Page 32 - ChipScale_Jul-Aug_2020-Digital

excessive heating at reflow. Overheating
can also be indicated by a goldish or
brownish appearance of the solder.
A continuous fillet around the entire
perimeter of the die is also indicative of a
good thermal profile. The cool-down time
to solder solidification is dependent upon
the mass of the tooling, components,
and weights, together with radiant and
conductive heat transfer to the water-
cooled process chamber surfaces. The
time above liquidus (TAL) is controlled
by the ramp-up, reflow, and natural
cooling rate of the assembly of tooling,
components, and weights. By carefully
designing, programming, and monitoring
these reflow parameters, an optimal
profile can be created, which will produce
high-quality, low-void solder interfaces.
Figure 5: Thermal conductivity inside the chamber as a function of the pressure level. Acceptance criteria are primarily based
on either X-ray or scanning acoustic
nitrogen gas is introduced into the There are four key parameters to microscope (SAM) scan verification after
chamber. This high pressure gas will control in a vacuum reflow die attach die attach reflow. A visual appearance
collapse or reduce the size of trapped application: 1) ramp-up time, 2) reflow of the solder with shiny bright color and
voids, and in most cases, eliminate the temperature, 3) cool-down time, and 4) excellent fillets is also essential and goes
remaining voids. This technique uses total time above liquidus (TAL) [4]. hand in hand with low-void content, as a
Boyle’s Law (P 1 V 1 = P 2 V 2 ) to reduce The actual programmed ref low proof of a well-designed reflow profile.
void sizes. temperature of the graphite tooling is In addition to the previously discussed
In this application, P 1 is the low typically much higher than the melting component metallization pitfalls, inferior
nitrogen level (<400Torr), and V 1 is temperature of the solder preform quality solder preforms will also result
the original volume of the void. Upon material. The purpose of this higher in poor wetting of the molten solder
rapid introduction of P 2 high-pressure tooling temperature is to ensure proper to the components’ gold surfaces. We
(30-50psig), the volume of void V 2 solder wetting and sealing. A shiny, light- typically recommend that the preform
will be reduced by the ratio of P 1 / colored metallic surface finish on the be sized to 95% of the designed die area
P 2 . This is equivalent to reducing or solder material usually means a proper to allow sufficient solder to form good
compressing the void by a factor of 7.5X, reflow temperature profile (see Figure 7). fillets without undesired solder outflow to
or a compression/reduction by 3 to 4 A dull gray appearance is indicative of adjacent components. The solder preform
atmospheres (refer to Figure 6). The must also be free of
loose or adhered surface
contaminants and oxides.
The amount of applied
force for each joint is
also critical. The weight
is designed to provide
a sufficient amount of
force to assure good
contact between the die/
component, the preform,
Figure 6: Boyle’s Law for void reduction.
a n d t h e pa c k ag e/
Figure 7: Close up of the soldered GaN die soldered to the package.
application of high pressure when the
preform is in a liquid state is critical for
void reduction/removal. This pressure
rise should occur near the end of the
high-temperature reflow soak and must be
held constant until the solder solidifies.
The exhaust portion of the profile should
be programmed only to occur after the
solder has gone well below the solidus
temperature or else voids could reappear Figure 8: a) Typical X-ray view of a GaN die soldered with AuSn to a copper heat sink; and b) A typical void
or expand back. level calculation.

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