Characterization of lead-free solders and under bump metallurgies for flip-chip package (original) (raw)
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Thermo-mechanical FEM analysis of lead free and lead containing solder for Flip Chip applications
2005
A variety of lead free alloys have been developed to replace the commonly used tin lead solder. At present, the leading candidates are SnAgCu, SnAg and SnCu(Ni) solder alloys. Nevertheless, a major concern regarding the rather new alloys is the fatigue reliability. The lack of reliability data of these solder materials makes every step towards the replacement of lead containing solder uncertain. A 3D Finite Element Model (FEM) was used to simulate the visco-plastic constitutive behavior of the solder in a f lip chip package when submitted to a thermal cycle test. When the solder is subjected to cyclic stresses generated during the thermal cycling, the reliability of the solder joint depends on its resistance to fatigue. The goal of the thermo-mechanical analysis in the electronic industry is to be able to predict, before extensive testing, the reliability of the solder joints. This paper focuses on predicting the thermomechanical behavior of fine pitch flip-chip packages using SnAgCu and SnPb solder alloys. Three different sizes and 2 different pitches were analyzed. The number of cycles to failure was correlated to the accumulated creep strain using an empirical relationship found in literature. Moreover, the results also indicate that this lead free alloys may be used as alternative solder to improve the resistance to fatigue when compared with standard lead containing solder.
High temperature reliability of lead-free solder joints in a flip chip assembly
Journal of Materials Processing Technology, 2012
The visco-plastic behaviour of solder joints of two models of a flip chip FC48D6.3C457DC mounted on a printed circuit board (PCB) via SnAgCu solder is investigated using Anand's model. While the bumps of one of the models are realistic with 6 m thickness of intermetallic compound (IMC) at interconnects of solder and bond pads, the other are made up of conventional bumps without IMC at these interconnects. The solder bump profiles were created using a combination of analytical method and construction geometry. The assembled package on PCB was accelerated thermally cycled (ATC) using IEC standard 60749-25. It was found in the result of the simulation that IMC does not only impact solder joint reliability but also is a key factor of fatigue failure of solder joints. The IMC sandwiched between bond pad at chip side and solder bulk is the most critical and its interface with solder bulk is the most vulnerable site of damage. With reference to our results, it is proposed that non inclusion of IMC in solder joint models composed of Sn-based solder and metalized copper substrate is one of the major causes of the discrepancy on solder joint fatigue life predicted using finite element modelling and the one obtained through experimental investigation.
Journal of Electronic Materials, 2002
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 lm 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 re owing 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 reowing depending on solder materials and re ow temperatures. It was found that the P-rich Ni layer contains Ni, P, and a small amount of Sn (,7 at.%). Further cross-sectional transmission electron microscopy (TEM) analysis conrmed 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 re ow. 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 rst 1 min of solder re ow, followed by a reduced growth step, andnally 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.
2001 Proceedings. 51st Electronic Components and Technology Conference (Cat. No.01CH37220), 2001
The electroless deposited Ni-P(Phosphorus) under bump metallurgy (UBM) layer was fabricated 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. And the interfacial reaction at the electroless Ni UBM/solder interface was investigated in this work. The intermetallic compound(IMC) formed at the interface during solder reflowing was mainly Ni 3 Sn 4 , and a P-rich Ni layer was also formed as a by-product of Ni-Sn reaction between the Ni-Sn IMC and the electroless Ni layer. A 1~4 µm of Ni 3 Sn 4 IMC and a 1800~5000 Å of P-rich Ni layer were formed in less than 10 minutes of solder reflowing depending on solder materials and reflow temperatures. However, less than 1 m thickness of the electroless Ni layer was consumed. It was found that the P-rich Ni layer contains Ni, P and a small amount of Sn (~7 at%). The atomic ratio of 3Ni:1P indicates that there is Ni 3 P phase in the P-rich Ni layer which was verified by the X-ray analysis. And no Sn was detected at the electroless Ni layer located just below the P-rich Ni layer. Therefore, the P-rich Ni layer, a by-product layer of Ni-Sn interfacial reaction, is not appropriate for a Sn diffusion barrier at the electroless Ni UBM and Sn containing solders.
Electromigration in flip chip solder bump of 97Pb–3Sn/37Pb–63Sn combination structure
Acta Materialia, 2004
Electromigration damage in the flip chip solder bump of 97Pb-3Sn/37Pb-63Sn (numbers are all in wt% unless specified otherwise) combination structure was studied after current stressing at 140°C with a density of 2.55 Â 10 4 A/cm 2 for up to 20 h. The under bump metallurgy for the 97Pb-3Sn solder on the chip side was TiW/Cu/electroplated Cu while the bond-pad for the 37Pb-63Sn solder on the printed circuit board (PCB) side was electroless Ni/Au. We observed in the thermo-electromigration test that failure occurred at the top of the bump with a downward electrical current flow while there was no failure in the opposite current polarity. The Pb atoms were found to move in the same direction as with the electron current flow. Therefore, in the case of the downward electron flow, the composition of the upper solder bump changed from 97Pb-3Sn to 83Pb-17Sn and it enabled the Cu 6 Sn 5 phase to precipitate onto the chip side. Due to the precipitation and growth of the Cu 6 Sn 5 intermetallic compound, the Cu under bump metallurgy was quickly consumed and the subsequent void formation induced failure.
Microelectronics Reliability, 2008
In this study, we evaluated the mechanical reliability of Sn-rich, Au-Sn/Ni flip chip solder bumps by using a sequential electroplating method with Sn and Au. After reflowing, the average diameter of the solder bump was approximately 80 lm and only a (Ni,Au) 3 Sn 4 intermetallic compound (IMC) layer was formed at the interface. Due to the preferential consumption of Sn atoms within the solder matrix during aging, the solder matrix was transformed sequentially in the following order: b-Sn and g-phase, g-phase, and gphase and e-phase. In the bump shear test, the shear force was not significantly changed despite aging at 150°C for 1000 h and most of the fractures occurred at the interfaces. The interfacial fracture was significantly related to the formation of brittle IMCs at the interface. The Sn-rich, Au-Sn/Ni flip chip joint was mechanically much weaker than the Au-rich, Au-Sn/Ni flip chip joint. The study results demonstrated that the combination of Sn-rich, Au-Sn solder and Ni under bump metallization (UBM) is not a viable option for the replacement of the conventional, Au-rich, Au-20Sn solder.
IEEE Transactions on Electronics Packaging Manufacturing, 2002
The choice of solder joint metallurgy is a key issue especially for the reliability of flip-chip assemblies. Besides the metallurgical systems already widely used and well understood, new materials are emerging as solderable under bump metallization (UBM). For single chip bumping Pd stud bumps form a solid core under the solder layer. These hard core solder bumps are an adequate solution if single dies are available only and the chosen assembly technology is flip chip soldering. The scope of this paper is to summarize the results from aging of lead/tin solder bumps on Palladium. The growth of intermetallic and its impact on the mechanical reliability are investigated.
IEEE Transactions on Components and Packaging Technologies, 2006
The effects of under bump metallurgy (UBM) microstructures on the intermetallic compound (IMC) growth of electroplated and stencil printed eutectic Sn-Pb solder bumps were investigated. The process parameters and their effects on UBM surface morphology and UBM shear strength were studied. For the electroplating process, the plating current density was the dominant factor to control the Cu UBM microstructure. For the stencil printing process, the zincation process has the most significant effect on the Ni UBM surface roughness and Ni grain sizes. In both processes, the good adhesion of UBM to aluminum can be obtained under suitable UBM processing conditions. Samples with different UBM microstructures were prepared using the two processes. The resulting samples were thermal aged at 85 C, 120 C, and 150 C. It was observed that the Cu UBM surface roughness had larger effect on the IMC growth and solder ball shear strength than the Ni UBM surface roughness. The thickness of Cu 3 Sn and Cu 6 Sn 5 IMC depended strongly on the UBM microstructure. However, for Ni/Au UBM, no significant dependence was observed. More likely, the thickness of Au-Ni-Sn IMC near the IMC/solder interface was controlled by the amount of gold and the gold diffusion rate in the solder. Shear tests were performed after thermal aging tests and thermal/humidity tests. Different failure modes of different sample groups were analyzed. Electroless Ni UBM has been developed because it is a mask-less, low-cost process compared to electroplated Cu UBM. This study demonstrated that the process control was much easier for Ni UBM due to its lower reactivity with Sn material. These properties made Ni UBM a promising candidate for the lead-free solder applications.
Characteristics of Sn-2.5Ag flip chip solder joints under thermal shock test conditions
Journal of Mechanical Science and Technology, 2009
The interfacial reaction between Cu pad coated with Au/Ni and solder bump of flip chip package, using Sn97.5wt.%-Ag2.5wt%, was studied under thermal shock stress. All joints were subjected to thermal shock test with -65℃/+150℃ temperature range. For the Sn-2.5Ag solder, a scallop-like (Cu,Ni) 6 Sn 5 intermetallic compound was formed in the solder matrix after 20 cycles of thermal shock. (Cu,Ni) 6 Sn 5 was detached from the interface as (Ni,Cu) 3 Sn 4 grew underneath the (Cu,Ni) 6 Sn 5 IMC(Intermetallic Compound), whereas the elements of Sn, Ni and Cu were moved by interdiffusion at the interface between solder alloy and Cu pad. The composition of the IMCs in the solder joints and elemental distribution across the joint interfaces were quantitatively measured with EPMA (electron probe micro analysis). Finally, it was found that the crack initiation point and its propagation path could be influenced by the thermal shock conditions, two underfills, and their properties.