Function Of Reflow Time Research Articles

Dissolution and precipitation kinetics of Cu6Sn5 intermetallics in Cu/Sn/Cu micro interconnects under temperature gradient

Multiple reflows are often required in 3D packaging. To elucidate the effect of temperature gradient during subsequent reflow on existing intermetallic compounds (IMCs), Cu6Sn5 IMC layers were initially formed in Cu/Sn/Cu micro interconnects. Upon subsequent reflow, synchrotron radiation real-time imaging technology was used to in situ study the dissolution and precipitation behavior of the pre-formed Cu6Sn5 under different temperature gradients. The pre-formed Cu6Sn5 IMC at the cold end continued to grow linearly with increasing aspect ratio, whereas that at the hot end dissolved linearly and then maintained a critical thin layer. The thick pre-formed Cu6Sn5 IMC at the hot end significantly hindered the dissolution of the neighboring Cu substrate until a dynamic equilibrium between chemical potential gradient and temperature gradient was satisfied. The thermomigration of Cu atoms from the hot end towards the cold end was responsible for the asymmetrical evolution of the interfacial Cu6Sn5 between the cold and hot ends. A theoretical model was proposed based on Cu diffusion flux to calculate the IMC thickness at the both ends as a function of reflow time and the equilibrium IMC thickness at the hot end under temperature gradient.

Spreading Behaviour and Joint Reliability of Sn–0.3Ag–0.7Cu Lead-Free Solder Alloy on Nickel Coated Copper Substrate as a Function of Reflow Time

Solder joint directly interfaces electrically, mechanically and thermally with numerous electronic components in electronic assemblies. Sn–Ag–Cu lead-free solder alloys for electronic assembly are being driven by the environmental issues concerning the toxicity of lead. In the present study, spreading behaviour, evolution of intermetallic compounds during solidification of Sn–0.3Ag–0.7Cu solder alloy on Ni coated Cu substrate and the related joint strength were studied as a function of reflow time. Experiments were carried out for various reflow times of 10, 100, 300 and 500 s at a reflow temperature of 270 °C. The solder alloy exhibited improvement in wettability on the substrate at longer reflow times. An increase in the IMC (CuNi)6Sn5 thickness was observed for samples reflowed up to 300 s and the thickness decreased for samples reflowed for 500 s. IMC layer formed were about 0.3, 1.15, 2.03, 1.94 μm during 10, 100, 300 and 500 s of reflow time respectively. The joint shear test was performed to assess the integrity of the Sn–0.3Ag–0.7Cu solder solidified on Cu substrates. The maximum joint strength was observed for samples reflowed for 100 s.

Wetting Kinetics and Joint Strength of Sn-0.3Ag-0.7Cu Lead-Free Solder Alloy on Copper Substrate as a Function of Reflow Time

Solder plays a vital role in the interconnection of electronic devices in electronic assemblies. As an interconnection material, the solder joint executes electrical, mechanical and thermal functions. The use of lead bearing solders in electronic products is banned due to the toxicity and environmental risks coupled with lead. In the present study, wetting kinetics, interfacial reactions and the formation of intermetallic compounds (IMCs) during solidification of Sn-0.3Ag-0.7Cu solder alloy on Cu substrate and the corresponding joint strength were studied as a function of reflow time. Experiments were carried out at various reflow times of 10, 100, 300 and 500s. The reflow temperature was maintained at 270°C. The solder alloy showed enhanced wettability on the substrate at longer reflow times. The thickness of IMC layer formed during a reflow time of 10s was 1.67μm and the thickness increased to 2.20μm, 2.85μm, 2.91μm during 100s, 300s and 500s of reflow time respectively. The joint shear test was performed to assess the integrity of Sn-0.3Ag-0.7Cu solder solidified on Cu substrates using Nordson DAGE 4000 Plus bond tester. The joint strength increased with the increase in reflow time up to 300s and the maximum joint strength was observed for samples reflowed for 300s. Although the samples reflowed for 500s samples showed good wettability, they exhibited lowest joint strength.

Assessment of Joint Reliability of Sn–2.5Ag–0.5Cu Solder/Cu as a Function of Reflow Time

Solder interconnect reliability is exceedingly influenced by the solder material properties, the intermetallics formed during soldering, and the environmentally imposed loads. The current study involves the study of the wetting behaviour, interfacial reactions and growth of intermetallic compounds (IMCs) in Sn–2.5Ag–0.5Cu lead free solder solidified on Cu substrate as function of reflow time. The reflow temperature was maintained at 270 °C and reflow time was varied from 10 to 10,000 s. The solder alloy showed better wettability on the substrate with the increase in reflow time. A growth in Cu6Sn5 IMC thickness was observed up to a reflow time of 500 s. IMC layer formed during a reflow time of 10 s was 3.09 μm and its thickness increased to 4.40, 8.21, 10.96 μm during 100, 300 and 500 s reflow time respectively. A decrease in the thickness of IMC was observed for samples reflowed for 1000 s. The IMC thickness increased with further increase in reflow time. The joint reliability of Sn–2.5Ag–0.5Cu solder solidified on copper substrate surfaces was assessed by performing bond shear test. The average shear strength decreased with an increase in reflow time after an optimum value.

Interfacial reactions and joint reliability of Sn–9Zn solder on Cu or electrolytic Au/Ni/Cu BGA substrate

The interfacial reactions and shear strength of joints between Sn–9Zn solder and two different kinds of BGA (ball-grid-array) substrates (Cu and Au/Ni–P/Cu) were investigated for reflow conditions of 230 °C up to 60 min. For the Cu substrate, only Cu 5Zn 8 intermetallic compound was observed and other Cu–Sn IMCs were not observed. For the Au/Ni/Cu substrate, an AuZn 3 IMC layer formed at the interface due to the fast reaction between Au and Zn. In addition, some of the AuZn 3 IMC layer was detached from the interface after reflow. The thickness of the AuZn 3 layer did not change with reflow time because the Au was consumed during the initial reflow process. After being reflowed for 60 min, a Ni 5Zn 21 IMC formed between AuZn 3 IMC and Ni substrate. Also, many voids existed inside the upper AuZn 3 IMC layer. Ball shear tests for the Cu substrate showed the shear strength significantly decreased after reflow for 60 min. A Cu 5Zn 8 IMC was also observed on the fracture surface. For the Au/Ni/Cu substrate, the shear strength value did not change much as a function of reflow time and fractures occurred within the solder.

IMC morphology, interfacial reaction and joint reliability of Pb-free Sn–Ag–Cu solder on electrolytic Ni BGA substrate

The interfacial reaction, morphology of intermetallic compound (IMC) and shear strength between the Sn–3.5Ag–0.7Cu (in wt.%) solder and Ni ball grid array (BGA) substrates during reflow at 255 °C for up to 20 min were investigated. The reaction between the molten solder and Ni layer resulted in the formation of two IMCs such as (Cu, Ni) 6Sn 5 and (Ni, Cu) 3Sn 4 at the interface. The (Ni, Cu) 3Sn 4 IMCs were needle-like type, while the (Cu, Ni) 6Sn 5 IMCs were cylinders with a hexagonal cross-section and pointed tips. Compared to the quenched sample, the (Cu, Ni) 6Sn 5 IMCs nucleated heterogeneously on the (Ni, Cu) 3Sn 4 layer during the solidification of the solder. In the ball shear test, the shear strength value did not change much as a function of reflow time. In all samples, the fracture occurred in the bulk solder. The results of interfacial reaction and shear test in this study show that the Sn–Ag–Cu/Ni BGA solder joint has a desirable joint reliability.

Effect of reflow time on interfacial reaction and shear strength of Sn–0.7Cu solder/Cu and electroless Ni–P BGA joints

The interfacial reaction and shear strength between the eutectic Sn–0.7wt.% Cu solder and two different kinds of ball grid array (BGA) substrates (Cu and Au/Ni–P/Cu) during reflow at 255°C for up to 30min were studied. With the solder joints between the Sn–0.7Cu solder and Cu substrates, a Cu6Sn5 intermetallic compound (IMC) was formed at the interface. In the case of the electroless Ni–P substrate, the IMC formed at the interface was mainly (Cu, Ni)6Sn5. Also, a P-rich Ni (Ni3P) layer was observed as a by-product of the Cu–Ni–Sn reaction. The Ni3P layer was between the (Cu, Ni)6Sn5 IMC and the electroless Ni–P layer. The thickness of the Cu6Sn5, (Cu, Ni)6Sn5 and Ni3P layer was found to increase with the reaction time. The growth of the Cu6Sn5 IMC layer for Cu substrate was faster than that of the (Cu, Ni)6Sn5 IMC layer for electroless Ni–P substrate. The Ni content in the (Cu, Ni)6Sn5 IMC layer formed at the interface increased with increasing reaction time. In the ball shear tests, the shear strength value did not change much as a function of reflow time. In all samples, the fracture mainly occurred in the bulk solder. However, for the Sn–0.7Cu/electroless Ni–P joint reflowed for 10min, the fracture occurred in ductile and slightly brittle mode.

Reaction characteristics of the In-15Pb-5Ag solder with a Au/Ni/Cu pad and their effects on mechanical properties

Reaction characteristics of the In-15Pb-5Ag (wt.%) solder with a Au/Ni/Cu pad during reflow soldering and aging treatment were examined. Interfacial reaction during reflow resulted in either an AuIn2 or Ni28In72 layer, depending on reflow time. The AuIn2 layer became thinner and disappeared from the interface, and only the Ni28In72 layer grew with the progress of aging treatment at 130°C. Based on those observations, the dissolution rate of the Au top layer was estimated, and the behavior of the AuIn2 layer during reflow and aging treatment was discussed. In addition, peak shear load and fracture energy of the solder bump were measured as a function of reflow time and aging treatment. The results were compared with those measured with the Sn-37Pb solder bump.

Study of the effect of reflow time and temperature on Cu–Sn intermetallic compound layer reliability

In terms of various reflow time and temperature, an analysis of Cu–Sn intermetallic compound (IMC) layer reliability is presented in this paper. Temperature cycling test data reported in existing publications for the solder paste material of 63Sn/37Pb eutectic alloy is used to model the probability distribution functions of solder joint lifetime due to the IMC layer fatigue. The relationship of the IMC layer thickness as a function of reflow time and temperature is studied. A reliability and mean time to failure function of the IMC layer in terms of various reflow time and temperature are presented. Calculation suggests that to achieve a higher IMC layer reliability, a shorter reflow time and a lower reflow temperature should be used, while lowering reflow temperature may be more efficient than controlling reflow time. In general, a reflow temperature ranged by 240–280 °C should be avoided. For a specified reliability goal, how to choose proper reflow time and temperature is discussed.

Metallurgical reaction and mechanical strength of electroless Ni-P solder joints for advanced packaging applications

We have studied the metallurgical reaction and mechanical strength of the electroless Ni-P solder joints as a function of reflow time at 220 °C. It is found that both Ni3Sn4 intermetallics and Ni3P are formed due to the solder reaction-assisted crystallization. However, after the first 15 min of reflow, an unusual depression of Ni3Sn4 growth has been observed. A detailed description of the diffusion mechanism has been presented to explain the prohibition of the Ni3Sn4 growth. It is found that the growth of Ni3Sn4 and Ni3P may have a mutual effect on each other during the solder reaction since there is a direct correlation between the depression of the Ni3Sn4 growth and the ending of Ni3P growth. The characteristic of the mechanical strength of electroless Ni-P solder joints has been demonstrated. A correlation between the mechanical strength and the interfacial metallurgical reaction has been discussed. Also, it is found that different reflow times will result in different fracture interfaces of the sheared electroless Ni-P solder joints. The detailed explanation of the fracture surface morphology has been explored.

Interfacial reaction and wetting behavior in eutectic SnPb solder on Ni/Ti thin films and Ni foils

Ni-based under bump metallization (UBM) is of interest in low cost flip chip technology primarily due to a slower chemical reactions with high-Sn solders such as eutectic SnPb as compared to Cu-based UBM. We studied wetting behaviors and interfacial reactions of the eutectic 63Sn–37Pb on Ni foils and Ni/Ti thin films using transmission electron microscopy (TEM), scanning electron microscopy, and energy dispersion x-ray analysis. Wetting angle, morphology of solder surface, and the rate of consumption of Ni have been studies as a function of reflow time at the temperatures of 200, 220, and 240 °C. From the TEM analysis, we found that Ni forms a single layer of scallop-type Ni3Sn4 compound with the eutectic SnPb. During the isothermal annealing, we observed the spalling of Ni3Sn4 compound from the Ni/Ti thin films. The spalling phenomenon is similar to that of Cu6Sn5 from the Cu/Cr thin films, yet the rate is slower. The spalling of Ni–Sn compound eventually caused dewetting of the molten solder from the Ti surface.

Rate of consumption of Cu in soldering accompanied by ripening

What is the rate of consumption of Cu in soldering reactions has been a critical question in electronic packaging technology. The Cu films are consumed by Cu–Sn compound formation. Because the Cu thickness is limited and the rework of a solder joint requires a layer of unreacted Cu, the loss of Cu in soldering must be under control. At the solder interface, Cu–Sn intermetallic compounds do not form layered structures. Rather, the Cu6Sn5 phase grows as scalloplike grains into the molten solder and ripening occurs between the grains. Therefore, it has been difficult to determine the compound growth rate, and in turn the Cu consumption rate. Using cross-sectional and top-polished samples, we have measured the total volume of Cu–Sn intermetallic compounds formed between eutectic SnPb alloy and Cu substrate as a function of reflow time and temperature. We have deduced that after 1 min reflow, for example, the thickness of Cu consumed was about 0.36, 0.47, and 0.69 μm at 200, 220, and 240 °C, respectively.