Studies of electroless nickel under bump metallurgy—Solder interfacial reactions and their effects on flip chip solder joint reliability

Abstract

The electroless-deposited Ni-P under bump metallurgy (UBM) layer was fabricated on Al pads for Sn containing solder bumps. The amount of P in the electroless Ni film was optimized by controlling complexing agents and the pH of plating solution. The interfacial reaction at the electroless Ni UBM/solder interface was investigated in this study. The intermetallic compound (IMC) formed at the interface during solder reflowing was mainly Ni 3 Sn 4 , and a Prich Ni layer was also formed as a by-product of Ni-Sn reaction between the Ni-Sn IMC and the electroless Ni layer. One to four microns of Ni 3 Sn 4 IMC and a 1800-5000 Å of P-rich Ni layer were formed in less than 10 min of solder reflowing depending on solder materials and reflow temperatures. It was found that the P-rich Ni layer contains Ni, P, and a small amount of Sn (ϳ7 at.%). Further cross-sectional transmission electron microscopy (TEM) analysis confirmed that the composition of the P-rich Ni layer was 75 at.% Ni, 20at.%P, and 5at.%Sn by energy-dispersive x-ray spectroscopy (EDS) and the phase transformation occurred in the P-rich Ni layer by observing grain size. Kirkendall voids were also found in the Ni 3 Sn 4 IMC, just above the P-rich Ni layer after extensive solder reflow. The Kirkendall voids are considered a primary cause of the brittle fracture; restriction of the growth of of the P-rich Ni layer by optimizing proper processing conditions is recommended. The growth kinetics of Ni-Sn IMC and P-rich Ni layer follows three steps: a rapid initial growth during the first 1 min of solder reflow, followed by a reduced growth step, and finally a diffusion-controlled growth. During the diffusion-controlled growth, there was a linear dependence between the layer thickness and time 1/2 . Flip chip bump shear testing was performed to measure the effects of the IMC and the P-rich Ni layers on bump adhesion property. Most failures occurred in the solder and at the Ni 3 Sn 4 IMC. The brittle characteristics of the Ni-Sn IMC and the Kirkendall voids at the electroless Ni UBM-Sn containing solder system cause brittle bump failure, which results in a decreased bump adhesion strength.

Failure mechanisms of lead-free chip scale package interconnections under fast mechanical loading

“…Especially, the reactions between Ni(P)|Au-coated soldering pads and solders have been studied extensively. [8][9][10][11][12][13][14][15][16][17] Likewise, the cracking of interconnections soldered on the Ni(P) metallization has also been reported in these papers and the occurrence of the failure is typically explained by the oxidation or contamination of the plating bath that affects the quality of the metallization.…”

Effect of Bath Life of Ni(P) on the Brittle-Fracture Behavior of Sn-3.0Ag-0.5Cu/ENIG

“…5,9 To observe the nano-voids formed in the Ni-Sn-P layers of the 0 and 3 MTO samples, the interfaces of the 0 and 3 MTO samples were observed by cross-sectional TEM; the results are shown in Fig. 10.…”

Mechanical strength of thermally aged Sn-3.5Ag/Ni-P solder joints

“…The formation mechanisms of Kirkendall voids have been discussed in detail in our previous work [3]. Jeon et al [27] used shear testing with a SnPb solder ball on Ni-P UBM and found Kirkendall voids on the fracture surface after reflow at 250C for 16 minutes. So, they attributed the decrease of solder joint strength to the existence of Kirkendall voids.…”