Why stack-up?

Planning optimal multilayer stack-up is one of the most important elements in determining the Electromagnetic Compatibility (EMC) performance of a product. A well-designed layer stack-up can both minimize the radiation and can stop circuit from being interfered by external noise sources. Well-stacked PCB substrates can also reduce signal cross talk and impedance mismatch issues. However, an inferior stack-up may get EMI (Electromagnetic Interference) radiation rising, because reflections and ringing in the system as a result of impedance mismatch can dramatically lower products' performance and reliability. This article then focuses on layer stack up definition, its designing rules and essential considerations.

What is layer stack-up?

Stack-up refers to the arrangement of copper layers and insulating layers that make up a PCB prior to board layout design. While a layer stack-up allows you to get more circuitry on a single board through the various PCB board layers, the structure of PCB stack-up design confers many other advantages:

• A PCB layer stack can help you minimize your circuit's vulnerability to external noise as well as minimize radiation and reduce impedance and crosstalk concerns on high-speed PCB layouts. 
• A good layer PCB stack-up can also help you balance your need for low-cost, efficient manufacturing methods with concerns about signal integrity issues
• The right PCB layer stack can enhance the Electromagnetic Compatibility of your design as well.

Prepreg is the common term for a reinforcing fabric which has been pre-impregnated with a resin system. This resin system (typically epoxy) already includes the proper curing agent. The main function of prepreg is to stack all layers into a whole board by high temperature. The following table shows physical and chemical attributes of main categories of Prepreg, that is, 7628, 2116 and 1080.

Based on chemical and physical attributes of prepreg, you have access to estimate thickness or copper weight of final circuit board through an easy calculation.To obtain specific thickness of prepreg, different types of prepregshave to be combined together to achieve required thickness.