Hi K epoxy or also known as high electrically and thermally conductivity epoxy be able to provide superior heat dissipation for wide range of electronics application. The characteristic of electrically and thermally conductivity epoxy can be changed by adjusting the metallic or ceramic fillers to adhesive formulation. The type of filler’s features such as concentration of particles, size and shape will determine the thermal conductivity of the product. However, the particles type will normally determine the cost of the material.

The components of high thermally conductivity material is filled up by silver (Ag) whereas, the non-conductivity material is filled up by Aliminum Oxide (Al2O3). The characteristic of the epoxy is normally based on several items such as :

  • Conductivity
  • Low outgassing
  • Low resin bleed
  • Manufacturability ( BLT, Voids, high temp resistant, oven cure profile)
  • Reliability
  • CTE mismatch resistance
  • Floor life
  • Shrinkage

Several important terminologies in epoxy :

Modulus of elasticity (K) : Normally units used is MPa and GPa. An elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance’s resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is defined as the slope of its stress–strain curve in the elastic deformation region. A stiffer material will have a higher elastic modulus.

Glass Transition Temperature (Tg) : Normally units used is degree Celcius (deg C). The glass transition temperature, often called Tg, is an important property when considering polymers for a particular end-use. Glass transition temperature is the temperature, below which the physical properties of plastics change to those of a glassy or crystalline state. Above Tg they behave like rubbery materials. Below the Tg a plastic’s molecules have relatively little mobility. Tg is usually applicable to wholly or partially amorphous plastics. A plastic’s properties can be dramatically different above and below its Tg.

Viscosity: Normally units used is degree Centipoise (Cps). Viscosity can be conceptualized as quantifying the internal frictional force that arises between adjacent layers of fluid that are in relative motion. For instance, when a fluid is forced through a tube, it flows more quickly near the tube’s axis than near its walls. In such a case, experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow through the tube. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion: the strength of this force is proportional to the viscosity.

Thixotrophic Index : Thixotropic Index is a ratio of a material’s viscosity at two different speeds, generally different by a factor of ten. A thixotropic material’s viscosity will reduce as agitation or pressure is increased. This index indicates the material’s ability to hold its shape. Normally, in epoxy of semiconductor, it will use .5/5 rpm.

Die Attach Adhesives and Materials

Die attach materials or adhesives do more than attach the die to the die pad, substrate, or cavity. They also provide thermal and/or electrical conductivity between the die and the package, essentially affecting the performance of the device while operating in the field. As such, proper selection of the most suitable die attach material for a semiconductor product and application is very important.

Polymer adhesives, which are organic adhesives that consist of long chains of thermosetting polymers, are widely used as die attach material. Most polymer adhesives used in semiconductor assembly also contain a large percentage of silver fillers for electrical conductivity.

Polymer adhesives initially contain solvents that allow easy dispensation, and are cured by exposure to high temperature in the presence of catalysts. Curing basically consists of two steps: 1) outgassing of the solvents and 2) polymerization of the paste into a cross-linked thermoset material.

‘Thermosetting’ refers to the characteristic of a material to remain solid once cured even if subjected further to high temperatures. On the other hand, ‘thermoplastic’ materials have the ability to reflow at a high enough temperature, even if curing has previously occurred.

Polymer adhesives also come in several forms, two of which are epoxy and polyimides.

Polymides are high temperature cure, highly rigid polymer adhesives that is formed from polyamic acid through condensation polymerization. Polyimides contain a large percentage of solvent and generally need at least two steps of curing at different temperatures for effective cross-linking.

Epoxies are low temperature cure, high strength polymer adhesives that allow a one-step, quick cure. One drawback of epoxies though is its relatively higher ionic content. Polyimides, on the other hand, apply more stress to the die than epoxies.

Source : www. eesemi. com

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