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Automating RF PA device manufacturing to accelerate

5G wireless rollout

By Limin Zhou, Julius Ortega [MRSI Systems, Mycronic Group]
T he successful rollout of 5G bands [2]. According to Yole’s report [3], achieving a good die bonding process

wireless depends on the
deployment of a significant the total GaN RF market will increase using a fully-automated solution in order
to enable multi-process and multi-die
from $740 million in 2020 to more than $2
quantity of base stations with a higher billion by 2025, with a compound annual manufacturing required for RF PA device
density of radio frequency (RF) power growth rate (CAGR) of 12%. Telecom production for 5G base stations. The
amplifiers (PA) compared to the older infrastructure and military radar are the sections below discuss these challenges.
4G technology. Each 5G base station has main drivers for RF GaN. The telecom RF PA chip handling. Electrostatic
more channels, and each channel needs infrastructure market segment for 5G discharge (ESD) is a major factor that
one RF PA device. These devices are wireless will enjoy a 15% CAGR. can potentially destroy or damage the RF
key components that boost the RF power This article discusses an essential PA chips. To solve this problem, the die
signals in base stations. element in automating RF power amplifier bonder should be properly grounded and
Before the 5G era, silicon-based device manufacturing—a fully automated periodically tested for ESD compliance
lateral double-diffused metal oxide die bonding solution to support the flexible, for all work surfaces that contact the
semiconductor (Si-LDMOS) was the high-precision, high-volume manufacturing product to minimize or eliminate ESD
mainstream solution in the RF PA required for large-scale deployment of RF risk during production.
market for 4G long-term evolution PA devices in 5G applications. These RF PA chips have unique
(LTE) technology. Those devices are characteristics—they may be larger
now regarded as almost commodities Manufacturing requirements than other chips, with an aspect ratio of
with a high level of technical maturity. and challenges more than 10. They are long and thin
Traditional Si-LDMOS performs well at RF PA devices have two major die dies (thickness can be as little as 30µm),
3.5GHz and below, but is unable to meet bonding assembly methods: eutectic and with air bridges or sensitive structures on
the higher frequency requirements for epoxy. The eutectic process, which is the the top that can be damaged if excessive
5G applications. The operating frequency most common die bonding technique for force or stress is applied to the top. These
of gallium arsenide (GaAs) applications power electronic devices, has historically characteristics require delicate handling
is mainly within 8GHz, suitable for been the only method for making higher including placement force control that is
medium- and low-power devices for power RF PA devices used in base only achieved with real-time force feedback.
5G cell stations. In the high-power RF stations. The eutectic alloy between the The die pick and place process uses vacuum
application, gallium nitride (GaN) has die and the heat sink is typically gold- collets that hold the chip edges. GaAs and
obvious advantages and is a necessary silicon (AuSi) or gold-tin (AuSn), which GaN are naturally brittle materials. The
material for 5G macro stations. GaAs gives the best thermal conductivity and bonding force ranges from 10g to 100g,
and GaN are on the rise to replace silicon lowest possible void rate after bonding. which must be tightly controlled to avoid
as the backbone of power switching On the other hand, the traditional epoxy damage and for the best bonds.
technology thanks to better power process may be cheaper, but has lower GaAs and GaN eutectic processes.
systems efficiency, performance, and thermal conductivity and a higher void Gold-tin (AuSn 80/20) is the alloy most
cost, of GaAs and GaN. As wide-bandgap rate after bonding. This may be sufficient commonly used in the industry for bonding
(WBG) semiconductor materials, for some low-power/low-reliability GaAs and GaN chips, because of its
both GaAs and GaN devices are more devices. In recent years, some new die compatibility with gold-based components
efficient than Si. GaAs/GaN devices are bonding adhesive materials and processes and its long-term stability. The backsides
replacing Si-LDMOS devices in 5G base have been developed to replace AuSi/ of GaAs and GaN chips are plated with a
stations, radar, and avionics, as well as AuSn solder for cost reduction for high- gold layer to provide a good thermal and
other broadband applications. In future power devices. For example, the pressure- electrical interface and allow flexibility
network designs, GaAs/GaN and other less nanosilver sintering materials in the die attachment method. The solder
WBG materials will replace most of are expected to have good thermal preforms (12~50µm in thickness) are often
the existing Si-LDMOS devices due to conductivity for high-power devices and used in this process. The solder preform
limitations of their physical properties [1]. to be able to adapt the standard epoxy size is determined experimentally. Al 2 O 3 ,
Generally speaking, 5G base stations will dispensing equipment and process. The Cu-W, Cu-Mo, and Si and SiC, etc., are the
incorporate GaAs/GaN-based PAs for the materials and processes have not yet common materials for substrates.
higher frequencies, while Si-LDMOS will matured enough for RF PA devices. The lowest melting point of the AuSn
remain just a part of the mix for lower There are five major challenges to 80/20 alloy is 280°C. A 2% decrease of

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