A reliability comparison of electroplated and stencil printed flip-chip solder bumps based on UBM related intermetallic compound growth properties (original) (raw)
Related papers
Journal of Electronic Materials, 2005
Using the screen-printed solder-bumping technique on the electroless plated Ni-P under-bump metallurgy (UBM) is potentially a good method because of cost effectiveness. As SnAgCu Pb-free solders become popular, demands for understanding of interfacial reactions between electroless Ni-P UBMs and Cu-containing Pb-free solder bumps are increasing. It was found that typical Ni-Sn reactions between the electroless Ni-P UBM and Sn-based solders were substantially changed by adding small amounts of Cu in Sn-based Pb-free solder alloys. In Cu-containing solder bumps, the (Cu,Ni) 6 Sn 5 phase formed during initial reflow, followed by (Ni,Cu) 3 Sn 4 phase formation during further reflow and aging. The Sn3.5Ag solder bumps showed a much faster electroless Ni-P UBM consumption rate than Cu-containing solder bumps: Sn4.0Ag0.5Cu and Sn0.7Cu. The initial formation of the (Cu,Ni) 6 Sn 5 phase in SnAgCu and SnCu solders significantly reduced the consumption of the Ni-P UBM. The more Cu-containing solder showed slower consumption rate of the Ni-P UBM than the less Cu-containing solder below 300°C heat treatments. The growth rate of the (Cu,Ni) 6 Sn 5 intermetallic compound (IMC) should be determined by substitution of Ni atoms into the Cu sublattice in the solid (Cu,Ni) 6 Sn 5 IMC. The Cu contents in solder alloys only affected the total amount of the (Cu,Ni) 6 Sn 5 IMC. More Cu-containing solders were recommended to reduce consumption of the Ni-based UBM. In addition, bump shear strength and failure analysis were performed using bump shear test.
IEEE Transactions on Components and Packaging Technologies, 2003
Electroplating solder bumping process offers fine pitch, highly reliable, and cost effective advantages for flip-chip technology. In this technology, under bump metallization (UBM) is required for chemical solder deposition and mechanically reliable solder contact to Al pads. The evaporated Cr/phased CrCu/Cu structure UBM has been used with 95Pb/5Sn and also with 37Pb/63Sn solder for flip-chip interconnection. In this study, the intermediate CrCu layer is modified using various sputtering techniques, and the underlying Cr adhesion layer is compared with TiW. Six UBM systems were selected, and their interfacial reaction and bump shear strength were investigated using 100 m and 50 m size electroplated Pb/63Sn solder bumps. The results demonstrate that the final Cu layer should have a minimum thickness, more than 0.8 m, for interface stability on CrCu based UBMs. Intermetallic compound growth and CrCu layer interface changes are more severe after 20 min reflow at 210 C compared with 1000 h aging at 125 C. Especially for small size bumps, the more stable interface between UBM and solder bump is required.
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.
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.
Microelectronics Reliability, 2008
This paper examines various aspects of SAC (Sn-3.8Ag-0.7Cu wt.%) solder and UBM interactions which may impact interconnection reliability as it scales down. With different solder joint sizes, the dissolution rate of UBM and IMC growth kinetics will be different. Solder bumps on 250, 80 and 40 lm diameter UBM pads were investigated. The effect of solder volume/pad metallization area (V/A) ratio on IMC growth and Ni dissolution was investigated during reflow soldering and solid state isothermal aging. Higher V/A ratio produced thinner and more fragmented IMC morphology in SAC solder/Ni UBM reflow soldering interfacial reaction. Lower V/A ratio produced better defined IMC layer at the Ni UBM interface. When the ratio of V/A is constant, the IMC morphology and growth trend was found to be similar. After 250 h of isothermal aging, the IMC growth rate of the different bump sizes leveled off. No degradation in shear strength was observed in these solder bump after 500 h of isothermal aging.
2001 Proceedings. 51st Electronic Components and Technology Conference (Cat. No.01CH37220)
Although the primary driver for the current interest in developing lead-free soldering is global market pressure for more environmentally friendly products, the main concern continues to be lead contamination from end-of-life electronic products in landfill sites. In response to existing and impending legislation in Europe and Japan for the elimination of lead from electronic products, the industry has embarked on a number of studies in search of suitable lead-free alternatives. Several reports [1,2] have been published, but there are as yet no drop-in solutions with respect to reflow temperature, joint reliability and assembly costs. Our survey show that the SnAg -Cu alloy is one of the promising lead-free alloys currently being evaluated by industry. There are however a number of issues regarding the use of SnAg -Cu alloys, including the solderability and long-term reliability of the solder joints, which require further study. The lower solderability of SnAg -Cu solder can alter the interface and microstructure of the solder joint formed because of the differing reaction rates between the molten solder and substrate surface. This also has an impact on the nature and extent of the intermetallic compounds formed at the interface, as the intermetallic is generally more brittle than the base metal. This can negatively impact the solder joint reliability. In this paper we report a study on the effect of solder volume on intermetallic layer formation and thickness. For lead-free soldering this could prove to be very important, as a wide range of devices and components of varying joint size, e.g. plastic quad flat pack (PQFP), ball grid array (BGA), chip-scale packaging (CSP), and flip chip, may need to be assembled on a typical board. This means that the nature and thickness of the intermetallic layer formed for each joint size will be different. In the study, solder joints of different sizes representing different devices were used for evaluating the effect of solder volume on intermetallic compound formation. The layer thickness and microstructure were analyzed using scanning electron microscopy (SEM). SEM analysis was also carried out on joint micro-sections, which has undergone temperature cycling to evaluate the effect of intermetallic layer the joint reliability. Our results show that increasing the solder volume (and solder joint size) does not significantly affect the growth of the intermetallic layer thickness. Therefore the intermetallic layer thickness provides the lower limit for solder joint design for ultra-fine pitch flip-chip applications.
Materials Science and Engineering: B, 2006
The interfacial reactions between Sn-0.4Co-0.7Cu eutectic alloy and immersion Au/electroless Ni(P)/Cu substrate were investigated after reflow soldering at 260 • C for 2 min. Common Sn-4.0Ag-0.5Cu and eutectic Sn-0.7Cu solders were used as reference. Two types of intermetallic compounds (IMC) were found in the solder matrix of the Sn-0.4Co-0.7Cu alloy, namely coarser CoSn 2 and finer Cu 6 Sn 5 particles, while only one ternary (Cu, Ni) 6 Sn 5 interfacial compound was detected between the solder alloy and the electroless nickel and immersion gold (ENIG) coated substrate. The same trend was also observed for the SnAg -Cu and Sn-Cu solder joints. Compared with the CoSn 2 particles found in the Sn-Co-Cu solder and the Ag 3 Sn particles found in the SnAg -Cu solder, the Cu 6 Sn 5 particles found in both solder systems exhibited finer structure and more uniform distribution. It was noted that the thickness of the interfacial IMCs for the Sn-Co-Cu, SnAg -Cu and Sn-Cu alloys was 3.5 m, 4.3 m and 4.1 m, respectively, as a result of longer reflow time above the alloy's melting temperature since the SnAg -Cu solder alloy has the lowest melting point.
Investigation of flip chip under bump metallization systems of Cu pads
IEEE Transactions on Components and Packaging Technologies, 2002
In this study, UBM material systems for flip chip solder bumps on Cu pads were investigated using the electroless copper (E-Cu) and electroless nickel (E-Ni) plating methods; and the effects of the interfacial reaction between UBMs and Sn-36Pb-2Ag solders on the solder bump joint reliability were also investigated to optimize UBM materials for flip chip on Cu pads. For the E-Cu UBM, scallop-like Cu 6 Sn 5 intermetallic compound (IMC) forms at the solder/E-Cu interface, and bump fracture occurred along this interface under a relatively small load. In contrast, at the E-Ni/E-Cu UBM, E-Ni serves as a good diffusion-barrier layer. The E-Ni effectively limited the growth of the IMC at the interface, and the polygonal-shape Ni 3 Sn 4 IMC resulted in a relatively higher adhesion strength compared with the E-Cu UBM. As a result, electroless deposited UBM systems were successfully demonstrated as low cost UBM alternatives on Cu pads. It was found that the E-Ni/E-Cu UBM material system was a better choice for solder flip chip interconnection on Cu pads than the E-Cu UBM.
Correlation between interfacial reactions and mechanical strengths of Sn(Cu)/Ni(P) solder bumps
Journal of Electronic Materials, 2004
The correlation between interfacial reactions and mechanical strengths of Sn(Cu)/Ni(P) solder bumps has been studied. Upon the solid-state aging, a diffusion-controlled process was observed for the interfacial Ni-Sn compound formation of the Sn/Ni(P) reaction couple and the activation energy is calculated to be 42 KJ/mol. For the Sn0.7Cu/Ni(P), in the initial aging, a needle-shape Ni-Sn compound layer formed on Ni(P). Then, it was gradually covered by a layer of Cu-Sn compound in the later aging process. Hence, a mixture layer of Ni-Sn and Cu-Sn compounds formed at the interface. For the Sn3.0Cu/Ni(P), a thick Cu-Sn compound layer quickly formed on Ni(P), which retarded the Ni-Sn compound formation and resulted a distinct Cu-Sn compound/Ni(P) interface. The shear test results show that the mixture interface of Sn0.7Cu bumps have fair shear strengths against the aging process. On the contrast, the distinct Cu-Sn/Ni(P) interface of Sn3.0Cu solder bumps is relatively weak and exhibits poor resistance against the aging process. Upon the reflowing process, the gap formation at Ni(P)/Cu interface caused a fast degradation in the interfacial strength for Sn solder bumps. For Sn0.7Cu and Sn3.0Cu solder bumps, Ni 3 P formation was greatly retarded by the self-formed Cu-Sn compound layer. Therefore, Sn(Cu) solder bumps show better shear strengths over Sn solder bump.
Characterization of lead-free solders and under bump metallurgies for flip-chip package
2001
A variety of Pb-free solders and under bump metallurgies (UBMs) was investigated for flip chip packaging applications. The result shows that the Sn-0.7Cu eutectic alloy has the best fatigue life and it possess the most desirable failure mechanism in both thermal and isothermal mechanical tests regardless of UBM type. Although the electroless Ni-P UBM has a much slower reaction rate with solders than the Cu UBM, room temperature mechanical fatigue is worse than on the Cu UBM when coupled with either Sn-3.8Ag-0.7Cu or Sn-3.5Ag solder. The Sn-37Pb solder consumes less Cu UBM than all other Pb-free solders during reflow. However, Sn-37Pb consumes more Cu after solid state annealing. Studies on aging, tensile, and shear mechanical properties show that the Sn-0.7Cu alloy is the most favorable Pb-free solder for flip chip applications.