Page 22 - ChipScale_Jul-Aug_2020-Digital

Materials and solutions for power package die bonding

Jonathan Abdilla [Besi Austria GmbH] and Maurizio Fenech [MacDermid Alpha Assembly Solutions]
P ower electronics packaging market pushing for solutions capable of attach materials commercially available

is entering a new era of rapid
— well above the melting point of most
advancement because of the withstanding high operating temperatures for power packaging, and their typical
niche position with respect to power level.
increased market demand in almost all areas cost-viable solder alloys — sintering is
of power applications such as automotive, gaining popularity. An overview of the Technical drivers
consumer, portable, home and high- process and concept of pressure-less As a power die heats up during
performance computing, renewable energy, sintering and pressure sintering will be operation and the die attach layer
and medical. Power packaging requirements presented together with a material-oriented subsequently approaches its melting
vary significantly depending on the type knowledge content. This article aims to temperature (T M ) or its glass transition
of package and end-product specifications, give an overview of the above packaging temperature (T G ), creep performance
with different emphasis placed on the core solutions for a better understanding of the of the layer reduces dramatically. This
requirements of power density, thermal benefits and drawbacks of each. is clearly shown in Figure 2, where
dissipation, operating temperatures, SAC 405 solder with a TM of 217°C is
frequency of operation, parasitic losses, Introduction subjected to different static stresses at
inductance, mechanical robustness, The plethora of requirements in the temperatures up to 200°C.
lifetime, reliability, size, integration power packaging market shows that no In order to have a useable and
capability and cost among others. one bonding solution is best or suitable for predictable lifetime from any die bonding
Therefore, several different packaging all applications. With the introduction of material, the homologous temperature
solutions and materials exist to cater to the ultra-thin vertical silicon power dies and (T Hj ) is defined to determine a safety
multitude of different scenarios. This article the advent of high band gap materials, margin bet ween the die junction
aims to present the major power packaging extreme power densities dictate that legacy temperature (T j ) and the T M of the die
solutions for die bonding. Soft-solder die bonding solutions would not always meet attach material.
attach has been the workhorse for power the engineering requirements. Novel
packaging for many years. It is widely used materials that can provide the necessary
in the semiconductor market and takes the thermal and mechanical performance
main forms of preforms, wire/ribbon or under cyclic loading are therefore required. It is generally accepted that creep
paste. Eutectic die bonding is a category of Figure 1 gives an overall summary of effects on any bonding material become
soft solder whereby alloys are synthesized the packaging technologies, the main die too much when T Hj surpasses 0.6. As
at their respective eutectic temperatures in
order to obtain a faster process cycle time
because solidification occurs in a very short
period of time. Conductive adhesives are
used widely in low-power packaging and
offer the advantages of utilizing a standard
die attach flow of dispensing, die placement
and batch cure. These conductive adhesives
fall into two main categories of anisotropic
conductive adhesives and isotropic
conductive adhesives.
A solution to avoid issues of voiding,
lack of coverage and epoxy creep, among
other issues, is to utilize conductive die
attach films. Conductive epoxies offer
the lowest performance of all the power
solutions in terms of power packaging
requirements already mentioned, however,
enhancements have been done in the form
of hybrid epoxies, whereby the conductive
filler shape is mixed between flakes and
balls, in order to maximize conductive
material percentage loading. With the
Figure 1: Die attach material performance vs. power level.

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