In a well-known poem titled “The Road Not Taken,” Robert Frost describes the apprehension that can accompany the choice to follow the most common course or chart a new path. Choosing the latter, Frost says, “has made all the difference.” Although the poem leaves us to wonder about the outcome of the journey, history tells us that all great breakthroughs and achievements, especially in science and technology, usually begin with the same choice to follow the herd or chart a new course.
As an engineer or PCB designer, you are constantly challenged to achieve a better outcome or break through a “problem” to find a new solution. Sometimes the problem requires new functionality or a choice between a custom or module-based PCB design. In either case, special or uncommon board types are often necessary due to the ever-increasing demand to fit electronic capabilities like high power or high speed into smaller and sometimes oddly shaped spaces. As we will see, there may be valid causes for apprehension; however, if you choose the right contract manufacturer (CM), your board fabrication and PCB assembly can follow an optimal path. To help you decide, let’s first define PCBs based on their electrical and mechanical properties, which can be used to specify your board requirements. Then, we will discuss the impact of manufacturing PCBs with special board types on turnaround time and cost.
PCB Electrical Properties
In most cases, PCBs are selected from a list of common sizes, shapes and materials provided by the CM. You can typically find a suitable board with desired properties from this list of recommendations. However, when your design has requirements that fall outside of those for commonly used boards, you need to determine whether your CM is capable of manufacturing special PCB board types. To ensure your CM is adequately informed, you need to define requirements in terms of board properties, which can be categorized as electrical or mechanical. Important electrical properties for PCBs include:
- Laminate dielectric constant, dk – This is used to determine impedances and trace dimensions.
- Dissipation factor, df – The loss-rate of energy for oscillatory signals, which is very important at high frequencies.
- Dielectric loss – The amount of energy loss, which is dependent on dk and df. This dominates PCB performance at high frequencies.
- Conductor loss, Ploss – The signal attenuation along the trace. For AC signals, this is dependent upon the power level and frequency of the signal. For DC, this is dependent on the conductor resistance.
- Volume resistivity, 𝝆 – The ability of the board to resist insulation or current flow. This is highly susceptible to temperature changes and moisture.
- Surface resistivity, 𝝆S – The board’s surface resistance is the ability to prevent insulation or current flow.
- Electrical strength – This is the resistance of a PCB material to electrical breakdown.
The properties above are used to determine impedances, trace sizes (widths and thicknesses), copper weight, spacing and finishes for your PCB.
PCB Mechanical Properties
The electrical properties of PCB board types are often neglected during design, especially for low-frequency design. Mechanical properties, on the other hand, are typically given more consideration. The important mechanical properties for PCBs are:
- Copper peel strength – The bond strength between the conductor and the substrate.
- Flexural or bend strength – Most applicable to flex or rigid-flex boards, this is the amount force that can be applied to the board before fracturing occurs.
- Size – The width, length and thickness of PCBs are variable but most CMs have maximum limitations.
- Shape – Usually, any board shape can be accommodated.
- Stackup – This is the number of layers that can comprise your PCB. Most CMs have limitations on these.
- Finish – The choice of finish can be very important, depending on the environment in which your board will be operating. Typically, there are various options available.
Besides the properties listed above, the thermal properties of PCB board types should also be considered.
Thermal or thermo-mechanical properties of PCBs are a major factor in determining how a board will perform in adverse temperature environments. Important properties are:
- Decomposition temperature, Td – The temperature at which decomposition of a PCB substrate starts.
- Glass transition temperature, Tg – The temperature at which a PCB begins to soften due to excessive heat.
- Coefficient of thermal expansion, CTE – When exposed to high heat, a PCB substrate begins to expand. The CTE defines the expansion rate. As the CTE for substrates is usually higher than for copper traces, this can be the source of connectivity problems.
- Thermal conductivity, k – This is the rate of heat transfer. The higher this property, the better the PCB dissipates heat.
These thermal properties can significantly impact the manufacturing of your board as the application of heat is essential to PCB assembly.
Optimizing the Manufacturing Process for Special PCB Board Types
The properties for all PCB board types are primarily defined by the materials used. Therefore, selecting the best PCB material is an essential aspect of PCB design. If you have board requirements which are not satisfied by the PCB board material selection, you must explore the properties requiring specialization and determine whether your CM can meet your requirements. The following table lists potential specializations your CM should be able to meet and the additional turnaround time and cost involved in implementing them.
|MANUFACTURING CONSIDERATIONS FOR SPECIAL PCB BOARD TYPES|
|Additional Turnaround Time||Additional Cost|
|Trace Width |
(< 0.003 in)
Light Copper Weight
Heavy Copper Weight
(> 2 oz)
(> 6 in x 6 in)
(round, triangle, octagon, etc.)
(> 8 layers)
1 Most CMs offer a form of FR4 as the standard material. Other options will most likely involve additional cost.
2 Most finishes require additional turnaround time and cost, except possibly Electroless Nickel Immersion Gold (ENIG).
The table above is not all-inclusive but provides a list of specializations and how specific PCB board types can impact the manufacturing process in terms of turnaround time and cost.
As we have seen, most special PCB board types result in increased time to receive boards, along with additional costs. To minimize delays and cost, you need to coordinate with your CM to determine which specializations are actually required to meet your design needs. By engaging your CM early in the design process, you will be able to incorporate DFM specifications that not only align with your CM’s equipment and capabilities but also facilitate an efficient board manufacturing process.
|Tempo‘s Custom PCB Manufacturing Service|
The capabilities of your CM determine the level of customization and variety of board properties you can expect. At Tempo Automation, the industry leader in fast turnkey prototype PCB manufacturing, we specialize in custom board fabrication and PCB assembly.
We offer a wide range of capabilities that will probably cover any special PCB board types you may need. However, if your requirements do fall outside of our stated capabilities, simply contact us and we will work to accommodate whatever your design requires.
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.