Evaluating Desorption Baking Cycles for Moisture Sensitive Integrated Circuits
Baking moisture sensitive ICs requires matching package thickness and carrier heat ratings to balance desorption speed against intermetallic lead oxidation risks.
Cu6Sn5
Intermetallic Classification
This stoichiometric compound represents the hexagonal crystal phase formed by the chemical reaction between copper and tin substrates within electronic interconnects during soldering operations. The substance cu6sn5 grows through a diffusion mechanism at the interface where molten tin contacts a copper surface. A layer of this brittle material thickens over time as atoms migrate across the boundary.
This process follows a parabolic rate law where thickness increases with the square root of time at elevated temperatures. Engineers monitor this growth because excessive accumulation causes mechanical weakness in solder joints. Such joints fail under thermal shock when the brittle layer reaches a critical dimension that prevents plastic deformation.
The International Electrotechnical Commission provides standards for testing the longevity of these connections under mechanical stress.
Thermal Drift
The formation of this phase occurs even at room temperature through solid state diffusion. Copper atoms move into the tin lattice while tin atoms occupy positions within the copper structure. This redistribution changes the local electrical resistance of the joint during service.
A technician measures these changes by applying a controlled current across the contact point while observing voltage drops. Any deviation from expected resistance values signals the presence of an unwanted phase growth. Measurements require high resolution equipment because the change in resistance remains subtle during initial stages of development.
Calibration of the test apparatus relies on reference specimens with known thickness of the intermetallic layer. Factors like vibration and humidity introduce noise during measurement sessions.
Mechanical Specification
Integrity of the physical bond depends on the uniform distribution of this intermetallic layer. A continuous film ensures proper adhesion between the base metal and the solder bulk. Voids or cracks within the layer indicate improper wetting during the initial assembly cycle.
Inspection protocols demand cross section analysis using scanning electron microscopes to quantify the regularity of the interface. Specifications set by industrial bodies dictate the maximum permissible thickness before a component reaches the end of its operational lifespan. Failure to meet these dimensional requirements results in rejected batches during quality control audits.
Manufacturers adjust the cooling rate after soldering to control the final grain structure and thickness of the layer.
Environment Interaction
Corrosive agents in the operating environment accelerate the degradation of junctions containing this compound. Oxygen and sulfur molecules react with the exposed edges of the joint to form oxides or sulfides. These chemical products expand and exert internal pressure on the rigid intermetallic structure.
Damage propagates from the edge toward the center of the connection until conductivity stops. Engineers shield the joint with conformal coatings to isolate the contact from atmospheric contaminants. Regular thermal cycling tests ensure that the coefficient of expansion mismatch between the copper and the intermetallic phase does not cause spontaneous fracturing.
The durability of an electronic device under field conditions depends entirely on the stability of the chemical bond formed by this copper tin phase.
Moisture Sensitivity Level Three Parts Arriving without a Dry Pack
Compromised MSL 3 packaging instantly voids floor life: quarantine components, verify humidity cards, and require J-STD-033 dry baking before SMT reflow.