Analysis of intermetallic compound formation in an In-Sn solder on copper reveal

Question

  Analysis of intermetallic compound formation in an In-Sn solder on copper revealed that the growth rate at temperatures above 90 C was governed by an activation energy of 5.3 kcal/mole. At this temperature the growth rate was 1x10-9 cm/s, while below 90 C an activation energy of 18.6 kcal/mol controlled the compound growth. Use

            dx/dt =(Ao)e-Q/RT x-n    and n=0.5, R=1.9872 Cal/mol/K and

            Ao3 = 5.7 x 10-10 m2/s                

a. Sketch the compound growth rate on an Arrhenius plot spanning temperatures from 25 C to 120 C. What compound thickness can be expected after a year at a temperature of 30 C?                                                                                         

b. Is it common for high temperature reactions to occur at lower activation energies than low temperature reactions?

c. Provided that log normal behavior is obeyed for intermetallic compound formation described above, what minimum median lifetime is required for devices to function 40 years and suffer only 100 FITs if s=1.0?

Explanation / Answer

Ans (b) High temperature reactions require an input of energy to initiate the reaction, which is called activation energy EA. Activation energy is the amount of energy required to reach the transition state,means it is the minimum amount of energy required for the reaction to occur.

High temperature reactions have a net release of energy but still they require a SMALL AMOUNT OF ENERGY INPUT BEFORE THEY CAN PROCEED WITH THEIR ENERGY RELEASING STEPS. It is very common

Some high temperature reactions are energy realesing reactions, these are negative Delta G reactions. The main reason , why these reactions require lower activation energy to proceed lies in the steps that takes place during a chemical reaction. At high temperature molecules gain energy and move faster , therefore the greater the temperature the higher the probablity that molecules will be moving with lower activation energy for a reaction to occur upon collision.

Once the Activation Energy Requirement is fullfilled by an outside source, the reaction proceeds to break bonds and form new bonds and energy is released as the reaction takes place. This results in a net gain in the energy in the surrounding system and a net loss in the energy from the reaction system and hence the Gibbs free energy is negative.

So it is very common for some high temperature reactions to occur at lower activation energies than low temperature reactions.

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