PWB surface finish process development to enhance the reliability of the solder joint strength

PWBs for mobile phones have widely applied electroless nickel/immersion gold plating (EL Ni/Au) for keypads. However, the solder joint strength tends to be weak when EL Ni/Au plating is applied to the PWB pads, especially BGA/CSP pads. The fracture mode of intermetallic 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 phosphorus content of the nickel film. We found that the fracture mode of the intermetallic layers appears when the phosphorus content of the nickel film is too high. We also found that the immersion gold bath condition is another factor. We have been using this information to improve our EL 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 newly established surface finish consists of selectively coating OSP over those copper pads where reliability of the solder ball joint strength is needed. The EL Ni/Au is plated on keypads. We call this process "selective OSP". This paper introduces the reliability data for solder joint strength, the design rules, and production results of selective OSP.