67021-ultraviolet-led-multi-chip-module-based-on-ceramic-substrate.pdf - 第7页

References [1] M. S. Shur, R. Gaska, “Deep-Ultraviolet Light- Emitting Diodes”, I EEE Trans. Electro n De - vices, Vol. 57, Nu m. 1 , pp. 12 – 25, J an 2010; doi:10.110 9/ TED.2009. 2033768. [2] S.L. M cDermott, J. E. Wa…

Optical Measurements

Optical power measurements and wavelength shift

are observed using an integrating sphere. A setup of

an Ulbricht sphere that allows for the mounting of

the proposed ceramics substrate attached to a heat

sink is constructed. This structure permits the oper-

ation of the LED-chips at high power ratings while

diffusing the light for an integrating measurement

of total power without concerns about the direc-

tional characteristics of the LED assembly. The de-

sign of the heat-sink and the substrate clamping

lead to approx. 30% NA-loss during measurements.

A calibration of the Ulbricht sphere with respect to

this fact is used to correct the results.

The results of optical output power of the four die-

attach variants – adhesive 1, adhesive 2, SnPb

solder, and AuSn solder – are shown in Figure 11.

Measurement has been conducted using one chan-

nel of 1×7 chips per substrate and for operational

currents of 350 mA, 700 mA and 1050 mA. A sig-

nificant improvement of output power is observed

for AuSn die-attach. Adhesive 2 shows a decrease

of output power with increased current.

Figure 11: Optical output power of one channel

(1×7 chips) for four die-attach variants.

Using both channels of 2×7 chips an optical output

power of 7.7 W is achieved at 1050 mA for AuSn

soldered assemblies. The resulting peak irradiance

is 30.8 W/cm² at the LED surface with respect to

the footprint and pitch of the attached chips. The

optimized cooling of AuSn-soldered LED-chips is

demonstrated by the increased optical output power

compared to adhesively joined LEDs (Figure 12).

Figure 12: Optical output power of both

channels (2×7 chips) for the gold-tin solder

(AuSn) and adhesive (Adh2) die-attach.

Additionally the thermally induced shift of emis-

sion wavelength can be used to compare different

die-attach technologies with respect to thermal

transfer capabilities. Better cooling corresponds to a

lower chip temperature and thus to a lower wave-

length shift. Both soldering techniques show

improved behavior compared to the adhesives

(Figure 13). Gold-tin solder layers show a lower

drift than the void-prone SnPb solder.

Figure 13: Wavelength shift over current for

four die-attach variants.

Summary and Outlook

The assembly of a UV-LED multi-chip module

using different die-attach technologies has been

demonstrated. Finite element analysis proves ac-

ceptable thermo-mechanical stresses and the reali-

zation of thermally high conductive joints using

solder alloys and new highly conductive silver-

loaded adhesives if thin layers could be processed.

The design of a multi-chip sub-module is flexible

and versatile to use. Line-shaped light sources are

possible by stackable modules. A six-module

assembly is demonstrated. Easy scaling of optical

output power as well as a simplified servicing by

changing single modules add to the customer

benefit. Measurements of optical output power and

low thermally induced wavelength drift as well as

analysis of solder layer structure show the

performance of thin gold-tin solder layers. A 14

chip module attains 7.7 W optical output power and

a respective peak irradiance of 30.8 W/cm². Long

term behavior and the investigation of reliability

over life time are the next steps to confirm the

usability of the proposed module design and die-

attach technologies.

Acknowledgements

The work presented in this paper was funded by the

TAB (Thüringer Aufbaubank) with funds provided

through EFRE (Europäische Fonds für regionale

Entwicklung) OP 2007-2013 under project FK

2009VF0003. The authors would like to thank G.

Leibeling, T. Feigl and A. Joswig for sample prepa-

ration, P. Kühmstedt for micro-tomography ana-

lysis and R. Schmidt for equipment manufacturing.

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IMAPS/ACerS 8th International CICMT Conference and Exhibition (2012) | April 16-19, 2012 | Erfurt, Germany

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