Stepmania music packs. Unsure of your conductor spacing requirements in high voltage PCBs? An IPC-2221 calculator can help.
The working clearance in front of the equipment on which you're working is also dependent on the composition of the facing wall and whether it houses live electrical parts. As you'll note in the Table, there are three conditions noted for two voltage levels 150V or less and 151V to 600V-to-ground.
There are further requirements for the clearance between live parts and surrounding fences or vehicles driving through the substation. All relevant clearances in high voltage substations can be calculated easily and quickly using primtech. Air Insulated Substation 3D. For example, a Primary Circuit to a Primary Circuit requires Functional Insulation. The Insulation Calculator will automatically determine the insulation. Notes are also provided as called out in Table 2G. Applicable Standards Insulation, Clearance, and Creepage are in accordance with: IEC 60950 Third Edition, 1999-04 IEC 60950-1, 2001 EN 60950. IPC-2221 is generally accepted in electronic industry as a generic PCB design standard. However, when it comes to distances between the PC traces, in my view, the IPC-2221 table 6-1 stepwise limits are mostly baseless: the curve for spacing vs. Voltage should be linear.
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PCB design and assembly standards aren’t there to constrain your productivity. Instead, they’re there to help create uniform expectations of product designs and performance across multiple industries. With standardization comes the tools for compliance, such as calculators for certain design aspects, processes for auditing and inspection, and much more.
In high voltage PCB design, the important generic standard for PCB design is IPC-2221. Many important design aspects are summarized in this design standard, some of which boil down to simple mathematical formulas. For high voltage PCBs, an IPC-2221 calculator can help you quickly determine appropriate spacing requirements between conductive elements on your PCB, which helps ensure your next high voltage board will remain safe at its operating voltage. When your design software includes these specifications as automated design rules, you can remain productive and avoid making layout mistakes as you build your board.
What is IPC-2221?
IPC-2221 is a generally accepted industry standard that defines a multitude of PCB design aspects. Some examples include design requirements on materials (including substrates and plating), testability, thermal management and thermal reliefs, and annular rings, to name a few. In high voltage PCB design, important design requirements are specified in the IPC-2221B standard. Note that IPC only specifies minimum conductor spacing values for voltages up to 500 V. Simple formulas are provided for calculating minimum conductor spacing values for voltages above 500 V.
Among the list of minimum electrical conductor spacing values defined in IPC-2221, there is one set of requirements that is intended to prevent metal migration failure. Metal migration is one of many failure mechanisms in high voltage designs with high conductor density. When two conductors are brought up to a high potential, electrochemical growth of metallic dendrites can occur when the conductors contain residues with water-soluble salts; an SEM image of dendritic growth between two solder balls is shown below. These metallic dendrites can short out two points on a high density PCB. Visecad viewer free license. This is actually an electric field effect, which explains why there is a minimum spacing requirement; increasing the spacing between conductors for a given potential difference reduces the field between the conductors, which inhibits dendrite growth.
SEM image showing dendritic growth between two solder balls. Image source.
In the case that your product is not covered by the more stringent UL or IEC standards (see more on this below), there is a simple set of data and a formula you can use to determine the minimum conductor spacing in your PCB. This data is a function of the peak voltage being used and whether a coating is present on your board, as shown in the following table. If you need to determine the minimum conductor spacing between two voltage values, you can use a linear approximation (interpolation) between two successive entries.
IPC-2221B conductor spacing requirements.
If you look throughout the electronics standards landscapes for different industries and applications, you’ll find a variety of safety standards that prescribe different conductor spacing requirements. IPC-9592B standard provides conductor spacing requirements for power conversion devices. These standards are quite consistent when graphed alongside the required conductor spacing specified in IPC-2221B. The table below specifies the spacing requirements under IPC-9592B. Note that this defines the minimum required trace spacing as a function of peak voltage values.
IPC-9592B conductor spacing requirements for power conversion devices.
Both sets of standards include formulas that can be easily programmed into a calculator application. You can find many IPC-2221 calculator applications on the internet, including IPC-2221B trace spacing calculators.
Working With an Integrated IPC-2221 Calculator
High Voltage Clearance Calculator Standard
When your design software includes an IPC-2221 calculator as part of your design rules, you won’t have to manually calculate trace widths or clearances to prevent breakdown. This type of software encodes the required clearance as a design rule, which is automatically checked as you create your PCB layout. If you are looking to ensure a higher level of safety for your new product, you can encode a larger required trace and/or component spacing in your design rules.
As you place components and route traces in your high voltage PCB, clearances can be checked automatically as you build your board. The ability to automatically check clearances between traces and components saves you from making a mistake and reduces the extent of any required placement changes. Some design rules won’t perform these important checks until you’ve finished your layout, putting you at risk of extensive redesigns when components are placed too closely together. Working with design tools that allow you to specify clearances reduces these risks and helps you remain productive.
Note that the IPC-2221 standards are totally voluntary. However, for products covered by legal safety standards, creepage and clearance requirements in the relevant UL or IEC standard are mandatory. As an example, the relevant set of safety requirements on IT and telecom products with AC mains and battery power can be found in the IEC-60950-1 standard (2nd edition). In terms of creepage, the spacing specified under IPC-2221B depends on RMS working voltage, pollution degree (numbered 1 through 3), and material group. The definitions of the latter two terms can be found in the UL 60950-1 standards. Whether you need to comply with IEC, IPC, or other required safety standards, you can specify your design requirements as design rules when you use the right PCB design software.
The CAD tools and routing features in Altium Designer® are built on a unified rules-driven design engine that automatically checks your layout as you create your board. Rather than using a manual IPC-2221 calculator, these design features help save time and sanity as you create your next high voltage PCB. You’ll also have access to a full set of documentation features that help you prepare for manufacturing and assembly.
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Now you can download a free trial of Altium Designer and learn more about the industry’s best layout, simulation, and production planning tools. Talk to an Altium expert today to learn more.
Several years ago, Sierra Circuits published a guide toward the design and manufacture of high voltage circuit boards, with potentials up to 40,000 V.
Considering Creepage and Clearance
Much of the advice in that brief regards a narrow project category, and I have an idea that a review of the basic layout precautions for any board that would carry what is considered hazardous voltage—greater than 30 VRMS, or 60 V dc—would have a much wider audience. There’s no harm reiterating such rules now and again, which could save a lot of headaches and perhaps liability down the road.
If the system or product for which your board is destined must meet IEC or UL standards, the minimum distance permitted between conductors that are subject to hazardous voltage will include a wide margin for safety, which depends on the peak working voltage, the laminate material, the operating environment (moisture, particulates, altitude), the circuit location relative to human access or proximity, and whether the traces involved are on an internal or outer layer. The spacing required on outer layers also depends on whether traces are coated or bare.
If the clearance is too small, a transient over-voltage event can result in arcing between the conductors. This is a virtually instantaneous fault that does not recur until another such over-voltage event. Faults resulting from insufficient spacing for creepage can take much longer to occur.
Determining what spacing to use is simple if your system or product requires compliance with international safety standards. Find the standard relevant to the product category and adhere to the tabulated values with respect to the details of your application as noted. For example, IEC-60950-1 (second edition) is the document to consult for most IT products to be sold internationally that are powered from ac mains, or batteries, and applies both to the primary side and the dc secondary circuits within those products.
Starting with section 2.10.3, midway through the document, the standard defines what clearances are required so that over-voltages, including transients and peak voltages that may be generated within the equipment, cannot cause faults. A series of tables, 2J through 2M, explains how to calculate clearances with respect to the details of your application. Next, section 2.10.4 explains how to determine the spacing required for creepage.
Pcb High Voltage Clearance
Clearance vs. Creepage
What’s the difference between clearance and creepage? Clearance is the shortest distance between two conductors measured through air. Creepage is the shortest distance between two conductors, measured along the surface of the insulation separating them. Consider the ways an electrical fault between conductors can occur if the clearance or the creepage spacing is too small. If the clearance is too small, a transient over-voltage event can result in arcing between the conductors, especially if there is dust in the air or humidity. This is a virtually instantaneous fault that does not recur until another such over-voltage event. Faults resulting from insufficient spacing for creepage can take much longer to occur.
Over time, usually stemming from dust buildup and moisture, a path (in other words, a track) for current to leak from one conductor to another can develop and cause a gradual breakdown of the surface of the insulating material between them. The mechanism for this is not a voltage spike—though that could contribute—but the continual presence of high voltage combined with an insulating material whose Comparative Tracking Index (CTI) is too low.
CTI is a measure of a material as an electrical insulator and is expressed as the voltage at which the surface of a material breaks down under a standardized test. The manufacturers of PCB laminates publish CTI figures for their products. For convenience, the breakdown voltages are lumped into six categories: The highest rating is 0 for withstand values of 600 V and greater and the lowest is 5, for less than 100 V. For example, conventional FR4 has a Comparative Tracking Index of from 175 to 249 volts, which places it in rating category 3.
Conventional FR4 has a Comparative Tracking Index of from 175 to 249 volts.
The paramount objective of all electrical safety standards is of course to prevent any danger of shock. If no one will ever be near a circuit while it is powered, the clearance and creepage allowances among its conductors that are subject to hazardous voltage can be the minimum that ensures the circuit will function and not deteriorate. IEC-60950-1 specifies five increasing levels of mandatory insulation related to the physical location of a circuit relative to human contact, the voltage, and the operating environment. Naturally, the best tactic to avoid the danger of shock or circuit degradation is to space traces as far apart as possible but very often that distance will not meet creepage requirements. What can be done in those cases?
If turning to a material with a better CTI is not feasible, routing a slot in the space between two traces can increase the creepage distance. Leakage from one trace to the other along the laminate surface would have to travel around the air gap. Alternatively, a vertical barrier of insulating material could be placed in the space, which would increase both the creepage and clearance distances.
Keep in mind the spacing on assembled boards when you consider creepage and clearance requirements for layout. The distance between an uninsulated edge of a charged component and the edge of an adjacent component could be too small to meet a clearance requirement, even though the creepage distance between their traces is sufficient. Moreover, there may be some operations needed during assembly to meet clearance requirements: for example, applying potting compound to isolate the leads on the package of a power semiconductor from some nearby component.
If your project does not require compliance with a particular safety standard, you can rely on the latest version of IPC-2221, the generic standard for PCB design, for clearance and creepage guidance. In any case, consultation with your PCB fabricator and assembler regarding suitable materials (you can read our article about choosing the right material) and design alternatives will help you achieve a safe product that meets your performance objectives at the lowest possible cost.
Our quick trip through the basics of laying out boards that involve hazardous voltage amounts to just a first step, simply an opportunity to point to out some helpful and hopefully familiar guideposts as you start your journey.