PWB Surface Finish Process Development to Enhance the Reliability of the Solder Joint Strength

摘要

PWB's for mobile phones have widely applied Electroless Nickel and Immersion Gold plating (E.L. Ni/Au) for key pads. However, the solder joint strength tends to be weak when E.L. Ni/Au plating is applied on the PWB pads, especially BGA/CSP pads. The fracture mode of inner-metallic layers between solder and nickel is observed on weakened pads. We have been analyzing the factors of this problem, and were able to resolve it to some extent. The solder joint strength depends on the phosphorous content of the nickel film. We found that the fracture mode of the inner-metallic layers appears when the phosphorous content of the nickel film is too high. We also found that the Immersion Gold bath condition has the factor. We have been using this information to improve our E.L. Ni/Au line. We have also developed a new surface finish utilizing copper, which is known to enhance the reliability of the solder ball joint strength. The established new surface finish consists of selectively coating OSP over those copper pads where the reliability of solder ball joint strengths are needed. The E.L. Ni/Au will be plated on key pads. We call this process "Selective OSP". Today we would like to introduce to you the reliability data of solder joint strength, the design rule, and the production result of selective OSP.