New Problems Caused by Lead-free Solder
Conventional solder, which is an alloy of tin and lead, has a low melting point, is inexpensive and easy to work with, but it is also an environmental pollutant that is harmful to humans. Therefore different types of lead-free solder composed of tin (Sn), silver (Ag), and Copper (Cu) or similar but no lead are now being used as a replacement. However, lead-free solder has a higher Young's modulus than conventional lead-based solder, making it more susceptible to expansion and contraction, and it is also harder and more brittle. Therefore, when the printed circuit board on which the chip components are mounted is subject to bending stress due to warping or other influences, the solder joint deteriorates and cracks can appear. This phenomenon is called solder cracking.
Another defect that can occur with lead-free soldering is the so-called Kirkendall voids which are microscopic cavities, also causing reduced cohesion. When two different types of metal that are in close contact are heated, atomic dispersion occurs. After the name of the scientist who discovered it, the effect is known as the Kirkendall effect. As the speed of the dispersion differs depending on the type of atom, repeated thermal cycling involving heating and cooling tends to cause the formation of voids which eventually lead to solder cracks. The temperature in the engine compartment of a running car regularly reaches 100 degrees centigrade and higher. This causes populated circuit boards to expand and contract, leading to bending stress that can result in cracks and voids in solder joints, thereby lowering the joint reliability.