Masazumi Amagai

Mechanical characterization of Sn–Ag-based lead-free solders

Abstract Recently, preventing environmental pollutions, lead-free (Pb-free) solders are about to replace tin–lead (Sn–Pb) eutectic solders. However, the mechanical properties of Pb-free solders have not been clarified. Hence, the following study was conducted; first, a rate-dependent plasticity was characterized to represent the inelastic deformation behavior for Sn–Ag-based lead-free solders. The material parameters in a constitutive model were determined in a direct method combining both rate-dependent and rate-independent plastic strains. The constitutive model unifies both rate-dependent creep behavior and rate-independent plastic behavior occurring concurrently at the same time in the solders. Secondly, the strength of solders with a variety of plating materials was studied. Intermetallic compounds (IMC) between solder and electrical pads are formed during reflow process and gradually grow in service. By using the Cu-plates on which Cu or Ni or Ni/Au plating was deposited, the specimens of solder joints were fabricated with Sn–Ag-based lead-free solders. After aging the specimens in an isothermal chamber, tensile tests were performed. From scanning electron microscope (SEM) microscope observation and EDX microprobe analysis, the growth and components of the IMC layer were also examined. Based on the experimental tests, the relations between solder joint strength and the aging period were discussed. Furthermore, the validation of fracture strength of solder joints resulting from the tensile tests was verified with package-mounted board level reliability tests.

A study of nanoparticles in Sn-Ag based lead free solders

Tin–lead (Sn-Pb) solder alloy has been widely used as an interconnection material in electronic packaging due to its low melting temperatures and good wetting behavior on several substrate platings such as Cu, Ag, Pd, and Au. Recently, because of environmental and health concerns, a variety of new lead-free solders have been developed. Lead-free solders lack the toxicity problems associated with lead-containing solders. However, unlike lead solders, the recently employed lead-free solders do not have a long history and manufacturing process, and also board level reliability has not been established well. Especially, drop test performance is a serious concern for mobile products such as cellular phones, cameras, video, and so on. Sn–Ag–Cu alloys are leading candidates for lead-free solders.

High drop test reliability: lead-free solders

Recently, preventing environmental pollution, lead-free (Pb-free) solders are about to replace tin-lead (Sn-Pb) eutectic solders. Sn-Ag-Cu alloys are leading candidates for lead free solders. Sn-Ag-Cu-P was developed last year, but Sn-Ag-Cu-P alloys were not satisfactory to meet severe customer requirements like thermal aging process followed by drop tests. To improve drop test performance after the thermal aging process, a combination of indium and nickel was investigated. After the effect of indium and nickel on Kirkendall voids and Cu/sub 3/Sn growth thickness was studied, the weight ratio of indium to nickel was optimized. Based on the results of board level reliability tests, a new lead free solder has been developed. This paper presents the optimum nickel and indium included in Sn-Ag-Cu based solder alloys.

Characterization of chip scale packaging materials

Abstract The mechanical stability of Chip Scale Packages (CSP) used in surface mount technology is of primary concern. The dominant issues are package warpage and solder fatigue in solder joints under cyclic loads. To address these issues, molding compound and die attach film were characterized with finite element method which employed a viscoelastic and viscoplastic constitutive model. The model was verified with experiments on package warpage, PCB warpage and solder joint reliability. After the correlation was observed, the effect of molding compound and die attach film on package warpage and solder joint reliability was investigated. It was found that package warpage tremendously affected solder joint reliability. Furthermore, a die attach film was developed based on results of the modeling. CSP with the developed die attach film are robust and capable of withstanding the thermal stresses, humidity and high temperatures encountered in typical package assembly and die attach processes. Also, a lead free solder is discussed based on the results of creep testing. This paper presents the viscoelastic and viscoplastic constitutive model and its verification, the optimum material properties, the experimental and simulated reliability and performance results of the u∗BGA packages, and the lead free solder creep.

High solder joint reliability with lead free solders

Recently. preventing environmental pollutions. lead-free (Pb-free) soldcrs are about to replace tin-lead (Sn-Pbj eutectic solders. Sn-Ag-Cu alloys arc lcading candidatcs for lead free solders. However. Sn-Ag-Cu alloys were not satisfied to meet sever custoiner requirements. At first, optimum silver wt%

Chip scale package (CSP) solder joint reliability and modeling

A viscoplastic constitutive model was used to analyze the thermally induced plastic and creep deformation and low cycle fatigue behaviour of the solder joints in chip scale packages (CSP) mounted on PCBs. The time-dependent and time-independent viscoplastic strain rate and plastic hardening work factors of solder material were used in 2D plane strain finite element models. The viscoplastic strain rate data was fitted to the viscoplastic flow equation. The plastic hardening factors were considered in the evolution equation. Finite element models, incorporating the viscoplastic flow and evolution equations, were verified by temperature cycling tests on assembled CSPs. The effect of the cyclic frequency, dwell time, and temperature ramp rate on the response of the viscoplastic deformation was studied for a tapeless lead-on-chip (LOC) CSP and a flexible substrate CSP. The ramp rate significantly affects the equivalent stress range in solder joints, while a dwell time in excess of 10 minutes per half cycle does not result in increased strain range. The failure data from the experiments was fitted to the Weibull failure distribution and the Weibull parameters were extracted. After satisfactory correlation between experiment and model was observed, the effect of material properties and package design variables on the fatigue life of solder joints in CSPs was investigated, and the primary factors affecting solder fatigue life were subsequently presented. Furthermore, a simplified model was proposed to predict the solder fatigue life in CSPs.

A study of nanoparticles in SnAg-based lead free solders for intermetallic compounds and drop test performance

Co, Ni, Pt, Al, P, Cu, Zn, Ge, Ag, In, Sb or Au including in Sn-Ag based lead free solders were evaluated to study if these nanoparticles can reduce the growth of intermetallic compounds after 4 time reflow processes and thermal aging. Also, these nanoparticles were studied if they can reduce the frequency of occurrence of intermetallic compound fractures in high impact pull tests. In addition to intermetallic compound analyses, these nanoparticle effects on solder ball hardness were studied if nanoparticles affects solder hardness and displacement in drop tests. Finally, these nanoparticle effects on drop test performance were studied. This study found that Co, Ni and Pt were very effective for the growth of intermetallic compounds and drop test performance compared to Cu, Ag, Au, Zn, Al, In, P, Ge and Sb

Mechanical reliability in electronic packaging

Abstract The dramatic increase in the number of devices and functionality of the latest ultra large scale integration designs have resulted in increasing chip size. Concurrently, to achieve higher circuit board component densities, package dimensions have been shrinking. These two competing trends are leading to ever more rigorous requirements on the mechanical characteristics of the packaging technology. The dominant issue in component level reliability is delamination and cracks initiated at the interface between dissimilar materials. In board level reliability, solder joint reliability is a primary issue. This paper describes the methodology of prediction and the explanation for interfacial delamination, cracks at the top of the interfaces and the edge of corner, and also solder joint reliability. This paper furthermore presents the role of the chip backside contamination affecting interfacial delamination, the surface characterizations and an explanation of the interface chemistry, and the strength of solders with a variety of plating materials for Sn–Ag-based lead free solders.

The effect of polyimide surface morphology and chemistry on package cracking induced by interfacial delamination

The increasingly severe demands of concurrently increasing die size while reducing package size have made the mechanical stability of novel surface mount technologies a primary concern. The dominant issue is device failure due to package cracking caused by interfacial delamination between the polyimide-coated chip surface and the epoxy molding resin. To investigate the effect of polyimide surface morphology and chemistry on molding resin adhesion, devices were fabricated with different types of epoxy molding resins, polyimides, reactive ion etch (RIE) treatments, cure temperatures, cure atmospheres, and cleaning chemicals. The samples were characterized with scanning acoustic tomography (SAT), X-ray photo emission spectroscopy (XPS), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and various package-level reliability tests. The results of the characterizations and an explanation of the primary factors affecting interfacial adhesion are presented in this paper.<<ETX>>

A study of package warpage for package on package (PoP)

Package warpage is a primary concern in a package-on-package. To enhance the accuracy of modeling prediction, viscoelastic parameters, the change of material properties after injection mold cure (IMC) and post mold cure (PMC) temperature and its time, and cure shrinkage were studied with a dynamic modulus analysis (DMA) and a thermal mechanical analysis (TMA) for a mold compound. A nano-indentation tool was used to characterize a viscoelasticity of underfill material. Material properties obtained from the TMA, DMA and nano-indentation tools were introduced to finite-element-based models. The validation of models was verified with a shadow moiré for package warpage.