Insertion Bonding: A Novel Cu-Cu Bonding Approach for 3D Integration

Abstract A novel low temperature Cu-Cu bonding approach called the insertion bonding technique has been developed. This technique leverages on the initiation of high shear stresses at metal-metal contact interface, thus resulting in high plastic deformation, which is essential for strong bond formation. Through finite element studies, it is observed that the insertion bonding technique result in significantly larger plastic deformation in comparison to the conventional bonding technique under the same bonding conditions. Experimental studies of the insertion bonding technique were performed and it is observed that a seamless bond interface is achieved, even at a low bonding temperature of 100°C. Bonding at room temperature (RT) in the presence of a surface cleaning agent resulted in an improved bond interface. Resistance measurement of the samples bonded at 100°C revealed that an electrical contact is achieved between the stacked dies. This shows that the insertion bonding techniques holds much promise for low temperature Cu-Cu bonding

Creep behavior of mixed SAC 405/SnPb soldered assemblies in shear loading

In this work, the creep behavior of mixed lead free (Sn4%Ag 0.5%Cu) (SAC 405) and eutectic tin lead (Sn37%Pb) solders has been studied. A double lap shear configuration was used to study the creep response of the mixed solder joints, the dimensions of which ranged from 300 to 500 μm. The tested solder specimens were wafer level chip scale packages (WLCSP) bumped with preformed solder spheres. The volume ratios of the two solders were controlled by using preformed solders spheres in the size range of 300 to 450 μm. All the specimens were reflowed at a profile with 260°C peak temperature resulting in a complete mixing of the two solder alloys. The creep tests were done at constant stress levels by applying a constant load to the specimen. The displacement of the joints was recorded as a function of time from which the steady state strain rates were determined. In the present experiments, these ranged from 1E-3/s to 1E-9/s. while the applied stress levels ranged from 5 MPa to 40 MPa. The tests were repeated for 3 different temperatures: 40°C, 70°C and 100°C The stress - strain rate data seems to fit well to the SINH creep model and the obstacle controlled model. The fitting to the power law shows different slopes above and below 10 MPa stress levels. In addition, the effect of isothermal aging at 125 °C on the creep behavior was also studied. In general, the mixed solder joints creep faster than SAC 405 and their deformation rate lies in between that of the eutectic SnPb and SAC 405 solders. © 2007 IEEE.status: publishe

Influence of intermetallic properties on reliability of lead-free flip-chip solder joints

Electroplated pure tin bumping as a lead-free alternative for ultra fine pitch applications is a relatively easy process and has provided us with comparable results to eutectic Sn/Pb for thermal cycling reliability. Experimentally, it has been reported that a significantly higher (~40%) thermal cycle fatigue life is seen with the use of cobalt under bump metallization (UBM) instead of copper UBM for a flip-chip device assembled on an alumina substrate. In the current approaches used to estimate fatigue life of solder joints, the solder joint is treated as a homogenous material and modeled as such. However, the smaller joint sizes and higher reactivity of Sn implies that a larger amount of intermetallics are formed as a percentage of bump volume. The existing approach cannot account for the influence on the fatigue behavior of these intermetallic layers within the solder joint. In order to investigate if a simplified engineering approach can provide some insight into this issue, we have attempted to explicitly model the intermetallics as a continuous but separate part of the solder joint. The main damage parameter investigated is the accumulated inelastic strain in a single thermal cycle. From the results, it is clear that the Young's modulus of the intermetallic layer plays an important role, more so when the ratio of intermetallic thickness to the solder joint standoff increases. Thickness of the intermetallic layer also influences the overall strain accumulation in the same manner. The CTE of the intermetallic layer has a relatively lesser influence on the strain accumulation. Both the experimental and FE results suggest that changing the UBM from copper to cobalt can improve the fatigue life by 20%-30%.status: publishe

Thermal cycling reliability of SnAgCu and SnPb solder joints: a comparison for several IC-packages

This paper deals with a comparison study between SnPb and SnAgCu solder joint reliability. The comparison is based on non-linear finite element modellin. Three packages have been selected: silicon CSP, underfilled flip chip and QFN package. Also the effect of thermal cycling conditions has been investigated. Comparing the induced inelastic strains in the solder joint, the leadfree SnAgCu generally scores better thanks to the lower creep strain rate. On the other hand for the CSP and flip chip package, SnAgCu scores worse for the more extreme loading conditions when the inelastic dissipated energy density is selected as damage parameter. The main reason is that due to the lower creep strain rate, the stresses become higher for SnAgCu resulting in higher hysteresis loops with more dissipated energy per cycle. For the QFN package, SnAgCu scores much better. 1

A novel Cu-Cu bonding technique: the insertion bonding approach

A novel low-temperature Cu-Cu bonding approach called the insertion bonding technique has been developed. This technique hinges on the introduction of a tangential pressure at the metal-metal interface, which leads to a high localized plastic deformation that is essential for bond formation. Through finite element modeling studies, it is observed that the insertion bonding technique results in a significantly larger plastic deformation in comparison to the conventional bonding technique under the same bonding conditions. First experimental studies of the insertion bonding technique were performed and it is observed that an electrically yielding Cu-Cu joint is achieved at a low bonding temperature of 100°C. This shows that the insertion bonding technique holds much promise for low-temperature Cu-Cu bonding. © 2011 IEEE.status: publishe

A novel mechanism of embrittlement affecting the impact reliability of Tin-based lead-free solder joints

This work addresses a new mode of brittle failure that occurs in the bulk of tin-based lead-free solder joints, unlike the typical brittle failures that occur in the interfacial intermetallics. Brittle failures in the joint bulk result from the low-temperature ductile-to-brittle transition in the fracture behavior of beta-tin. The bulk embrittlement of these joints is discussed by referring to the results of impact tests performed on both solder joints and bulk solder specimens. The mechanism of bulk embrittlement is largely explained based on the results of a fractography study performed on the bulk joint failures using scanning electron microscopy.status: publishe

Low-temperature embrittlement of lead-free solders in joint level impact testing

The low-temperature embrittlement of tin-based (Snbased) lead-free (Pb-free) solder alloys has been previously reported for bulk specimens tested in impact by means of a conventional Charpy V-notch test setup. [1][2] The brittle failure of Sn-based Pb-free solders at low temperatures (i.e. below -30ºC) is of great concern for electronic assemblies that operate at harsh conditions, because it can lead to occurrences of early failures. A miniaturized impact test setup has been devised to perform a scaled-down version of the conventional Charpy impact test on specimens, which lie in the size range of 300 μm to 3 mm. In this work, the impact tests have been carried out in the temperature range between 23°C and - 110°C. The specimens consisted of SuperBGA® (SBGA®) substrates bumped with 4 different solder alloys: Sn3%Ag0.5%Cu (SAC 305), Sn4%Ag0.5%Cu (SAC 405), Sn3.5%Ag, and Sn37%Pb. The energy absorbed during the fracture of the solder joints was seen to decrease as a function of temperature for the SAC 305 and SAC 405 alloys, while for the Sn37%Pb and Sn3.5%Ag this trend was less pronounced. The ductile-to-brittle transition in the fracture behavior of Sn-based Pb-free solders was confirmed by scanning electron microscopy (SEM) investigations. These investigations showed the progressive bulk embrittlement of the Sn matrix of SAC 305 and SAC 405 solder alloys. The onset of the Sn embrittlement was observed between -40ºC and -50ºC leading to the complete brittle fracture of the solder joint at around - 70ºC.status: publishe