Designing robust circuit-board products for military and aerospace applications

Military and aerospace designs are demanding more product functionality than previous generations; consequently there is ever-more pressure on making printed-circuit-board (PCB) assemblies robust enough to comply with growing functionality.

Feb 1st, 2007

By Zulki Khan

Military and aerospace designs are demanding more product functionality than previous generations; consequently there is ever-more pressure on making printed-circuit-board (PCB) assemblies robust enough to comply with growing functionality. That calls for implementing specific design and layout techniques, as well as 100 percent design-for-test (DFT) coverage. Thermal management also needs special attention, while high-frequency RF requires special PCB design and layout techniques.

There are several key design and layout considerations to make military and aerospace PCBs more robust. Those include using correct mil-spec components, designing in an extra cushion of current, maintaining the right aspect ratios, separating power and ground planes, keeping signals clean and properly shielded, verifying impedance calculations and performing pre-layout simulations, using proper termination techniques, adding an extra cushion for mechanical holes and dimensions, maintaining stack up considerations during PCB layout, keeping immaculate assembly notes, specifying an exact drill chart, and defining the proper board material.

Design and layout

Electronics manufacturing services (EMS) providers working with military and aerospace contractors must clearly understand that these demanding PCB designs must be populated with mil spec components and not commercial grade ones. Mil spec components have tight tolerances of 1 to 2 percent, whereas commercially available ones have tolerances of 5 to 10 percent.

As for beefing up the current in military and aerospace PCB circuitry, it is a good idea to factor in an extra cushion of current. If the military and aerospace PCB application calls for a maximum load of two amperes, then PCB designers should design the circuitry so it can handle three amperes. This extra margin is important in case of an excessive spike or another anomaly not anticipated.

Aspect ratio plays a hand in creating robustness as well. It deals with how thick the PCB bare board is compared to how thin the vias are drilled. An aspect ratio of 1 to 10 is acceptable or anything less is also fine. However, anything greater, like 1 to 15, for instance, puts reliability in jeopardy. Also, higher-aspect-ratio PCBs are more difficult and expensive to fabricate. They require greater precision, yet introduce the probability of failure, especially in extreme temperatures and humidity.

Power and ground planes must be kept separate in these designs. Also, low-frequency components must be separate from high-frequency ones. Oscillating components at higher frequencies generate waveforms that affect lower frequency components. In turn, those effects degrade the signal and introduce noise, which is unacceptable in a high reliability military and aerospace product.

Clock signals must be kept clean and shielded. Clock and physical shielding can be implemented in the design stage. An aluminum or similar material enclosure is used for physical shielding and complements the clock design shielding.

Verifying impedance calculations and conducting pre-layout simulations are critical simply because it is difficult to test military and aerospace applications in real environments. The next best thing is a high-quality software simulation program to check out the PCB design. It will verify the different loads at specific locations, and will provide recommendations for layout alterations so that the design performs optimally. Simulation creates as close an operational environment as possible short of taking the product into the actual field.

PCB trace routing has to be maintained with no greater than 45-degree angles and preferably curved traces. This crucial design technique keeps current transmitted smoothly throughout the circuitry. Trace angles of 90 degrees are verboten because the flow of current hits the top of the 90° turn and reflects backward, creating a ripple effect and subsequently, an unclear signal.

Mechanical holes and other dimensions should have an extra cushion to provide greater efficiency and strengths when required. For example, a cable cut-out can use extra wiggle room in case the cable is damaged or accidentally pulled, the PCB incurs little to no damage.

“Stack-up” considerations exist during PCB layout. The rule of thumb is to avoid extremely thin cores, material used in the internal layers of the PCB. Specifically, two to three mil cores, as internal stack-up layers, need to be avoided.

Thinner cores are difficult to fabricate. Plus, there are registration issues leading to possible long-term product reliability. Tolerances must be controlled very tightly. It is not good practice for military and aerospace PCBs to use thin cores.

Avoiding the wrong or impractical components like thin cores must be fully documented in a robust military and aerospace PCB design. Comprehensive and clearly explained assembly notes are beneficial. These notes identify specific PCB materials and finish surfaces, stack-up information, the number of PCB layers the product calls for, impedances, and other key aspects of a given project.

High-caliber assembly notes rank a product as a well thought-out one. They leave no room for guesswork.

DFT is vital. All major modules of a military and aerospace PCB must be covered. Each module should be designed so it can be independently tested outside the system, itself. Also, PCB fabrication must be taken into account as part of a design since board materials can affect design and layout. Here, the correct high temperature material follows mil spec fabrication level P55-110.

When specifying the drill chart, all possible issues should be resolved, for example, making sure plated and not nonplated holes are used, and there are plus and minus tolerances for each via size.

Lastly, in regard to PCB materials, FR4, G10, polyimide or Cynanate Ester can be used for low-frequency applications. High-frequency applications, on the other hand, require Rogers Series RO4003, Duroids, Polyimide or other Teflon-based materials.

Thermal management

Component usage is the first area to scrutinize. For example, light-emitting diodes (LEDs) are a prime source of generating considerable heat and power. If the PCB initially calls for LEDs, it is prudent to search for alternatives, such as low-power LEDs. If ball-grid arrays are used in the application, they should have a metal top instead of ceramic or plastic. A metal top traps the heat and allows it to dissipate within the component.

Excessive heat is also generated by the circuitry. In the case of an analog/digital PCB, it is critical to separate analog from digital circuitry. The reason is waveforms generated by the high current analog circuits require adversely affects the digital circuits’ waveforms. To eliminate that problem, analog and digital need to be properly sandwiched between ground planes so that a potentially damaging signal to digital circuitry can be suppressed.

Horizontal heat sinks traditionally are used to help dissipate heat. Vertical heat sinks are also now becoming more popular. The advantage they offer is their ability to dissipate heat from both sides of the heat sink as opposed to a horizontal heat sink, which dissipates heat from the surface of the PCB only. Cooling fans can sometimes be an option in military and aerospace applications. If not, heat can be dissipated through a metal enclosure by connecting the heat sink to the chassis, thereby making it a big heat sink.

Thermal management also involves careful selection of materials and components that can withstand extreme heat. Metal-core PCBs, for instance, are used in military and aerospace applications that generate considerable power and heat. One side of the PCB is epoxy-based material like FR4; the other side is aluminum. There can also be another type material which acts to dissipate heat throughout the PCB’s surface. The downside is it cannot be used for component placement.

A component surface mount has to be used on one side of the board, thus vertical heat sinks are used more often. But if the option is available, metal core PCBs can be used advantageously where one side of the PCB is used to dissipate the heat.

Laser drilling can be a method to create more real estate on the board. If more PCB surface area is necessary, laser drilling can be used to move from 10 to 15 mil vias down to 5-mil vias to save PCB real estate. That extra space can be used to extend the ground plane on the surface, which can be used as a device to dissipate heat. It can also be used as exposed copper on the PCB surface to dissipate heat.

Handling RF

Radio frequency (RF) refers to the 500 MHz to 2 GHz frequency bandwidth. It has come represent signals sent at high frequency over a controlled impedance line, using ground or shielding to prevent signal degradation. Coaxial cable is an example of a more generic definition where RF is carrying the current to wires. It becomes a transmission line with electromagnetic fields. The result of the fields is of minor importance at lower frequencies, but at higher frequencies it becomes significantly important. These fields and related effects such as return loss, VSWR (Voltage Standing Wave Ratio), and insertion loss are important when designing for robustness.

Connectors are critical to a successful transmission line because they must perform the mating segment or assignment without degradation of the signal itself.

For separate RF connections it’s even more important. When matching circuitry impedances, it is vital to make sure that when the signal goes to the power supply outside the PCB, minimal signal loss is being incurred by using the non-performing RF cable or shield.

Transmission signal length is important to avoid a mismatch between the transmission and reception. If there’s a mismatch, it’s going to create a nonbalanced return signal, which bounces back to the lines, produces an inbound wave, and may create waveform distortion, degrading the signal.

If the mismatch is high, the incoming signal will be lost because the return signal is so strong it cancels it completely. For RF applications, transmission and return lines must be equal. They need to be as short as possible, and go from point A to point B without many curves or bends. The RF signal needs to be shielded properly because they are frequency sensitive. It’s easy to create a ripple effect or bounce back, or skin effect. Ground bounce is generated to create undesirable noise or cross-talk, thereby reducing transmitting signal strength.

To minimize and eliminate these issues to make RF more robust, proper shielding must be strategically placed on the PCB. At times, antennas need to be created, and in a shielding pattern separate from the noise generating portions of the circuitry.

Zulki Khan is president and founder of Nexlogic Technologies Inc. in San Jose, Calif. Contact him by e-mail at zulki@nexlogic.com, or online at www.nexlogic.com.

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