ABSTRACT
The resin systems that are used today to produce printed circuit laminates vary with the design requirements, intended application, end item requirements, and cost. For many commercial and consumer applications (e.g., automotive, appliances, toys) that do not require high-performance resins, the board manufacturer will usually use the lowest-cost resin system. However, for the medical, military, aerospace, and high-reliability fields, high-performance resin systems (i.e., the high-Tg epoxies, polyimides, and cyanate esters) are required. Before deciding which resin system to use, consider the possible reasons for choosing one type over another:
Processability — The resin should perform well within the existing established processes and parameters.
Flammability — Because of the Underwriter Laboratory’s (UL) flammability requirements, almost all laminates and prepregs made today are required to pass the UL 94 V-O specification. This means that some type of flame retardant chemical (bromine, antimony oxide, phosphorous, etc.) must be included in the selected resin. This usually results in a compromise in some of the properties of the resin such as dielectric loss or long-term thermal stability.
Chemical resistance — Laminates are subjected to many chemicals, solvents, moisture, and high temperatures during fabrications and assembly. The resin system that is selected must be capable of surviving all these conditions with minimal deterioration of the resin properties.
Electrical properties — Electrical properties are one of the most important factors in selecting a particular resin. Resins with a low Dk and Df are critical for high-speed digital or microwave applications. The changes in electrical property values as a result of frequency, moisture content, and temperature can limit the use of some resins. High breakdown voltages may be required for some types of buried capacitors.
22Thermal stability — The thermal stability of the resins is an important consideration especially for high-temperature applications and high-layer-count multilayer boards. Sometimes assembly-related factors (e.g., wire bonding to a circuit, rework) can cause copper cracking in the plated through hole. Using a resin with a high glass transition (Tg) can minimize these problems. The Tg can be thought of as a temperature at which the resin begins to act as a rubbery solid rather than a rigid solid. Above the Tg, the rate of thermal expansion increases more rapidly than it does below the Tg. High-Tg materials will exhibit less z-axis expansion and will be less likely to cause cracking of the copper in the plated through hole. Also important is the thermal decomposition temperature (Td), which is an indicator of the long-term serviceability of the resin at elevated temperatures.
