Page 23 - ChipScale_Jul-Aug_2020-Digital

circuit components. An ideal conductive The challenge in formulating an ICA,
adhesive should exhibit a long shelf life therefore, is to maximize conductive
(good room temperature latency), fast filler content to achieve a high electrical
cure, relatively high T G , low moisture conductivity and improve thermal
pickup, and good adhesion [2]. conductivity without adversely affecting
These pastes consist of a mixture the mechanical properties. In a typical
of organic resins and inorganic fillers. ICA formulation, the volume fraction of
Resins provide the necessary mechanical the conductive filler is about 25–30%.
stability, however, because these are The deposition of the adhesive can
often insulators, the inorganic fillers are be performed by dispensing, dipping
required to provide the necessary thermal or printing (stamping, stencil or inkjet
and electrical properties. Silver is the printing), depending on the positioning
most popular filler as it has the highest accuracy and volume control required.
room temperature electrical and thermal For chips thinner than 50µm, either
conductivity among all the metals, special dispensers for controlled, small
while also having the added advantage volumes or stencil/screen printing have
of having a conductive oxide (Ag 2 O). to be chosen. Another way to ensure full
In addition to the resin and filler, other bond line control and handle thin dies is
components like hardeners, thinners to use conductive films.
and curing agents are added to modify Films are composed of organic
cure rates, viscosity, and other mixture resin that may be thermoplastic or
properties of the isotropic conductive thermosetting, mixed with a conductive
adhesive (ICA). filler, and subsequently dried into sheets/
I C A e l e c t r i c a l a n d t h e r m a l film of thickness typically ranging
Figure 2: Creep performance of SAC 405 solder conductivity can be increased by between 10-40µm [3]. Being a dry
alloy under a constant stress of a) (top) 10MPa and b) increasing the filler loading levels, but medium, common issues with liquids
(bottom) 15MPa [1]. this should be done with care. Electrical such as voids, high fillet heights and die
wide bandgap materials such as SiC and resistivity drops dramatically above tilt are easily limited with film.
GaN gain larger market share and can a critical filler concentration called
easily operate at temperatures of 200°C the percolation threshold. At this Solder materials
or beyond, the homologous temperature concentration, all the conductive particles Conventional soldering consists of
T H200 of a typical solder material such as form contact with adjacent ones and a having a low T M component such as a Sn
SAC 405 can easily be 0.96, resulting in three-dimensional conductive network alloy, which is heated above its liquidus
severe stability issues and pushing the is established. The bulk resistivity temperature to wet surfaces that are
power electronics industry to look beyond decreases only slightly with further to be joined together. Upon cooling,
legacy solutions where a higher T j is not increases in the filler concentrations as a degree of intermetallic phase (IMP)
traded for reliability. shown in Figure 3. If more filler is added is formed at the interfaces, while the
Reducing bond line stresses can control majority of remaining liquid forms a
creep and fatigue to some extent. This conformal solid joint [4]. For die bonding
can be achieved by limiting the operating applications, wafer back metallization
temperature, matching substrate and is required to form the bond between
die coefficient of thermal expansions, the solder and the wafer. Temperatures
increasing bond line thicknesses and/or used in solder die attach range from
having a low Young´s modulus (Y) in the 260°C to 345°C, depending on the solder
bond material. Switching to a different metallurgy used.
bond material with a higher T M is also a Flux is normally added to solder stock
solution. However, the key to a successful and is an integral part of solder pastes.
die bonding material solution is making Fluxes are inert at room temperature
sure that the higher melting temperature but become strongly reducing at higher
can still be handled by established temperatures. In general, flux serves a
production line equipment. threefold purpose:
Figure 3: Effect of filler volume fraction on the
Conductive pastes/films resistivity of ICA systems (electrical conductive • Removing any oxides from the
adhesives with nanotechnologies) [2].
For a paste to qualify as conductive, surfaces to be soldered;
its minimum thermal conductivity • Improving wetting characteristics
as specified by Method 5011 of MIL- and the load-bearing resin fraction is of liquid solder by facilitating
STD-883 is 1.5W/mK. Conductive pastes decreased further, deterioration of bulk amalgamation; and
form electrical connections between elasticity and mechanical strength can • Sealing out air, thereby preventing
lead frames, input and output leads, occur with no apparent added advantages further oxidation [5].
passive and active devices, and other in resistivity properties.

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