In order to facilitate soldering and prevent oxidation, surface finish is usually carried out after PCB manufacturing, where HASL finish is favored for its cost-effectiveness and wide applicability. However, improper operation can sometimes lead to exposed copper on the solder pad surface and within the plated through-holes, which is an unacceptable defect that may impact the soldering quality, environmental sensitivity, and even cause PCB short circuit. To address this issue, TechSparks has conducted an analysis of the causes of copper exposure in the HASL process, aiming to help readers improve their PCB manufacturing quality.
Many small manufacturers, in order to expedite the production process, often neglect PCB cleaning procedures. Instead, they may simply inspect the PCB surface and proceed directly to the HASL process, which is evidently an incorrect practice. Impurities can come from various sources, including residual flux during the manufacturing process, airborne particles, and fingerprint oils from workers. Not all impurities can be easily detected with the naked eye, but these residues can be a major cause of HASL failure and copper exposure.
Essentially, HASL involves coating the PCB with a layer of smooth solder to achieve this. Typically, the PCB is immersed in a molten solder bath, where the molten solder, usually a tin/lead alloy, adheres to the exposed copper surface and, under the influence of gravity, naturally sinks and flows to achieve a smooth finish. It’s a straightforward process, but the prerequisite for successful solder adhesion is a clean and tidy PCB surface.
This is the same as the copper exposure problem caused by insufficient cleaning, i.e., inadequate solder adhesion. To address this issue, mild etching with sulfuric acid-hydrogen peroxide is usually performed. However, sometimes insufficient pre-treatment can occur due to batch-related reasons, such as a decrease in etching solution concentration or insufficient etching time.
For such problems, the most noticeable feature is a widespread distribution of exposed copper spots, with the edge area being more severe. To improve pre-treatment processes:
- Check whether the etching solution concentration for the batch with copper exposure issues is appropriate, and inspect the quality of the second acid wash solution, replacing overly contaminated solutions promptly.
- Check if the spray system is unobstructed to ensure that the liquid can cover the entire board evenly and effectively.
- Moderately extend the etching time, but avoid excessive prolongation.
- For PCB that need rework, treat them in a 5% hydrochloric acid solution after hot air leveling to improve processing quality.
Insufficient Flux Activity
In the HASL process, the flux serves multiple purposes, including activating the exposed copper surface on the PCB, improving solder wetting on the copper surface, and providing anti-oxidation properties for the solder during cooling after leveling. When selecting flux for the HASL process, attention should be paid to the acidity not being too high, to avoid copper bite, and not too low, to prevent insufficient activity leading to copper exposure. Choose flux for the HASL process with the following characteristics:
- Non-toxic, harmless, good water solubility, and biodegradable.
- Good thermal stability to avoid thermal decomposition during the process.
- A certain viscosity for better adhesion to the PCB surface.
- The pH value of the selected flux is recommended to be around 2.5-3.5, which is compatible with both activity and solubility.
The above three are common and easily overlooked causes of copper exposure in the HASL process. In addition to these reasons, factors such as improper process parameter settings and uneven coating may also lead to copper exposure. When problems arise, timely feedback and root cause analysis are essential to optimize manufacturing solutions and upgrade manufacturing processes to avoid the recurrence of similar issues in future projects.
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