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How to Choose Lead-free Surface Finish Techniques

by: Dec 24,2018 645 Views 0 Comments Posted in PCB Manufacturing Information

Surface Finish

Introduction of Application of Lead-free Surface Finish Techniques:

Whether lead-free HASL can be popularized will depend on the results of the scientific study and its promotion and influence, but its smooth surface and high temperature process problems may be obstacles. OSP technology is widely used when a flatter surface is required, long shelf life is not required, and multiple heating applications are not required. OSP is likely to replace HASL technology in a large number of general application fields, because OSP costs less than HASL and has made good progress in its weakest heat resistance without the disadvantages of high-temperature processing.

ENIG technology is most often used when protection requirements are higher, when contact and bonding applications are required, and when quality and integration applications are required. But pressures of cost and the rise of I-Ag could replace ENIG to some extent. If the I-Ag technology can be adopted by users in bonding applications, its cost advantage will put pressure on ENIG. Some of the higher-quality products may be more inclined to use I-Ag technology, and sometimes ENIG technology. The following table is a comparison of the performance of several surface treatment processes.

image.png

Influence of Reflow Times on PCB Performance of Surface Finish Process:

The weldability of ENIG did not change much in the repeated reflow welding, but the weldability of HASL, OSP and Ag declined sharply. It was found that after 4 reflowing, the average diameter which is generally 2.696um of image.png particles at the OSP interface was 30% higher than that at the I-ag interface which is generally 1.945um, mainly due to the interface a large number of thin sheet image.png blocks are generated to weaken the mutual diffusion between Cu and Sn in the alloy, thus thinning the thickness of the interface layer. The solderability of Sn decreases most rapidly due to the formation of Sn-Cu compounds and the oxides formed by Sn.

image.png

Influence of Nitrogen Protection on PCB Performance of Surface Finish Process:

For the combination of Sn-Pb solder and PCB surface protection layer, image.png protection has a significant role in promoting wetting and spreading. For lead-free solders, nitrogen protection does not necessarily have the same effect. For example, solderability of ENIG, OSP and I-Ag are all enhanced in image.png atmosphere, but among these, OSP and I-Ag vary little and vary with the test method. If HASL (Sn-Cu and SAC) and l-Sn is with a volume fraction of image.png or more in oxygen, it would increase with the decrease of residual oxygen content, while If HASL (Sn-Cu and SAC) and l-Sn is with a volume fraction of which is less image.png in oxygen, it would not change much. Generally speaking, the activity of solder has the greatest influence on wettability of solder, but the effect on ENIG coating is not very prominent. On the contrary, it shows better weldability in nitrogen environment. Therefore, image.png protection in wave soldering is better than that in high-activity solder. The following sheet is the spreadability of solder alloy on different coating surfaces after different reflowing times under the air image.png environment.

image.png

The following sheet is the wettability of solder alloy on different coating surfaces after different reflowing times under the air image.png environment.

image.png


As can be seen from the table, when the number of reflowing is small, the spreadability of the surface coating mainly depends on the coating itself, image.png protection does not have a great impact. When the number of reflowing is large, image.png protection has a certain impact, and it also shows that the failure of surface finish with Sn is the fastest.

Conclusion:

The solderability of I-Sn coating degrades rapidly and "whisker" phenomenon is easy to occur. Compared with lead-free solder, there is little difference in solderability and little spread rate, while there is a great spread rate for solder with lead. HASL coating is limited by poor surface smoothness and "whisker" problem. But due to SAC and Sn-Ag was often chosen in the lead-free surface finish, which was used for both good tolerance and low defect rate, mainly used in non-fine-spacing assembly. ENIG coating is widely used in fine spacing, bonding, high reliability and multiple reflux processes due to its good weldability, low dependence on interfacial compounds and smooth surface. However, due to the high cost, complicated process and the phenomenon of "black welding plate", the I-Ag coating has been widely used. OSP coating short-term storage, the lowest cost, mainly used in simple products.




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