Shinichi Fujiwara

Deterioration mechanism of flip chip attachment using an anisotropic conductive film and design technology for high reliability

The flip-chip technique, in which a bare chip is directly connected to a substrate, has become a key technology in producing compact electronic products, including cellular phones. In particular, the technique of using Au bumps to connect the bare chip with the substrate, with the aid of an anisotropic conductive film (ACF), is one of the most useful technologies. The most serious problem with ACF bonding technology today is that the deterioration mechanism of interconnections is not clear. This study is motivated to clarify the mechanism of deterioration and to establish the method of obtaining reliability in the design of interconnections for which ACF is used.

Bondability of Mg2Si element to Ni electrode using Al for thermoelectric modules

The purpose of this study has been to develop a low cost bonding technique for thermoelectric Mg2Si/Si-Ge modules that provides reliable bonding. Aluminum was chosen as an alternative material to conventional silver alloy braze because of its cost advantage and bondability. The shear strength of an aluminum joint between a Mg2Si element and nickel electrode was 19 MPa. The generation capacity of a prototype Mg2Si/Si-Ge twin couple module was about 20% higher than that of a conventional Si-Ge/Si-Ge twin couple module at 923 K (ΔT = 620 K).

Electromigration in Copper-Core Solder Ball Joints During Thermal Cycle Tests

Electromigration current densities in Cu and Al lines on a silicon die exceed 1.0 × 106 A/cm2 . However, solder joints can only withstand electromigration current densities below about 1.0 × 104 A/cm2 . Thus, electromigration in solder joints will become a problem in semiconductor packages in the near future. Previous studies demonstrated that Cu-core solder balls increased the electromigration lifetime and led to better current stability at temperatures below 423K. This is because electrons flow through the Cu cores, reducing the current density on the cathode side, which is where electromigration occurs. In the present study, we forcused on the reliability of solder joints in a combined environment by examining the effect of thermal cycle tests on the current in a new test sample. A new test sample for the evaluation of joining reliability by using Cu-core solder balls in a combined enbironment was made. In initial tests, this test sample exhibited similar results to those observed in previous studies. Cu-core solder balls subjected to cyclic testing at 233/398K and a current density of 1.0 × 104 A/cm2 exhibited lower reliabilities than when there was no current. Examination of cross-sections of the solder balls after reliability testing revealed that the combined environment accelerated growth of intermetallic compounds and cracks in the joining region. In a combined environment, Cu-core balls were converted into intermetallic compounds on the anode side. This phenomenon is thought to occur due to the different electrical resistivities of Cu-Sn intermetallic compounds.Copyright

Development of Highly Reliable BGA and Flip-Chip Structures by Using Cu-Cored Solder Ball

A Cu-cored solder joint is a micro-joint structure in which a Cu sphere is encased in solder. It results in a more accurate height and has low thermal and electrical resistance. In a previous paper, we examined the thermal fatigue life of a Cu-cored solder ball grid array (BGA) joint through actual measurements and crack propagation analysis. As a result, we found that the thermal fatigue life of a Cu-cored solder BGA joint is about twice as long as that of a conventional joint. In this paper, we describe the impact strength of a Cu-cored solder BGA joint determined by conducting an impact bending test. This test is a technique to measure the impact strength of a micro-solder joint. This method was developed by Yaguchi et al., and they confirmed that it is an easier and more accurate method of measuring impact strength than the board level drop test. First, we simulated the impact bending test by finite element analysis (FEA) and calculated solder strains of both Cu-cored solder joints and conventional joints. The results indicated that the maximum solder strain of a Cu-cored solder joint during the impact bending test was slightly smaller than that of a conventional joint. The solder volume of the Cu-cored solder joint was also smaller than that of a conventional joint. On the other hand, joint stiffness of the Cu-cored solder joint was larger than in a conventional joint. The former increases the solder strain of the Cu-cored solder joint, and the latter decreases it. By balancing these phenomena, it is possible to obtain a maximum solder strain in the Cu-cored solder joint that is slightly smaller than in a conventional joint. Based on these phenomena, the impact strength of the Cu-cored solder joint is predicted to be the same as or higher than that of a conventional joint. Therefore, we measured the impact strengths of a Cu-cored solder joint and a conventional joint using the impact bending test. As a result, we confirmed that the impact strength of the Cu-cored solder joint was the same as or higher than that of a conventional joint. Accordingly, a Cu-cored solder BGA joint is a micro-joint structure that makes it possible to improve thermal fatigue life without decreasing impact strength. Moreover, we investigated whether the use of Cu-cored solder in a flip-chip (FC) joint improved its reliability. As a result, we found that the stress of an insulating layer on a Si die surface was reduced by using a Cu-cored solder FC joint. This is because bending deformation of the Cu land occurs, and the difference in thermal deformation between the Si chip and the Cu land becomes small. Accordingly, the Cu-cored solder FC joint is a suitable structure for improving reliability of a low-strength insulating layer.Copyright

Interconnection reliability and interfacial structure between Au alloy bump and Al pad using ultrasonic bonding

Abstract Flip chip technology with Au bumps on a substrace has been widely applied to electronic equipment such as smart phones. The purposes of this study are to examine the effect of Al pad thickness on the bondability of flip chip using ultrasonic bonding and to clarify interfacial structures between Au alloy bumps and Al pads by ultrasonic bonding compared before and after a thermal cycle test. Suitable Al thickness for excellent initial Au/Al bonding without chip cracking are 0.8 μm because a thin Al layer could not reduce stress to a chip under an Al pad during the ultrasonic bonding process. Intermetallic compounds between the Au alloy bump and chip after reflows consisted of five Au-Al layers, and a pure Al layer remained. On the other hand, after the temperature cycle test at 218/423 K, intermetallic compounds between the Au alloy bump and chip were changed into two kinds of Au-Al layers, so a pure Al layer did not exist. In addition, if thick intermetallic compound layers existed around the bonding region, bondability deteriorated easily by thermal stress due to a thermal cycle test, therefore the open failure rate was rising when the Au thickness was 1.2 μm.

Ultrasonic bonding of Cu/Ni and its thermal reliability

In this study, we ultrasonically bonded Cu and Ni sheets and evaluated the thermal reliability of the joints at 473 K by high-temperature testing. Furthermore, we observed the interfacial microstructures of the joint and evaluated their effects on the bondability of the joint. Cu and Ni sheets were metallurgically bonded under the optimum condition of ultrasonic bonding. Cu base metal strongly deformed compared with deformation of Ni base metal during ultrasonic bonding. The ultrasonically bonded interface of Cu and Ni sheets was composed of bonded region and unbonded region. The bonded region was composed of two types of microstructures. Cu and Ni were locally stirred and an ∼200-nm thickness of solid-soluted region of Cu and Ni atoms was formed in the bonded region. The Cu grains were fine; moreover, the Ni grains became fine from the bonded interface to the Ni side of 1–2 μm. These fine grains can be formed by deformation of bonded base metals during the ultrasonic bonding process, suggesting that plastic deformation was formed from the soft Cu base metal to the hard Ni base metal. The ultrasonically bonded joints showed good thermal reliability at 473 K through the holding time up to 1000 h. Some of the Cu grains became coarse in the Cu base metal. Phase separation of the solid-soluted region of Cu and Ni atoms may occur during the high-temperature test performed at 473 K.

Electromigration failure analysis of flip-chip solder joint by using void growth simulation and synchrotron radiation X-ray microtomography

Electromigration (EM) failures of flip-chip solder joints due to void growth, resulting from miniaturization of joint structure, have recently been reported. In addition, growth behavior of electromigration voids in solder joints has not been clarified. It is therefore difficult to predict electromigration failure life. A novel method for simulating growth behavior of an electromigration void in a solder joint was developed. This method was applied to predict failure lives of a conventional solder joint and a copper-cored solder joint. According to the simulation results, the failure life of the copper-cored solder joint is more than three times longer than that of the conventional joint. Moreover, failure life of each joint was measured by electromigration test, and the void shape was observed by synchrotron-radiation X-ray microtomography provided at SPring-8. The good agreement between the predicted growth behaviors and the measured and observed behaviors demonstrate the validity of the developed simulation method.

Bondability of Si-Ge thermoelectric element and molybdenum electrode using aluminum for thermoelectric module

A new method that is low cost and produces highly reliable refractory bonds was developed for bonding Si-Ge thermoelectric devices and Mo electrodes. Aluminum foil was chosen as an alternative material to conventional Ag braze alloy, because of its cost advantages and bonding ability. Good wettability of Al to both the Si-Ge devices and Mo electrodes was achieved at a bonding temperature of 953 K. Molten Al reacted with the Mo electrode and caused partial dissolution of the Si-Ge device. Si-Ge thermoelectric devices could be bonded to Mo electrodes in vacuum, pure nitrogen, and in nitrogen with 4% hydrogen. Mcroscopic observations of cross-sections were conducted to investigate cracking in the bonded joints. The coefficient of thermal expansion (CTE) of Mo (4.5 ppm/K) is similar to that of Si-Ge (4.0 ppm/K), so that the use of a thin Al bonding layer results in sufficiently low thermal stress and allows crack-free bonding. In addition, for 12.5-μm-thick Al foil, almost no degradation of the joint strength occurred after heat treatment at 823 K for 5 h, because the reaction of the thin Al bonding layer to Si-Ge was completed during the bonding process.

Bondability of Cu wire on Cu substrate with Sn plating by ultrasonic

This study evaluated both the joint strength of copper wire on a copper substrate with tin plating and the joint reliability of copper wire bonding after heat treatment. The suitable tin thickness and bonding conditions, which are stage temperature, wire bonding power and bonding time, were chosen by the peel test after copper wire bonding. Tin thickness of 10 m showed a high bonding rate under the conditions of stage temperature 373 K, bonding power 500–700 mW and bonding time 30 50 ms. Before heat treatment, the peel strength of the copper wire on the copper substrate with tin plating conditions was weaker than that of gold wire on a gold substrate. After heat treatment for more than 70 h at 298 K, the peel strength of the copper wire became higher than that of the gold wire and twice as high as the initial bonding strength. The tin layer remained between the copper wire and copper substrate before heat treatment. When the samples were held at 298 K, tin reacted with copper and turned into a Cu–Sn intermetallic compound. Upon completion of this reaction at 298 K for over 70 h, the soft tin layer between the copper wire and copper substrate disappeared. Therefore, the peel strength of copper wire after heat treatment increased. These results were observed by scanning electron microscope images of the interface between the copper wire and copper substrate before and after heat treatment.