Introduction to the Application of Lead-free Surface Finish Techniques:
The widespread adoption of lead-free HASL will depend on scientific studies, its promotion, and its influence. However, the smooth surface finish and the challenges of high-temperature processing may present obstacles. OSP (Organic Solderability Preservative) technology is commonly used when a flatter surface is required, there is no need for extended shelf life, and multiple heating applications are not necessary. OSP is poised to replace HASL in many general applications due to its cost-effectiveness and recent improvements in heat resistance, without the drawbacks of high-temperature processing.
ENIG (Electroless Nickel Immersion Gold) technology is often employed in applications where higher protection is needed, such as for contact and bonding requirements, as well as in quality-sensitive and integrated applications. However, the increasing pressure from cost concerns and the rise of I-Ag (Indium-Silver) technology could reduce the use of ENIG. If I-Ag technology is adopted for bonding applications, its cost advantage may push ENIG aside. Some high-quality products may prefer I-Ag technology over ENIG in certain instances.
Below is a comparison of the performance of several surface treatment processes.
Effect of Reflow Times on PCB Surface Finish Performance:
The solderability of ENIG remains largely unaffected by repeated reflow processes. In contrast, the solderability of HASL, OSP, and Ag deteriorates significantly with each reflow cycle. After four reflows, the average particle diameter at the OSP interface is 30% larger than at the I-Ag interface. This increase is due to the formation of numerous thin sheet blocks that hinder the mutual diffusion of Cu and Sn in the alloy, thus thinning the interface layer. The solderability of Sn decreases most rapidly due to the formation of Sn-Cu compounds and the oxides created by Sn.
Influence of Nitrogen Protection on PCB Surface Finish Performance:
When combined with Sn-Pb solder and a PCB surface protection layer, nitrogen protection plays a significant role in improving wetting and spreading. However, for lead-free solders, nitrogen protection does not necessarily yield the same benefits. For instance, the solderability of ENIG, OSP, and I-Ag all improve in a nitrogen atmosphere, though OSP and I-Ag show minimal variation and depend on the testing method. In contrast, when HASL (Sn-Cu and SAC) and l-Sn are exposed to oxygen levels higher than a certain threshold, the solderability improves as the residual oxygen content decreases. However, when exposed to lower oxygen levels, no significant change occurs. In general, the activity of the solder has the most significant impact on wettability, but the effect on ENIG coating is not as prominent. ENIG performs better in a nitrogen environment, showing improved weldability.
Protection during wave soldering is superior compared to high-activity solder. The table below illustrates the spreadability of solder alloy on different coating surfaces after varying reflow times in an air environment.
The following table shows the wettability of solder alloys on different coating surfaces after varying reflow times in an air environment.
As shown in the table, when the reflow cycles are minimal, the spreadability of the surface coating is mainly determined by the coating itself. Protection factors do not significantly influence this behavior. However, as the number of reflow cycles increases, the protective coating starts to show some impact. This also demonstrates that surface finish failures with Sn-based coatings occur more rapidly.
Conclusion:
The solderability of I-Sn coatings degrades quickly, and the “whisker” phenomenon is more likely to occur. In comparison to lead-free solder, I-Sn coatings show little difference in solderability with minimal spread rate, while lead-based solder exhibits a significantly higher spread rate. HASL coatings suffer from poor surface smoothness and the “whisker” issue. SAC and Sn-Ag alloys, often chosen for lead-free surface finishes, offer good tolerance and low defect rates, making them ideal for non-fine-pitch assemblies. ENIG coatings are widely used in fine-pitch bonding, high-reliability applications, and multiple reflow processes due to their excellent weldability, low reliance on interfacial compounds, and smooth surface. However, ENIG coatings are more expensive and have a complex process, with a potential issue of “black pad” formation. I-Ag coatings have gained popularity as an alternative. OSP coatings, while offering the lowest cost and short-term storage capabilities, are primarily used in simpler products.
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