Keith Sweatman

The influence of solder composition on the impact strength of lead-free solder ball grid array joints

This study aims to investigate the shear and tensile impact strength of solder ball attachments. Tests were conducted on Ni-doped and non-Ni-doped Sn–0.7wt.% Cu, Sn–37wt.% Pb and Sn–3.0wt.% Ag–0.7wt.% Cu solder ball grid arrays (BGAs) placed on Cu substrates, which were as-reflowed and aged, over a wide range of displacement rates from 10 to 4000 mm/s in shear and from 1 to 400 mm/s in tensile tests. Ni additions to the Sn–0.7wt.% Cu solders has slowed the growth of the interface intermetallic compounds (IMCs) and made the IMC layer morphology smooth. As-reflowed Ni-doped Sn–0.7wt.% Cu BGA joints show superior properties at high speed shear and tensile impacts compared to the non-Ni-doped Sn–0.7wt.% Cu and Sn–3.0wt.% Ag–0.7wt.% Cu BGAs. Sn–3.0wt.% Ag–0.7wt.% Cu BGAs exhibit the least resistance in both shear and tensile tests among the four compositions of solders, which may result from the cracks in the IMC layers introduced during the reflow processes.

Effects of element addition on the β→α transformation in tin

The influence of 1 wt.% Pb, Cu, Ge and Si on the transformation kinetics of the β-Sn→α-Sn transformation in high-purity powder mixtures was examined using synchrotron powder X-ray diffraction. Pb and Cu were found to be effective in suppressing α–tin growth. Ge and Si inhibited the β→α transformation. The kinetics of the transformation are largely determined by the nucleation of α-Sn.

The effects of precipitation strengthening and solid solution strengthening on strain rate sensitivity of lead-free solders: Review

Electronic devices are increasingly portable, and often susceptible to damage during handling, transportation, and service. Because of the great variety of stresses to which solder joints are subjected, the mechanical properties of the solder need to be evaluated under a wide range of conditions. The strain rate sensitivity is an essential parameter in understanding the mechanical behaviour of lead-free soldered joints in microelectronics. This literature review is focused on the strain rate sensitivities of lead-free solders and how they are influenced by strengthening mechanisms, including precipitation and solid solution strengthening. Most of the tensile tests and microstructure analysis reviewed showed the dependence of strength on strain rate. However, a clear relationship between solid solution hardening or precipitation hardening and strain rate sensitivity was not revealed in the existing literature.

The beneficial effect of Zn additions on the microstructure of SnCu and SnCuNi solder joints to Cu substrates

The commercial success as Pb-free solders of alloys based on a trace addition of Ni to Sn-0.7Cu and the academic recognition of the metallurgical effects that deliver the excellent performance in soldering processes and in service, means that Sn-Cu-Ni is now well established as one of the main alloy systems on which the electronics industry depends for the assembly of reliable circuitry. One important effect of the Ni addition is the stabilization of the hexagonal form of the Cu6Sn5 phase that forms at the interface between the solder and the Cu substrate. However, with the limited amount of Ni available in a solder joint, growth of the intermetallic layer by reaction with a copper substrate can result in the Ni level in the Cu6Sn5 falling below that required for stabilization. Increasing the Ni in the solder alloy beyond the level required for optimization of properties such as fluidity that are important in soldering processes can result in the appearance of undesirable high melting point Sn-Ni phases, It would therefore be useful if a second stabilizing element could be introduced to the system to support the effect of the Ni. Ni works as a stabilizer by partially substituting for Cu in the Cu6Sn5 so potential additives were considered on the basis of reports of their partially replacing either Cu or Ni in that crystal structure. Zn was selected for this study because it can partially substitute for Sn in the Cu6Sn5 crystal, potentially complementing the role of Ni. The effect of a Zn addition was studied in the basic Sn-0.7Cu and when there was also an addition of Ni. By itself a Zn addition did not promote a eutectic microstructure in the way that a Ni addition does so powerfully but alone or with Ni it substantially reduced undercooling and in the alloy with both Ni and Zn there was an overall refinement in the near completely eutectic microstructure. In combination with the Ni the Zn significantly refined the grain size of the Cu6Sn5 that formed at the solder/Cu interface. By itself the Zn did not suppress the formation of Cu3Sn at the interface between the Cu and the Cu6Sn5 in the way that Ni does but it does not interfere with that beneficial effect of the Ni. Diffraction studies with high energy synchrotron-generated X-rays confirm that as well as stabilizing the hexagonal form of the Cu6Sn5, Zn reduces the anisotropy of this phase and increases the randomness of the crystal orientation in the intermetallic layer. These effects should result in a stronger intermetallic layer that is less likely to degrade during thermal cycling. The combination of all of these effects mean that an addition of Zn to the widely used Sn-Cu-Ni lead free solders should result in an overall increase in reliability of the solder joint, particularly in harsh environments.

The impact of process sequence on the quality of pressurized sintered silver joints

Sintered silver is being increasingly recognized as an ideal method of joining in situations where a relatively low temperature process is required to form a bond that can survive service at elevated temperatures, e.g. in the mounting of power semiconductor dies. In the optimization of this process much consideration is given to the parameters of the sintering stage but the integrity and reliability of the bond achieved during sintering can also be affected by the preheat parameters. Since the goal is minimum organic residues in the finished bond it is essential that excess solvent added to create the rheology required for the application method, e.g. printing, dispensing, be removed before sintering begins. However, the solvent also plays a role in holding the particles together to maximize interparticle contact, imparting tackiness to hold the die in place during handling and protecting the particles from oxidation. To achieve the best result the optimum level of residual solvent has to be achieved during the preheat stage to provide the best preparation for the sintering stage of the bonding process. An important consideration is whether the preheat is conducted before or after the die is placed since that affects the ease with which the solvent can evaporate. In the experiments reported in this paper the effect of temperature and time on the final bond strength and microstructure was determined for preheat conducted before and after die placement. The conclusion was that the best bond strength and a wider process window could be achieved if the silver paste is preheated before die placement.

Study of Pb-free HASL PCB surface appearance impact on 2nd level interconnect solderability

Different surface appearances were detected on printed circuit board (PCB) pads with a lead-free (Pb-free) Hot Air Solder Leveled (HASL) surface finish although all were processed in the same solder machine at the same time. The appearance of these HASL finished pads could be categorized into two main groups: shiny surface and dull surface. A useful feature of the HASL finish that distinguishes it from other PCB finishes is that its appearance provides a good indication of its solderability. Surface appearance is, therefore, a primary quality criterion with a smooth shiny finish the target. Experiments were carried out to identify the root causes of this difference in solder surface appearance and its impact on the solderability of these PCB pads in downstream processes such as surface mount technology (SMT) and wave soldering. Counter measures to achieve uniform shininess across the PCB panel are proposed.

Ga-Based Alloys in Microelectronic Interconnects: A Review

Gallium (Ga) and some of its alloys have a range of properties that make them an attractive option for microelectronic interconnects, including low melting point, non-toxicity, and the ability to wet without fluxing most materials—including oxides—found in microelectronics. Some of these properties result from their ability to form stable high melting temperature solid solutions and intermetallic compounds with other metals, such as copper, nickel, and aluminium. Ga and Ga-based alloys have already received significant attention in the scientific literature given their potential for use in the liquid state. Their potential for enabling the miniaturisation and deformability of microelectronic devices has also been demonstrated. The low process temperatures, made possible by their low melting points, produce significant energy savings. However, there are still some issues that need to be addressed before their potential can be fully realised. Characterising Ga and Ga-based alloys, and their reactions with materials commonly used in the microelectronic industry, are thus a priority for the electronics industry. This review provides a summary of research related to the applications and characterisation of Ga-based alloys. If the potential of Ga-based alloys for low temperature bonding in microelectronics manufacturing is to be realised, more work needs to be done on their interactions with the wide range of substrate materials now being used in electronic circuitry.

Role of Bi in microstructure formation of Sn-Cu-Ni based BGAs on Cu metallizations

There is ongoing research seeking solders with improved performance under harsh environmental conditions, elevated operation temperatures, and higher mechanical loads while, at the same time, meeting new emerging requirements for the continuous miniaturization of electronics. Bi has been identified as one of the advantageous alloying elements that significantly improve solder performance. The present investigation explores the influence of Bi on microstructure formation in Sn-Cu-Ni-Bi/Cu joints containing 0–14 wt% Bi. It is shown that Bi additions have no catalytic effect on βSn nucleation, however, they alter βSn growth textures in Sn-Cu-Ni-Bi/Cu joints causing a transition from a columnar grain growth to a virtually single grain structures as the Bi content increases above 8wt%. Bi additions are demonstrated to reduce the thickness of the Cu6Sn5 IMC layer and to cause formation of non-equilibrium grain boundary βSn + (Cu, Ni)6Sn5 + (Bi) eutectic in joints with Bi contents ≥2wt% Bi.

Influence of Bi additions on the distinct βSn grain structure of Sn-0.7Cu-0.05Ni-xBi (x = 0–4wt%)

Effects of Bi additions to Ag-containing lead-free solders have been the focus of a considerable amount of past investigation. However, the influence of Bi on Sn-Cu-Ni solders has not been studied extensively. In the present study, we explore the influence of Bi on microstructure formation of Sn-0.7Cu-0.05Ni/Cu solder joints both in the bulk and at the interface. It is shown that (i) Sn-0.7Cu-0.05Ni solidifies to produce a markedly different grain structure to Ag-containing lead-free alloys, with 5-8 independent βSn grains in each joint; (ii) Bi additions to Sn-0.7Cu-0.05Ni maintain this distinct βSn grain structure and had no discernible effect on the (Cu,Ni)6Sn5 interfacial intermetallic layers or primary intermetallic crystals.

Effect of intermetallic stabilization on the impact resistance of joints to BGA packages

To the extent that it provides a potential low energy route for crack propagation the intermetallic compound that forms at the interface between a tin-based solder and the substrate can play a critical role in determining the likelihood of joint failure in impact loading such as can occur when portable devices are accidentally dropped. While it has been clearly documented in the standard equilibrium phase diagrams the implication of the allotropic change in the Cu6Sn5 intermetallic from a close-packed hexagonal to monoclinic crystal structure at 186°C with an accompanying 2.15% increase in volume has been found to have implications for the mechanical integrity of the interfacial intermetallic that were not previously recognized. In this paper the authors report the effect of a trace addition of Ni in permanently stabilizing the high-temperature form of the Cu6Sn5 to room temperature on the response to high speed shear of Sn-0.7Cu spheres reflowed to a copper substrate. By reducing the incidence of incipient cracking in the intermetallic layer the stress required to initiate cracking and the energy required for crack propagation are significantly increased with consequent benefits for the impact resistance of the solder joint.