Francis M Mutuku | Binghamton University (original) (raw)

Papers by Francis M Mutuku

Journal of Electronic Materials, Nov 28, 2016

The number, and the spacing, of Ag 3 Sn precipitates in SnAg -Cu/Cu solder joints were related to... more The number, and the spacing, of Ag 3 Sn precipitates in SnAg -Cu/Cu solder joints were related to separate processing parameters. The mechanical properties of an individual solder joint were directly related to the resulting distribution of different dispersoids in the joint. As the number of Ag 3 Sn precipitates increased, so did solder joint strength and shear fatigue lifetime. The room-temperature shear fatigue lifetime was inversely correlated with the separation between Ag 3 Sn precipitates. Bi and Sb solid solution strengthening was found to result in significantly larger values of shear strength and shear fatigue lifetime for one Pb-free solder. Room-temperature shear fatigue lifetime tests were identified as a relatively straightforward, yet sensitive means to gain insight into the reliability of SnAg -Cu (SAC) solder joints.

EDFA Technical Articles

IBM engineers recently conducted a study to better understand and control the reliability of copp... more IBM engineers recently conducted a study to better understand and control the reliability of copper pillar solder joints in 2.5-D packages. Here they describe their approach and the results they obtained. They explain how they created test samples to evaluate different solder compositions, pillar geometries, and thermal histories and assess their effect on microstructure, precipitate morphology, intermetallic layer thickness, and shear strength. They also present thermal cycling test results comparing the performance of silicon and glass interposers.

The desire for smaller, lighter and faster products drives the development of 2.5D/3D integration... more The desire for smaller, lighter and faster products drives the development of 2.5D/3D integration technologies that can utilize tens of thousands of connections per die. Micro copper (Cu) pillar geometries have been widely adopted because their small size and fine pitch provides high thermal conductivity, higher input/output (I/O) density and resistance to deleterious electromigration effects. In micro Cu-pillars, SnAg solder is electroplated on top of a Cu pillar. Because of the small volume of solder employed, intermetallic compounds comprise a significant fraction of the resulting solder joint, and very fine Ag3Sn precipitate morphologies can occur. Thus, the microstructure of SnAg solder/Cu pillar microstructures varies significantly from that of larger solder joints such as flip chip solder joints. Furthermore, 2.5D applications include interposers of distinctly different materials, such as Si or glass. The different properties of these materials such as coefficient of thermal ...

2016 IEEE 66th Electronic Components and Technology Conference (ECTC), 2016

With the challenges of moving to 2.5/3D packaging structures, it has become imperative to improve... more With the challenges of moving to 2.5/3D packaging structures, it has become imperative to improve our understanding of the materials science of fine pitch Pb-free solder joints. The use of Cu pillars capped with thin layers of SnAg solder provides for tighter bump pitches reducing the chance of solder bridging at chip joining. However, changes in geometry, materials and processes associated with 2.5 D packaging create new materials challenges. The thinner solder regions mean that a larger volume fraction of joints is consumed by the formation of intermetallic compounds at the pillar/SnAg solder interface. The final concentration of Ag in the joint can vary, and the Ag3Sn precipitate morphology in the solder joint may change, directly affecting the reliability of the joint. This can occur through the formation of Ag3Sn plates, or simply because of different distributions of much smaller Ag3Sn precipitates. Or, the entire solder joint may be transformed into intermetallic compounds during assembly or operation of fine pitch joints. The presence of interposer materials with a different CTE compared to FR-4 laminates may also affect the lifetime of the package during drop/shock or thermal fatigue. In the current study, relations between processing, microstructure and reliability of assemblies enabled through Cu pillar/interposer technology were examined. The effects of solder cap composition, thickness and volume on microstructure of assemblies on Si and glass substrates were examined. Effects of multiple reflows on the microstructure of solder joints were also studied. Significant variation in Ag3Sn precipitate morphology was observed under nominally identical fabrication conditions. These were correlated with relatively large variations in mechanical behavior, for instance in measured values of shear strength. Large variations in Ag3Sn precipitate size and number were also observed with changes in composition and upon aging, as would be expected. Cu pillar assemblies revealed small, but continuous solder layers. After failure during ATC, cracks were found to have propagated through these continuous solder layers.

2015 IEEE 65th Electronic Components and Technology Conference (ECTC), 2015

An improved understanding of how materials and processing affect solder joint microstructure prov... more An improved understanding of how materials and processing affect solder joint microstructure provides insight into how to enhance the fatigue performance of Pb free solder joints. Our previous results showed that differences in solder volume can alter the Sn grain morphology of a near eutectic, SnAgCu Pb-free solder joint and change the lifetime in thermal cycling test. Solder composition and PCB surface finish were also shown to influence Pb free solder joint microstructure and the lifetimes of package interconnects in an ATC test. However, the effects of those parameters on failure mechanisms, particularly recrystallization behavior and crack propagation, are not well understood. Thus, in the current work, Pb-free solder joints were assembled with a range of materials, and the microstructure and reliability of those solder joints were characterized and correlated with the different fabrication parameters. In each case, the failure mechanism was examined. Solder joints of various sizes were assembled onto test boards with Cu-OSP, ENIG or ENEPIG PCB surface finishes using commercial solders alloys with different microalloying elements, such as Ni and Mn. For comparison, Sn3Ag0.5Cu (SAC 305) and eutectic SnPb alloys were included in the sample set. Accelerated thermal cycling testing was performed on the assembled vehicles. Microstructural analysis was performed on as-reflowed, on partially cycled samples, and on samples after electrical failure, to understand the initial microstructure and its evolution during the test. Thus, selected samples were taken out of the thermal cycling chamber at specific intervals of characteristic life (i.e., 20% and 50% of their characteristic life) for microstructural analysis. Polarized light microscopy and electron backscatter diffraction (EBSD) techniques were used to assess the Sn grain morphology of solder joints. The effect of solder composition, Sn grain morphology and PCB surface finish on solder joint microstructure and lifetime was evaluated. The effect of microstructure on the recrystallization behavior and on crack initiation, and thus the failure mechanism of joints in thermal cycling test is reported.

2014 IEEE 64th Electronic Components and Technology Conference (ECTC), 2014

The effects of changes in thermal history (changes in the reflow profile) on solder joint microst... more The effects of changes in thermal history (changes in the reflow profile) on solder joint microstructure are examined. The effects of variations in the reflow temperature, and in the cooling rate from the melt, on the microstructure of near eutectic lead free SnAgCu solder joints were investigated. Changes in precipitate or Sn grain morphologies have previously been correlated with changes in the failure rates in these Pb free alloys. Thus correlations were sought between changes in reflow parameters and in both precipitate and Sn grain morphologies. Precise reflows were conducted in a differential scanning calorimeter and it was found that changes in microstructure were correlated with large changes in the reflow temperature, and that the precipitate microstructure was a strong function of the cooling rate. This effect was similar in magnitude to that observed in relatively large changes (1 to 3wt%) in the Ag concentration of these near eutectic SnAgCu alloys.

2014 IEEE 64th Electronic Components and Technology Conference (ECTC), 2014

Thermal fatigue and room temperature isothermal mechanical performance of various Pb-free and SnP... more Thermal fatigue and room temperature isothermal mechanical performance of various Pb-free and SnPb solder joints were examined. Various solder alloys doped with Ni (SN100C) and Mn (SAC105-Mn and SACM) were evaluated and compared to SAC105, SAC 205 SAC 305 and SAC 405 and eutectic SnPb alloys. Solder spheres ranging from 10 to 20 mils were reflowed on various printed circuit board (PCB) surface finishes such as copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG). The mechanical behavior of these solder joints was evaluated in low speed and high speed shear tests, and also in shear fatigue test. The effect of isothermal aging was examined. Custom made ball grid array (BGA) packages with the same alloys and sizes were tested in accelerated thermal cycling (ATC) test. The results from room temperature mechanical tests were correlated to package level ATC results obtained for the thermal cycling profile of -40/125°C. The effect of solder volume and composition on the solidification temperature of each solder joint was carefully measured by differential scanning calorimeter (DSC). Precipitate sizes and distributions were analyzed using backscattered scanning electron microscopy (SEM). Sn grain morphology was characterized by polarized light microscopy (PLM) and electron backscatter diffraction (EBSD). Investigation of the lifetimes of various solder joints in room temperature fatigue and accelerated thermal cycling tests showed distinct dependences of lifetime on solder composition. Distinct increases in lifetimes with increases in Ag content were observed. Results suggested the recrystallization and failure mechanism in Pb-free solder joints are strongly affected by Ag3Sn precipitates. Combination of Cu/ENIG surface finishes generally resulted in some improvement in thermal fatigue performances. Results also showed that solder volume can greatly affect the microstructure and performance of SAC solder joints in mechanical and ATC tests. Larger samples generally solidified at higher temperatures and revealed different Sn grain morphologies than smaller samples, which generally undercooled more. Addition of dopants generally reduced the undercooling, resulting in different solder joint microstructures. The effect of variation in solder composition and volume and PCB surface finish on solder joint microstructure and lifetime was carefully evaluated.

2017 IEEE 67th Electronic Components and Technology Conference (ECTC), 2017

The effect of the variation of processing parameters such as thermal history, or the composition ... more The effect of the variation of processing parameters such as thermal history, or the composition of Sn-Ag-Cu solder joints, on microstructure and reliability performance depends strongly on joint geometry, in particular length scale. The advent of 2.5/3D packaging technologies in microelectronics has further decreased joint length scales and changed interconnect aspect ratios to reduce I/O pitch. While fifty to one-hundred-micron diameter joints are becoming more common, the diameter of some SnAg based solder joints has been reduced to as little as ten microns, affecting the solidification microstructure and the formation of intermetallic compounds at solder/metallization interfaces. This study investigates these effects and correlates them with shear strength and reliability performance in accelerated thermal cycle test. Comparisons with results from solder joint studies at much larger length scales (ball grid array) are reported. Copper pillars having Sn caps with controlled variations of Ag content, height, and diameter were joined to silicon interposer substrates with Cu based metallizations. The microstructures of such Cu pillar to interposer solder joints, as well as those of reflowed (but unjoined) copper pillar structures were examined using optical and electron microscopy. The size and number density of Ag3Sn precipitates were quantified for a number of different processing conditions. The shear strength of solder caps as-reflowed on Cu pillars was measured. Accelerated thermal cycling test was the primary means of reliability testing for the interposer joint structures.

Alloy composition, reflow profile and thermal history are all known to affect the microstructure ... more Alloy composition, reflow profile and thermal history are all known to affect the microstructure and mechanical properties of Sn-Ag-Cu based solder joints. Secondary precipitates such as Ag3Sn and Cu6Sn5 interact with dislocations in the Sn grain matrix and thus affect the creep resistance of Sn-Ag-Cu solder alloys. The size, number and arrangement of these strengthening precipitates determine the thermomechanical response of a Sn-Ag-Cu solder joint in service. During typical service, solder joints age with time, temperature and cyclic loading. As the secondary precipitates coarsen, their number density decreases, increasing their spacing and reducing their effectiveness at impeding dislocation motion. This microstructural evolution with aging can degrade the mechanical performance of solder joints and the reliability of the solder interconnects. Sn-Ag-Cu solder strengthening can also be achieved through solid solution alloying elements such as Bi and Sb. Such solid solution strengt...

International Symposium on Microelectronics

A novel lead-free solder alloy 90.6Sn3.2Ag0.7Cu5.5Sb, designated as Indalloy276, was developed ta... more A novel lead-free solder alloy 90.6Sn3.2Ag0.7Cu5.5Sb, designated as Indalloy276, was developed targeting for high reliability with a wide service temperature capability. The alloy exhibited a melting temperature range of 223 to 232°C, reflowable at profile with peak temperature 245°C and 255°C, with ambient temperature Yield Stress 60MPa, UTS 77 MPa, and ductility 28%, and a higher stress than both SAC305 and SACBSbN, the latter two alloys were used as controls. When tested at 140°C and 165°C, the die shear stress of 276 was comparable with SACBSbN but higher than SAC305, and the ductility was higher than both SACBSbN and SAC305, with SACBSbN exhibited distinct brittle behavior. When aged at 125°C and 175°C, the die shear strength of 276 was comparable or higher than both controls. When pretreated with a harsh condition, TST (−55°C/155°C) for 3000 cycles, the die shear strength of 276 was 8 times of that of SACBSbN and SAC305. When pre-conditioned at TCT (−40°C/175°C) for 3000 cycle...

International Symposium on Microelectronics

The desire for smaller, lighter and faster products drives the development of 2.5D/3D integration... more The desire for smaller, lighter and faster products drives the development of 2.5D/3D integration technologies that can utilize tens of thousands of connections per die. Micro copper (Cu) pillar geometries have been widely adopted because their small size and fine pitch provides high thermal conductivity, higher input/output (I/O) density and resistance to deleterious electromigration effects. In micro Cu-pillars, SnAg solder is electroplated on top of a Cu pillar. Because of the small volume of solder employed, intermetallic compounds comprise a significant fraction of the resulting solder joint, and very fine Ag 3 Sn precipitate morphologies can occur. Thus, the microstructure of SnAg solder/Cu pillar microstructures varies significantly from that of larger solder joints such as flip chip solder joints. Furthermore, 2.5D applications include interposers of distinctly different materials, such as Si or glass. The different properties of these materials such as coefficient of therma...

2014 IEEE 64th Electronic Components and Technology Conference (ECTC), 2014

2015 IEEE 65th Electronic Components and Technology Conference (ECTC), 2015

Journal of Electronic Materials, Nov 28, 2016

EDFA Technical Articles

2016 IEEE 66th Electronic Components and Technology Conference (ECTC), 2016

2015 IEEE 65th Electronic Components and Technology Conference (ECTC), 2015

2014 IEEE 64th Electronic Components and Technology Conference (ECTC), 2014

2017 IEEE 67th Electronic Components and Technology Conference (ECTC), 2017

International Symposium on Microelectronics

The desire for smaller, lighter and faster products drives the development of 2.5D/3D integration... more The desire for smaller, lighter and faster products drives the development of 2.5D/3D integration technologies that can utilize tens of thousands of connections per die. Micro copper (Cu) pillar geometries have been widely adopted because their small size and fine pitch provides high thermal conductivity, higher input/output (I/O) density and resistance to deleterious electromigration effects. In micro Cu-pillars, SnAg solder is electroplated on top of a Cu pillar. Because of the small volume of solder employed, intermetallic compounds comprise a significant fraction of the resulting solder joint, and very fine Ag 3 Sn precipitate morphologies can occur. Thus, the microstructure of SnAg solder/Cu pillar microstructures varies significantly from that of larger solder joints such as flip chip solder joints. Furthermore, 2.5D applications include interposers of distinctly different materials, such as Si or glass. The different properties of these materials such as coefficient of therma...

2014 IEEE 64th Electronic Components and Technology Conference (ECTC), 2014

2015 IEEE 65th Electronic Components and Technology Conference (ECTC), 2015

— With the challenges of moving to 2.5/3D packaging structures, it has become imperative to impro... more — With the challenges of moving to 2.5/3D packaging structures, it has become imperative to improve our understanding of the materials science of fine pitch Pb-free solder joints. The use of Cu pillars capped with thin layers of SnAg solder provides for tighter bump pitches reducing the chance of solder bridging at chip joining. However, changes in geometry, materials and processes associated with 2.5 D packaging create new materials challenges. The thinner solder regions mean that a larger volume fraction of joints is consumed by the formation of intermetallic compounds at the pillar/SnAg solder interface. The final concentration of Ag in the joint can vary, and the Ag3Sn precipitate morphology in the solder joint may change, directly affecting the reliability of the joint. This can occur through the formation of Ag3Sn plates, or simply because of different distributions of much smaller Ag3Sn precipitates. Or, the entire solder joint may be transformed into intermetallic compounds during assembly or operation of fine pitch joints. The presence of interposer materials with a different CTE compared to FR-4 laminates may also affect the lifetime of the package during drop/shock or thermal fatigue. In the current study, relations between processing, microstructure and reliability of assemblies enabled through Cu pillar/interposer technology were examined. The effects of solder cap composition, thickness and volume on microstructure of assemblies on Si and glass substrates were examined. Effects of multiple reflows on the microstructure of solder joints were also studied. Significant variation in Ag3Sn precipitate morphology was observed under nominally identical fabrication conditions. These were correlated with relatively large variations in mechanical behavior, for instance in measured values of shear strength. Large variations in Ag3Sn precipitate size and number were also observed with changes in composition and upon aging, as would be expected. Cu pillar assemblies revealed small, but continuous solder layers. After failure during ATC, cracks were found to have propagated through these continuous solder layers.