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46

Chip Scale Review May • June • 2018

[ChipScaleReview.com]

of finding substitutes for components

like conductive silver flakes in die attach

adhesives. Again, finding a comparable

green material with similar electrical and

mechanical properties that interacts well

with the organic matrix compounds is the

issue, so to speak.

Recyclable epoxy or thermoset

materials

As a l r e a dy h i n t e d , t h e o r g a n i c

constituent in most plastic packaging

mater ials—molding compound, die

attach adhesive, chip underfill—is an

epoxy thermoset material. For example,

the current types of epoxy materials

used in commercial plastic packaging

materials cannot be recycled or reused

easily and ends up being disposed of as

industrial waste, whether in landfills or

in a chemical incinerator [8,9]. There has

been recent research in this area in pursuit

of recyclable, biodegradable thermosetting

materials [10]. Some of the thermosetting

materials discussed tended to degrade

starting around 200°C, which might prove

problematic during high-temperature

solder reflow processes.

Table 3

compares

two reworkable thermosetting underfill

materials, specifically, adhesives based on

maleimides and with/without acetal ester

linkages, subject to die shear testing after

curing, both at room temperature and after

heating for rework [10]. Thermosetting

materials utilizing maleimides have found

uses in electronic packaging applications,

partly due to their rapid cure and an ability

to be reworked. The first cured material

in

Table 3

, with the acetal ester linkages,

shows the potential to be reworked after

exposure to heat. On the other hand, the

second material retains much higher

adhesive strength even after heat exposure,

suggesting poor reworkability.

More broadly, there has been work done

towards commercialization of a recyclable

epoxy system for industrial and consumer

products, like surfboards and snowboards

[11, 12]. For example, the RECYCLAMINE

proprietary epoxy hardener, which allows

for thermoset composites to be recycled

without pyrolysis (breaking down plastics

with elevated temperatures and typically in

the absence of oxygen to prevent combustion

[13]). In the case of RECYCLAMINE, the

epoxy can be converted into a thermoplastic

material that can be reused after a prolonged

soak in a heated weak acid (vinegar)

bath. Whether something similar can

be found that can meet a semiconductor

package’s performance and reliability

requirements is not yet known but appears

worth investigating.

Table 1:

Partial list of reliability tests for semiconductor packages [3].

Table 2:

Features of biodegradable plastics [7].