Ohm’s law: it’s a wonderful tool we have for analyzing all types of circuits. This simple relation applies to so many devices that it’s quite easy to explain many aspects of component behavior with this single equation. With high voltage PCBs, however, we have to use other tools in addition to Ohm’s law to understand some important aspects of circuit behavior. Bring in Paschen’s law and Kirchoff’s laws, and you have everything you need to understand the operating principles of high voltage PCBs.
One important effect that occurs at high voltage is PCB leakage current. This effect is explained quite simply using Ohm’s law: if there is a potential difference between two points on your board, the current between these two points will be lower when the resistance is higher. As your PCB is placed into operation, the leakage current can change for a number of reasons. Your job as a designer is to anticipate these problems and choose appropriate materials to minimize leakage current.
What is PCB Leakage Current?
In the high voltage design world, whether we’re talking generally about PCBs or high voltage systems design, leakage current results from a DC potential difference between two points. On a PCB, two conductors with a potential difference are separated by an insulating substrate, and some current can flow through the substrate between these two conductors. A ~10 V potential difference is enough to produce ~10 nA of leakage current, depending on the conductivity of the substrate.
The porosity of fiber weave substrates and solder mask materials causes them to uptake water during fabrication, and this water uptake continues over time during operation. Moisture can be present in the epoxy glass prepreg material and in any micro-cracks in the substrate before fabrication. Water and other liquids can be absorbed during the wet manufacturing processes, and moisture can diffuse into the surface of the PCB during storage.
A PCB deployed in a high humidity environment will uptake water until the moisture content saturates. PCB substrates with higher moisture content will have higher leakage current as water and other liquids used during PCB manufacturing processes are polar, thus they tend to have high conductivity. Over time, the PCB leakage current across the board will increase, even if the board is prepared in a moisture-free environment and heavily outgassed prior to deployment. In addition to moisture, small dust particles can accumulate on the board, and dust will accumulate faster in areas where the electric field is larger. Moisture and dust both contribute to increases in the PCB leakage current over time. Moisture and dust accumulation also make the surface more susceptible to arcing, i.e., the breakdown field is lower across the surface of the board.
Dust can lead to increased PCB leakage current
A large leakage current between the nodes of a component with high impedance input can lead to a rather large drop in the input voltage seen by the component, similar to IR drop. As an example, consider a PCB leakage current of 100 nA diverted across the positive and negative leads of a component with 1 MOhm input impedance—according to Ohm’s law, this will decrease the input voltage by 0.1 V. This should be considered alongside the PCB leakage current when determining failure criteria for your high voltage board.
Creepage, Clearance, and Leakage Current
Leakage current can already occur across an insulating substrate simply due to a DC voltage difference, but leakage current also increases after an initial breakdown occurs between two charged conductors. In the event that breakdown between two conductors does occur, carbon can accumulate along the surface of the PCB. The track that forms along a carbonized surface is rather conductive, which increases the leakage current between two points on the board with a high potential difference. Extremely serious carbonization, such as breakdown in a carbon-rich atmosphere or repeated breakdown events, can effectively form a short circuit between two points on the board.
IPC 2221B is the general standard that covers creepage and clearance distances as a function of voltage, elevation level, and coating. Although this standard specifies these distances as a function of elevation, the real parameter that determines breakdown field is the atmospheric pressure for air between conductors (according to Paschen’s law). The moisture content in air will also affect the breakdown field as well as the potential for leakage current to increase over time. These factors also affect creepage and clearance requirements; high voltage systems should generally be over-designed for safety purposes and to reduce leakage current.
If your board will be deployed in a humid environment, there’s almost no point in removing any moisture from your finished board as it will just be re-adsorbed back into the board once it’s placed in operation. There are some insulating conformal coatings for humidity protection that are designed for high voltage PCBs.
For boards with dust problems or with leftover residues, a simple washing procedure is sufficient to remove contaminants from your PCB. This involves brushing the boards with isopropyl alcohol, followed by rinsing with deionized water and baking the board at 85 °C for a few hours. You should still be careful when using solvents on boards with water-soluble ﬂuxes; mixing these materials can leave behind salt deposits after the board is dried and baked.
You shouldn’t clean your high voltage PCBs this way...
The CAD tools and routing features in Altium Designer® are ideal for creating PCBs for any application. These design features automatically check your layout against important design rules, ensuring your board complies with important creepage and clearance standards, as well as PCB leakage current standards. You’ll also have a full set of documentation features that help you prepare for manufacturing and assembly.
About the AuthorMore Content by Zachariah Peterson