Mamoru Mita

Drop-on-Demand Laser Sintering With Silver Nanoparticles for Electronics Packaging

This paper proposes a “dry” laser-sintering method and discusses characteristics of a laser-sintered silver thin film on a polyimide or a copper substrate. This novel technology consists of the following processes: first, ink-jet printing of metal nanoparticles with dispersants and solvents for minute patterning; second, short preheating to remove organic substances in the ink; and finally, millisecond-order laser-beam irradiation under atmospheric conditions with the flow of argon gas for metallization. Regarding the wiring, visible lasers with high absorption on the ink develop rapid metallization and activate solvent evaporation, resulting in a rough surface with large pores. Interface adhesion is increased by the anchoring effect in the course of laser irradiation. In contrast, near-infrared lasers with low absorption heat the ink from the polyimide interface, yielding a dense, low-specific-resistance structure. Regarding pad formation on the copper leadframe without any surface pre-treatments, interdiffusion takes place at the Ag/Cu interface and increases adhesivity. The structural quality of the laser-sintered silver pad is almost the same as that of an electroplated one, so that no difference in good wire-bondability is obtained when the near-infrared continuous-wave laser is irradiated for a short time of a millisecond order per lead.

Laser sintering of Ag nanopaste film and its application to bond-pad formation

A novel coating technology by means of sintering the silver NanoPastereg by a laser beam has been proposed. The novelty lies in the use of a conventional NdYAG laser to metalize nanoparticles as an alternative to furnace sintering. Silver nanoparticles with 5 nm in average diameter dispersed in the the silver NanoPaste are successfully sintered on the Cu substrate by the proposed method. Laser sintering proceeds from the paste surface into the substrate, and the thickness of a peeling-free film is around 0.2 mum. Multi-step sintering by the repletion of spin coating and laser metallization enables us to make a thicker Ag pad of around 1 mum. No peeling of the laser-sintered Ag film from the substrate was observed. Its adhesive strength is higher than that of a furnace-sintered sample. The Ag film thus fabricated can be used as pads for wire bonding, being an alternative to electroplating.

Packaging of electronic modules through completely dry process

In order to establish technology of packaging electronic modules, we investigated conditions for laser sintering of Ag nanoparticles, and evaluated characteristics of the sintered film. First, we plotted minute patterns on a copper substrate by ink-jet printing, and then employed an NdYAG laser to metalize the nanopaste in a short time. The Ag nanoparticles (5 nm in average diameter) dispersed in organic solvents were solidified to form coarse agglomerates of about 0.05-0.5 mum with a pulsed laser, or about 0.05 mum by CW mode. We carried out a bend-peel test to find that no separation occurred at the interface between the sintered Ag film and the substrate. Adhesive strength of the laser-sintered pattern on the Cu substrate is higher or equal to than that obtained by furnace sintering. An SIM observation of FIBed cross-sections revealed that the laser-sintered film is as thin as less than 0.5 mum, and has a porous structure. As for wiring a polyimide substrate, the use of water-repellant is indispensable for ink-jet printing. Three-step laser sintering enables us to make Ag wires on the polyimide film at a low laser power, which leads to less thermal damage to the substrate.

Influence of wavelength on laser sintering characteristics of Ag nanoparticles

The purpose of this study is to develop a dry process for minute wiring or interconnections without any wet process such as chemical etching and electroplating. The process is based on the use of metal nanoparticles and their metallization by laser sintering, which will bring a desktop manufacturing of electronic circuits in view. The present paper investigates the influence of wavelength on laser sintering characteristics of Ag nanoparticles, in which a CW Nd:YAG laser (1064 nm in wavelength), a CW laser diode (980 nm), a CW green laser (532 nm) and a CW Ar+ laser (488 nm) are employed. Sintering mechanism as well as adhesion to a polyimide substrate is extensively investigated by FIB observation and tensile-shear testing. As a result, the lasers with visible wavelength are preferable from the viewpoint of laser output and adhesion strength because of a high absorbance of light in the NanoPaste® containing Ag nanoparticles, a dispersant and solvents. On the other hand, the near-infrared lasers yield a denser sintered structure, which leads to lower specific resistance, although it is a couple of times high compared to furnace curing.

Laser Sintering of Silver Nanoparticles for Electronic Use

The silver NanoPaste® having silver nanoparticles with 5 nm in average diameter, coated either on a polyimide substrate or a copper one, is successfully sintered with CW lasers. A rapid metallization from the paste surface with the visible laser makes the evaporation of solvent and dispersant difficult, resulting in an insufficient sintering with large pores. In contrast, the near-infrared laser with a little absorption in the paste heats the substrate first, and then develops metallization up to the paste surface, so that an easy evaporation makes the structure denser. No peeling was observed at the interface of the laser-sintered Ag film and the substrates. Its adhesive strength is nearly equal to that of the furnace-sintered sample.

On-Demand Infrared Laser Sintering of Gold Nanoparticle Paste for Electrical Contacts

This paper discusses the formation of a conductive film from noble metal nanoparticles as an alternative to conventional electroplating for electrical components, such as connectors, switches, and memory cards. The proposed method consists in dispensing with nanoparticle paste followed by laser sintering. The aims are fourfold: 1) to establish sintering technology for gold nanoparticles placed on a nickel-electroplated phosphor-bronze substrate; 2) to characterize the laser-sintered film; 3) to discuss the laser sintering mechanism; and 4) to examine applicability to industry. The major results obtained are as follows: the laser sintering formed a gold film with a diameter of 0.3-0.8 mm and a thickness of 0.3-0.5 μm on the nickel-electroplated phosphor-bronze substrate; a laser with a wavelength of 915 nm enabled instantaneous sintering within 1 s in air; the laser-sintered gold nanoparticle film had such a high adhesion to the substrate that no separation occurred after 90° -0.5R bend-peel tests; the high adhesion was attributed to interdiffusion of gold and nickel in the course of sintering; optical properties of the gold nanoparticle paste depend on preheat conditions. A relatively high-preheat temperature around 523 K for 60 s produced a paste surface with a suitable absorbance of the infrared laser; a primary sintering of the preheated gold nanoparticles with a small amount of solvents followed by an auxiliary sintering from the substrate side made possible an efficient sintering of the nanoparticles as well as high adhesion to the substrate with a high thermal conductivity; and the film possesses such a good electrical property as that of the electroplated one in reciprocating abrasion tests.

Advanced TAB/BGA multi-chip stacked module for high density LSI packages

A newly advanced design for a TAB/BGA multi-chip stacked module has been developed for high density LSI packages. The configuration of this module is designed to reduce package body size and to lighten the weight. Electrical and thermal performance of this module was carefully considered and estimated by simulation or experiments. This module is called a COCB module which means the Chip On Chip Ball Grid Array module. Two chips are mounted on both top and bottom sides of a small substrate by TAB technology using a new Au-Sn (Au10-40%-Sn) eutectic micro-connection method. Two chips are electrically connected by routing wires and through holes of an interposed substrate respectively. This module is useful when packages are mounted on a board with solder paste, directly to its surface, with some discrete packages on the assembly line. The authors propose the concept of the micro bare chip module.<<ETX>>

Temperature soak reliability of laser-sintered Ag pads for wire bonding

A small amount of Ag nanoparticles (5 nm in average diameter) was supplied to the tips of Cu lead-pins by ink-jet printing, and then metallized by laser irradiation to form circular wire-bonding pads between 2 and 3 µm thick and φ150 µm. A continuous-wave Nd:YAG laser (1064 nm in wavelength and 150 W in maximum power output) was employed in an argon atmosphere. A φ25 µm gold wire was bonded between the adjacent pads using an ultrasonic wire-bonder. The wire-bonded parts were placed in an atmospheric electric furnace for temperature soak tests in dry conditions at 150°C for up to 1000 h. Metallographic changes in the junctions were investigated by SEM, FIB, and EDX, while mechanical strength was examined by pull tests. No metallographic changes were observed in the 500 h specimen, whereas a slight solid-phase diffusion took place at both Cu/Ag and Ag/Au interfaces in the 1000 h specimen. The Ag pad formed on the Cu lead, above 2 µm in thickness, also prevented the degradation of pull strength; an average of 8.8 cN was maintained at both soak times. No separation from the first (ball) and second (wedge) bonds occurred, but the wire fractured close to the hook of the tester.

New design for a lead frame used for high pin counts and high-power LSI package

To meet the requirement of high speed driving LSI package with powerful output, combining flexible printed circuits (FPC) with the lead frame has been designed, and has been manufactured using TAB tape carriers by chemical etching. The back side plain metal laminated with high modulus adhesive allows for high speed wire bonding on it and used as electrical grounding and a heat spreader. The Au/Sn eutectic microsoldering method by heat press is applied for combining FPC and lead frame. The combining lead frame has some excellent characteristics. Electrical impedance of the lead can be easily controlled by the rearrangement of lead width, gaps, and dielectric insulation etc. The use of thin copper foil results in increasing pin count and narrow pitch lead frame. The heat spreading effect by plain metal with apparent thermal resistance is excellent. Insulation level of the dielectric is kept stable for a long time because of the selection of materials which has resistivity to electro-migration.

Laser-based conductive film forming with gold nanoparticles for electrical contacts

The present study discusses the formation of a conductive film from noble metal nanoparticles as an alternative to conventional electroplating for electrical components, such as connectors, switches, and memory cards. The proposed method consists of inkjet printing with nanoparticle paste followed by laser sintering. The aims are fourfold: to establish sintering technology for gold nanoparticles placed on a nickel-electroplated-phosphor bronze substrate, to characterize the laser-sintered film, to discuss the laser sintering mechanism, and to examine applicability to industry. The major results obtained are as follows: the laser sintering formed a gold film with a diameter of 0.3-0.8 mm and a thickness of 0.3-0.5 μm on the nickel-electroplated phosphor-bronze substrate; a laser with a wavelength of 915 nm enabled instantaneous sintering within one second in an atmosphere; the laser-sintered gold nanoparticle film had such a high adhesion to the substrate that no separation occurred after 90°-0.5R bend-peel tests; the high adhesion was attributed to interdiffusion of gold and nickel in the course of sintering; optical properties of the gold nanoparticle paste depend on preheat conditions. A relatively high-preheat temperature around 523 K produced a paste surface with a suitable absorbance of the infrared laser; and a primary sintering of the preheated gold nanoparticles with a small amount of solvents, followed by an auxiliary sintering from the substrate side made possible an efficient sintering of the nanoparticles as well as high adhesion to the substrate with a high thermal conductivity.