Reliability study of the electroless Ni–P layer against solder alloy

Copper (Cu) has been widely used in the under bump metallurgy of chip and substrate metallization for chip packaging. However, due to the rapid formation of Cu–Sn intermetallic compound (IMC) at the tin-based solder/Cu interface during solder reaction, the reliability of this type of solder joint is a serious concern. In this work, electroless nickel–phosphorous (Ni–P) layer was deposited on the Cu pad of the flexible substrate as a diffusion barrier between Cu and the solder materials. The deposition was carried out in a commercial acidic sodium hypophosphite bath at 85 °C for different pH values. It was found that for the same deposition time period, higher pH bath composition (mild acidic) yields thicker Ni–P layer with lower phosphorous content. Solder balls having composition 62%Sn–36%Pb–2%Ag were reflowed at 240 °C for 1 to 180 min on three types of electroless Ni–P layers deposited at the pH value of 4, 4.8 and 6, respectively. Thermal stability of the electroless Ni–P barrier layer against the Sn–36%Pb–2%Ag solder reflowed for different time periods was examined by scanning electron microscopy equipped with energy dispersed X-ray. Solder ball shear test was performed in order to find out the relationship between the mechanical strength of solder joints and the characteristics of the electroless Ni–P layer deposited. The layer deposited in the pH 4 acidic bath showed the weak barrier against reflow soldering whereas layer deposited in pH 6 acidic bath showed better barrier against reflow soldering. Mechanical strength of the joints were deteriorated quickly in the layer deposited at pH 4 acidic bath, which was found to be thin and has a high phosphorous content. From the cross-sectional studies and fracture surface analyses, it was found that the appearance of the dark crystalline phosphorous-rich Ni layer weakened the interface and hence lower solder ball shear strength. Ni–Sn IMC formed at the interfaces was found to be more stable at the low phosphorous content (∼14 at.%) layer. Electroless Ni–P deposited at mild acidic bath resulting phosphorous content of around 14 at.% is suggested as the best barrier layer for Sn–36%Pb–2%Ag solder.

Influence of megasonic agitation on the electrodeposition of high aspect ratio blind vias

This paper presents preliminary studies in electroplating using megasonic agitation to avoid the formation of voids within high aspect ratio microvias that are used for the redistribution of interconnects in high density interconnection technology in printed circuit boards. Through this technique, uniform deposition of metal on the side walls of the vias is possible. High frequency acoustic streaming at megasonic frequencies enables the decrease of the Nernst diffusion layer down to the sub-micron range, allowing thereby conformal electrodeposition in deep grooves. This effect enables the normally convection free liquid near the surface to be agitated. Higher throughput and better control of the material properties of the deposits can be achieved for the manufacturing of embedded interconnections and metal-based MEMS. For optimal filling performance of the microvias, a full design of experiments (DOE) and a multi-physics numerical simulation have been conducted to analyse the influence of megasonic agitation on the plating quality of the microvias. Megasonic based deposition has been found to increase the deposition rate as well as improving the quality of the metal deposits.

Megasonic agitation for enhanced electrodeposition of copper

In this paper we propose an agitation method based on megasonic acoustic streaming to overcome the limitations in plating rate and uniformity of the metal deposits during the electroplating process. Megasonic agitation at a frequency of 1 MHz allows the reduction of the thickness of the Nernst diffusion layer to less than 600 nm. Two applications that demonstrate the benefits of megasonic acoustic streaming are presented: the formation of uniform ultra-fine pitch flip-chip bumps and the metallisation of high aspect ratio microvias. For the latter application, a multi-physics based numerical simulation is implemented to describe the hydrodynamics introduced by the acoustic waves as they travel inside the deep microvias.