D. R. Frear

Electromigration of eutectic SnPb and SnAg3.8Cu0.7 flip chip solder bumps and under-bump metallization

The electromigration damage in flip chip solder bumps of eutectic SnPb and SnAg3.8Cu0.7 was studied after current stressing at 120 °C with 1.5 A. The diameter of the bumps was about 200 μm. The under-bump metallization (UBM) on the chip side was electroless Ni and on the board side was electroplated Cu. Surface marker motion was used to measure the atomic flux driven by electromigration and to calculate the effective charge number, Z*, of the solder. For eutectic SnPb, Z* is about 36–100 after 39.5 h of electromigration, but for SnAg3.8Cu0.7 the marker movement was too small to measure Z* even after 200 h of current stressing. While the effect of electromigration in SnAg3.8Cu0.7 is much smaller than that in eutectic SnPb, hillocks of intermetallic compounds at the anode have been observed in the former. An extensive growth of Ni–Cu–Sn intermetallic compounds in the matrix of both solder bumps was found. A localized dissolution of electroless Ni UBM was observed under current stressing, yet no dissolution ...

Morphology, kinetics, and thermodynamics of solid-state aging of eutectic SnPb and Pb-free solders (Sn–3.5Ag, Sn–3.8Ag–0.7Cu and Sn–0.7Cu) on Cu

Intermetallic compound (IMC) growth during solid-state aging at 125, 150, and 170 °C up to 1500 h for four solder alloys (eutectic SnPb, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and Sn-0.7Cu) on Cu under bump metallization was investigated. The samples were reflowed before aging. During the reflow, the solders were in the molten state and the formation of the IMC Cu 6 Sn 5 in the cases of eutectic SnPb and Sn-3.5Ag had a round scallop-type morphology, but in Sn-0.7Cu and Sn-3.8Ag-0.7Cu the scallops of Cu 6 Sn 5 were faceted. In solid-state aging, all these scallops changed to a layered-type morphology. In addition to the layered Cu 6 Sn 5 , the IMC Cu 3 Sn also grew as a layer and was as thick as the Cu 6 Sn 5 . The activation energy of intermetallic growth in solid-state aging is 0.94 eV for eutectic SnPb and about 1.05 eV for the Pb-free solders. The rate of intermetallic growth in solid-state aging is about 4 orders of magnitude slower than that during reflow. Ternary phase diagrams of Sn-Pb-Cu and Sn-Ag-Cu are used to discuss the reactions. These diagrams predict the first phase of IMC formation in the wetting reaction and the other phases formed in solid-state aging. Yet, the morphological change and the large difference in growth rates between the wetting reaction and solid-state aging cannot be predicted.

Morphology of interfacial reaction between lead-free solders and electroless Ni–P under bump metallization

The morphology of interfacial reaction products between four lead-free solder alloys on electroless Ni–P was characterized. The four Pb-free solders were 99.3Sn0.7Cu, 95.5Sn3.8Ag0.7Cu, 96.5Sn3.5Ag, and 96Sn2Ag2Bi (in wt%) alloys. After reflow, the interfacial intermetallics in the first two solders that contain Cu (99.3Sn0.7Cu and 95.5Sn3.8Ag0.7Cu) had good adhesion with electroless Ni–P. However, the 96.5Sn3.5Ag and 96Sn2Ag2Bi alloys formed interfacial intermetallics with a needle shaped morphology that spalled off the surface of electroless Ni–P. This difference is attributed to the role of Cu in the solders (which modified the chemical potential of the interfacial intermetallics), the volume change that occurs during intermetallic formation, and the interfacial properties of the compound. In solid state aging experiments, the consumption of electroless Ni–P by intermetallic growth was not significant (approximately 1 μm) and all the intermetallics had good adhesion to the electroless Ni–P. The electrol...

Electromigration of eutectic SnPb solder interconnects for flip chip technology

The electromigration of eutectic SnPb solder interconnects between a Si chip and a FR4 substrate was studied at 120 °C for up to 324 h with current stressing of 104 amp/cm2. Hillocks were observed at the anode and voids at the cathode. The dominant diffusing species was found to be Pb, confirmed by its accumulation at the anode. Diffusion markers were used to measure the electromigration flux and calculate the effective charge of atomic diffusion in the solder. Extensive microstructural evolution was also observed in the two-phase solder alloy that occurred by a ripening process.

Wetting reaction versus solid state aging of eutectic SnPb on Cu

The reaction kinetics of eutectic SnPb solder on Cu were studied and compared in the liquid state at 200 to 240 °C and in the solid state aged at 125–170 °C. The ternary phase diagrams of SnPbCu, the morphology of intermetallic compound (IMC), and the kinetics of growth of the intermetallics were used in the comparison. The temperature difference between these two reactions is only 30 °C, but the kinetics of reaction, as well as the morphology of IMC formation, are very different. The kinetics in the wetting reaction is four orders of magnitude faster than that in solid state aging. The Cu6Sn5 intermetallic morphology in solid state aging is a layer type, but it has a scallop-type morphology in the wetting reaction. The morphology strongly affects the kinetics. While the kinetic difference can be attributed to the difference in atomic diffusivity between the liquid state and the solid state, it is the morphology that determines the kinetic path in these reactions. We conclude that a fast rate of reaction,...

Impact of flip-chip packaging on copper/low-k structures

Copper/low-k structures are the desired choice for advanced integrated circuits (ICs). Nevertheless, the reliability might become a concern due to the considerably lower strength and greater coefficient of thermal expansion (CTE) of the low-k materials. To ensure successful integration of the new chips within advanced packaging products, it is essential to understand the impact of packaging on chips with copper/low k structures. In this study, flip-chip die attach process has been studied. Multilevel, multiscale modeling technique was used to bridge the large gap between the maximum and minimum dimensions. Interface fracture mechanics-based approach has been used to predict interface delamination. Both plastic ball grid array (PBGA) and ceramic ball grid array (CBGA) packages were evaluated. Critical failure locations and interfaces were identified for both packages. The impact of thin film residual stresses has been studied at both wafer level and package level. Both PBGA and CBGA packaging die-attach processes induce significantly higher crack driving force on the low-k interfaces than the wafer process. CBGA die-attach might be more critical than PBGA die-attach due to the higher temperature. During CBGA die-attach process, the crack driving force at the low-k/passivation interface may exceed the measured interfacial strength. Two solutions have been suggested to prevent catastrophic delamination in copper/low-k flip-chip packages, improving adhesion strength of low-k/barrier interface or adding tiles and slots in low-k structures to reduce possible area for crack growth.

Interfacial reaction of eutectic AuSi solder with Si (100) and Si (111) surfaces

The dissolution behavior of Si (100) and (111) dies by eutectic AuSi solder was investigated. On the Si (100) surface, the dissolution primarily occurred by the formation of craters resulting in a rough surface. The dissolution of the Si (111) resulted in a relatively smooth surface. The morphology of the Si (100) surface during a AuSi soldering reaction exhibited more time-dependent behavior and the etching craters on a Si (100) surface grew larger with time whereas Si (111) did not significantly change. This difference was ascribed to the surface energy differences between Si (111) and (100) surfaces that resulted in the two- and three-dimensional dissolution behaviors, respectively. This difference plays an important role in the formation of voids during the AuSi die bonding. The etching craters on Si (100) act as a AuSi solder sink and the regions surrounded by etch pits tend to become voids. For Si (111), flat surfaces were observed in the voided regions. Cross section analysis showed that no solder ...

Lead-free flip chip interconnect reliability for DCA and FC-PBGA packages

A variety of Pb-free solders and under bump metallurgies (Cu, Ni, NiP) were investigated for flip chip packaging applications. The result shows that Sn-0.7Cu exhibits the most desirable mechanical properties (shear, tensile, aging, etc.) during deformation under a variety of stress conditions and has the most favorable failure mechanism under both mechanical and thermomechanical stress testing regardless of UBM type. The eutectic Sn-0.7Cu failed through bulk solder while the eutectic Sn-37Pb, Sn-3.5Ag and Sn-3.8Ag-0.7Cu failed at the solder and UBM interface, involving their respective intermetallic compounds. Cu UBM is more favorable for better reliability than NiP UBM from both interface IMC morphology and electromigration points of view. The current carrying capability for all alloys had no failures when stressed up to 2,338 hours at 2.6/spl times/10/sup 4/ A/cm/sup 2/ and 170/spl deg/C. However, when stressed at 5.1/spl times/10/sup 4/ A/cm/sup 2/, there is a significant migration of Pb toward the anode, creating a multiple layered Pb-rich and Sn-rich microstructure. An observation of excessive Ni migration away from the NiP UBM towards the anode after only 30 hours of current stressing at 5.1/spl times/10/sup 4/ A/cm/sup 2/ and 150/spl deg/C raised the reliability concern for solders with NiP UBM, especially for high power applications. The Sn-0.7Cu/Cu UBM and Sn0.7Cu/Ni UBM exhibit greater than 5,300 cycles of thermal fatigue characteristic life under -55/spl deg/C 1+150/spl deg/C and -40/spl deg/C /+125/spl deg/C air-to-air thermal cycling conditions, respectively.

Characterization of lead-free solders and under bump metallurgies for flip-chip package

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.

Integration of SAW RF Rx filter stacked on a transceiver chip in a QFN package

Enhanced integration of mobile phone components is driven by demands for reduced form factor and cost. Because SAW filters must be fabricated on piezoelectric substrates, they are difficult to monolithically integrate on semiconductor chips. Here we report on the integration of a compact wafer-scale packaged SAW filter stacked over a transceiver chip in a quad flat-pack no-lead (QFN) package. An integrated passive device interposer provided redistribution and matching. We demonstrated the successful integration of both EGSM and DCS filters in such modules. SAW compact models based on the coupling of modes model were developed to facilitate system design.