Different substrate materials have widely divergent properties.
Physical Properties ||Alumina ||FR-4 Density g/cm(E3)* ||3.7 ||1.1 - 1.4 Coeff of Linear Expansion 10(E-6)/� C ||2.5 ||10 - 13 Thermal Conductivity W/m/�C ||26 ||0.1 - 0.3 Electrical Resistivity ohm m ||>1,000 G ohm ||100 M ohm Dielectric Constant ||9.5 ||4.9 Loss Factor ||5 - 20 ||200 * E indicates exponent
Continuing: * Spacing between same plane traces effect the electical parameters between the traces. * Dielectric materials used between different plane traces effect the electical parameters between the traces. Read my review of "Printed Circuit Assembly Design" L Marks & JA Caterina McGraw-Hill 2000  in a recent SMTnet Newsletter. The book is a good introduction to circuit board design.
The following are design standards for boards: * 6011 - Generic Performance Specification for Printed Boards * 6012A-AM - Qualification and Performance Specification for Rigid Printed Boards-includes Amend 1 * 6013-K - Qualification & Performance Specification for Flexible Printed Boards & Amend 1 * 6015 - Qualification & Performance Specification for Organic Multichip Module Mounting and Interconnections * 6016 - Qualification & Performance Specification for High Density Interconnect (HDI) Layers or Boards * 6018A - Microwave End Product Board Inspection and Test