Tom
10-11-2002, 08:27 PM
IPC Advanced Study Guide Page Reference: Page 128, Section 2.8
How does CTE and Tg affect the dielectric constant?
The CTE characteristics in the X and Y plane in ppm/ºC are controlled, in most instances, by the reinforcement used in building the base material. The ranges shown are mostly for glass epoxy woven cloth, thus a good comparison exists between these properties and their variation is the contribution that the resin makes to the movement of the material in the X and Y plane. This characteristic has the greatest influence on the reliability of the solder joint to withstand the temperature cycling of the assembly.
The plated-through hole is not without its consideration. Z-axis expansion (which is accounted for mostly by the type of resin) has an impact on the strain in the barrel of the plated-through hole. As the resin expands, the ductility of the copper must be able to withstand the expansion rate of the resin system.
Another factor that impacts the mechanical properties of the dielectric materials chosen for printed circuit fabrication is the COLOR=green]glass transition temperature[/COLOR]. This term is defined as a "temperature at which an amorphous polymer or the amorphous regions in a partially-crystalline polymer, changes from being in a hard and relatively brittle condition to being in a viscous, or rubbery, condition." The term "glass transition" has nothing to do with the glass cloth reinforcement used in the printed board.
It has mainly to do with a temperature at which the characteristics of the Z-axis expansion change dramatically when reaching a particular temperature. Up to the temperature shown in the previous table, the material acts relatively uniformly. It expands at a certain rate in a relatively straight line characteristic. Once reaching the Tg temperature, however, the material properties change dramatically and the expansion rate is much greater. This means that the Z-axis expansion places a greater stress on the barrel of the plated-through hole.
The higher the temperature shown in the table, the more resistant is the board to Z-axis expansion based on the temperature environment inside of the unit where it resides.
How does CTE and Tg affect the dielectric constant?
The CTE characteristics in the X and Y plane in ppm/ºC are controlled, in most instances, by the reinforcement used in building the base material. The ranges shown are mostly for glass epoxy woven cloth, thus a good comparison exists between these properties and their variation is the contribution that the resin makes to the movement of the material in the X and Y plane. This characteristic has the greatest influence on the reliability of the solder joint to withstand the temperature cycling of the assembly.
The plated-through hole is not without its consideration. Z-axis expansion (which is accounted for mostly by the type of resin) has an impact on the strain in the barrel of the plated-through hole. As the resin expands, the ductility of the copper must be able to withstand the expansion rate of the resin system.
Another factor that impacts the mechanical properties of the dielectric materials chosen for printed circuit fabrication is the COLOR=green]glass transition temperature[/COLOR]. This term is defined as a "temperature at which an amorphous polymer or the amorphous regions in a partially-crystalline polymer, changes from being in a hard and relatively brittle condition to being in a viscous, or rubbery, condition." The term "glass transition" has nothing to do with the glass cloth reinforcement used in the printed board.
It has mainly to do with a temperature at which the characteristics of the Z-axis expansion change dramatically when reaching a particular temperature. Up to the temperature shown in the previous table, the material acts relatively uniformly. It expands at a certain rate in a relatively straight line characteristic. Once reaching the Tg temperature, however, the material properties change dramatically and the expansion rate is much greater. This means that the Z-axis expansion places a greater stress on the barrel of the plated-through hole.
The higher the temperature shown in the table, the more resistant is the board to Z-axis expansion based on the temperature environment inside of the unit where it resides.