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A direct and deleterious effect on packaging reliability has been observed during elevated temperature isothermal aging for fine-pitch ball grid array (BGA) packages with Sn-1.0Ag-0.5Cu (SAC105), Sn-3.0Ag-0.5Cu (SAC305), and Sn-37Pb solder ball interconnects. Package sizes ranging from 19 mm with 0.8-mm-pitch BGAs to 5 mm with 0.4-mm-pitch μ BGAs with three different board finishes (ImSn, ImAg, and SnPb) were evaluated. The aging temperatures were 25°C, 55°C, 85°C, and 125°C, applied for a period of 6 mo. Subsequently, the specimens were thermally cycled from -40°C to 125°C with 15-min dwell times at the high temperature. Weibull analysis of failures versus cycle number show a ~ 50% reduction in package lifetimes when aged at 125°C compared to the same at room temperature, with less dramatic but measurable reductions in lifetime at 85°C and even 55°C. In contrast, the reliability performance of Sn-37Pb is much more stable over time and temperature. The degradation was observed for both SAC alloys on all tested package sizes and board finishes. For the 19-mm SAC105 case, for example, there was a 53% (32%) reduction of characteristic lifetime at 125°C (85°C) compared to room temperature aging. The trends were in the expected directions; namely, the reliability was reduced when using higher aging temperatures, smaller solder balls, and SAC105. The dominant failure mode can be associated with the growth of Cu6Sn5 intermetallic compounds during the aging, particularly on the pad side. [ABSTRACT FROM AUTHOR]

The global transition to lead-free (Pb-free) electronics has led component and equipment manufacturers to transform their tin-lead (SnPb) processes to Pb-free. At the same time, Pb-free legislation has granted exemptions for some products whose applications require high long-term reliability. However, due to a reduction in the availability of SnPb components, compatibility concerns can arise if Pb-free components have to be utilized in a SnPb assembly. This compatibility situation of attaching a Pb-free component in a SnPb assembly is generally termed "backward compatibility." This paper presents the results of microstructural analysis of mixed solder joints which are formed by attaching Pb-free solder balls (SnAgCu) of a ball-grid-array component using SnPb paste. The experiment evaluates the Pb phase coarsening in bulk solder microstructure and the study of intermetallic compounds formed at the interface between the solder and the copper pad. [ABSTRACT FROM PUBLISHER]

In solder ball grid array (BGA) technology, solder joint reliability is one of the critical issues in microelectronics manufacturing industries. In this reliability aging study, Sn3.5AgO.7Cu solder joints were subjected to accelerated temperature cycling (ATC) test in TBGA assembly. Fatigue fracture occurred, very close to the solder/intermetallic compound (IMC) interface, at the TBGA component side due to the larger coefficient of thermal expansion (CTE) mismatch compared to the PCB side. During reflow, needle-type and scallop-type morphologies of (Cu,Ni)6Sn5 IMCs were formed at the TBGA component and PCB interfaces. In the process of thermal cycling, a layer of (Ni,Cu)3Sn4 IMC grew beneath the (Cu,Ni)6Sn5 IMC due to the out diffusion of Ni from the under bump metallization (UBM). After extended thermal cycling aging, Ni-Sn-P IMC was found between the (Ni,Cu)3Sn4 IMC and the In3P layer at the printed circuit board (PCB) interface. Grain ripening and spalling of (Cu,Ni)6Sn5 IMC grains into the solder joint was also observed in the process of thermal cycling. The spalling phenomena of (Cu,Ni)6Sn5 IMCs was caused by interface structure change and cyclic shear stresses and strains incurred during temperature cycling. [ABSTRACT FROM PUBLISHER]

The effect of different reflow profiles on the reliability of lead-free (LF) Sn-3.0 Ag-0.5 Cu (wt.%) (SAC 305) ball grid array (BGA) devices assembled with a SnPb eutectic paste was investigated. The memory modules in a back-to-back configuration were reflowed on standard graphic cards finished with immersion silver (IAg) or hot air solder leveling (HASL) coatings. The reflow peak temperatures ranged from 209 degC to 227 degC, while the time above liquidus (TAL) varied from 45 to 80 s. Depending on the reflow conditions, the solder interconnects displayed varied degrees of SnPb and LF solders intermixing. It was established that in order to receive a homogeneous solder alloy, the reflow peak temperature had to be in the 218 degC-222 degC range. The reliability of solder interconnects of memory modules was assessed by subjecting the cards to 1500 cycles of accelerated thermal-cycling with a profile from 0 degC to 100 degC. It was found that the control SnPb/SnPb assemblies displayed superior reliability to that of the mixed assemblies. Regardless of the degree of homogeneity of the BGA balls, the predominant failure mode of the mixed solder joints was interfacial cracking through a Pb-rich phase near the intermetallic layer. In contrast, only partial cracks propagating diagonally through the bulk solder were present on the control boards. It was concluded that a combination of state of stress and segregation of the Pb-rich phase at the interface was responsible for the shortened thermal-mechanical fatigue life of the mixed solder interconnects [ABSTRACT FROM PUBLISHER]

Environmental and health concerns, due to the leaching of lead from landfills into ground water, have necessitated legislation that restricts the use of lead in electronics. The transition from the eutectic tin-lead composition used in electronic solders to lead-free solder is imminent. Understanding the impact of this transition on lead-free wave soldering is crucial because a large segment of printed circuit boards (PCBs) used in electronic home appliances are wave soldered. The usage of volatile organic compound (VOC)-free flux chemistry is expected to gather momentum in conjunction with lead-free wave soldering because of process requirements and environmental considerations. A thorough review of published literature indicated that there is limited information available on the application of water-soluble VOC-free flux chemistries for lead-free wave soldering. Consequently, the objectives of this research were to select a preferred VOC-free water-soluble flux chemistry, understand the process window of wave soldering using a 96.5Sn/3.0Ag/0.5Cu lead-free alloy, and study the impact of the lead-free wave soldering process on different surface finishes. Many of the earlier process recommendations were based on a solder pot temperature of 260 degC and higher. However, the packaging of through hole components may not withstand the higher pot temperature and longer contact time. Hence, assembly at a lower solder pot temperature and shorter contact time are highly desired. Consequently, experiments were conducted to verify the feasibility of reducing the solder pot temperature (250 degC) and the contact time (2 s). Three VOC-free fluxes, from three different vendors, were evaluated and the best flux chosen was used to establish the process window. It was demonstrated through this study that it is possible to wave solder, using a lead-free solder with low silver content, different surface finishes with a wide process window. The effect of lead-free process temperatures - - on the cleaning of the VOC-free water-soluble flux residues was evaluated. Residues were washed and cleaned using the existing equipment sets without any change in the process parameters. The process developed based on the initial set of experiments has been validated by the successful assembly of a number of lead-free prototype assemblies [ABSTRACT FROM PUBLISHER]

We have recently developed a thermodynamic database for micro-soldering alloys, alloy database for micro-solders (ADAMIS). ADAMIS which consists of the elements Ag, Bi, Cu, In, Pb, Sb, Sn, and Zn has been constructed by the calculation of phase diagrams (CALPHAD) method. The thermodynamic parameters for describing the Gibbs energy of the liquid and solid phases have been evaluated by optimizing the experimental data on phase boundaries and thermo-chemical properties. In this paper, the phase equilibria and the related thermodynamic properties pertaining to the Sn-Ag-X (X=Bi, In, Cu, and Zn) alloys are examined using ADAMIS. Typical examples of the isothermal and vertical section phase diagrams, liquidus surface, etc. for these promising lead-free solders are presented. In addition, ADAMIS is also applied to calculate the nonequilibrium solidification process using the Scheil model. [ABSTRACT FROM PUBLISHER]

In this paper, we report development of a new process for lead-free solder bumping with no flux residues, using stencil printing and hydrogen radicals, which can lessen both the environmental load and the manufacturing cost of solder bumping. In this process the reduction of the hydrogen radicals, instead of the flux, will eliminate the surface oxides of the stencil printed lead-free solder paste. Sn-3.0Ag-0.5Cu lead-free solder paste, which contains no residue flux was printed on an 8-in wafer. Hydrogen plasma was radiated for a minute during the reflow process, and the printed paste formed a bump. Reflow experiments without hydrogen radicals treatment were also carried out for comparison, where no successful reflow could be observed. [ABSTRACT FROM PUBLISHER]

Pb-free solder is one of the biggest issues in today's electronic packaging industry. This paper introduces a newly developed Sn/3.5Ag alloy plating process for wafer level bumping. The effects of Under Bump Metallization (UBM) on the process, interfacial reaction, and mechanical strength have been investigated. Four different types of sputtering-based UBM layers-TiW/Cu/electroplated Cu, Cr/CrCu/Cu, NiV/Cu, and TiW/NiV-were fabricated with eutectic Pb/63Sn and Sn/3.5Ag solder. The result shows that the Sn/Ag solder gains Cu or Ni from UBM's and becomes Sn/Ag/Cu or Sn/Ag/Ni during reflow process. Sn/Ag solder has higher reactivity with Cu and Ni than Pb/63Sn. The Intermetallic Compound (IMC) spalling from the interface between UBM/solder has been observed on Cr/CrCu/Cu and TiW/NiV UBM's. However, the IMC spalling phenomena did not decrease the bump shear strength with a bump size of 110 μm, whereas a size of 60 μm brought a decrease in shear value and failure mode change. [ABSTRACT FROM PUBLISHER]

The microstructure, joint strength and failure mechanisms of SnPbAg, SnAg and SnAgCu solders on Cu/Ni/Au BGA pad metallization were investigated after multiple reflows or high temperature aging. In the SnPbAg system, a two-layer structure, i.e., Ni3Sn4 and (Au, Ni)Sn4, was formed at the solder-substrate metallization interface after aging at 125, 150, and 175°C. However, such structure was not present in the two Pb-free solder systems. Only a layer of Ni3Sn4 intermetallic compound in the SnAg system and a layer of Cu-Sn-Ni-Au intermetallic compound in the SnAgCu system were found at respective interfaces, even after the two solder systems had been heat treated for 1000 h at the afore-mentioned temperatures. The formation of the (Au, Ni)Sn4 ternary compound in the SnPbAg system was due to re-settlement of Au at the interface which led to brittle failure in this system during ball shear testing. In contrast, SnAg and SnAgCu systems failed exclusively inside the solder ball during shear testing after aging at 150°C for up to 1000 h. The two Pb-free solder systems showed good resistance to thermal aging with the solder ball shear strength being maintained at 1.60 to 1.70 kgf. The SnPbAg system degraded in mechanical performance with aging time and had strength as low as 1.20 kgf after aging at 150°C for 1000 h. The growth rates of intermetallic compound layers at 125, 150, and 175°C aging temperatures and the activation energy for the formation of different intermetallic compound layers were also determined in this investigation. [ABSTRACT FROM PUBLISHER]

For a successful transition to Pb-free manufacturing in electronics assembly, it is critical to understand the behavior of Pb-free solders (in bulk and paste form) and their interaction with the Pb-free printed wiring board (PWB) finishes. This paper presents the results obtained from solder paste spread tests and wetting balance experiments with several Pb-free solder alloys and Pb-free PWB finishes. The solder alloys studied were Sn3.4Ag4.8Bi, Sn4.0Ag0.5Cu, Sn3.5Ag and Sn0.7Cu. Eutectic Sn37Pb was used as a reference. The PWB surface finishes were Sn, NiAu, Ag and OSP. Wetting balance experiments were conducted in air while the spread tests were performed in air and nitrogen to understand the effect of reflow atmosphere on the spreading. Surface analysis techniques such as Nomarski phase contrast microscopy, Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to characterize the as-received PWB finishes. Sequential electrochemical reduction analysis (SERA) was also performed on the as-received PWB test coupons and on the Sn test coupons after multiple reflow cycles. The effect of multiple reflow cycles on the wetting performance, spreading and the surface composition of the PWB finishes was studied. [ABSTRACT FROM PUBLISHER]

First, the light intensity (luminance) distributions reflected from quad flat package (QFP) gull wing lead solder fillets were calculated by a simple geometrical model, using a modified diffusion equation. The dependence of the microscope angle θ (= incident angle of light) on the highlight distribution from specular surfaces was obtained. Luminance distributions from top to toe fillets were experimentally observed, using Sn-Pb eutectic solder and two kinds of Pb-free solder (Sn-Ag-Cu and Sn-Zn-Bi systems) and the characteristics were compared with the simulation results. As the Sn-Pb solder fillet has a specular (or smooth) surface, it is not so difficult to identify the profiles of Sn-Pb fillets by changing θ. On the other hand, the luminance distributions for Pb-free solder were strongly affected by the surface roughness. However, it is not impossible to extract the characteristics of the fillet profile if wetting properties of Pb-free solder are taken into consideration. It was found for Pb-free solder that the fillet formation characteristics extraction becomes easy if elements of Bi and In are doped because the wetting properties were improved. In addition, some problems with characteristics extraction of fillet formation for Pb free solder are discussed from the viewpoint of an external feature inspection system [ABSTRACT FROM PUBLISHER]

This paper examines the assembly process for flip chip die with SnAgCu solder bumps and the results of liquid-to-liquid thermal shock testing. The SnAgCu alloy required a thicker dip layer of flux to achieve good wetting compared to the SnPb eutectic alloy. A liquid spray flux yielded more consistent solder wetting with the SnAgCu alloy. With both fluxes, a nitrogen reflow atmosphere was necessary with the SnAgCu alloy. A peak reflow temperature of 246°C was used for the assembly of the SnAgCu thermal shock test vehicles. A lower peak temperature of 235°C did not yield sufficient solder wetting. Liquid-to-liquid thermal shock testing was performed from -40°C to +125°C. The SnPb alloy performed slightly better than the SnAgCu and the dip flux was better that the spray flux. The degree of delamination with the SnAgCu alloy was significantly higher than with the SnPb alloy. Cracks in the underfill between adjacent solder balls were observed. The SnPb alloy extruded into these cracks more readily than the SnAgCu and created electrical shorts [ABSTRACT FROM PUBLISHER]

There is tremendous interest at present with Pb-free solder assembly in the surface mount assembly industry in response to recent Japanese and European initiatives and proposed governmental restrictions regarding Pb usage and disposal. Many different solder alloys have been proposed as potential Pb-free solder replacements and the most promising of these fall into the general alloy families of tin-silver (Sn-Ag), tin-silver-copper (Sn-Ag-Cu) and tin-silver-bismuth (Sn-Ag-Bi). Published melting point data on some of these alloys indicates that they should be capable of reduced reflow temperatures relative to the commonly available Sn-3.5Ag alloy, which melts at 221°C. Differential scanning calorimetry (DSC) and reflow visualization was used to characterize the melting and wetting of the Pb-free alloys and generate the practical reflow temperature requirements. This was compared to the DSC data to gain insight on the meaning of the DSC melting data for surface mount applications. The results show that, in general, the wetting performance of the Sn-Ag-Bi alloys are more similar to Sn-Ag and Sn-Ag-Cu than would be predicted by the major onset melting temperature data as measured by the DSC [ABSTRACT FROM PUBLISHER]