My introduction to electronics was not at a terminal, but at a job site. Starting out as an electronics technician allowed me to see how systems worked, why they failed, and probably most important, how they were put together. It was easy to recognize bad designs due to the difficulty encountered when troubleshooting problem systems or boards. One clear indicator of a designer with limited knowledge of the product’s manufacture was spending excessive amounts of time removing components that rarely, if ever, needed replacement, simply to access components that required preventive or periodic maintenance. From that experience, I learned that the best PCB designers are the ones who at least somewhat understand how their designs are made.
This correlation between design quality and ease of manufacture or service extends across virtually every industry, especially PCB design and manufacture. The more you know about the processes and materials used to make your boards, the better equipped you are to design high-quality boards that can be manufactured efficiently. Designing from this perspective requires an awareness and understanding of PCB material properties and how they impact the manufacture of your boards.
PCB Material Properties
Before tackling the impact of PCB material properties on your board’s manufacture, we need to know what they are. The significant properties may be classified as mechanical, electrical, thermal or chemical.
- Bending (Flexural) Strength
The ability of your board to resist deformation or breakage when under bending stress. Standards for rigid, flex and rigid-flex must be adhered to according to IPC-6013C.
- Time to Delamination
A measure of how long a PCB’s layers will remain attached when exposed to temperature changes or moisture.
- Density (g/cm3)
The amount of mass per volume for a dielectric.
- Peel Strength
The ability of copper layers and dielectrics to remain bonded under thermal and chemical stress.
- Dielectric Constant, dk
Affects the signal integrity and impedance of a PCB material. Should be constant over a broad range of frequencies for high-speed applications.
- Relative Permittivity, Er
Referred to as relative dk.
- Surface Resistivity, 𝛒S (Ω-m)
A measure of a dielectric material’s surface resistance to the flow of electricity. Susceptible to change with temperature and moisture. Should be high to facilitate good signal integrity.
- Volume Resistivity, 𝛒 (Ω/m2)
Measure of a dielectric’s volume resistance. Preferably high to maintain isolation between layers.
- Dissipation Factor, Df
A measure of signal or power loss of a PCB material. Not a factor for digital signals, but can be significant for analog signals, especially for frequencies above 1GHz as Df increases with increased frequency.
- Electrical Strength (V/mil)
The ability of a PCB to resist electrical breakdown in the z-direction.
- Glass Transition Temperature, Tg (°C)
The temperature at which a PCB substrate will reversibly change from hard to soft. Should be lower than solder temperature.
- Decomposition Temperature, Td (°C)
The temperature at which a PCB substrate will permanently decompose. Should be lower than solder temperature.
- Coefficient of Thermal Expansion, CTE (ppm)
The rate of expansion for a PCB in response to heat. Generally should be kept as low as possible.
- Thermal Conductivity, k (W/m)
The rate at which heat is transferred through a material. Higher for conductors as compared to insulators.
- Methylene Chloride Resistance, MCR (%)
Gives the percentage absorption of methylene chloride, a volatile toxic solvent used in PCB production. Typically, ≤ 0.2% for dielectrics.
- Water Absorption (%)
This value gives the amount of moisture that will be absorbed by the PCB material when submerged. For most materials, this value is ≤ 0.2%. May effect electrical and thermal properties of a dielectric.
According to the Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances (UL 94), there are time limits on how long a material may burn in response to combustion. PCB materials are limited to 10s per specimen.
As shown above, there are many PCB material properties to consider. Now, let’s see how to use this information to optimize your board’s manufacturing process.
Managing the Impact of PCB Material Properties on Your Board’s Manufacture
Once your board is manufactured, the PCB material properties presented above become fixed parameters that cannot be manipulated or altered. These parameters will determine how your PCB will function as well as your board’s quality and reliability. Therefore, it is necessary to define them during design so they can be incorporated into the manufacturing process. In most cases, it is difficult and unnecessary to explicitly define your material properties. Instead, ensuring that the properties are within acceptable ranges for your design will suffice. This can be achieved by following these procedures:
1. Select the best materials for your board design
Board material requirements are greatly influenced by the functionality and signal types of your board. Therefore, it is important to know the correlation between board type and material properties in order to choose materials that are capable of withstanding your design requirements.
2. Determine your board’s stackup
In addition to selecting the materials that will comprise your PCB’s construction, you also need to determine its vertical layout or PCB stackup. Especially important are the thickness values for the layers which will impact PCB material properties.
3. Understand the board manufacturing process
In order to select a contract manufacturer (CM) that is capable of meeting your quality control, turnaround time or critical system capability, you need to understand the PCB manufacturing steps and how your choices impact them.
|Tempo‘s Custom PCB Manufacturing Service|
A good understanding of PCB material properties and how they impact the manufacture of your boards will assist you in making the best material selection for your design. At Tempo Automation, we will work with you to so that the fabrication and assembly of your PCBs will incorporate materials and properties that support your design and requirements.
And to help you get started on the best path, we furnish information for your DFM and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on PCB material properties or the impact on your board’s manufacture, contact us.