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Sub-micron gold materials offer bonding versatility

Some of the fastest growing segments of the semicon-ductor industry reside in power electronics, specifi-cally in industrial and mili-tary applications such as

telecommunications, microwave, RF and energy management.

These applications deal with high currents of electricity and generate large amounts of heat.

A crucial materials choiceA crucial material choice for manufactur-ers in these segments is the semiconductor package’s die-attach material.

Rather than traditional power semiconduc-tor materials such as AuSn, high Pb and other low thermal epoxies, many are opt-ing for sub-micron gold as the material of

Michael ThuressonTanaka KikinzokuInternational (America)ml.tanaka.co.jp

choice, which brings the benefits of low temperature bonding, minimal outgas and zero material bleed-out.

Commercial applicationsGold’s high-thermal conductance, com-bined with a low resistivity, makes it ide-al for high-power commercial applications with demanding reliability requirements.

At the same time, demand is also strong for this material in high-powered LED appli-cations, such as commercial and industrial lighting.

One of the unique technical challeng-es for manufacturers of high-power LEDs is that the power of LED com-ponents steadily declines as the tem-perature inside the package heats up.

Thermal dissipation is vitalThis has a dramatic effect on the life-time of an LED, and as a result, ther-mal dissipation is vital.

See next pageLED module using conventional wire bonding

LED flipchip module using conventional gold-tin sol-der

sub-micron gold (from 12)

There may be a drastic effect on perfor-mance if recommended LED operating tem-peratures are exceeded. The result may be reduced light outputl for a given voltage and current.

Correct temperature vitalAccordingly, LED chip manufacturers care-fully choose their recommended operating temperatures.

The traditional method of wire bonding to interconnect LEDs makes it difficult for heat to escape outside the component.

A Thermal challengeThe crowded structure, with the LED light-emitting surface sitting at the top and the bonding wire separating it from the sub-strate, imposes a thermal challenge. There is limited space for the heat to escape.

This limitation has caused many LED chip-makers that use gold or silver wire bonding to pay careful attention to the purity of the

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wire. The more impurities in the wire, thegreater the wire’s resistivity.

The thermal dissipation requirements of the LED packaging industry have been driving bonding wire manufacturer to improve the purity rating of their wire.

Face-down LED packaging Even then, many high-power LED manu-facturers are taking a page from the semi-conductor industry and electing a flip-chip style design, in part to deal with the ther-mal limitations of traditional wire-bonding design.

An increasing number of suppliers are using sub-micron gold mate-rial to replace bulky wire bonds, allowing for di-rect bonding between LED chip and substrate.

This space-saving move enhances thermal dis-sipation, while also im-proving electrical char-acteristics

See next page

LED flip-chip module using conductive gold-sintered material for improved thermal dissipation

LED flip chips mounted on a module

Sub-micron gold (from 13)

There are cost-saving benefits to having the face-down structure on a metal sub-strate as well.

A conventional bonding technology that uses AuSn solder requires a substrate con-taining aluminum nitride, an expensive ma-terial.

Relieves stressUsing a gold sub-micron material elimi-nates this cost and allows for directing bonding to the metal substrate, releasing the stress between the device and the sub-strate caused by differing rates of thermal expansion.

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Gold bump jointsThe superior resistivity of sintered gold ma-terial compared to solder alloys also gives it great appeal as a metal-to-metal joint in face-down LED packaging, and it sinters at 200 degree C without requiring any clean- up.

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Sub-micron gold (from 14)

The gold’s low electrical resistance (~<5 x 10-6 ohm cm) combined with its high-ther-mal conductance (~>150W/mK), makes it especially attractive in high-powered LED applications such as automotive illumina-tion.

Flip-chip alternativeThe trend towards increasing power output from a smaller LED device also has driv-en need for an alternative to solder-bump flip-chip, which is an inferior electrical and thermal conductor compared to gold bump technology.

In addition, the malleability of gold is an added advantage: the bumps can be shaped

into a flatter profile, which reduces device size and further aids thermal dissipation.

Better heat resistanceFurthermore, using gold increases the heat resistance of flip-chip joints (mp: 10630C) allowing for higher temperature operation of LED devices and increasing light power output.

See next page

Gold bump joint with gold-sintered materials on an LED flip chip

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Sub-micron gold (from 15)

Meanwhile, MEMS wafer-level vacuum packaging with electrical interconnection is another application for sub-micron gold material.

MEMS challengesThere are many challenges with MEMS packaging. These include decreasing pack-age size requirements and a correspond-ing increase in package densities, all while achieving targets for hermetic sealing.

Achieving all this while managing costs has driven demand for sub-micron gold mate-rial.

The material can be bonded using thermo-compression to achieve the dense struc-tures required in MEMS packages.

Low-temperature bondingIn recent years, low temperature bonding has become important for reducing post-bonding residual stress in the mechanical structures of MEMs.

This practice also prevents the bowing or

cracking that can occur with materials that have dissimilar coefficients of thermal ex-pansion (CTE).

A key feature of the gold particles is their surface activity at low temperatures. For in-stance, it has been shown that at 99.95%wt purity, spherical gold particles with diame-ters in the range of 0.2um to 0.5um exhibit excellent bonding at temperatures as low as 200C, and also have a high adaptability to the roughness of an alumina surface.

Stencil printingAn example of the fabrication process is forming a rim structure on the Si wafer using a dry-etching process, applying the sub-micron Au particles coating using sten-cil printing and then heating at 2000C for two hours using Ar-4%H2 gas. This is fol-

followed by thermo-compression bonding at 2000C for 30 minutes in a vacuum chamber under a pressure of 200MPa.　

The bonded laminates maintain deflection in the diaphragm, and the effectiveness of the seal is shown by the helium (He) leak rate.

The maximum leak rate of the bonded lam-inates, sealed by Au particles, has been measured in the range of 10-14 Pa∙m3/s (He),sufficient for many MEMS applications.

ConclusionThe combined thermal and conductivity re-quirements of power semiconductors and LED packaging make submicron gold par-ticles an increasingly attractive option. its mechanical benefits are added feature for the MEMS industry.

See next page

The strong bonding characteristis of sintered gold materi-als ensures hermetic sealing in a MEMS package.

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Sub-micron gold (from 16)

Mr. Thuresson is an account executive at Tanaka Kikinzoku International (Ameri-ca), responsible for sales and marketing of bonding wire and semiconductor mate-rials in the U.S. market.

He has a B.A. from Claremont McKenna College/ He earlier held product manage-ment posts at Molex Japan and NTT Elec-tronics. He returned to the U.S. recently after working in Japan for eight years.