Friday, March 29, 2024

Ensuring Quality and Reliability of Electronic Systems

Learn about the design steps to be followed to make a quality and reliable product -- K. Sita Rama Rao

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Printed circuit board layout and analysis. PCB layout pattern and design play an important role in the reliable performance of an electronic system. This is particularly true at high frequencies, where PCB tracks behave like transmission lines. This condition happens if signal transmission delay along the length of the PCB track is comparable with the rise and fall times of pulses. Pulses at the receiver will be distorted with ringing and not received by the receiver correctly. Pre-layout and post-layout signal integrity (SI) analysis using EDA tools needs to be done to examine this problem. Terminating resistors, if required, need to be added at appropriate places.

Care should be taken so that the loop area enclosed by signal current’s send and return paths is minimised. Larger loop area results in electromagnetic interference (EMI) to nearby circuits through mutual inductive coupling. It also makes the circuit under operation susceptible to EMI from nearby circuits.

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Signal return current path is normally the circuit’s signal reference ground. This ground is shared by many signals. Ideally, ground should have zero impedance at all frequencies. Practically, ground tracks will have non-zero impedance (inductive), especially at higher frequencies. This results in non-zero voltage drop across ground, causing the transmitted signal to be received erroneously by the receiver. Care should be taken to ensure near-zero-impedance ground at direct currents to frequencies of interest. To achieve this, a large ground area should be used. If necessary, use a multilayer PCB with separate planes for ground and power supply.

Keep the following things in mind:
1. Use separate grounds for sensitive analogue circuits and noisy digital circuits.

2. Physically separate sensitive analogue circuits from noisy digital circuits.

3. The rise and fall times of pulses should not be less than required. Pulses with short rise and fall times will have a high frequency content and increased electromagnetic emissions.

4. Use power supply decoupling capacitors—one for every four to five digital integrated circuits (ICs). These are required to counter inductive voltage drop developed by switching currents in digital ICs.

5. Select the track width for signal and power tracks such that these can sink the required current.

6. Provide appropriate spacing between signal tracks to avoid cross-talk.

7. Select pads suitable to component dimensions.

8. Allow adequate clearances between tracks and through-hole component pads so that components can be easily soldered.

9. Use heat-sinks for high-power-consuming components like power amplifiers and power supplies.

10. Perform thermal analysis with EDA tools to find out hotspots on PCBs.

11. Only general guidelines are provided here. Study PCB layout literature for more details.

Packaging
Any electronic product normally has a number of PCBs connected by a motherboard. All the PCBs are placed in a metallic enclosure. The enclosure has connectors fixed to it for external communication of signals. Wiring harness runs from connectors on the chassis to the PCBs/motherboard.

Unit packaging is to be designed carefully such that the unit can withstand thermal, mechanical, electromagnetic and electrostatic discharge (ESD) stresses.

Thermal stress. Performance of many electronic components and systems degrades with temperature. This happens more when subjected to high temperatures for a long duration. Electronic components can also get mechanically damaged when temperature changes fast from high to low and vice versa.
The most common method of thermal control is use of heat-sinks. By employing a heat-sink, a low thermal resistance path is provided from the component to the air. Heat conducts from the component to the heat sink. Heat sink is cooled by convection, with air serving as the thermal reservoir.

Other techniques are:
1. In a forced-air-cooled package, spread the heat-dissipating parts uniformly along the cold wall.
2. Do not place thermally sensitive or highly dissipating parts close to each other.
3. Do not place thermally sensitive parts next to hotspots.
4. In free-convection-cooled equipment, do not place parts directly above highly dissipating parts; stagger them horizontally.
5. For contact interfaces, use as much contact area as possible.

Mechanical stress. Excessive mechanical stresses, such as vibration and shock, generated during operation of electronic sub-systems (for example, launch of missile systems) can cause chafed wiring, loose fasteners or components, intermittent electrical contacts, deformed seals, failed components, and cracked and/or broken structures.

Protection against mechanical stresses is generally achieved by suitable packaging, mounting and structural techniques. Two approaches used are mechanically isolating the equipment and building the equipment to required strength.

Packaging analysis using CAD tools is done to determine natural frequencies and mechanical stresses within components, produced by shock and vibrations. If stresses exceed safe levels, corrective measures such as stiffening and incorporation of further support members are required. An isolation system can be used at the source of vibration in addition to isolating the protected component. Damping devices can be used to reduce peak oscillations.

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