Influence Of Palladium Addition On The Reliability Of Copper Wire Bonding

Miniaturization,multi-function and low-cost are the trend of microelectronic packaging,which requires advanced packaging technologies and reliable packaging materials progressively.The electrical conductivity and thermal conductivity of gold(Au)are lower than that of copper(Cu),which increases its application challenges in high density and fine pitch packaging.Cu wire with high electrical conductivity and thermal conductivity,as well as low-cost,has gradually become a new bonding wire material.Considering the problems of oxidation,corrosion and reliability of Cu wire,palladium-coated Cu(Pd-coated Cu)wire became a new bonding wire in industrial manufacturing.However,there are several problems that is not clear enough in the application of Pd-coated Cu wire.Firstly,the effect of Pd distribution in the free air ball(FAB)on the bonding reliability.Secondly,the final state of FAB determines the initial state of the first bond,so the mechanism of Pd distribution and state on the interfacial microstructure and bonding strength needs to be clarified.Thirdly,the smaller the diameter and pitch of the bonding wire,the larger the current density,temperature gradient and stress gradient within the bonding wire and the first/second bonds.The electromigration reliability of Pd-coated Cu wire should be investigated and compared with other bonding wires to improve its application.This paper focues on the influence of Palladium addition on the reliability of copper wire bonding,to clarify the involved mechanism during interfacial reaction and reliability test.The effect of firing current and firing time on Pd distribution at the FAB is studied,the mechanism of different distribution and state of Pd is clarified,the electromigration reliability of Pd-coated Cu wire,Cu wire and Au wire(all in 20?m diameter)after molding is compared,and the electromigration lifetime and failure mechanism are studied in detail.Firstly,the distribution of Pd on the surface of FAB under different EFO current and firing time was analyzed.When the ratio of FAB diameter to bonding wire diameter was fixed to 1.5,with the increase of EFO current from 40mA to 90mA,the exposed Cu regions over FAB surface increased,and the exposed Cu regions was asymmetric at the top of FAB.Through calculation,about 44.5%of the spherical FAB top surface will take part in the bonding process to form a bonding interface.When the EFO current is above 70mA,part of exposed Cu regions will contact with epoxy molding compound(EMC)directly,which will increase the corrosion probability by Cl-in EMC.Then,the movement and distribution of Pd over the surface of FAB with the increased firing time were analyzed.At a short firing time(<180 ?s at an EFO current of 80 mA),a Pd-rich layer was formed on the FAB surface.When the firing time was increased to 260 ?s,PdCu alloy could form over the entire surface of the FAB due to inter-diffusion,which acts as a protective shield layer against corrosion attack by halogen ions in molding compounds.Therefore,two existence states of Pd can be obtained in the firing stage,Pd layer and PdCu alloy on the surface of FAB.Compared to the bare Cu wire bonding,Pd addition can increase the ball shear strength by>50%,from 12 g to over 20 g.The average shear strength of Pd-alloyed Cu wire bonding(19.7 g)is marginally lower than that of Pd-coated Cu wire bonding samples(20.4-21.3 g),which were fabricated at EFO current of 50-90 mA.In order to further clarify the mechanism of Pd addition in interfacial reaction,ultrasonic welding(USW)was used to simulate the quick reaction process during wire bonding.The pure Cu,Pd-coated Cu and Pd-alloyed Cu plates were bonded on Al substrate using USW.Pd layer acted as a diffusion barrier layer at the Pd-coated Cu interface.Pd-alloyed Cu promoted the generation and growth of interfacial IMCs.At the same time,Pd in Pd-alloyed Cu also accelerated interfacial void formation compared to the pure Cu system.Cracks appeared at 2 wt%PdCu/Au interfaces during high temperature annealing.Thickness of IMCs at pure Cu/Al and 0.3 wt%PdCu/AI interfaces during annealing has a linear relationship with the square root of time,which means that the growth of IMCs is controlled by volume diffusion.For 0.3 wt%PdCu/Al system,the estimated diffusion activation energy for the growth of Cu9Al4,CuAl and CuAl2 was 46.08 kJ/mol,52.88 kJ/mol,and 65.35 kJ/mol,respectively.For pure Cu/Al system,the estimated diffusion activation energy for the growth of Cu9Al4,CuAl and CuAl2 was 60.24 kJ/mol,58.12 kJ/mol,and 82.94 kJ/mol,respectively.Meanwhile,0.3 wt%PdCu decreased the percentage of Cu9Al4 at the interface,which can reduce IMCs corrosion by halogen from EMC.Among IMCs in 0.3 wt%PdCu/Al system,Cu9Al4 was the dominant phase which has the highest percentage,while the percentage of CuAl and CuAl2 changed obviously at different temperatures.The percentage of CuA12 was higher than that of CuAl when annealing temperature was 250?,while became lower than that of CuAl when annealing temperature increased to 300?.Pd element in 0.3 wt%PdCu accelerated the void formation at 300? and the Cu diffusion was blocked y voids,then partial CuAl reacted with A1 to form CuAl2 again at the interface.Electromigration(EM)reliability and failure analysis of Pd-coated Cu wire,Cu wire and Au wire bonded on A1 substrates after molding were investigated and compared systematically.According to the resistance change as a function of EM time,there were four stages during the EM test for wire bonding,which can be tentatively named as the fluctuating resistance stage,stable resistance stage,rapidly increasing resistance stage and quick failure stage.When the applied current was 1.8 A in a wire with a diameter of 20 ?m(at a current density of 5.7 × 105A/cm2),the lifetimes of the Pd-coated Cu wire,Cu wire and Au wire bonding structures were 353 h,1032 h and 119 h,respectively.Two kinds of failure modes were observed for all bonding wires,wire fracture and first bond fracture.Almost 90%of the failure samples belonged to wire fracture mode.After the EM test,the thickness of intermetallic compounds(IMCs)at the anode interface was similar to that at the cathode interface for all bonding wires,which means that the normal polarity effect during electromigration did not occur in this study.However,the thickness of the IMCs at the Au/Al bonding interface was much thicker than that at the Pd-coated Cu/Al and Cu/Al interface after EM tests.It is concluded that the Cu wire bonding process had a slower thermal diffusion and EM flux than those of the Au wire bonding process,because the diffusion coefficient,effect charge(Z*)and electrical resistivity of Cu are smaller than that of Au.Due to a decrease in vacancy flux within the Cu wire bonding structure,the cracks were also suppressed,which greatly extends the lifetime of the Cu wire bonds.Compared with Cu wire,the presence of Pd increased the appearance of interface voids,promoted the growth of IMCs,hence reduced the lifetime of Pd-coated Cu wire.