Selecting the surface finish and getting the design optimized are important steps in ensuring that your product performs well, but is that the end of the process? No, you also have to ensure that the PCB base material specified is available within the factory and that the factory is UL approved to support such materials.
NCAB knows that there are many options concerning base materials and with our technical knowledge we can help guide and assist you in terms of material selection and material specifications.
How should material be specified?
Our recommendation is not to specify one specific brand or type of material wherever possible, 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 factories have several material brands or preferences 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 product, then it can simply be referenced with a comment stating “or equivalent” and NCAB’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 IPC 4101 (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 base material, 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.
If you would like more information on IPC 4101 or material specification methods, then please contact us and we will be happy to help.
Key factors in specifying material characteristics
When considering the performance characteristics of the base material, 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 V”0.
Key material characteristics
- CTE – Z axis (Co-efficient of thermal expansion): This is a measure of how much the base material will expand when heated. Measured as PPM/degree C (both before and after Tg) and also in % over a temperature range.
- Td (Decomposition temperature): This is the temperature at which material weight changes by 5%. This parameter determines the thermal survivability of the material.
- Tg (Glass transition temperature): The temperature at which the material stops acting like a rigid material and begins to behave like a plastic / softer.
- T260 (Time to delamination): This is the time it take for the base material to delaminate when subjected to a temperature of 260 degrees C.
- T288 (Time to delamination): This is the time it take for the base material to delaminate when subjected to a temperature of 288 degrees C.
- 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.
- 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.
|Tracking Index (V)||PLC|
|600 and greater||0|
|400 through 599||1|
|250 through 399||2|
|175 through 249||3|
|100 through 174||4|
The table below is an extract of certain characteristics from IPC-4101 classifications, highlighting some of the details already referenced.
|Tg (min) C||150||110||110||150||170||110||150||170||170|
|Td (min) C||325||310||310||325||340||310||325||340||340|
|CTE Z 50-260 C||3,5%||4%||4%||3,50%||3%||4%||3,50%||3,50%||3%|
|T260 (min) minutes||30||30||30||30||30||30||30||30||30|
|T288 (min) minutes||5||5||5||5||15||5||5||15||15|
|Fillers > 5%||Yes||Yes||NA||NA||Yes||Yes||Yes||NA||Yes|
New opportunities with insulated Metal Substrate
For greater amounts of energy or local thermal loads, e.g in modern constructions with high intensity LEDS, IMS technology can be used. The abbreviation, IMS, stands for “Insulated Metal Substrate.” This is a PCB built on a metal plate – normally aluminium – on which a special prepreg is applied, the primary qualities of which are an excellent capacity for heat dissipation and great dielectric strength against high voltages. Together with EBV and a number of other companies, NCAB has taken part in the development of a demo product. The aim here is to attract the market´s attention to the opportunities for combining high intensity LEDs with IMS technology.
The most important constituent – the heat-conducting prepreg – is a ceramic or boron-filled material, specially produced to be able to dissipate large amounts of heat. Its heat conductivity is often 8-12 times greater than that of an FR4.
The advantages of IMS PCBs for heat dissipation
An IMS PCB can be designed with a very low thermal resistance. If, for example, you compare a 1.60 mm FR4 PCB to an IMS PCB with a 0.15 mm thermal prepreg, you may well find the thermal resistance is more than 100 times that of the FR4 PCB. In the FR4 product it would be very difficult to dissipate any larger amount of heat.
NCAB can offer a wide range of materials to meet almost all of the customers’ needs – be this with the exact brand or as referenced above, an equivalent based upon IPC-4101 classification / material characteristics. The materials available are categorized into four sections – standard (widely available), advanced (specific to a smaller number of factories), flex and IMS.
Another solution is to combine the FR4 material with vias, for example, which are plugged with thermally conductive paste, giving the PCB better thermal qualities than normal. This is often a more cost-effective solution, as traditional FR4 technology is used.
TO THINK ABOUT:
When considering the performance characteristics of the base material, 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.
If you have any specific questions about base materials, please contact your local NCAB Group company.
Material recommendations for different conditions and technologies can be seen below. However it is important to note that they should be viewed as very “rough” recommendations. Equally, we recommend that the customer evaluate their process and define the factors that the materials are required to withstand, such as peak temperature, time above liquidous and take into account the demands for Td, T260 and T288.
Materials according to IPC 4101/121 (min. Tg 130 Deg. C)
|Total thickness||≤ 1.60mm|
|Number of layers||1 to 4|
Material according to IPC 4101/99 or /124
|Total thickness||≤ 2.40mm|
|Number of layers||6 to 12|
|Blind / Buried vias / µvias|
Material according to IPC 4101/126 or /129
|Number of layers||12+|
|Blind / Buried vias / µvias|
At NCAB, 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 NCAB has approved in our material 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.