Based on the increasing awareness of ecological protection in the West and Japan, the European Union's (Ec) wEEE/ROHS Directive (WEEE Directive refers to the EC Directive on waste Electrical and Electronic E-qlJipment, ROHS Directive refers to the EC Directive on Restriction 0f Hazardous Material), which bans the use of lead (Pb) and other hazardous substances, has come into effect. With the entry into force of the WEEE/ROHS Directive and the imminent implementation of the Chinese government's own "RoHS" regulation, pure tin plating is now a widely accepted alternative to tin-lead plating in the integrated circuit/leadframe, passive component and connector industries. However, for pure tin plating of connectors, there is often a problem with tin discolouration when the co-flow temperature is increased from 235°C to 260°C, see Figure 1, and the discoloured tin plating will have a poor appearance, resulting in the product being unacceptable to the user.
Although pure tin has become a widely accepted alternative to tin-lead solderable surfaces, there is a difference between pure tin plating processes in the automotive and consumer industries. Bright tin plated connectors are widely used in computers, mobile phones and other consumer electronics due to their better appearance and better solderability after heat aging. However, in the automotive industry, matte tin (non-glossy tin) is preferred over bright tin for reasons of product reliability. In short, even with matte and bright tin, the discolouration of the tin plating after reflow is unacceptable. Therefore, the author has effectively improved the problem of reflow discolouration of pure tin on the surface of the nickel layer by using a nickel layer surface and then plating a layer of nickel phosphorus (Ni-P) alloy to form a Ni/Ni-P double layer plating process to meet the needs of customers.
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2 Analysis of reflux discolouration of tin
In simple terms, the reflow discolouration of tin is very much related to the thickness of the tin oxide layer. When different heat treatments are carried out on bright tin, different thicknesses of oxide layers are produced due to different heat treatment conditions. Figure 2 shows a photograph of the connector at different treatment temperatures and its Oxygraph.
As can be seen in Figure 2, the samples did not change colour after dry baking, the samples turned yellow after the steam ageing test and the samples turned purple after refluxing 3 times at 260°C. The Osher plot in Figure 2 shows that the thickness of tin oxide produced after 24h of dry baking at 150°C was approximately 50Å, after 12h of steam ageing the tin oxide was approximately 250Å and after 3 reflows at 260°C the oxide film in the plating was over 500Å. This suggests that the discolouration after tin treatment is related to the formation of tin oxides and that the discolouration is due to interference with the oxide layer. The dry baking was carried out at a relatively low temperature and under water vapour, so that less oxide was produced. After the 12h steam ageing test, however, the oxide layer was thicker due to the higher humidity. The Oechtogram shows that the oxide layer has a constant sn/0 ratio, which indicates that a large number of crystalline particles on the surface have undergone oxidation reactions. During reflow, the sample passes through a temperature zone above the melting point of tin, where the coating reaches a molten state. The Oxygraph shows that although there is a relatively low level of oxygen in the oxide layer, it is present deeper into the plating. This indicates that the oxidation reaction during reflow occurs mainly in the blemish area and at the grain boundaries.
3 Process research to prevent discolouration of the tin plating layer
3.1 Adjustment of grain size and carbon content
It was found that larger crystalline grain size, lower grain boundary density and co-deposited carbon content all helped to control the reflow discolouration of tin. Therefore, the new process uses a method of adjusting the grain size and carbon content. Figure 3 compares the crystal orientation structure of the tin layer produced by the new process with that of the conventional bright tin layer. As can be seen in Figure 3, the brightness of the tin layer resulting from the new tin treatment process is comparable to that of conventional bright tin, despite the larger crystalline grains.
The new semi-bright tin exhibits less reflow discolouration problems due to its larger crystalline grain size and lower grain boundary density (see Figure 4). As can be seen from the Osher electron longitudinal analysis profile in Figure 4, after three reflows at 260°C, the conventional bright tin surface produces an oxide layer of 400-500 Å, whereas the corresponding semi-bright tin produces a surface oxide layer of less than 150 Å.
Under certain conditions, such as aging of the bath, high operating current density and multiple reflow cycles, the semi-bright tin produced by the new process can still produce tin discolouration. In order to effectively control the oxidation of tin during reflow, a process was developed to replace the single Ni-P coating on the base layer of the tin coating with a Ni-P alloy coating, with satisfactory results. The isochronous discolouration of tin plating on different substrates is shown in Figure 5.
In fact, the reflow discolouration of tin is closely related to the formation of different metal intercalation compounds (IMc). Tin plating on copper and its alloys, for example, does not have significant discolouration problems when reflowed. However, tin plating on nickel substrates will usually exhibit a reflow discolouration of tin when reflowed. This is due to the formation of different metal intercalations such as cusn and NiSn. The reflow properties of tin can be improved by covering the nickel layer with a thinner Ni-P layer. This is probably due to the altered morphology of the metal intercalation (IMc) formed at the Ni/Sn junction, which speeds up the crystallisation of tin from the molten state during reflow and reduces the discolouration problems caused by oxidation.
3.3 Controlling oxidation on the surface of the tin layer through post-treatment
Common problems on tin surfaces include defects in grain alignment, bubble scars or pinholes, which can exacerbate the oxidation of the plated layer. Research has shown that a new post-treatment process can reduce the risk of oxidation of surface defects. The traditional alkaline post-treatment process is designed to neutralise acidic residues on the surface of the plated part so that other residues can be rinsed off the plated part. The new post-treatment process removes surface blemishes and thus reduces the diffusion of oxidation through the blemishes during high temperature reflow, while at the same time removing residues from the surface of the plated part. The results of the two different post-treatment processes are shown in Figure 6. Figure 6 shows that the new post-treatment process is effective in improving the reflow discolouration of the tin plating.
3.4 Application results
The above new process has been applied on a pure tin plating line for connectors. Samples produced by this process have passed the JE.DEc tin whisker test (shown in Figure 7) experiment.
It has been reported in the literature that bright tin has a high potential to produce tin whiskers. However, the risk of whisker growth on both bright and non-bright tin can be controlled if dry baked, or plated with a nickel base layer. Figure 7 verifies the results.
4 Conclusion
The problem of co-flow discolouration of tin plating has been effectively controlled by using a range of different methods. These include the use of semi-bright tin with larger crystalline grain size and smaller grain boundary density instead of conventional bright tin with very fine crystalline grain size, resulting in improved reflow discolouration of the tin layer; tin plating with nickel phosphate (Ni-P) as the substrate can form metal intercalations during tin reflow, which improve the crystalline properties of tin and reduce reflow discolouration; after This improves the crystalline properties of the tin and reduces reflow discolouration; afterwards, the surface blemishes are selectively removed by an acid-based post-treatment process, reducing the oxidation reaction exacerbated by defects during reflow.
The entire process, comprising semi-bright tin plating, Ni-P substrate and acid post-treatment process, complement each other to reduce oxidation and reflow discolouration of tin plating in connector applications to satisfactory effect. Working with connector manufacturers has proven that the entire new production process can control reflow and discolouration during tin plating.
JEDEc whisker testing has also demonstrated that the risk of whisker in tin layers produced using this process can be controlled.
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