Eutectic and Eutectoid Systems

A positive heat of mixing in the solid phase tends to lower the solidus curve and will eventually result in solid immiscibility at low temperatures. If this heat of mixing is large enough for the solid-solid immiscibility to occur in the vicinity of the melting points of the pure components, a eutectic reaction may occur in which a melt at a certain composition and temperature may transform into two immiscible solids or s5mibolically L Si + S2 or L - a + (3. A similar reaction called a eutectoid reaction can also occur in the solid state in which -y a -T (3. 

As temperature is increased, the liquid free energy curve drops because of the —TAS term in the free energy, and encoimters the mutual tangent line at T. At this temperature, three phases can coexist, a liquid plus the two solid phases. The Gibbs phase rule tells us that, at constant pressure, we have no additional degrees of freedom therefore, there is only one composition and temperature for which this situation can occur. We call this particular combination of temperature and composition the eutectic point. This particular temperature is called the eutectic temperature and the composition is called the eutectic composition. Since temperature and composition are fixed at this particular point, it is called an invariant point. At this invariant point we say a eutectic reaction occurs. We will encounter other invariant points characterized by different invariant reactions. 

This lowering of the melting temperature is exploited to form low melting point alloys such as Pb-61 wt% Sn which melts at 183°C whereas, the melting point of pure Pb is 327.4°C and pure Sn is 231.9°C. A similar situation is seen in the Ag-Cu system as shown 

If the solid has a large enough positive heat of mixing, it will tend to separate into an A-rich a phase and a B-rich P phase when it solidifies. At the eutectic point, these two phases are in equilibrium with the melt. The eutectic temperature Tg wiU be lower than the melting point of either pure A or B. 

This means a Si chip can be bonded to a Au-coated ceramic substrate by raising the temperature to 370°C at which point a melt will form with the eutectic composition. When cooled back down, even though the melt solidifies back into almost pure Au and pure Si because there is practically no solid solubility between the two phases, there will be a fusion weld in which there is intimate contact between the two components. Thus we have a practical, low temperature method for bonding Au interconnects to Si chips or Si chips to Au-coated ceramic substrates. 

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