How should you select the materials for your PCB

The materials used to manufacture Printed Circuit Boards (PCB) encompass a set of insulating/dielectric and conductive materials used to construct circuit board interconnects. There is an extensive selection of material choices to choose from to meet varying performance and budget requirements. The type of material used to manufacture a PCB is a key factor in the durability and functionality of the PCB assembly. Selecting the correct PCB material requires a knowledge of the available materials and their physical characteristics and how they align with the required functionality of the board.  

Types of Printed Circuit Boards

There are 4 main types of PCBs:
  1. rigid – a solid, inflexible, single or double-sided PCB
  2. flexible (flex) – often used when the PCB cannot be confined to a single plane or sits a non-planar position
  3. rigid-flex – is a combination of rigid and flex PCBs, where the flex board is joined with a rigid board(s)
  4. high-frequency – these are PCBs that are usually used in applications requiring special signal transmission between a target and a receiver.
The chosen PCB material needs to optimize the performance of the final printed circuit board assembly. So, it is critical to consider the performance and environmental requirements for the circuit assembly.

Material characteristics to consider when selecting the PCB material

Four main characteristics (from the IPC 4101 – Specification for Base Materials for Rigid and Multilayer Printed Boards) of the PCB material are critical in helping define the performance of your base material. 1. CTE – the coefficient of thermal expansion is a measure of how much the material expands when heated. This is critical in the z-axis. Typically the expansion is greater above the decomposition temperature (Tg). If the CTE is insufficient or too high in the material, then failures can occur during assembly since the rapid expansion of the material above Tg. 2. Tg – the glass transition temperature of the material is the temperature at which the material changes from a rigid glass-like material to a more elastic and bendable rubbery like material. At temperatures above the Tg materials, the rate of expansion increases. Keep in mind that materials can have the same Tgs but different CTEs. (A lower CTE is preferable). 3. Td – the decomposition temperature of the laminate material. It is the temperature at which the material decomposes. Reliability is compromised, and delamination may occur at the point which the material discharges 5% of its original weight. Higher reliability PCBs or PCBs that operate in demanding conditions will require a TD greater than or equal to 340°C. 3. T260/T288 – Time to Delamination at 260°C and 280°C – cohesive failure of the laminate material resulting from the thermal decomposition (Td) of the epoxy matrix – when the thickness of the PCB is irreversibly changed. To select the most applicable laminate material for your PCBs it’s important to know how you want the material to perform. One of the objectives in material selection is to have the thermal properties of the laminate material closely aligned with the components that will be soldered to the board. (to be continued).