and adjust it promptly to the standard range to provide a suitable environment for gold layer deposition;
Check the concentration of conductive salts to ensure it meets process requirements, thereby guaranteeing uniform current distribution.
If none of the above checks reveal any abnormalities, an AA analyser may be used to conduct a precise analysis of impurity levels in the gold bath solution to identify hidden contamination issues. Furthermore, the filter cotton cores in the gold bath must be replaced regularly to prevent blockages that result in inadequate filtration of the solution, which in turn causes surface defects in the gold layer. This is a detail that is easily overlooked in daily production and requires enhanced control.
The issue of blackening in the gold plating layer on PCBs is primarily attributable to the quality of the underlying nickel layer and the operational status of the relevant plating baths. In actual production, by strengthening daily monitoring of the nickel and gold plating tanks, strictly controlling the thickness of the nickel layer in accordance with process requirements, and standardising the maintenance of the plating solutions, the occurrence of this problem can be effectively reduced, thereby ensuring the quality stability of PCB products.
