Strain Gauge Adhesive Selection for Structural Strain Transfer
Structural strain transfer requires minimal bondline thickness, high shear modulus adhesives, and precise surface roughness matching to prevent measurement lag.
Water Plasticization
Material Softening
Moisture absorption by polymer substrates causes a reduction in the mechanical strength and stiffness of circuit board laminate materials. This physical transformation, known as water plasticization, occurs when absorbed water molecules disrupt the intermolecular hydrogen bonds within the epoxy resin matrix. The process increases the free volume within the polymer, which makes the material more flexible and susceptible to deformation.
It is a common concern in the design of boards that operate in humid environments.
Thermal Shift
The absorption of moisture has a direct impact on the glass transition temperature of the epoxy laminate. When water plasticization occurs, the glass transition temperature drops substantially, causing the material to transition from a rigid state to a rubbery state at lower operating temperatures. This change increases the coefficient of thermal expansion, which puts greater strain on copper vias and solder joints during thermal cycles.
This added mechanical stress increases the rate of via cracking and joint fatigue.
Assembly Precaution
Baking printed circuit assemblies before high-temperature assembly processes like reflow soldering represents the primary method to mitigate the effects of absorbed moisture. This baking process drives out the accumulated water molecules, restoring the polymer matrix to its original glass transition temperature. Without this precaution, the high heat of reflow soldering can cause the trapped water to rapidly vaporize, leading to delamination or measling within the board layers.
This pre-assembly dry-bake is standardized in IPC guidelines to ensure board reliability. In high-reliability electronics manufacturing, boards are stored in dry cabinets or moisture-barrier bags after baking to prevent the material from reabsorbing water before assembly is complete. The tracking of exposure time ensures that the moisture level remains below the threshold required to avoid delamination.