Research On The Process Of Electroplating Tin On Lead-Frame Alloys And Mechanism Of Tin Whisker Growth

In this thesis, two kinds of lead-frame alloys (FeNi42 and C194)aere used as plating substrate. Electroplating Sn (bright Sn and matte Sn) films were deposited onto the substrates, with two kinds of Ni (flat Ni and Ni nano-cone) films as the barrier layer to prevent the growth of Sn whisker. The electroplated samples were put into the thermal-humidity chamber for whisker evaluation and mechanism research.It was found that the current density for electroplating bright Sn and matte Sn should be controlled at around 10A/dm~3 and 1A/dm~3 respectively. The whiskers grown in the C194 system were longer and larger than those in the FeNi42 system. Whisker growth in the bright Sn system is easier than that in the matte Sn system. Increasing the thickness of matte Sn film will restrain whisker growth while the thickness of bright Sn has no much effect on it. Annealing could restrain whisker growth. Surface damage or corrosion will induce whisker growth. Both Ni barriers are effective in preventing the whisker growth since no whisker was found after up to 8000 hours of evaluation.Mechanism of tin whisker growth was investigated through considering many aspects including the IMC effect, type and thickness of coatings and annealing. A following whisker growth mechanism is proposed. Sn atoms usually diffuse along the direction of lower chemical potential. If the Sn coating absorbs energy from the outside, the migration will be accelerated. During the migration, Sn atoms may meet a resistance or obstacle. If the resistance is smaller than the power, Sn whiskers will grow out. This basic principle can rationally explain Sn whisker growth rule in many ways. Some effective mitigation methods were provided finally.