Thursday, March 28, 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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Humidity. Many electronic systems need to be stored and operated in coastal areas, which are highly humid. Humidity can cause corrosion, loss of mechanical strength, change of electrical properties and increased conductivity of insulators, thus degrading the performance of electronic systems.

Protection against moisture can be provided by applying protective coating, using hermetically sealed components and encapsulating materials with moisture-resistant waxes.

Electromagnetic interference (EMI). Electronic systems’ operation could be affected by electromagnetic interference caused by electromagnetic wave radiations. Sources of electromagnetic wave radiation include communication transmitters, radio broadcasting transmitters or radiation from circuits operating within a unit itself. Examples of EMI problems include noise created on a television set when a nearby grinder is operating, intermittent resetting of computers and noise heard in a radio set when a fluorescent lamp is switched on.

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EMI problem also occurs when an electronic unit shares its source of power with other electronic units. This situation is called conducted susceptibility.

Effects of EMI can be minimised by:
1. Providing a metallic enclosure as equipment casing that isolates the circuits within from electromagnetic radiation
2. Using shielded cables
3. Using EMI gaskets to cover discontinuities in metallic enclosure at the places of connectors and cable entry points
4. Using EMI filters

Electrostatic discharge. When certain types of material come into contact and separate, electrons pass back and forth between them. One material gets positively charged and the other negatively charged, for example, when walking on a carpet or rubbing a plastic comb against dry hair. This phenomenon is called electrostatic discharge (ESD). The amount of charge and voltage developed depends on the materials, area of contact, speed of separation and relative humidity. The voltage developed could be as high as 20,000V.

Static voltage is generated in work areas by people during their normal movement. This voltage can cause malfunction of parts, assemblies and equipment or even damage them. Certain devices and technologies are more sensitive to ESD than others. Components that include field-effect transistors (FETs) such as complementary metal-oxide semiconductor (CMOS), high-frequency parts produced by Schottky barrier process and micro-circuits that utilise small active junctions are sensitive to ESD.

Failures caused by ESD could be intermittent, catastrophic or latent. In case of latent defect, the damaged device remains operational and fails some time later in its use.

System enery test bench
System enery test bench

General protection techniques to be followed to provide immunity against ESD are:
1. Select components that have inbuilt protection against ESD, or build protection in circuit design by using diodes, silicon-controlled rectifiers, metal-oxide varistors, etc.

2. Enclose the circuit in a metallic box and establish wire connection from the box to ground. Any static discharge will transfer from the box through the wire connection to ground. If a plastic enclosure is used, coat its interior with a conductive paint.

3. During production and assembly, components and sub-assemblies should be handled such that these are not exposed to ESD. This can be achieved by doing assembly in an ESD-controlled work area using ESD work benches with ESD straps and seats. These ensure that the operator or user is grounded.

4. Personnel doing assembly should wear ESD clothing to minimise ESD generation.

5. All components and assemblies should be stored in an ESD-controlled area.

6. All components and assemblies should be handled and transported in conductive or ESD-protective bags.

 

Others. Manufacturability, testability and maintainability aspects should also be considered during PCB, module and package design. Specialists in all fields should be involved from initial stages of design, in addition to core functional designers. This is called concurrent engineering. Get the design reviewed by experts in relevant fields with respect to all the given aspects.

Documentation
Documents need to be prepared that give information on materials and components to be used, as well as electrical and mechanical drawings. Documents are also required for any special instructions for components assembly, cable harness and integration, assembly sequence, acceptance procedures for individual components, materials, assemblies/sub-modules and total unit.

Prototype and qualification testing
Build a prototype and perform testing as per the documented procedure. Initially, prove functional performance under ambient conditions. Later, prove performance under higher stresses than required during normal operation—a process called qualification testing.

Qualification testing establishes design margins above required operating stresses for a unit. A design having a higher margin will help reduce rejections in production and reliability for a longer duration in the field. Analyse any failure to meet any requirement during this phase. Modify the design to meet the requirement. Modification could be required either in the circuit or packaging.

Critical design review
Get the design reviewed again by experts in relevant fields with respect to prototype qualification test results. Relevant modifications need to be incorporated in design documents.

Pilot production and acceptance testing
Now go for production of ten to fifteen units with qualification-test-cleared design. Component values are spread randomly among the units within tolerance limits. Complete performance testing as per the documented procedure at application stresses. This process is called acceptance testing.

With all the design steps given earlier followed, it is much less likely that any design problem will occur. Manufacturing problems are, however, likely to surface. Identify performance problems at this stage as manufacturing or design. Manufacturing problems can be solved by repair or rework. If any design problem surfaces, modify the design and confirm solution. Modify design documents accordingly. Major design modification might require qualification testing again.

4A5_pic3

Production and quality control
The required level of quality and reliability is built into the product during design stage. It can be maintained in production only if correct materials and components are used, and correct assembly and manufacturing procedures followed as per design documents and drawings.

It is the responsibility of the quality control department to ensure that the design is translated correctly into a product during production by using right materials, components, assembly and manufacturing procedures as per design documents and drawings.

Failure reporting and corrective action
Any failure or non-conformance occuring during production quality control and testing is to be analysed by a failure analysis board to find out the root cause.

The cause needs to be traced to the manufacturing/assembly process or design. Required improvements are recommended and implemented to avoid recurrence of such failures.

Accelerated stress screening
A product is likely to fail in its initial stages of operation after it is delivered to the customer. This can happen if some manufacturing defects escaped industry quality control and testing procedures, or the unit was not subjected to testing under required operational stresses. These defects remain hidden in the product delivered to customers. Hidden defects precipitate under unit operational stresses at customer site for some time and lead to failure.

This situation can be avoided by testing the product at stresses higher than operational requirement but within design limits during manufacturing. Higher stresses will precipitate hidden defects in a shorter time than required under normal operational stresses at customer site.

Failed units can be reworked to remove manufacturing defects and delivered to the customer. This reduces the chance of the unit failing at the customer’s site in early stages of operation. This method of testing is called accelerated stress screening.

Reliability aspects
It is possible to estimate and predict the reliability of a design based on quality grades of components used and operational stresses. It is also possible to demonstrate reliability achieved by building certain number of units with the same design and using statistical methods. Reliability can also be estimated from field failure data.


The author is scientist-‘F’ and deputy director in Reliability and Quality Assurance Directorate of Research Centre Imarat, Defence Research & Development Organisation, Hyderabad

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