
NOVEL MATERIALS FOR IMPROVED QUALITY OF RF-PA
IN BASE-STATION APPLICATIONS
Z. Radivojevic1, K. Andersson1, L. Bogod1, M. Mahalingam2, J. Rantala1 and J. Wright3
1Nokia Research Center, P.O.Box 407, FIN-00045, Helsinki, Finland
2RF Division, Freescale Semiconductor, 2100 E. Elliot Rd, Tempe, AZ., USA.
3RF Division, Freescale Semiconductor, Kaapelitie 4, Oulu, Finland.
Abstract:
New materials and production technology have been introduced into LDMOS RF-PA transistors to provide
advanced thermal features and increased thermal conductivity (Kth). Recently Kth of WCu flanges has been
increased by nearly 25% from near 160 W/mK to near 200 W/mK. Further improvements in the latest generation of
the RF-PA utilize novel flange materials such as Cu-laminate with even higher Kth, by more than 25% compared to
WCu. The development of Cu-laminate flange structures, involved optimization between achieving higher Kth and
preserving desired mechanical properties for low stress and long-term reliability. Such optimization provided
desired flatness for the RF-PA; yielding in lower interfacial thermal resistance between the RF -PA transistor flange
and the next level heat sink. Furthermore, well characterized, highly thermally conductive, and very robust AuSi
die attach was employed for efficient and reliable thermal coupling. Constellation of such materials and production
technology improved overall quality of the RF-PA, enabling successful implementation in base-stations.
INTRODUCTION
The RF-PA components examined in this work belong to LDMOS (Laterally Diffused Metal Oxide
Semiconductor) family recently implemented in WCDMA BS. The LDMOS Si device technology has
many advantages such as high power gain, high RF efficiency, excellent linearity, ruggedness, need of
only a single voltage supply, and inherently better thermal structure. Hence, LDMOS device technology
has nearly replaced the previous Si bipolar technology for RF-PA applications in the cellular base-station
market. LDMOS device technology has been evolving since 1st generation (1993) to its current 6th
generation device structures. In the present work here, two types of RF-PA (WCu and Cu-laminate)
devices have been used with the novel packaging materials in accomplishing higher thermal performance
and high quality picture.
High power Radio Frequency Power Amplifier semiconductor devices (RF-PA) used in cellular base-
station (BS) infrastructure equipment dissipate substantial amount of power and consequently can reach
higher junction temperatures (Tj). Failure mechanisms are accelerated at higher temperatures and hence
proper attention must be paid to RF-PA’s thermal management and long-term reliability assessment.
Overall quality of an RF-PA is dictated by manufacturing technology, mounting method and materials
used to provide low junction-to-heat sink thermal resistances (Rjh). An integrated and comprehensive
approach from device layout, die thinning, package material selection, to manufacturing process has been
rigorously used to achieve low Rjh in the RF-PA.
Effective heat spreading in the Si through optimized device layouts is used to lower device thermal
resistance, while providing high RF electrical performance. Thermal resistance due to Si chip thickness
was reduced by thinning Si to 100 microns. Furthermore, a well characterized, highly thermally
conductive, and very robust AuSi die attach is practiced. Near void free and very thin bond line thickness
are key in reaching best in class die bond and enabling a lower thermal resistance. Further, AuSi die
attach does not show any hardening or fatigue behavior after long thermo mechanical cycling (being a
hard solder, behaves elastically). Understanding the stress created in the Si and designing the stress to be
well below the strength of Si is key in the successful implementation of AuSi die attach in concert with
the novel flange material and 100 micron thick Si chip. For such purpose, Finite Element (FE)
simulations have been combined with dedicated measurements to develop suitable model for stress
analysis. The validated FE simulations helped in correct understanding of the involved thermal resistance