Electricity is our friend. It provides us with light and energy that allows us to be maximally productive 24 hours a day and 365 days a year. Even so, it can be extremely dangerous, especially to those who do not understand its lethality. For example, it is not uncommon to see exposed electrical boxes that, if breached, could severely harm children or unwitting adults. Typically, these are well-labeled with warnings, such as “Danger High Voltage.” And it is true that educating the public about electrical hazards has improved in conjunction with easier access to information through the Internet. Yet, most non-electrical and/or electronic professionals probably do not realize that even relatively low currents (30mA AC or 0.5A DC) can be fatal.
Exposed electrical box
Electrical safety concerns are not relegated to electrical boxes or other high voltage equipment, including transmission and distribution lines, transformers, and caps. In fact, PCBAs, which comprise virtually all devices, appliances, and systems that we use, can be dangerous if a contingency occurs that exposes users or personnel to an excessive current level. This contingency is more likely to occur for industrial boards for which high current PCB design tips and guidelines should always be followed. Additionally, standard testing should be employed to ensure the boards can withstand the environmental conditions of factories or production facilities. A number of these tests measure the resistance of insulation to unwanted, alternative current paths or surface tracking. Let’s take a look at one of these, the comparative tracking test method, and how it is best applied.
What is the Comparative Tracking Index?
Similar to the high voltage arc tracking rate (HVTR), the comparative tracking index (CTI) measures or evaluates the ability of a PCB surface to resist tracking. Tracking, in this case, refers to the breakdown of the insulation material that exists between conductive surface elements (e.g. traces and pads). Typically, the insulation material is a type of solder mask; however, it may also include a PCB conformal coating type added to protect the board from contamination.
Why and When is the CTI Important?
HVTR testing and other standard high voltage arcing tests are performed on dry, solid insulation materials. By contrast, CTI testing is performed with liquid applied to the board’s surface. The objective is to evaluate the resistance of the board’s insulation to tracking in the presence of contamination, which is more likely for industrial boards, especially when embedded in production equipment, as found in advanced industrial processing environments. The CTI is important as it provides a quantitative metric by which different materials can be compared and a selection made based upon its probability to withstand its intended environment. Moreover, the CTI can be used to determine creepage and clearance distances for your board’s design.
What is the Best Comparative Tracking Index Test Method?
UL-746A Standard for Polymeric Materials - Short Term Property Evaluations gives the performance level categories (PLCs) and associated voltage ranges to rank and compare insulation materials, which are listed below.
|Comparative Tracking Index Category Ranges|
|Performance Level Category (PLC)||CTI Range Index (Volts)|
|1||400 - 599|
|2||250 - 399|
|3||175 - 249|
|4||100 - 174|
As shown, the voltage levels are what would be considered high for commercial circuit boards and are most applicable to industrial production environments. When developing boards for these environments, high voltage design considerations must be included to ensure your boards are ready for industrial PCB production. And optimizing your component layout, material, board protection and other development decisions requires that you incorporate CTI testing results.
In ASTM D3638 Standard Test Method for Comparative Tracking Index of Electrical Insulating Materials, a method of testing materials to determine the CTI is defined. This should serve as the guide for finding the index. Deviation from this standard may well result in erroneous data and unreliable performance once your board is deployed. Therefore, you should adhere to this standard irrespective of the testing options (listed below) that you opt to follow.
Options for Comparative Tracking Index Testing
- UL testing facility
Testing by a UL facility is the best option if testing is required. Not only can the CTI test be performed, but also related surface tracking evaluations.
- Other material testing facility
There are a number of companies that can perform material testing for you. The advantage here is that you are not required to have or acquire the necessary equipment or environment to meet standard requirements.
If you anticipate the need to perform this type of testing on a regular basis, it may be worth the expense to acquire the equipment and expertise to do so whenever required.
An additional option may be to utilize previous testing results, if available, and they are compatible with your design.
|Tempo's Industry 4.0 Custom PCB Manufacturing Services
If utilized accurately, CTI testing can aid you in selecting the best materials, PCB layout, and protective coating for your industrial board. However, any selections made must be done that can be implemented by your contract manufacturer (CM). At Tempo, we will work with you beginning on day one of design to ensure that your manufactured boards meet your design intent and can be built to a high quality.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.