Young-Doo Jeon

Studies on Ni-Sn intermetallic compound and P-rich Ni layer at the electroless nickel UBM-solder interface and their effects on flip chip solder joint reliability

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. 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/sub 3/Sn/sub 4/, and a P-rich Ni layer was also formed as a by-product of Ni-Sn reaction between the NiSn IMC and the electroless Ni layer. A 1-4 /spl mu/m of Ni/sub 3/Sn/sub 4/ IMC and a 1800-5000 /spl Aring/ 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 /spl mu/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 (/spl sim/7 at%). The atomic ratio of 3Ni:1P indicates that there is Ni,P phase in the P-rich Ni layer which was verified by the X-ray analysis. 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. Because of the fast diffusion of Sn into the P-rich Ni layer, a series of Kirkendall voids were found in the Ni/sub 3/Sn/sub 4/ IMC, just above the P-rich Ni layer during extended solder reflowing. The amount of the Kirkendall voids appeared to be proportional to the growth of the P-rich Ni layer determined by solder reflowing and subsequent annealing processes. Because the Kirkendall voids are considered to be the main cause of the brittle fracture, it is recommended to restrict the growth of the P-rich Ni layer by choosing proper processing conditions. 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.

Solder reflow process induced residual warpage measurement and its influence on reliability of flip-chip electronic packages

To meet the future needs of high pin count and high performance, package size of flip-chip devices is constrained to become larger. In addition, to fulfill the environment issues, lead free solders will be replacing lead contained eutectic (Sn/37Pb) in near future. Thus, in this work, the effect of residual warpage and consequent residual stress on the reliability of large flip-chip using lead free solder is examined. Several effective experimental approaches to accurately measure residual warpage, using Moire interferometry, shadow Moire, and image processing schemes, are introduced. Moreover, geometric, process, and material parameters affecting the residual warpage during reflow process are discussed and some modifications are suggested. Finally, it is verified that it is crucial to accurately quantify and control the residual warpage in order to guarantee the overall reliability of flip-chip packages regardless of presence of underfill.

Reduced thermal strain in flip chip assembly on organic substrate using low CTE anisotropic conductive film

Flip chip assembly directly on organic boards offers miniaturization of package size as well as reduction in interconnection distances, resulting in a high performance and cost-competitive packaging method. This paper describes the usefulness of low cost flip-chip assembly using electroless Ni/Au bump and anisotropic conductive films on organic boards such as FR-4. As bumps for flip chip, electroless Ni/Au plating was performed as a low cost bumping method. Effect of annealing on Ni bump characteristics informed that the formation of crystalline nickel with Ni/sub 3/P precipitation above 300/spl deg/C causes an increase of hardness and an increase of the intrinsic stress. As interconnection material, modified ACFs composed of nickel conductive fillers for conductive fillers, and nonconductive fillers for modification of film properties, such as coefficient of thermal expansion (CTE), were formulated for improved electrical and mechanical properties of ACF interconnection. Three ACF materials with different CTE values were prepared and bonded between Si chips and FR-4 boards for the thermal strain measurement using moire interferometry. The thermal strain of the ACF interconnection layer, induced by temperature excursion of 80/spl deg/C, was decreased according to the decreasing CTEs of ACF materials. This result indicates that the thermal fatigue life of ACF flip chip assembly on organic boards, limited by the thermal expansion mismatch between the chip and the board, could be increased by low CTE ACF.

Microwave model of anisotropic conductive film flip-chip interconnections for high frequency applications

Microwave model and high-frequency measurement of the anisotropically conductive film (ACF) flip-chip interconnection was investigated using a microwave network analysis. The test integrated circuits (ICs) were fabricated using a 1-poly and 3-metal 0.6 /spl mu/m Si process with an inverted embedded microstrip structure. As flip chip bumps, electroless Ni/Au plating was performed on Al input/output (I/O) pads of test IC chips, As an interconnect material, several ACFs were prepared and flip-chip bonded onto the Rogers(R) RO4003 high frequency organic substrate. S-parameters of on-chip and substrate were separately measured in the frequency range of 200 MHz to 20 GHz using a microwave network analyzer HP8510 and cascade probe, and the cascade transmission matrix conversion was performed. The same measurements and conversion were conducted on the test chip mounted substrates at the same frequency range. Then impedance values in flip-chip interconnection were extracted from cascade transmission matrix. The extracted model parameters of the 100 /spl mu/m/spl times/100 /spl mu/m interconnect pad show the resistance increases due to skin effect up to 8 GHz. Above this frequency, conductive loss of epoxy resin also increases. Reactance is dominantly affected by inductance of Ni/Au bumps and also conductive particles in the ACF interconnection over the measured frequency range. The inductance value of ACF flip chip interconnection is below 0.05 nH and the contact resistance is below 0.9 R. In addition, the effects of different ACF conductive particle materials on high frequency electrical behavior in GHz range were also investigated, Different ACF conductive particle materials show difference in the reactance, resistance, and resonance frequency behavior up to 13 GHz. Our results indicate that high frequency electrical performance of ACF combined with electroless Ni/Au bump interconnection is acceptable for use in the high frequency flip chip application up to 13 GHz. Finally, 80-ps rise time digital signal transmission with small dispersion low loss reflection was demonstrated through the flip-chip interconnection with combination of ACF and Ni/Au bump.

Comparison of interfacial reactions and reliabilifies of Sn3.5Ag, Sn4.0Ag0.5Cu, and Sn0.7Cu solder bumps on electroless Ni-P UBMs

Screen-printing of solder bumps combined with electroless Ni-P plating of UBMs is one of the most cost effective flip chip bumping technologies. As many Pb-free solder pastes such as ternary and quaternary alloys have been applied, demands for understanding of interaction between electroless Ni-P UBM and various Pb-free solders also are increasing. Because of higher process temperature and higher Sn content than Pb63Sn solder, faster interfacial reaction and faster consumption of the UBM occur resulting in reliability problems of solder joint. In addition, it is expected that small addition of Cu in solder would greatly affects the formation and growth of IMCs, and consumption of the electroless Ni-P UBM. In this paper, interfacial reactions and reliability of 3 widely used Pb-free solder bumps (Sn3.5Ag, Sn4.0Ag0.5Cu, and Sn0.7Cu) on electroless Ni-P UBMs were investigated. UBM consumptions, ball shear strengths, and fracture modes were compared with each solder alloy. Finally, the best compatible Pb-free solder alloy with electroless Ni-P UBM will be suggested. It was observed that consumption of Ni-P UBM in Sn3.5Ag solder bump was much faster than those in Cu-contained solder bumps (SnAgCu and SnCu). Due to the small addition of Cu in Cu-contained solders, firstly forming IMC at the interface was not Ni3Sn4 but Cu6Sn5. Therefore, formation of Cu6Sn5 IMC in SnAgCu and SnCu solders greatly reduced consumption of Ni-P UBM. Compared with Sn4.0Ag0.5Cu and Sn0.7Cu, higher Cu content was more effective to reduce consumption of the UBM. In order to estimate reliability of each solder bump, ball shear strengths and shear modes were measured and compared, respectively. Ball shear strength seemed to be very dependent to mechanical properties of solder bumps.

Interfacial reactions and bump reliability of various Pb-free solder bumps on electroless Ni-P UBMs

Electroless Ni-P UBMs combined with screen-printing of solder pastes are one of the lowest cost flip-chip bumping techniques. Pb-free solder bumps on electroless Ni-P UBM can be easily fabricated as various Pb-free solder alloy pastes are available. Therefore, interfacial reactions between electroless Ni-P UBM and Pb-free solders should be investigated, because they are greatly affected by small amounts of alloying such as Ag, Cu, and Bi in Pb-free solders. Reliability of Pb-free solder bumps can be greatly affected by intermetallic growth and P-rich Ni layer at the interface. Recently, Pb-free solder alloys such as SnAg, SnAgCu, SnCu, and SnAgBi have been suggested as promising candidates for substituting Sn37Pb solder. In this study, these four alloys were selected as solder bump materials for electroless Ni-P UBM. The effects of Ag, Cu, and Bi in Pb-free alloys on interfacial reactions and bump reliability at electroless Ni-P/solder interfaces were investigated. It was found that the consumption rate of Ni-P UBM was much slower in SnAgCu and SnCu alloys than in SnAg and SnAgBi alloys during solder reflow. SnAgCu and SnCu solders also showed lower Ni-P UBM consumption rate than SnAg and SnAgBi during aging. In particular, more Cu-containing Sn0.7Cu solder showed lower Ni-P UBM consumption than SnAg0.5Cu solder for the same heat treatment conditions. Consumption of Ni-P UBM can be reduced by adding Cu, as Cu addition initially causes (Cu,Ni)/sub 6/Sn/sub 5/ phase rather than Ni/sub 3/Sn/sub 4/ phase. Bi addition in Pb-free solder alloys did not affect interfacial reaction with Ni-P UBMs. However, higher mechanical properties and lower melting point of Pb-free solder alloys can be obtained by Bi addition. Bump shear test results showed that all failure occurred inside soft solders, and shear strength was proportional to ultimate tensile strength of solder alloys. However, because P-rich Ni layer has been reported as a brittle failure site, it is suggested that Cu-containing Pb-free solder alloys such as SnAgCu and SnCu showing lower interfacial reaction rate with Ni-P UBMs are preferable.

Stresses in electroless Ni-P films for electronic packaging applications

Electroless-plated nickel films for electronic packaging applications such as under bump metallurgy (UBM) and flip chip bumps are investigated in this study. Quantitative stress of an electroless-plated Ni-P film on an Al coated Si wafer has been measured using a laser scanning profiler and the Stoney equation. A tensile intrinsic stress was developed due to plating defects, and also a tensile extrinsic thermo-mechanical stress due to temperature change and the CTE mismatch of Ni film and Si substrate was observed. It was found that the extrinsic stress became more tensile as the phosphorus content of the electroless Ni film decreased. Therefore, it is necessary to reduce the amount of stresses developed at the electroless Ni film by controlling phosphorous content of the electroless Ni film for reliable electronic packaging applications.

Over 10 GHz equivalent circuit model of ACF flip-chip interconnect using Ni-filled ball and Au-coated polymer balls

In this paper, we firstly present the equivalent circuit model of an anisotropic conductive film (ACF) flip-chip interconnect using Ni-filled balls and Au-coated polymer balls. The models were extracted up to the microwave frequency region over 10 GHz. The extracted model parameters of the Ni-filled ball interconnect are compared to those of the Au-coated polymer ball interconnect with respect to impedance and resonance frequency. The ACF using the Ni-filled ball can be manufactured with reduced manufacturing cost and a simplified process, while it still has comparable electrical performance to that of the Au-coated polymer ball. Thus far, the Au-coated ball interconnect has been most widely used. S-parameter measurements and subsequent microwave network analysis were used for the extraction procedure for the impedance parameters.

Flip chip assembly on organic boards using anisotropic conductive adhesives (ACAs) and nickel/gold bumps

Flip chip assembly directly on organic boards offers miniaturization of package size and reduced in interconnection distances, resulting in a high performance and cost-competitive packaging method. This paper describes the investigation of alternative low cost flip-chip mounting processes using electroless Ni/Au bumps and anisotropic conductive adhesives/films as an interconnection material on organic boards such as FR-4. As bumps for flip chip, electroless Ni/Au plating was performed and characterized for plating speed, surface roughness, and elemental analysis as a function of plating condition. High plating rate and surface planarity of the electroless Ni were considered as requirements for ACA flip chip bumps. In order to obtain high plating rate and low surface roughness, plating conditions were determined by controlling complexing agents in electroless Ni solution. Annealing effects on Ni bump characteristics showed that the formation of crystalline Ni with Ni/sub 3/P precipitation above 300/spl deg/C causes an increase in hardness and intrinsic stress, resulting in reliability limitation. As an interconnect material, modified ACFs composed of Ni conductive fillers for electrical conductor and nonconductive inorganic fillers for modification of film properties such as CTE and tensile strength were formulated for improved electrical and mechanical properties of ACF interconnection. The thermal cycle life of ACAs flip chip on organic boards was usually limited by the CTE mismatch between chip and board. However, flip chip assembly on FR-4 boards using modified ACAs almost doubled the thermal cycle life.

Studies on the interfacial reactions between electroless Ni UBM and 95.5Sn-4.0Ag-0.5Cu alloy

Even though electroless Ni and Sn-Ag-Cu solder are widely used materials in electronic packaging applications, interfacial reactions of the ternary Ni-Cu-Sn system have not been known well because of their complexity. Because the growth of intermetallics at the interface affects reliability of solder joint, the intermetallics in Ni-Cu-Sn system should be identified, and their growth should be investigated. Therefore, in present study, interfacial reactions between electroless Ni UBM and 95.5Sn-4.0Ag-0.5Cu alloy were investigated focusing on morphology of the IMCs, thermodynamics, and growth kinetics.