Peter Bratin, Michael Pavlov and Eugene Shalyt
ECI Technology, Totowa, NJ, USA
Immersion gold coating is a one of the major metallic alternative finishes used to replace tin/lead hot air finish. The immersion process provides flat coating over the substrate and do not introduce any lead into the solder joint. Both these features make this coating extremely attractive to PCB manufacturers and assemblers. Despite its un-doubtful advantages, immersion processes reveal some problems known as “black nickel” and migration of nickel into the gold, which affect the quality of solder joints. The early detection of these problems and monitoring of surface conditions of immersion gold coatings is extremely critical for the formation of reliable solder joint with appropriate mechanical properties.
This paper extends our study of the Sequential Electrochemical Analysis (SERA) technique for evaluation of various alternative finishes and discuss the application of SERA technique to assess the surface conditions of immersion gold coatings.
The results of surface analysis and internal analysis obtained after different aging conditions will be presented. These results will be correlated to the Solderability properties of tested boards.
The search for Hot Air Solder Level (HASL) alternatives is being driven by technical, economic and environmental factors. The technical considerations include the need for flatter pads on boards where the new components such as BGA, COB and Flip Chip will be mounted. Since lead has been rated as one of top 17 chemical compounds that impose the greatest threat to human health, electronic industry intends to eliminate lead from production providing lead-free solder joint. The research is moving in two directions: developments of (I) lead free surface finishes and development of (II) lead free solders.
The existing alternative finishes can be separated into two major categories: metallic and organic coatings. The metallic coatings include precious metal coatings such as gold, palladium and silver as well as immersion tin finish. Immersion silver and tin coatings are placed directly over the copper whereas the gold requires the presence of underlying nickel. Nickel plays the role of barrier layer between gold and copper in order to avoid a formation of un-solderable intermetallic compounds during the mutual migration. The palladium coating has applicability as a surface finish with or without the need for a nickel intermediary. The precious metal coatings provide good solderability, are wire bondable and provide surface pads with required flatness. However, they also have some disadvantages. The nickel salts are carcinogenic and require special waste treatment, thus making them almost as environmentally unfriendly as lead. In addition, the price of these processes is relatively high, which will not make them cost-effective replacements for solder. The silver finish is less expensive and more user-friendly process than gold and palladium. However, the silver surface is quite reactive and can form tarnish film, which significantly reduces the solderability of coating . Another alternative to tin/lead is immersion tin coating. It obviously a desired metal for assemblers since it provides flat surface features and do not introduce additional components into the solder joint. However, since immersion tin coating is relatively thin , at certain conditions, tin layer degrades yielding thicker copper-tin intermetallic layer. If oxidized, intermetallics might create serious Solderability problem.
Another category of alternative finishes is organic coatings, also co-called Organic Solderability Preservatives (OSP). Most of the commercially available coatings are azole related compounds such as imidazole, benzotriazole (thin films) and substituted benzimidazole (thick films). Thick OSPs incorporate copper in the structure, which creates an organo-metallic coating. This provides a significant improvement in protection against copper oxidation at elevated temperature and humidity, which is important when the PC board goes through the complex assembly process. Regardless of thickness and composition, even the thick organic films can degrade in time or/and during assembly process.
The electroless nickel/immersion gold finish provides a solderable flat surface that does not oxidize or discolor. It has a long shelf life, and the precious metal layer provides excellent electrical conductivity. The nickel underlayer serves as a barrier against formation of unsolderable gold copper intermetallics. Typical failures observed with immersion gold coatings are associated with poorly formed joint at solder / nickel interface. The interfacial failure has been attributed to the presence of brittle gold/tin intermetallic compounds. In other cases, the problem is caused by absence of the reliable metallurgical bond formed between tin and nickel. When nickel is examined, it was found that the failed area reveal black color. This defect is “hidden” under the gold layer and cannot be detected unless boards are soldered. Another possible failure might occur due the oxidation of the nickel trough the pores in the coating causing higher concentration of nickel on the surface of coating.
The detection of such defect prior to the soldering operation or even immediately after manufacturing will allow to significantly reduce and possibly eliminate rejects caused by poor coating conditions. All solder wetting semi-analytical techniques such as wetting balance verify the ability of gold layer to be dissolved in the molten solder. It is obviously cannot simulate an actual formation of intermetallic layer on the interface nickel / solder. Another analytical techniques such as Auger / ESCA are too expensive and cannot be used in production.
SERA technique is successfully used for characterization of various coating including alternative finishes. In this work, we focused our investigation of the ability of SERA technique to monitor conditions of immersion gold coatings and use this information to predict the behavior of these coatings during soldering operation.
EXPERIMENTAL and RESULTS
SERA technique was used for characterization of immersion gold coatings produced from the same type of plating solution (the same chemical supplier). The boards have been collected and then split to two groups; one group was used for actual soldering cycle followed by bending-test; and second was submitted for SERA tests. All boards represent real commercial products and no coupons were tested. Based on the actual soldering experiments, selected boards were separated into three categories. The first group of boards represented ideal samples, which did not reveal any defects after soldering. The second group reveals significant number of defective boards mainly due to separation on the interface nickel / solder. The third group of samples revealed marginal quality and revealed similar defects to group #2, but in smaller quantity.
The selected boards were tested using two different SERA applications: surface analysis and internal analysis. The surface analysis gives the information about the presence of reducible species on the surface. The internal analysis reveals the information about the thickness of coating and its porosity.
Since the gold is a precious metal and it is not oxidizing, the surface test is very sensitive to the presence of reducible contaminants on the surface or other reducible species that might penetrate through the coating. During our previous work , it was found that pure NiO oxide cannot be detected in presence of gold metal due to the hydrogen evolution process taken place on the surface of gold. It is clear that clean surface has less chances to fail during assembly process.
The internal test was initially planned just for gold thickness determination. However, when the initial screening has been performed, it became clear that this test could give much more information about the conditions of immersion gold coating and interface between gold and nickel. Figures 1 and 2 reveal SERA curves obtained from two identical in design boards, where one of them failed the soldering / bending test. The failed board was also tested with ESCA/Auger techniques and SEM. It was found that the failed board reveals the presence of nickel on the surface while another one did not. From Figure 2, it is clear that transition region of SERA curve can be used for the monitoring of porosity of immersion gold coatings and prediction of their behavior after soldering operation.
After initial “screening experiments”, all selected boards were submitted to thorough investigations. It was fund that the results obtained from the same board are repeatable, which indicates that it is not necessary to perform too many tests to characterize the quality of the board. It was decided to perform tests in three locations for each board. Figures 3 and 4 reveal results of surface and internal tests for samples passed bending tests.
Figures 5 compares the results obtained from three groups of samples.
The obtained results are in good agreement and indicate that the poor Solderability of samples were caused by degradation of nickel / gold interface. The surface test detected reducible compounds on the surface of coating, which are the products of nickel and oxygen. This information was obtained based on ESCA analysis.
The conditions of immersion gold coating can be expressed quantitatively if the “stripping angle” will be calculated. The stripping angle indicates the slope of the SERA curve in its transition part.
In conclusion, it has been shown that the Sequential Electrochemical Reduction Analysis (SERA) technique can be a powerful tool for evaluating the surfaces of immersion gold coatings. Surface analysis in conjunction with internal analysis can be used for Solderability prediction of boards finished with immersion gold.
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