PCB Base materials

PCB Base materials

GWT recommends is not to specify one specific brand or type of PCB material wherever possible in PCB manufacturing, as this can ultimately limit the supply chain options in terms of who can support the project. The reason for this is that whilst there are many well-known brands of material widely in use within our factory base, there are occasions where some PCB factories have several PCB base materials’ brands or performances which achieve the required material specification. Availability and indeed price can then become factor as to which brand will be used.

This does not mean that you cannot specify known materials, far from it. If you have experience of a material which you know works for your project, then it can simply be referenced with a comment stating “or equivalent” and GWT’s technicians and procurement teams can review and offer you an alternative that will meet the functional needs without compromising performance.

Every well-known manufacturer of material will have their product categorized in accordance with IPC4101 (specification for base materials for rigid and multilayer printed boards) with the aim of this specification being to identify and categorize performance characteristics. Using this categorization approach is ideal as it defines the characteristics of the PCB base materials, in detail, and allowing the factory to follow the IPC-4101-xxx categorization enables them to select wisely thus ensuring performance is nothing less than expected.

Key Factors in Specifying PCB Material Characteristics

When considering the performance characteristics of PCB base materials, consideration should be given to both the mechanical properties (specifically in relation to how the material shall perform during heat cycling / soldering operations) and also the electrical properties associated with the material. These are typically regarded as the most common factors for selection of standard products. This comment is based upon all material being considered able to meet UL flammability rating “94v-0”.

PCB Base Materials Requirements with Lead Free Assembly Process

At GWT, practical experience shows that in a controlled assembly / solder environment and less advanced operating environments, material which falls under IPC-4101C/21 may, along with basic technology, also work in a lead-free environment. The lower grades of FR-4 which GWT has approved in our PCB base materials list at each factory will in principle meet /121 on most of the points. That is with the exception of time to delamination at T260 °C or T288 °C. At this point, it might be necessary to use high-grade materials.

Lead-free PCB assembly brings new challenges to the thermal resistance of PCB materials. In IPC-4101B/99, it adds 4 characteristics requirement on PCB materials for lead free assembly:

--Tg ≥150°C

--Td ≥325°C

--CTE – z axis ≤ 3.5% (at 50-260°C)

--Time to Delamination T288 ≥5minutes

Key Material Characteristics

1.CTE – Z axis (Co-efficient of thermal expansion): This is a measure of how much the PCB base materials will expand when heated. Measured as PPM/°C, both before Tg (Alpha 1) and after Tg (Alpha 2) and also in % over a temperature range.

2.Td (Decomposition temperature): This is the temperature at which material weight changes by 5%. This parameter determines the thermal survivability of the material.

3.Tg (Glass transition temperature): The temperature at which the material stops acting like a rigid material and begins to behave like a plastic / softer.

4.T260 (Time to delamination): This is the time it takes for the base material to delaminate when subjected to a temperature of 260°C.

5.T288 (Time to delamination): This is the time it takes for the base material to delaminate when subjected to a temperature of 288°C.

6.Dk (Dielectric constant): The ratio of the capacitance using that material as a dielectric, compared to a similar capacitor which has a vacuum as its dielectric.

7.94V-0: burning stops within 10 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed. (UL 94, the Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances testing, is a plastics flammability standard released by Underwriters Laboratories of the United States.)

8.CTI (Comparative tracking Index): A measure of the electrical breakdown properties of an insulating material. It is used for electrical safety assessment of electrical apparatus. Rating can be seen below.