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tin to 18% increases the melting point to heat as soon as possible. The bonding process usually is a 5-10 cycle scrubbing
350°C. The eutectic reflow temperature force of the eutectic process may be set movement under a certain amplitude and
should be 20-40°C higher than the to 30~100g. Scrubbing must be applied frequency in X-Y-Z directions. During
melting point. Because of reliability during die attachment to eliminate voids. the eutectic process, the scrubbing
concerns, a maximum reflow temperature Void-free processing. Solvent cleaning process distributes the solder and
of 320°C for a maximum of 30 seconds of solder preforms and substrates/ removes the gas bubbles that are among
is recommended for GaAs and GaN. packages is recommended to remove the main reasons for void generation.
The temperature profile is dependent on any existing surface contaminants (from Epoxy process. The epoxy process
the eutectic stage, the size and number machining, packaging, handling, etc.). has mostly been used for attaching
of dies, and the substrate, etc. The first GaAs and GaN can be cleaned using passive components, such as capacitors,
step of the process is to pick and place an oxygen or plasma cleaning process but not active components in RF PA
the solder preform while at a pre-heating before the eutectic process. During the devices. In recent years, new adhesive
temperature. The second step of the eutectic process, an inert gas (nitrogen or materials have been developed to replace
process is to pick and place the die. The nitrogen with 5-10% hydrogen) is used AuSn. Silver micron-particle sintering
die are placed on top of the solder preform to surround the parts and remove oxygen joining technology developed by Prof.
either at the reflow temperature, or just during reflow, preventing oxidation of the Katsuaki Suganuma of Osaka University
before reaching it. The die are placed with interconnect surface. has enabled pressureless die bonding
a force of 10~100g and scrubbed according Reliable eutectic die attach of high- in ambient temperature at a low cost.
to some preprogrammed parameters. In power/high-frequency devices requires Because this technology showed high
some cases, the preform is pre-deposited nearly a void-free attachment. A reliability at high temperatures over
on either the substrate or the PA chip. better than 95% void-free attachment 250°C, its use is spreading as a key next-
LDMOS eutectic process. AuSi is is achievable with a scrub assist. An generation power semiconductor die
used for ultra-high thermal demand oxygen level >10,000ppm contributes to bonding technology. There is a lot of
applications, such as silicon-based voiding poor quality solder joints. The ongoing research and development work
LDMOS. The packaging or substrate oxygen level in the eutectic chamber of on the pressureless nano Ag sintering
materials of LDMOS RF PA devices are <5000ppm is necessary to achieve a low materials and sintering processes [4].
Cu-W or Cu-Mo. With AuSi bonding, an void rate. The oxygen level should be Potentially, the new materials will be
additional alloy material is not included. controlled <1000ppm to have a buffer for used for high-power RF PA [5-7].
Instead, the process relies on the different eutectic processes. For process The thinner the epoxy bond lines,
diffusion of gold and silicon that occurs control, the amplitude of the scrub the higher their thermal conductivity.
at a high temperature. This process is cycle, time of scrub, and force, must all Achievi ng these thi n bond li nes
done in the presence of forming gas with be optimized to form a good void-free requires a careful design of the dispense
5-10% hydrogen. attachment. Scrub-assisted automatic pattern, die placement and process
AuSi eutectic bonding occurs at a high die attach is a common process that parameters during the epoxy process.
melting temperature of 363°C. In order to relies on “scrubbing” of the die into the A thicker bond line may help mitigate
maintain the LDMOS performance, the gold or package surface to break up the CTE mismatches between the chip
bonded chip-on-substrate (CoS) needs to oxidation layer and have good mixing to and the substrate, especially for GaN
be processed quickly and removed from form the eutectic alloy. The scrubbing dies (that can handle a slightly higher
mismatch), but the resulting increase
in thermal resistance may not be an
acceptable trade-off. High-performance
stamping and dispensing functions, as
well as bonding force control, provided
by the die bonder are necessary for a
satisfactory bond line control.

MRSI automation solution
MRSI has developed fully-automated
die bonding solutions to enable multi-
process and multi-die manufacturing
of RF PA devices for 5G base stations.
These systems have the flexibility and
high throughput required for this high-
mix and high-volume manufacturing
environment and solve the challenges
outlined above. The sections below
summarize the bonding solutions.
Automation solutions for RF PA.
Our new assembling solutions have two
platforms, the 1.5µm MRSI-H-LDMOS
Figure 1: MRSI’s “on-the-fly” tool changer.

30 Chip Scale Review March • April • 2021 [ChipScaleReview.com]
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