High Precision Solder Droplet Printing Technology: Principle and Applications

Solder droplet printing technology, which is low-cost, noncontact, flexible, data-driven, and environmentally friendly, has emerged as an enabling technology for precisely placing fine solder deposits on a variety of small substrates. It is suitable for a variety of applications including direct chip attach site preparation, 3D substrates, fine line interconnect, substrate via fill, optoelectronics and many others. It enables manufacturing techniques that are impossible or unfeasible with current technology, such as localized replacement of solder on board, depositing solder in different thicknesses on the same board, or using more than one type of solder on the same board. This makes the evaluation of solder droplet printing technology essential for the microelectronics industry. In this paper, the principle of the solder droplet printing technology is described, recent experimental results are included, and potential applications of the technology in the microelectronics industry are evaluated

Electroless nickel bumping of aluminium bondpads. Part 2 - electroless nickel plating

Electroless nickel has been used for many decades to provide a hard, corrosion resistant surface finish to engineering components. In recent years its application has been extended to the electronics industry for the production of solderable surfaces on printed circuit boards, which utilize a further thin gold coating to prevent oxidation of the nickel surface. The recent interest in the use of flip-chip technology in electronics manufacture has required the development of low cost methods for solder bumping of semiconductor wafers. The electroless nickel process has been considered as a suitable candidate for the deposition of a solderable under bump metallization (UBM) layer onto the Al bondpads. However, the extension of existing electroless nickel plating processes to this new application requires greater understanding of the technique. In particular, the coating of the small isolated bondpads on the wafer surface introduces difficulties that make the use of many commercially available solutions impossible. This paper reports the results of a number of experiments carried out to investigate the electroless nickel bumping of Al bondpads and highlights the issues that need to be considered when selecting materials and techniques

Microstructural Coarsening during Thermomechanical Fatigue and Annealing of Micro Flip-Chip Solder Joints

Microstructural evolution due to thermal effects was studied in micro solder joints (55 {+-} 5 {micro}m). The composition of the Sn/Pb solder studied was found to be hypereutectic with a tin content of 65--70 wt%.This was determined by Energy Dispersive X-ray analysis and confirmed with quantitative stereology. The quantitative stereological value of the surface-to-volume ratio was used to characterize and compare the coarsening during thermal cycling from 0--160 C to the coarsening during annealing at 160 C. The initial coarsening of the annealed samples was more rapid than the cycled samples, but tapered off as time to the one-half as expected. Because the substrates to which the solder was bonded have different thermal expansion coefficients, the cycled samples experienced a mechanical strain with thermal cycling. The low-strain cycled samples had a 2.8% strain imposed on the solder and failed by 1,000 cycles, despite undergoing less coarsening than the annealed samples. The high-strain cycled samples experienced a 28% strain and failed between 25 and 250 cycles. No failures were observed in the annealed samples. Failure mechanisms and processing issues unique to small, fine pitch joints are also discussed

The production of lead-free solder spheres for the electronics industry using materials jetting

This thesis describes an investigation into the production of precision dimension lead-free solder spheres using continuous-mode jetting equipment for the electronics manufacturing industry. The "Jetting Technology" has evolved from the ink-jet printing industry and has been adapted to cope with elevated material temperatures, which allow for a much wider range of materials to be investigated. A continuous-mode jetting head was licensed from the Massachusetts Institute of Technology and re-engineered to produce uniform solder droplets between 54–1152 μm diameter (± 5 μm tolerance), at operating temperatures up to 400°C. The necessary modifications to the MIT equipment to facilitate the research have been fully documented and include: orifice redesign; combined focusing and deflection hardware; 'in-situ' stream direction adjustment; droplet detection and the development of a novel algorithm to precisely control the size of the droplets. [Continues.

Materials jetting for advanced optoelectronic interconnect: technologies and application

This report covers the work carried out on Teaching Company Scheme No. 2275 "Materials Jetting for Advanced Interconnect" between February 1998 and February 2000. The project was conducted at the Harlow laboratories of Nortel Networks with the support of the Department of Manufacturing Engineering of Loughborough University. Technical direction and supervision has been provided by Mr Paul Conway, Reader, at Loughborough University, Professor Ken Snowdon and Mr Chris Tanner of Nortel Networks. The aim of the project was to produce and deposit minute and precise volumes of a range of materials, such as metallic alloys, glasses and polymers, onto a variety of substrates commonly used in the electronics and optoelectronics fields. The technology, which is analogous to ink-jet printing, firstly had to be refined to accommodate higher processing temperatures of up to 350°C. The ultimate project deliverable was to produce a specification for jetting equipment suited towards volume manufacturing. [Continues.

Electrodeposition and characterisation of lead-free solder alloys for electronics interconnection

Conventional tin-lead solder alloys have been widely used in electronics interconnection owing to their properties such as low melting temperature, good ductility and excellent wettability on copper and other substrates. However, due to the worldwide legislation addressing the concern over the toxicity of lead, the usage of lead-containing solders has been phased out, thus stimulating substantial efforts on lead-free alternatives, amongst which eutectic Sn-Ag and Sn-Cu, and particularly Sn-Ag-Cu alloys, are promising candidates as recommended by international parties. To meet the increasing demands of advanced electronic products, high levels of integration of electronic devices are being developed and employed, which is leading to a reduction in package size, but with more and more input/output connections. Flip chip technology is therefore seen as a promising technique for chip interconnection compared with wire bonding, enabling higher density, better heat dissipation and a smaller footprint. This thesis is intended to investigate lead-free (eutectic Sn-Ag, Sn-Cu and Sn-Ag-Cu) wafer level solder bumping through electrodeposition for flip chip interconnection, as well as electroplating lead-free solderable finishes on electronic components. The existing knowledge gap in the electrochemical processes as well as the fundamental understanding of the resultant tin-based lead-free alloys electrodeposits are also addressed. For the electrodeposition of the Sn-Cu solder alloys, a methanesulphonate based electrolyte was established, from which near-eutectic Sn-Cu alloys were achieved over a relatively wide process window of current density. The effects of methanesulphonic acid, thiourea and OPPE (iso-octyl phenoxy polyethoxy ethanol) as additives were investigated respectively by cathodic potentiodynamic polarisation curves, which illustrated the resultant electrochemical changes to the electrolyte. Phase identification by X-ray diffraction showed the electrodeposits had a biphasic structure (β-Sn and Cu6Sn5). Microstructures of the Sn-Cu electrodeposits were comprehensively characterised, which revealed a compact and crystalline surface morphology under the effects of additives, with cross-sectional observations showing a uniform distribution of Cu6Sn5 particles predominantly along β-Sn grain boundaries. The electrodeposition of Sn-Ag solder alloys was explored in another pyrophosphate based system, which was further extended to the application for Sn-Ag-Cu solder alloys. Cathodic potentiodynamic polarisation demonstrated the deposition of noble metals, Ag or Ag-Cu, commenced before the deposition potential of tin was reached. The co-deposition of Sn-Ag or Sn-Ag-Cu alloy was achieved with the noble metals electrodepositing at their limiting current densities. The synergetic effects of polyethylene glycol (PEG) 600 and formaldehyde, dependent on reaching the cathodic potential required, helped to achieve a bright surface, which consisted of fine tin grains (~200 nm) and uniformly distributed Ag3Sn particles for Sn-Ag alloys and Ag3Sn and Cu6Sn5 for Sn-Ag-Cu alloys, as characterised by microstructural observations. Near-eutectic Sn-Ag and Sn-Ag-Cu alloys were realised as confirmed by compositional analysis and thermal measurements. Near-eutectic lead-free solder bumps of 25 μm in diameter and 50 μm in pitch, consisting of Sn-Ag, Sn-Cu or Sn-Ag-Cu solder alloys depending on the process and electrolyte employed, were demonstrated on wafers through the electrolytic systems developed. Lead-free solder bumps were further characterised by material analytical techniques to justify the feasibility of the processes developed for lead-free wafer level solder bumping

Properties and behaviour of Pb-free solders in flip-chip scale solder interconnections

Due to pending legislations and market pressure, lead-free solders will replace Sn–Pb solders in 2006. Among the lead-free solders being studied, eutectic Sn–Ag, Sn–Cu and Sn–Ag–Cu are promising candidates and Sn–3.8Ag–0.7Cu could be the most appropriate replacement due to its overall balance of properties. In order to garner more understanding of lead-free solders and their application in flip-chip scale packages, the properties of lead free solders, including the wettability, intermetallic compound (IMC) growth and distribution, mechanical properties, reliability and corrosion resistance, were studied and are presented in this thesis. [Continues.

Evaluation of commercial electroless nickel chemicals for a low cost wafer bumping process

10.1088/0268-1242/17/9/302Semiconductor Science and Technology179911-917SSTE