Moisture Sensitivity Level Receiving Audit and Dry Bake Containment
Receiving verification of moisture barrier packaging and strict floor life tracking prevent reflow delamination yield losses in surface mount assembly.
Copper Tin Intermetallic Growth
Chemical Reaction
Solid-state diffusion between joined metals at high temperatures creates a transitional boundary layer with distinct chemical composition. This chemical process is copper tin intermetallic growth, which occurs when tin from the solder reacts with copper from the board pad. It establishes a necessary chemical bond but continues to expand over the life of the assembly.
The resulting layer consists of two distinct sub-layers known as Cu6Sn5 and Cu3Sn.
Structural Failure
As this chemical layer expands, it reduces the overall ductility of the solder joint. Because these intermetallic compounds are brittle, they are more susceptible to fracturing under shock and vibration. Large layers can lead to void formation at the copper interface.
This degradation weakens the joint and increases the electrical resistance over time.
Verification Testing
Quality engineers use cross-sectional scanning electron microscopy to measure the thickness of the transition layer. The test sample is cut, polished, and etched to reveal the metallic boundaries. Continuous thickness measurements across multiple points provide an average value for the growth rate.
This analysis helps determine the remaining service life of the solder joints under specified loads.
Environmental Influence
Storage and operating temperatures directly determine the speed of the chemical diffusion process. Higher temperatures accelerate the rate of copper tin intermetallic growth following the Arrhenius relationship. When assemblies are kept below seventy degrees Celsius, the growth remains slow and manageable.
This kinetic behavior means that thermal management is essential to prevent premature joint degradation in long-term field applications. Sustained exposure to high operating temperatures can cause the Cu3Sn layer to dominate, which is particularly prone to micro-voiding and mechanical failure.