Tom
10-09-2002, 07:09 PM
IPC Advanced Study Guide Page Reference: Page 7 & 8, Section 1.1
While there are many different material properties that can be measured in a laminate, only a few are of overriding importance in circuit design. These properties are:
Thermal expansion
Glass transition temperature (Tg)
Moisture absorption
Thermal Expansion
Thermal expansion is usually characterized in terms of changes to the X-Y plane, which is controlled primarily by the reinforcement of the material. It is desirable in certain circumstances to attempt to more closely match CTEs of the substrate with the components that are to be mounted on them to improve assembly reliability.
All thermal expansion is measured in parts/million/ change in temperature (°C).
Glass TransitionTemperature
Glass transition temperature (Tg) is that property of the material where the reinforcement and the resin systems deviate from their existing linear coefficient of thermal expansion and expand at a much higher rate based on a change of state in the resin. This usually occurs when the resin system exceeds its cured polymer state. It is usually an expansion in the Z-axis of the material and that the temperature stated expands at a faster rate, although still linear (mm/mm of thickness).
Thermal expansion can occur in all planes. The impact on the X-Y plane is controlled primarily by the reinforcement. The expansion in the Z-axis, is determined by the resin system, and is usually at a rate significantly larger than in the X-Y plane, particularly at temperatures above the Tg .
To the designer expansion is a concern from two perspectives, potential warpage of the assembly and damage to the plated through holes. High glass transition materials thus tend to be more forgiving.
Moisture Absorption
While there are exceptions, most organic materials are hygroscopic and can pick up moisture relatively rapidly. Moisture absorption is a concern to the designer because it changes the electrical characteristics in the material, can alter dimension configuration and can cause potential difficulties in assembly. The following table shows the water absorption rate by weight for the various materials.
The table also shows conditions for glass transition temperature and thermal expansion of various material types.
While there are many different material properties that can be measured in a laminate, only a few are of overriding importance in circuit design. These properties are:
Thermal expansion
Glass transition temperature (Tg)
Moisture absorption
Thermal Expansion
Thermal expansion is usually characterized in terms of changes to the X-Y plane, which is controlled primarily by the reinforcement of the material. It is desirable in certain circumstances to attempt to more closely match CTEs of the substrate with the components that are to be mounted on them to improve assembly reliability.
All thermal expansion is measured in parts/million/ change in temperature (°C).
Glass TransitionTemperature
Glass transition temperature (Tg) is that property of the material where the reinforcement and the resin systems deviate from their existing linear coefficient of thermal expansion and expand at a much higher rate based on a change of state in the resin. This usually occurs when the resin system exceeds its cured polymer state. It is usually an expansion in the Z-axis of the material and that the temperature stated expands at a faster rate, although still linear (mm/mm of thickness).
Thermal expansion can occur in all planes. The impact on the X-Y plane is controlled primarily by the reinforcement. The expansion in the Z-axis, is determined by the resin system, and is usually at a rate significantly larger than in the X-Y plane, particularly at temperatures above the Tg .
To the designer expansion is a concern from two perspectives, potential warpage of the assembly and damage to the plated through holes. High glass transition materials thus tend to be more forgiving.
Moisture Absorption
While there are exceptions, most organic materials are hygroscopic and can pick up moisture relatively rapidly. Moisture absorption is a concern to the designer because it changes the electrical characteristics in the material, can alter dimension configuration and can cause potential difficulties in assembly. The following table shows the water absorption rate by weight for the various materials.
The table also shows conditions for glass transition temperature and thermal expansion of various material types.