Klaus-Peter Galuschki

Effect of Ni barrier on the tin whisker formation of electroplating Sn on lead-frame alloy

Electroplating matte Sn has attracted much attention in the lead-free age. To prevent a growth of tin whiskers from lead-frame supported matte tin films, various mitigation methods have been reported. In this work, pure matte Sn was electroplated onto C194 alloy, and electroplating Ni film was used as barrier between matte Sn and lead-frame alloy. The microstructures of the Ni/Sn depositions and tin whisker growth were investigated through using scanning electron microscope (SEM). It was found that the Ni barrier layer played an important role in suppressing the growth of tin whiskers.

Ni Barrier-Induced Cracks in Matte Sn Films

A nickel barrier layer between a matte tin film and a C194 substrate has been used to prevent whisker formation on the tin surface by blocking the formation of Cu6Sn5 intermetallics, which is one of the root causes of whisker growth. However, the introduction of the Ni barrier to the matte Sn/C194 system greatly changes the mechanical properties of the bended leads. In this paper, the effect of a Ni barrier on the mechanical stability of matte tin films deposited onto C194 leads of an SOIC-8 package is investigated. Backscattered electron imaging and focused ion beam imaging of the cross-section samples indicate that, during the forming process, the surface cracks in the matte tin films often initiate and propagate from the Ni barrier, which lacks cooperative deformation ability in comparison with the ductile copper alloy substrate and the matte tin film. The thicker Ni barrier could induce considerable mechanical damage to the matte Sn films of the integrated circuit package after reflow treatment due to the coefficient of thermal expansion mismatch. The thermal-humidity testing of the Sn/Ni/C194 samples reveals that the surface cracks formed may be attributed to the built-in tensile stress arising from the formation of Ni3Sn4 intermetallic compounds (IMCs). A thinner Ni barrier layer can withstand the forming stress, thermal mismatch stress, and IMC-induced stress without causing cracks in the matte Sn films and does not promote Sn whisker growth.

Ni barrier for tin whisker mitigation

In the present work, a close-to-product investigation was conducted in evaluating tin whisker growth on electroplated matte Sn films. The matte Sn films were deposited on Cu leadframes (C194). Electroplating Ni film was used as a barrier between the Sn film and the substrate. The experimental results revealed that Ni barrier could completely restrain the tin whisker growth after 8,000 hours storage at 55°C/85%RH. However, it was found that cracks often formed from the surface during forming process, reflow process and even storage under thermal/humidity condition. Crack formation and corresponding stresses in different processes were investigated. The thicker Ni barrier could induce considerable mechanical damage to the matte Sn films of the IC leads. It was found that thinner Ni barrier layer could withstand the forming stress, thermal mismatch stress and IMC-induced stress without causing cracks in the matte Sn films and thus did not promote Sn whisker growth.

Tin whisker growth on bright Sn films supported by lead-frame alloy substrates

In this work, we fabricate bright tin films on two kinds of representative lead-frame substrates (C194 and FeNi42). Ni film was electroplated between Sn plating and the alloy substrate as a diffusion barrier. The samples were stored under 55 °C / 85% RH condition for up to 8,000 hours. The effect of alloy substrate, tin film thickness and Ni barrier on the tin whisker growth was investigated for a comprehensive understanding of the tin whisker growth behavior. It was found that the substrate type has great influence in the tin whisker density. Further, the density and average length of the tin whisker were related to the film thickness. Ni barrier was found strongly prevent Sn whiskers or hillocks growth even after thermal/ humidity storage for 8,000 hours. Surface and cross-sectional observations of IMC formed at the interface were also carried out using field emission scanning electron microscope (FE-SEM).

Microstructural characterization of electroplating Sn on lead-frame alloys

Electroplating Sn plays an important role in the lead-free age because of its excellent solderability and many other advantages. In this work, bright Sn was electroplated onto both C194 and FeNi42 alloys. The plating parameters, substrate effect, barrier layer effect and IMC formation were investigated through detailed microstructural characterization of the Sn films, barrier layers and cross-sections. It was found that the current density, plating time, substrate type and barrier layer play an important role in determining the fine structures of the Sn deposition. Interfacial reactions to form intermetallic compounds (IMC) was also observed. Replacing the traditional Ni barrier with Ni nanocone barrier was found to result in a quite different IMC formation/distribution.

Tin whisker formation on electroless tin films deposited on lead-frame alloys

Electroless Sn films have great potential in the lead-free age such as for high-density, fine-pitch, narrow soldering pad and bump interconnection applications. In the present work, electroless Sn were deposited onto lead-frame alloys (C194 and FeNi42). The microstructures of the electroless Sn films and tin whisker growth in thermal / humiditive chamber were investigated with scanning electron microscope and optical metallography. It was found that, in comparison with the FeNi42 substrate, the C194 substrate was more favorable for the deposition of electroless Sn coatings. The grain size increased with increase of the thickness of the Sn deposits. Within 45 min, autocatalytic activity still exists, which could keep the autocatalytic deposition of electroless tin on the C194 substrate. Tin whiskers growth was found only after 1000-hour exposure of the samples to the thermal-humiditive environment.