Optimizing Manufacturing Processes for Special PCBs

The increasing demand for higher functionality and smaller size requires Rush PCB to fit capabilities such as high speed or high power into small and often oddly shaped spaces. The reason may be to add new functionality or to make a choice between a module-based or a custom PCB design. Oftentimes, the challenge may lead to a better outcome or to a break-through in finding a new solution.

By selecting your contract manufacturer wisely, you can lead your board manufacturing and PCB assembly down the optimal path. However, this requires specifying the board requirements first, which again depend on the electrical, mechanical, thermal, and chemical properties of the PCB. Furthermore, manufacturing special board types can and often do impact the turnaround time and cost.

Electrical Properties

As a contract manufacturer, Rush PCB offers a list of PCB materials. Although it may be possible to find a suitable board that meets your specific properties from the recommended list, sometimes the design may call for boards with requirements falling outside those commonly available. To inform us of the special requirements, you need to define them as specific board properties. While some of these properties relate to general safety and reliability, others define how well the board will function at high speeds or high frequencies:

General Safety & Reliability

• Electrical Strength — measured across the z-axis of a PCB, this is the ability of a dielectric material to resist an electrical breakdown. It is common for PCBs to have an electrical strength between 800 and 1500 V/mil.
• Volume Resistivity — this is the ability of the dielectric material of the PCB to oppose current flow through it. Highly dependent on the presence of moisture and ambient temperature change, it is usual for PCBs to have a volume resistivity of between 10³ and 10¹⁰ MΩ-cm.
• Surface Resistivity — this is the ability of the dielectric material of the PCB to oppose current flow along its surface. Presence of moisture, temperature extremes, and surface contaminants impact this ability, lowering the ability. Common values for PCB materials are between 10³ and 10⁹ MΩ/sq.

Board Function at High Speed/Frequency

• Conductor Loss — this is the attenuation of the signal along the conductor or trace. For DC signals, conductor loss relates directly to the resistance of the trace. For AC or high frequency signals, it is dependent on the frequency of the signal.
• Dielectric Loss — the amount of energy lost in the dielectric. Depending on the dielectric constant of the laminate and its dissipation factor, it dominates PCB performance at high frequencies. For general-purpose PCB materials, the dielectric loss is around 0.02, whereas for high-end materials it may be around 0.001. Although consequential for analog signals of high frequency, it affects digital signals with frequencies beyond 1 GHz.
• Dissipation Factor — the amount of AC energy the dielectric absorbs from an electromagnetic field passing through the material. The parameter provides an important insight into the behavior of the PCB material when minimizing the signal distortion and preserving the signal integrity.
• Dielectric Constant — also known as the Relative Permittivity of the laminate of the PCB, it is a critical factor for performance at high frequencies. For most PCB materials, the dielectric constant is in the range of 2.4 to 4.5. Materials suitable for applications at high frequencies need to have dielectric constants remaining relatively unchanged over a wide frequency range covering 100 MHz to several GHz.

Mechanical Properties

While Rush PCB will accommodate most common width, length, and thickness requirements of PCBs, we also cater to most of the shapes you define. However, you must define certain parameters as these can affect the functionality of your product:

• Flexural or Bend Strength — Applicable to flex or rigid-flex boards, this is the ability of the PCB material to bend under physical stress before fracturing. It is measured in kilograms per square meter or pounds per square inch.
• Copper Peel Strength — this defines the ability of the copper layer in the PCB to bond to the dielectric. Thermal stress at high temperatures and exposure to moisture and chemicals may affect the copper peel strength.
• Stackup — this is the number of layers comprising your PCB. The density of the electrical circuit defines the stackup, and the number of layers may be affected by the technology you adopt, such as HDI and micro vias.
• Time to Delamination — this is the time required for layers to separate from one another when the PCB is exposed to temperatures beyond a certain threshold. Delamination can also occur when the PCB is exposed to moisture.
• Surface Finish — it is very important to define the surface finish as it depends on the environment in which the board will be operating. You can select from various options available.

Thermal Properties

Application of heat is essential to PCB assembly. Therefore, thermal properties of the PCB material can impact the manufacturing of boards significantly. Additionally, thermal properties are a major factor for determining the performance of a board in environments with extreme temperatures.

• Coefficient of Thermal Expansion — the rate at which the PCB material expands or contracts with change in temperature. Unless matched, substrates can expand or contract faster than copper traces do, and this may cause problems in connectivity.
• Thermal Conductivity — the rate at which PCB material can conduct heat. This is an important factor to consider when heat producing components are present on board.
• Glass Transition Temperature — when exposed to a temperature above a certain threshold, the PCB material softens, but hardens back to its natural state when the heat source is removed. The temperature at which this happens is the glass transition temperature and is important for PCB baking and assembly.
• Decomposition Temperature — beyond a certain high temperature, the PCB material will start decomposing, irreversibly losing about 5% of its overall mass. This temperature is the decomposition temperature. For reflow, the soldering temperature range is generally between 200°C and 250°C. Ideally, PCB substrates should have their glass transition temperature below this range and their decomposition temperature should be above.

Chemical Properties

During manufacturing and assembly, PCB materials are subject to several chemicals for cleaning and protection. It is necessary these chemicals not degrade the dielectric material or change its electrical and mechanical properties.

• Flammability — Materials used in PCBs must be flame retardant, meaning these must not support combustion even if these have caught fire for some reason. The Underwriters Laboratory or UL94 specify the flame-retardant properties of plastics, ranking them from the highest to the lowest. According to UL94 standards, specimens must not burn for more than 10 seconds with flaming combustion.
• Moisture Absorption — Most PCB materials show a moisture absorption value of between 0.01 and 0.20 percent, the amount of liquid the PCB material absorbs when submerged. The amount of moisture absorbed by the material influences its electrical and thermal properties.
• Methyl Chloride Resistance — the PCB surface is subject to different cleaning solutions during and after assembly. Its capability to resist these chemicals is measured by testing its absorption of Methyl Chloride. It is usual for PCB dielectric materials to show a Methyl Chloride resistance between 0.01 and 0.2 percent.

Optimizing Manufacturing Processes for PCBs

Some of your special requirements will require additional turnaround time and or involve additional cost. For instance, trace widths less than 0.003 in, heavy copper weight greater than 2 oz., stackup greater than eight layers, and surface finish call for additional turnaround time along with enhanced cost.

Specifying material other than FR4 will most likely not add to turnaround time, but may involve additional cost. Likewise, size greater than 6 in x 6 in, and light copper weight less than 0.5 oz. may only involve additional cost but may not increase the turnaround time.

Depending on your requirements, selecting the best substrates and copper is essential when designing your PCB. This is because the materials we use will define the properties for all PCB types. Discussing the PCB board material selection with us will determine if we can meet your requirements. Additionally, certain best practices help in selecting the best combination of substrate material and copper, ensuring a reliable and high quality product.

Matching Dielectric Constants — it is necessary to match the dielectric constants of the different dielectrics in a PCB to avoid problems.

Matching Coefficient of Thermal Expansion — the CTE between two substrates must match to allow them to expand or contract in unison. The CTE between substrate and copper layer must also match to prevent electrical discontinuity when expanding.

Use Smooth Foils — this is especially helpful at high frequencies, as the foil smoothness helps mitigate losses at such frequencies.

Use Foils of Good Conductivity — Right conductivity can help lower heat generation and ensure proper dissipation.

Conclusion

PCB performance is mostly based on its quality. We ensure quality by offering high quality, well-matched constituents. Substrates of good quality do not come cheap, and if you are willing to invest properly, you stand to gain far more down the line for the good quality of your product.