InHand Electronics is a designer and manufacturer of portable and handheld electronic devices, designed for inherently low-power operation and high computational performance. A particular area of design that fits well with InHand’s normal practices is an area known as intrinsic safety (IS). An intrinsically safe product is a product that is designed to operate in hazardous areas by limiting the energy available for ignition of dangerous explosive substances. Such products simplify deployment and reduce installation cost over other protection methods. These design methods are necessary since IS devices are certified to be used in areas with dangerous concentrations of flammable gases or dust, such as petrochemical refineries and mines. Any IS device must pass stringent qualification and design guidelines.
Several different agencies develop standards for intrinsic safety and evaluate products for compliance with standards; the key agency in North America is the Underwriters Laboratories (UL) and in the European Union, the ATEX directive. These and other certifying agencies have specific compliance and certification guidelines that products must meet before they can be stamped and labeled as compliant.
In normal use, electrical equipment often creates internal sparks in switches, connectors and in other places. Compact electrical equipment (inherent in handheld electronic products) generates heat as well, which under some circumstances can become an ignition source, especially if shorted. Electric arcing, in which a current jumps a circuit gap or between electrodes, is also a consideration. Although there are multiple ways to make equipment explosion-proof or safe for use in hazardous areas, IS is the only realistic method that allows for the use of functional handheld electronics. A device deemed intrinsically safe is designed and certified to be incapable of producing heat or spark sufficient to ignite an explosive atmosphere.
In order to achieve this certification and level of protection/functionality for a modern and sophisticated electronic device, there are several considerations in the system design that must be implemented. The key areas include: 1. the system needs to be designed to eliminate components or circuits that produce internal sparking, 2. component peak temperatures must be controlled in such a way to ensure that they cannot be a source of ignition and, 3. component spacing and PCB trace spacing must be done in a way to prevent arcing under all operational conditions. The problem with these guidelines is that the normal operational characteristics of many modern components make them unusable without additional precautions. These precautions consist of a variety of techniques, including component selection, circuit design, board layout, manufacturing methods, and software architecture.
Designing an Intrinsically Safe Device: Component Selection and Circuit Design
The primary concept behind intrinsic safety is the restriction of available electrical and thermal energy in the system so that ignition of a hazardous atmosphere (explosive gas or dust) cannot occur. This is achieved by ensuring that only low voltages and currents enter the hazardous area and that no significant energy storage is possible.
Elimination of spark potential within components is accomplished by a combination of techniques starting with the electrical design and component selection. The most common method for protection is to limit electrical current by using multiple series resistors (assuming that resistors always fail open) and limit the voltage with multiple Zener diodes to ground (assuming diode always fail shorted). Total capacitance of the device must also be kept to specified minimum values. Among other circuit design implementations, these practices required using LVDS (low-voltage differential signals) for the display, throttling processor speed to reduce power, developing custom SCR (silicon-controlled rectifier) circuits, reducing capacitance to a minimum level, while meeting signal integrity and filter specifications, and reducing boot time peak power inrush by controlling the booting sequence and speed of the processor and subsystems.
The challenge from a design and functionality perspective is that tradeoffs must be made to keep functionality, but meet the IS criteria. In addition to component selection, creative circuit design is necessary to limit system capacitance while keeping robust operation and meeting other system requirements such as performance (MIPS). In all cases, the design must use techniques to support keeping components from reaching junction temperatures sufficient for ignition. Safeguards must be employed to ensure that, in any circumstance. a component can never reach a temperature that could cause ignition of a combustible atmosphere.
Designing an Intrinsically Safe Device: Circuit Layout
Since the functionality of complex devices (such as InHand produces) requires certain capacitance and inductance that cannot be addressed solely by circuit design, other techniques must be employed at a system level to ensure that the temperature under all fault conditions (such as an internal short in a semiconductor device) can never rise to a value which may cause the ignition of explosive gasses. Typical component spacing for many components used in non-IS environments do not meet the criteria for IS environments, so other techniques are needed to create finished products that meet the guidelines and device performance characteristics. Therefore, in addition to circuit layout, in order to eliminate the possibility of an arc between components if dust or other particulate matter works into the circuitry, component spacing, isolation, and potting (or encapsulation) becomes important to the design. Further, during layout, careful rules must be followed to create the correct layer stack-up, keep traces and component spacing to specified values, and position components in logical groupings to allow the manufacturable implementation of conformal coating and selective encapsulation.
Designing an Intrinsically Safe Device: Conformal Coating/Encapsulation
A complete IS solution requires a system view, as tradeoffs can be made in different areas to meet the end goal. As an extreme, the entire board could be potted in a protective encapsulate, effectively creating a spark-proof container. Such a technique (while potentially meeting the IS guidelines) would be expensive, heavy, difficult to manufacture/integrate, and impossible to repair or troubleshoot. Therefore, a more effective method has to be employed. Once the component selection and circuit design have produced an IS compliant product that adheres to performance specs in an optimal way, the next step is to protect the resulting circuit in a manner consistent with IS requirements. In order to do this in a manufacturable way (while also allowing for repair of failed assemblies), in addition to layout and component placement, InHand is also involved in developing encapsulation and conformal coating methods to meet the UL/ATEX certification guidelines. After the circuit design and layout is optimized to meet the functional operation and space requirements of the end-device, InHand works with the contract assembly house to develop the techniques to ensure a manufacturable solution.
As an example from one of InHand’s own projects, for one class of components and surface traces, this was accomplished by using standard automated precision machine application of a polyurethane-type conformal coating over select areas of the board. By UL guidelines, machine application requires two passes. To stay out of the keep-out areas and keep the surfaces requiring encapsulation free from coating, precision application is required: The figure (right) shows two applications of the conformal coating over the specified area. Inspection of coverage can be verified using optical techniques under a UV light. Thickness can be verified though the use of special equipment such as a PosiTector 600 coating thickness gage.
However, for functional operation, some computing elements of the design may require the use of inductors, but these standard components would fail IS requirements (even with low-power implementation) without the addition of encapsulant/potting dams around the components and sealed to the PCB. Conformal coating alone may not be adequate for this requirement. By IS guidelines for these inductors, there must be a void-free area of at least 1mm of contiguous encapsulate anywhere between the protective component(s) and the outside edge of the encapsulant. (A void by UL definitions is considered any absence of material exceeding 2.5mm on its longest axis.)
InHand has worked with its contract manufacturing partners to develop techniques to apply various types of encapsulants to the select areas of a board and ways to inspect the encapsulation to the criteria set by UL guidelines. This required a challenging combination of material selection, curing, application technique, and inspection methodology. Through a combined approach between our customers, assembler, UL and InHand engineers, we arrive at manufacturable techniques that meet all requirements.
On one such design, a problem to overcome was compound application that would achieve the coverage and thickness requirements only over the local areas where it was needed; to avoid keep-out zones and have the necessary viscosity, we used the following technique:
We cut pieces of specialized tubing to predetermined lengths that could be placed over components on the board to serve as dams for the encapsulant.
By applying adhesive to the sleeves (Loctite 382 or 444) and flash curing with a UV light, it was then possible to fill the dammed areas with potting compound (Loctite 5091 or Dow 184 or 182) and cure with the same technique. By using clear/translucent compounds, IS inspection was possible to ensure UL requirements were met. The figure (right) shows the resultant solution with an immersed bubble clearly identified.
Designing an Intrinsically Safe Device: Embedded Software
Once through the maze of component selection, circuit design, board layout, conformal coating, encapsulation, and inspection, the last area of good IS design is the embedded software, as performance can be substantially improved through the optimization of the software. The hardware by design (as a standalone subsystem) is current-limited, so it cannot (under any operational mode) pull more current than for which it was designed. The board would either throttle or trip a fuse if it requested more power than its design limit.
One way to reduce the power consumption of the assembly is merely to reduce the clock speed of the processor to reach the necessary threshold. This brute force method is supported by most modern CPUs, but performance takes a corresponding hit by limiting the computational power of the device. Depending on the application, this can result in sluggish operation, from slow display refresh to decreased computational updates.
Further, peak current inrush typically occurs during boot or other specific use cases, but as the current limit must not be exceeded under any use case, setting this threshold (based on corner cases) brings down system performance as a whole. This is where InHand’s background in power management has helped tremendously to squeeze more computational bandwidth from an IS device. By spreading out the current-consuming spikes during the booting process and allowing the hooks to dynamically change the computational speed (and hence power consumption) during normal operation, InHand creates an operational environment that optimizes computational power from the device during all use cases and functional states.
Producing an end-to-end manufacturable product that offers the advantages of today’s modern handheld instrumentation for an IS environment requires a total system approach to the design. This starts with a good requirements capture that accurately defines the system specification and all use cases. It is followed by the proper component selection, circuit design, board layout, conformal coating, encapsulation, and excellent documentation with well-defined test and inspection techniques. Additionally, the integral approach to the embedded software design (for minimized peak and sustained current draw) creates a product with long battery life and impressive performance that suitable for use in stringent explosive atmospheres, meeting IS requirements, as defined by the UL and ATEX standards.