The most common surface treatment in ball matrix fabrication is the Ni/Au plating on the Cu substrate. The Au layer in the upper layer has an antioxidant effect, while the lower Ni layer serves as the diffusion layer. In this experiment, the interface reaction between Sn-Zn-Ag solder and Ni was observed at 250 degree of temperature and different time. Sn9ZnxAg lead free solders were used in the experiment, and X, 0.5, 1.5, 2.5 and 3.5 (wt%), respectively. The experimental results show that Ni5Zn21 intermetallic is generated on the interface, and the product in the solder is AgZn3. And with the increase of Ag component, the thickness of the metal Ni5Zn21 is thinner. Analysis of the results we can see that, in this system, the activity of Zn is higher than Sn, so that the generated compound is Zn containing ingredients, and has not seen any Sn compounds, and Ag has ability to inhibit growth of intermetallics. On the other hand, from the thickness of the reaction time and the growth of the intermetallic, we infer that the system may be diffusion controlled at 250 C.
Keywords: Sn9ZnxAg, lead free solder (lead-free, solder), diffusion control. 1. The most difficult part in the current process of microelectronic packaging is that the ball matrix packages (Ball-Grid-Array, BGA) with high input and output terminal densities are soldered to the printed circuit board (PCB). This packaging technology not only substantially increases the upper density limit of the input / output terminals, but also reduces the area of chip packaging and reduces the defect rate of solder joints by [1-19]. In the BGA packaging technology, the solder ball (solder balls) BGA welding to PCB, the map can clearly see the direct contact with the solder ball consists of metal layer and the printed circuit board of BGA package and BGA package on the corresponding cushion. In the BGA board, the conductive path material pads is generally the most common Cu, and these pads must first go through the surface treatment process (surface finish), so as to avoid surface oxidation and affect the properties of wetting. The most commonly used surface treatment layer at present is plating Ni first and then plating Au (Au/Ni). In Au/Ni on BGA, in general, the thickness of Ni is about 5 m, while the thickness of Au is thinner, about 1 m. On PCB, the thickness of Ni is about 3 m, and the Au layer is about 0.02~0.05 mu m. Among various lead-free solders, Sn9Zn series solder is another potential candidate material. Its main advantage is that its liquefaction temperature is lower (<200 DEG C) . But Sn9Zn solder has a major drawback, that is, this alloy is easier to be oxidized, and its poor corrosion resistance. But recent evidence shows that adding a small amount of Ag to Sn9Zn, will help enhance its antioxidant and anti corrosion ability (first discovered by Professor Lin Guanglong, the results of a large) significantly enhance the utility of Sn9ZnxAg solder. However, the data about the properties of Sn9ZnxAg (including Sn9Zn) are very scarce at present, especially in the literature, there is hardly any data about the reaction characteristics between the solder and the substrate surface treatment layer. Therefore, the main purpose of this project is to investigate the reaction between Sn9ZnxAg series solder and substrate finish (surface). This experiment chose Sn9ZnxAg and Ni instead of reason and Au reaction, is due to the literature clearly mentioned that during the reflow of the solder ball Ni reaction layer above the Au layer and the BGA board very quickly and in 10 seconds to start welding, Au layer has already been exhausted, and then the solder ball contact is Ni, so most of the time, and all solder layer in Ni reaction . On the other hand, the lab found 230 degrees in the next time after welding, the mechanical strength is still weak, and in the contact performance of 250 C next time after welding significantly improved many, so this plan will set the temperature to 250 DEG C to improve.
Figure 1. Relative positions of PCB, BGA plates and solder balls, two, experimental methods
The main purpose of this scheme is to investigate the reaction mechanism between the Sn9ZnxAg series solder and the surface of Ni substrate. For the four kinds of SnZnAg with different composition of solder (Table 1), reacted with Ni at a fixed temperature of 250 DEG C, and the analysis of the generated on the interface intermetallic compound (IMC) composition, and in different reaction time (1hr, 4hr, 9hr), grow the thickness of the intermetallic compound, and to investigate its growth kinetics. The experimental procedures are as follows:
Different solder composition
Table 1. Composition of Sn9ZnxAg (wt%)
In the Ni substrate preparation, the diameter of 5mm high purity Ni rod (99.999%) to the diamond cutting machine cut thickness of 0.5mm sheet, after grinding, polishing, so as to smooth trace metal surface Ni, after the Ni tablets with water and alcohol clean, clean in place reserve. A small metal solder sample tank every reaction were from 6G, the small sample tank placed in the heating bag, with a temperature of 250 DEG C heating, melting the solder to be stable for 25 minutes (25 minutes waiting for the purpose is to make the liquid solder everywhere in the concentration of each component can be uniform), immediately after the Ni sheet prepared with hydrochloric acid for 1 minutes, to remove the surface oxide formation, then quickly immersed in flux seconds to flux Ni chip micro dry, Ni immersed in molten solder reaction immediately, and in the heating bag on top of multilayer glass fiber light covered with insulation to do reduce heat loss. At the end of the reaction time, turn off the power to stop heating, gently remove the glass fiber, with the hair dryer by blowing cold evenly from all sides, to avoid the holes caused by uneven condensation, the solidification of the solder from the canister removed,