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What You Need to Know About UL Certification as an Electronics
Designer
Safety is always a concern in the back of an engineer’s mind. Even with all the
simulations in the world, you still wonder how your PCB will perform under a variety
of temperatures, environmental conditions, and handlings in its physical form. This
can only be verified with thorough testing. If you’re planning to design PCBs for
information technology, audio, video, medical, or appliance applications, then you’ll
need to be certified with the utmost safety standards. This means dealing with the
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details of UL certification. In this blog, we’ll be looking at what UL certification, what
kind of standards it requires for PCB designs, and how this certification affects your
job as an electronics design.
What is UL?
The Underwriters Laboratory (UL) was founded in 1894 by William Henry Merrill, an
American electrical engineer. UL is an independent, international organization that
tests products, certifies manufacturers and documents safety standards for a diverse
range of industries and applications.
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Underwriters Laboratory, making products safer since 1894. (Image source)
The UL Mark that Underwriters Laboratory offers is a well-known symbol of product
safety in the electronics manufacturing industry. There are more than 17 billion UL
marks added to new products every year, and their next mark might be on your board.
UL offers a rigorous set of standards that covers risk assessment for electronic
products, focusing on both fire protection and electrical safety. Since their founding
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in 1894, UL has evolved with electronics and electrical technology, offering safety
certifications for:
The first vacuum cleaner, electrical signal system, light shield for moving
pictures, and vehicle washer with electric lights in 1909.
The first automatic washing machine in 1937, which had several safety hazards
like water spilling into the motor.
The first black and white television in 1939. UL went on to work with TV makers in
1953 to produce protective shields and glass laminates to prevent imploding
picture tubes.
The first video game arcade machine, microwave oven, personal computer, and
solar collector between 1975 and 1980.
Today, UL is working on advancing safety requirements for lithium-ion batteries
through computational modeling, thermal analysis, and advanced testing approaches.
They’re also one of the leading certification groups to accelerate safety requirements
for electric vehicle batteries.
Did we mention UL now certifies hoverboards? (Image
source)
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These days UL has a very intimate relationship with the electronics design industry.
There’s a very high chance that somewhere along your career journey you’ll design a
product that requires the UL Mark of approval.
Why Should You Care About UL?
Many end product standards now require their PCBs to be built with UL safety
standards in mind. The following product categories are just a sample of the
applications that UL standards cover:
IEC 60065: Audio and Video Equipment
IEC 60960: Information Technology Equipment
IEC 60601: Medical Equipment
IEC 61010: Equipment for Laboratory Use
UL 508: Industrial Control Equipment
All of the applications above must be certified with a UL mark following three
standards, UL 796, UL 746, and UL 94. These are broken down into specific subcategories, which includes:
PCB Standard UL 796. This covers printed wiring boards (PWBs) guidelines for
standard rigid, metal-based, and high-density interconnect PCBs.
PCB Standard UL 796F. This standard evolved to cover flexible material electronics
including flex and rigid-flex PCBs.
PCB Standard UL 746E. This covers all industrial laminates and materials used in the
PCB manufacturing process.
PCB Standard UL 746F. This covers all flexible dielectric film materials used in the PCB
manufacturing process.
UL 94. This covers all flammability requirements for plastics and PCBs.
As part of UL 796, UL 746 and UL 94 standards, PCBs are thoroughly tested under a
number of conditions to evaluate the chances of electrical shock, flammability, and
mechanical strength. Here’s an overview of each category and the tests performed:
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MOT – Maximum Operating Temperature
DSR – Direct Support Requirement
RTI – Relative Thermal Index
Aside from verifying the safety of your electronic device, UL certification also offers
other benefits to end users and the industry as a whole, including:
Reach. UL certified PCBs are recognized globally and can help to extend the reach
of your product to new markets.
Standardization. PCB manufacturers can work from a common set of guidelines
to help standardize the fabrication and assembly process.
Trust. End users and consumers are provided a product that meets a rigorous set
of safety standards.
Types of UL PCB Certifications
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UL certifies PCBs under a PCB Recognition Program that comes in two flavors – Full
Recognition and Flame-Only Recognition. These break down to meet the following
requirements:
Types of PCB Recognition offered by UL. (Image source)
Flame Ratings
Flame ratings are typically specified by end product requirements and evaluate a
sample burn time when a PCB is subjected to a horizontal and vertical flame. These
flames are broken down into classes, which includes V-0, V-1, V2, VTM-0, VTM-1, and
VTM-2. Details on measured conditions for each class are shown below:
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UL flame rating classifications are broken down by criteria. (Image source)
Solder Limits
Solder limits test the kind of soldering processes that a PCB will be subjected to
during the component assembly process. These tests simulate a thermal shock and
maximum temperature to gauge how your PCB performs.
An example of how thermal shock is applied to solder, notice the cracks. (Image
source)
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Maximum Operating Temperature
Maximum Operating Temperature (MOT) defines a PCBs maximum continuous use at a
specific temperature. This test simulates the exposure of a PCB to normal operating
condition temperatures during a 10-56 day period, with temperatures based on your
manufacturer’s guidelines.
Direct Support Request
Direct Support Request (DSR) measures the performance of a PCB laminate when in
contact with 120V or less. This test is only performed on laminates rather than the
whole PCB, and each laminate receives a DSR compliant rating. During this testing, a
laminate will undergo several performance tests, including:
Dielectric strength
High current arc ignition
Hot wire ignition
Volume resistivity
Heat deflection
Comparative Tracking Index
The Comparative Tracking Index (CTI) also measures the performance of a PCB
laminate materials when subjected to a wet contaminant. CTI values are listed as
Performance Level Categories (PLC) as shown in the table below:
CTI range
PLC class
≥ 600
0
400
599
1
250
399
2
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175
249
3
100
174
4
< 100
5
In addition to Full-Recognition and Flame-Only Recognition, PCBs are broken down by
category to identify UL-specific requirements based on construction type. These
types include:
Single Layer PCBs
Full Recognition Requirements
Flame-Only Recognition Requirements
Minimum laminate thickness
Minimum starting copper foil thickness
Single or double-sided boards
Minimum laminate thickness
Single or double-sided boards
Multi Layer PCBs
Full Recognition Requirements
Flame-Only Recognition Requirements
Minimum dielectric build-up
Minimum dielectric build-up
thickness
thickness
Minimum laminate and prepreg sheet
Minimum laminate and prepreg sheet
thickness
thickness
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Minimum starting external copper foil
thickness
Maximum internal copper thickness
Metal Base, Single Layer PCBs
Full Recognition Requirements
Flame-Only Recognition Requirements
Minimum and maximum dielectric
thickness
Minimum and maximum dielectric
Minimum metal thickness
thickness
Minimum starting copper foil
Minimum metal thickness
thickness
Single or double-sided boards
Single or double-sided boards
Metal Base, Multi Layer PCBs
Full Recognition Requirements
Flame-Only Recognition Requirements
Minimum and maximum dielectric
Minimum and maximum dielectric
build-up thickness
build-up thickness
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Minimum individual sheet thickness
Minimum laminate and prepreg sheet
Minimum starting external copper foil
thickness
thickness
Minimum metal thickness
Maximum internal copper foil
Single or double-sided boards
thickness
Minimum metal thickness
Single or double-sided boards
High Density Interconnect PCBs
Full Recognition Requirements
Core Requirements
Flame-Only Recognition
Requirements
Core Requirements
Minimum dielectric build-up thickness
Minimum dielectric build-up
Minimum laminate and prepreg sheet
thickness
thickness
Minimum laminate and prepreg
Minimum eternal copper foil thickness
sheet thickness
when no HDI material is applied
Maximum internal copper thickness
HDI Requirements
HDI Requirements
Minimum and maximum HDI
layer thickness
Minimum and maximum HDI layer
Minimum and maximum number
thickness
of HDI layers applied
Minimum and maximum number of HDI
layers applied
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Maximum internal copper thickness
Minimum external copper foil thickness
Flexible PCBs
Flexible PCBs are defined by both their application type and construction type. You
can reference the above tables for a flexible PCB construction type with the following
application parameters:
Flexible PCBs are tested for dynamic and repeated bending.
Flex-to-Install PCBs are tested for flexing during installation and field servicing.
Rigid-Flex PCBs are assessed for a variety of applications, and construction
types gave their combination of flexible and rigid circuitry.
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What Does the UL Mark Mean On a PCB?
When your PCB receives a UL Mark from your manufacturer, it signifies that your
board meets all established safety and flammability requirements. Specifically, a UL
Mark signifies that:
The base material used in your PCB meets a specified flammability level per
UL94. For example, when FR4 is subjected to an open flame and then removed it
self-extinguishes within 10 seconds and does not drip any flaming particles.
Each base material that a manufacturer offers will go through this testing
scenario.
The base material used in your PCB meets the required level of ability to resist
ignition from external electrical sources.
The base material used in your PCB meets the required electrical breakdown
value. This is the voltage difference where the insulation properties of a base
material may start to break down and cause safety issues.
The base material in your PCB meets the performance levels for direct support of
current carrying capacities as specified in UL 796A.
When your PCB receives its UL Mark, it will be printed on your silkscreen layer as
shown in the image below. In this example, UL94V-0 signifies that this board meets
UL94 flammability requirements at flame class V-0. The E198312 is a unique UL Mark
identifier for a specific manufacturer, and ML signifies that this is a Multi-Layer PCB.
Example UL Mark on a
PCB. (Image source)
How Does UL Affect My Job as a PCB Designer?
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UL certification is driven by the end product requirements, and it’s up to you as the
PCB designer to know and understand these requirements. We recommend first
checking with your end product manufacturer before beginning your design process
to determine the specific requirements your PCB must meet. If you are designing any
of the UL specified products below then you’ll likely need to get your hands on UL
standards documentation:
UL 60950 – Information Technology Equipment
UL 60065 – Audio & Video Equipment
UL 62368 – ITE and Audio/Video Equipment
UL 60601 – Medical Equipment
UL 60335 – Appliances
UL 61010 – Industrial Control Equipment
UL 8750 – LED Equipment
Once you know your end product rating requirements, it’s time to get started on your
design. There are several things to keep in mind when designing a board for UL,
including:
Always Select UL Certified Parts
Make sure you always use UL listed parts whenever you have a line voltage present in
your design as this will reduce the cost of certification. If you don’t select UL listed
parts from your distributor, then someone down the line will have to do the
investigation and adjustments. Instead of risking a delay for your project, be sure to
look for UL certification marks when putting together your component list. You might
need to dig into the datasheet for each component to find the UL approval, as shown
for this Metal-Oxide Varistor below:
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Finding the UL approval for components can take some datasheet investigation.
(Image source)
Follow Clearance and Creepage Distances On Your Board Layout
For high voltage design with operating voltages between 30VAC and 60vDC, you’ll
need to pay particular attention to your clearances and creepage when setting up
your design rules. There is some confusion between these two rules. Most design
tools, including EAGLE, categorize both creepage and clearance rules under a general
Clearance category. However, there’s an important distinction to make between both:
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Rules for pad-to-pad, pad-to-trace, or trace-to-trace, spacing are considered
creepage rules. These rules define the shortest path between two conductive parts.
Rules that define the spacing between two conductive elements measured through
the air are considered clearance rules.
The difference between clearance and creepage per
IEC guidelines. (Image source)
Clearance and creepage requirements will be determined by your specific design
requirements, and there are no general rules to offer here. Instead, be sure to use
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this Creepage and Clearance Calculator which meets specifications for UL 60950 and
UL 60960-1 among other popular IEC guidelines.
Specify Dielectric Materials for Your Manufacturer
For some end products, UL certification can take weeks or months to secure as your
device goes through a series of qualifying tests. Instead of allowing your
manufacturer to select any fiberglass material, be sure to specify your dielectric
material requires in your design documentation. This will help to avoid any delays in
your certification process and costly respins.
Getting Your UL Marking
The good news is that getting a UL Marking on a PCB is a relatively straightforward
process. The bulk of UL requirements rest on the shoulders of your manufacturer as
part of a UL compliance program. Under this program, your manufacturer is audited
for UL compliance four times per year. For manufacturers producing Full Recognition
PCBs, random production boards will also be subject to annual testing.
When getting a quote for the fabrication of your PCB most manufacturer websites will
have a section where you can select your needs for a UL Marking. In the image below
we were able to add a UL Mark through the pcb.cart.com quote tool.
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Safety First
Whether you’re designing medical equipment, an IT device, or even a PCB for an
appliance, chances are you’ll need to have your board UL certified. The bulk of the
certification process is already handled by your manufacturer as they work with UL to
test materials and components. However, as a PCB designer, you still need to take the
time to research and define the requirements of your circuit board to meet your end
product needs. Once your requirements are defined, you’ll need to select UL certified
parts during your schematic design and follow creepage and clearance distances on
your board layout. Most importantly, when documenting your board, it’s critical to
specify the right dielectric material that will meet your end product requirements.
Ready to design your first UL recognized PCB? Subscribe to Autodesk EAGLE today!
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