Soldering technology plays a key role in various levels of electronic packaging, such as flip-chip connection (or C4), solder-ball connection in ball-grid-arrays (BGA), or IC package assembly to a printed circuit board (PCB). Solder joints produced in the electronic packages serve critically as electrical interconnection as well as mechanical/physical connections. At present, Sn-Pb solder of various compositions are used in electronic packaging. Among them, the 60Sn-40Pb eutectic is the most widely used. Since lead is detrimental to human health and environments, recent trend on solder alloys is toward reducing the lead content in Sn-Pb solder or developing Pb-free solder alloys. In this regard, 80Sn-20Pb alloy can be an alternative of 60Sn-40Pb because of its low Pb content and superior mechanical properties. And, among the Pb-free solders which was investigated and developed as alternative of Pb-Sn solder, Sn-Bi alloy are already used in some applications because of specific advantages they offer over eutectic Sn-Pb, such as lower process temperature, compatibility with particular substrates, or superior reliability under certain conditions.
In the electropackaging process, the solder bump and coating are produced by elecroplating process having merits on the reliability, accuracy and economics. So, it is necessary to develop the electroplating process of 80Sn-20Pb and Sn-Bi alloy, and to study on the properties of electrodeposits.
In this work, effects of plating condition on composition, morphology, and solder properties of 80Sn-20Pb and Sn-Bi electrodeposited from methane sulfonate bath were studied.
Effects of the Plating Condition on Morphology of the 80Sn-20Pb electrodeposited from methane sulfonate bath and the growth of the intermetallic compounds.
Alloy deposits of 80Sn-20Pb were electroplated on the Cu from an organic sulfonate bath using dc or pulse currents, and aged at 100℃ to 18℃ to form Cu-Sn intermetallic compounds at deposit/substrate interface. Influences of plating parameters on the microstructure of the 80Sn-20Pb deposits, effects of deposit morphology on the formation of intermetallic compounds and their influence on the fracture resistance of the electrodeposits were investigated.
Addition of grain refiner to the bath increased the cathodic polarization 640 mV greater than that in the bath without the grain refiner (additive) at cathodic current density of 10 A/＄dm^2＄, thereby producing an extremely fine and round deposit. The grain morphology of electrodeposit produced by pulse currents was also extremely fine at low duty cycle (10%) and low frequency (25Hz), but became coarser with increasing the duty cycle and/or the frequency, approaching that of electrodeposit formed by dc current. The electrodeposits of 80Sn-20Pb was identified a mixture of Pb and β-Sn crystals by X-ray diffraction. The Pb crystals were randomly oriented, but β-Sn crystals exhibited {200} preferred orientation in deposit formed from the bath without additive and {220} preferred orientation in that formed from the bath with additive. In pulse plating, β-Sn crystals exhibited {200} preferred orientation which was gradually transformed to {220} one with decreasing the duty cycle and/or the frequency.
When deposits of 80Sn-20Pb were aged at 150℃, the growth of Cu-Sn intermetallic compound layer ＄[\varepsilon(Cu_3Sn＄)+n'(Cu_6Sn_5)]$ were showed a parabolic time dependence, and the growth rate of that depended significantly on the microstructure of Sn-Pb electrodeposits which was influenced by deposition condition; it was fastest in as extremely fine deposit formed using pulse current in bath without additive, but slowest in deposit formed using dc current in bath containing the additive in spite of equally fine structure. The additive incorporated at grain boundary of electrodeposit appears to delay the diffusion rate of Sn stoms across the intermetallic/deposit interface on aging, resulting in slow growth of intermetallic layer in thickness direction but substantial growth in lateral one. The apparent activation energy of intermetallic compound growth in deposit formed using dc current in bath containing the additive was 43.32 kJ/mol, that in deposit formed using dc current in bath without additive was 34.36 kJ/mol, and that in deposit formed using pulse current was 30.35 kJ/mol
The occurrence frequency of surface crack, formed when the aged samples were subjected to the 90-bending test, were found to be associated with the growth rate of the intermetallic compound layer and ductility of deposit layer. For the same aged samples, the occurrence frequency of surface crack was the least in sample deposited in bath containing the additive because of the slowest growth rate of the intermetallic compound layer and the highest ductility, and the most is that formed using pulse current in bath without additive because of the fastest growth rate of the intermetallic compound layer and the lowest ductility of deposit layer.
Effects of the plating condition on the composition and morphology of Sn-Bi electrodeposited from methane sulfonate bath, and Effects of the electrodeposit composition on the growth of the intermetallic compounds.
sn-Bi alloy were electroplated on the Cu plate from an methane sulfonate bath with the additive using dc or pulse current, and aged at 80℃ to 130℃ to from Cu-Sn intermetallic compounds at deposit/substrate interface. Influences of plating parameters on the microstructure and composition of the Sn-Bi deposits, and effects of alloy composition on the formation of intermetallic compounds were investigated.
Addition of grain refiner to the bath increased the cathodic polarization and reuced the difference of the deposit potential between Sn and Bi. So, Sn-Bi alloy can be electrodeposit with fine and rounded grains in bath containing the additive. The Sn content in the deposits was increased according to a parabolic shaped curve with increasing Sn content in the bath. The preferred deposition trend of Sn was stronger than that of Bi, because the deposit potential of Sn was more positive than that of Bi bath with the additive. The Sn content in the deposits was decreased with increasing the current density and the grains of deposit became finer because of the increased cathodic overpotential. When Sn-Bi alloy was electrodeposited using pulse current at 4 A/d㎡ peak current density, the Sn contents in the deposit was increased with increasing the pulse frequency and decreasing the duty cycle. The morphology of Sn-Bi deposits bacame finer with decreasing pulse frequency and increasing duty cycle. The electrodeposits of Sn-Bi was identified a mixture of Bi and β-Sn crystals by X-ray diffraction.
When deposits of Sn-Bi were aged at 80℃, the growth of Cu-Sn intermetallic compound layer [ε(＄Cu_3Sn＄) + n' (＄Cu_6Sn_5＄)] were showed a parabolic time dependence, and the growth rate of that depended significantly on the composition of Sn-Pb electrodeposits; it was 90Sn-10Bi electrodeposit because of its high Sn contents, and slowest in 42Sn-58Bi electrodeposit. The apparent activation energy of intermetallic compound growth in 42Sn-58Bi electrodeposit was 52.48 kJ/mol, that in 70Sn-30Bi electrodeposit was 42.32 kJ/mol, and that in 90Sn-10Bi was 41.35 kJ/mol.
Effects of the alloy composition and the intermetallic compounds on the solder properties of Sn system solder alloy electrodeposited from methane sulfonate bath.
Sn-Bi alloy were electroplated on Cu plate from methane sulfonate bath and, then reflowed at 250℃. Two Cu plates electrodeposited with Sn-Bi or Sn-Pb were clamped each other and soldered at 220℃ to produce solder joints. The solder joints were pulled to failure at strain rate ＄3.3\times10^｛-4}＄ to ＄3.3\times10^｛-2}＄/sec to measure the shear strength of the solder joints. 42Sn-58Bi solder joints were aged at 80℃ during 6∼120 hours and then pulled to failure at strain rate $3.3\times10^｛-4}＄. Influences of alloy composition on the wettability and the shear stress of the solder joint, and effects of the growth of the intermetallic compounds on the shear stress of 42Sn-58Bi solder joint were investigated.
The wetting angle of Sn-Bi solder was increased with increasing Sn contents in the solder. This is due to the surface energy of Sn-Bi solder.
The increment of Sn increased the surface energy of Sn-Bi solder, and decreased the wettability
At low strain rate, The shear stress of Sn-Bi solder joint was decreased with Sn contents increment in the solder, but, at high strain rate, increased. It is due to the brittleness of Bi. The shear stress of the 80Sn-20Pb solder joint was higher than that of the 60Sn-40Pb solder joint at all strain rate. The shear of the 42Sn-58Bi solder joint was higher than that of the 80Sn-20Pb at low strain rate, but, lower at high strain rate.
The shear stress was increased with increasing againg time until 24 hours because the rough surface of the intermetallic compounds at intermetallic compound/solder interface acted as wedge. After 24 hours, the increment of thickness of the brittle intermetallic compound layer decreased the shear stress of the 42Sn-58Bi solder joint.
