PCB (printed circuit board) screw terminals are a fundamental component in electronics, providing a secure and reliable method for connecting wires and cables. Their versatility and ease of use make PCB screw terminals popular among design engineers and hobbyists for prototyping, testing, and production.
PCB screw terminals enable fast installation and wiring replacement without soldering. Their modular design also allows multiple wiring configurations to be changed out swiftly during product development or repairs. This article will explore what PCB screw terminals are, the advantages they offer, types and specifications, and how to select the right one for your application.
What are PCB Screw Terminals?
A PCB screw terminal is an electro-mechanical component that provides an interface for connecting individual wires or cables to a printed circuit board. It comprises a plastic housing that mounts directly onto the PCB and metal screw terminals for the wire connections.
The housing is soldered to pads on the PCB to create a solid mechanical and electrical connection. The metal screw terminals provide a clamping mechanism to secure the stripped ends of wires inserted into the terminal block. Tightening the screw terminal creates a solid metal-to-metal connection between the wire conductor and the terminal.
PCB screw terminals are often arranged in single or multiple rows in the housing. Multiple positions allow for connecting several wire conductors and signals in a compact footprint. Standard configurations are 2, 3, 4, 6, 8, and 10, but blocks with over 50 positions are readily available.
Benefits of Using PCB Screw Terminals
Several vital benefits make PCB screw terminals a staple wiring interface for engineers:
Quick and Simple Wiring
PCB screw terminals allow wires to be securely fastened without soldering or crimping. Wire preparation only requires stripping a short length of insulation. Insert the stripped wire end into the terminal block opening and tighten the screw to clamp down on the conductor.
This straightforward wiring process enables faster assembly and lower production costs than soldering. Wiring errors are also reduced.
Flexible and Adaptable
One of the significant advantages of PCB screw terminals is their modular design. Swapping out terminal blocks or rearranging wire positions can easily change wiring configurations. This adaptability streamlines prototyping, testing, repairs, and modifications during a product’s life cycle.
Terminal blocks commonly have a standard pitch (distance between terminals), making mixing and matching different blocks straightforward. Spacing options like 3.5mm, 5mm, 7.5mm, 10mm, etc., allow designers flexibility for their layout and wiring density requirements.
Reliable and Vibration Resistant
The metal-to-metal screw clamping mechanism creates a gas-tight connection between the wire conductor and the terminal. This ensures a low resistance and robust connection over thousands of mating cycles. PCB screw terminals maintain integrity in high-vibration environments where solder joints could fail prematurely.
Unlike soldering, PCB screw terminals don’t require handling hot tools or exposure to lead fumes. The housing insulation protects against accidental contact with live terminals. These safety advantages make PCB screw terminals a good choice for novice hobbyists.
Easy Maintenance and Troubleshooting
Individual wires can be quickly accessed, removed, replaced, or reused without desoldering or damage to the PCB or adjacent wires. This facilitates maintenance, upgrades, troubleshooting, and field repairs. Being able to isolate and swap out a lousy wire saves considerable time compared to soldered connections.
Types of PCB Screw Terminals
There are several joint PCB screw terminal types available to suit the wiring requirements, PCB space, operating voltages/currents, and cost considerations of a design:
Rising cage PCB screw terminals are one of the most popular types. They utilize a spring-loaded cage under the screw head that rises as the screw is tightened. This allows wires to be easily inserted into the open cage. Tightening the screw then forces the cage down to clamp the wire.
Rising cage terminals are affordable, easy to use, and reliable. They accommodate a wide range of wire sizes from 24 AWG to 10 AWG. Current ratings up to 20A per terminal are typical. Voltages up to 600VAC/DC can be handled.
Screw and Clamp
Screw and clamp PCB terminals provide a slim, space-saving footprint. Wires pass through a vertical channel in the housing. A separate horizontal clamp piece has an opening that matches the wire size. Tightening the screw compresses the wire between the clamp and a metal contact in the housing.
These economical terminals work well for smaller gauge wires from 22 AWG to 16 AWG. Ratings are usually 10A at 300VAC/DC or less. Their compact design allows packing many positions into a small PCB area.
Piercing screw PCB terminals offer fast installation by eliminating wire stripping. Instead, turning the screw forces pointed metal contacts through the wire insulation to make an electrical connection to the inner conductor.
Piercing terminals accommodate 18 AWG to 24 AWG solid core or stranded wires with thin insulation. The current per terminal is generally 10A or less. These simple terminals provide low-cost wiring in high-volume products like appliances.
Push-in screw terminals speed up wiring by eliminating the need to turn screws. Wires are pushed directly into the spring-loaded terminal. The terminal’s clamping mechanism secures the wire without tools.
Push-in terminals allow quick connection changes by depressing the spring tab to release wires. They handle light-duty wiring from 12 AWG to 26 AWG if rigidity requirements are met. The current per terminal is typically 10A or less.
Pluggable screw terminals add modular convenience for connecting cables or wires to boards. They utilize detachable plugs that hold multiple wire terminations. The plugs mate to a header soldered on the PCB.
This arrangement allows plugs to be installed, removed, or replaced without desoldering. Pluggable terminals easily accommodate wiring modifications or replacements. They work well for signal-level connections under 3A.
Feed-through screw terminals facilitate joining two separate wires or cables. Wires are inserted from opposite sides of the terminal housing and clamped together with the screw.
Feed-through blocks allow flexibility in tying various circuits together or splitting connections. They have similar electrical specs to the rising clamp and screw and clamp terminals. Feed-through types come in pluggable versions too.
Ground screw terminals provide a convenient connection point to ground wires and cables safely. The terminal screw clamps the wire to the grounded mounting frame or chassis.
Including dedicated ground terminals on the circuit board simplifies meeting safety requirements. Ground terminals help minimize noise by giving shielded cables a low-impedance path to the chassis ground.
PCB Screw Terminal Specifications
Several key specifications should be evaluated when selecting a PCB screw terminal block for an application:
- Pitch refers to the center-to-center distance between adjacent terminal screw positions in a terminal block.
- It is a critical dimension that impacts wiring density, current capacity, and overall footprint.
- Common standard pitch sizes include: 2.5mm, 3.5mm, 3.81mm, 5mm, 5.08mm, 7.5mm, 7.62mm, 10mm
- Larger pitch sizes:
- Allow the use of thicker wire gauges
- Enable higher current capacity per terminal
- Reduce the risks of overheating
- Provide more space for tool access during wiring
- Can lower wiring density on the PCB
- Require a larger overall footprint
- Smaller pitch sizes:
- Allow the use of smaller gauge wires
- Reduce current capacity per terminal
- Increased risks of overheating
- This can make accessing terminals difficult
- Maximize wiring density on the PCB
- Minimize overall footprint
- Help where PCB space is very constrained
- Pitch size compatibility:
- Using terminals with standard pitches enables mixing and matching different blocks as needed
- Standards like 5mm or 10mm pitch aid interoperability
- Consistent pitch allows flexibility in PCB layouts
- When selecting a pitch, consider the following:
- Expected wiring densities
- Anticipated operating currents
- Available PCB area
- Need for future expandability
- Mixing terminal types
- Ease of wiring access
- Optimizing pitch size for an application helps maximize the performance and reliability of PCB screw terminals.
Number of Positions
- The number of positions refers to the quantity of individual terminal screws inside the terminal block housing.
- Each position provides a separate electrical connection point for a discrete wire.
- The typical range is from:
- 2-position up to
- Over 50-position
- Benefits of more positions:
- Allows interfacing more wired circuits and signals
- Reduces the need for additional terminal blocks
- Consolidates connections in one footprint
- Simplifies wiring architecture
- Drawbacks of more positions:
- Requires larger overall PCB area
- It can make tracing wires more difficult
- Increases complexity if any position fails
- The current rating indicates the maximum steady-state current each terminal position can carry without exceeding temperature limits.
- Typical per terminal current rating ranges from:
- 1A up to
- Over 30A
- Factors influencing current rating:
- Contact metal thickness and conductivity
- Cross-sectional wire engagement area
- Housing thermal dissipation ability
- Benefits of higher current rating:
- Allows using thicker wire gauges
- Handles more power-hungry loads
- Reduces risk of overheating failures
- Drawbacks of higher current rating:
- A larger terminal size may be required
- Can limit wiring density
- Often has a higher per-unit cost
- The voltage rating specifies the maximum working voltage that can be applied between terminal connections without risk of arcing or electrical breakdown.
- Standard voltage ratings include: 300V, 600V, 1000V
- Factors influencing voltage rating:
- Spacing between current carrying components
- Creepage distance along insulating materials
- Dielectric strength of insulating materials
- Benefits of higher voltage rating:
- Allows interfacing higher voltage circuits
- Reduces risk of arcing and short circuits
- Provides hazard protection from shocks
- Drawbacks of higher voltage rating:
- Increased size to accommodate the spacing
- Higher cost components
- Reduced wiring density
Wire Size Capability
- The wire size range specifies the minimum and maximum conductor gauge that the terminal can securely clamp.
- More diminutive gauge wire compatibility:
- Typically from 24 AWG to 12 AWG
- Best for signal, data, and light power wires
- More extensive gauge wire compatibility:
- Usually from 10 AWG up to 500 MCM
- Used for higher power and load currents
- Benefits of more extensive wire capability:
- Allows terminating thick cable and bus bars
- Reduces voltage drop over long cable runs
- Handles higher power levels
- Drawbacks of more extensive wire capability:
- Increased terminal size required
- Reduced wiring density
- Higher cost
- PCB screw terminals come in two main mounting styles:
- Surface mount
- Through-hole mounting:
- Terminal blocks have metal pins
- Pins are inserted into plated thru-holes on the PCB
- Soldered to copper pads underneath the board
- It provides a solid mechanical connection
- Benefits of through-hole mounting:
- Very secure mechanical attachment
- More straightforward visual inspection of solder joints
- No specialized PCB finish required
- Drawbacks of through-hole mounting:
- Requires plated PCB holes
- More challenging to automate the assembly
- Not suitable for double-sided boards
- PCB screw terminals can have different wire orientation options:
- Vertical (180°)
- Horizontal (90°)
- Angled (45°, 60°, etc.)
- Vertical wire orientation:
- Wires exit PCB straight up
- It takes up the least amount of space
- This can make accessing wires more difficult
- Horizontal wire orientation:
- Wires exit the PCB parallel to the board
- It provides the easiest access to wires
- Requires space around the perimeter of the PCB
- Angled wire orientation:
- Wires exit at 45°, 60° angles
- Balances space and access considerations
- Useful for wires routed to the board edge
Material and Plating
- PCB terminal housings are typically:
- Thermoplastic or thermoset polymers
- Withstand soldering temperatures
- Provide electrical insulation
- Terminal metals are usually:
- Brass or steel alloys
- Plated in tin, nickel, gold
- Over copper base material
- Benefits of plating metals:
- Resists corrosion and oxidation
- Creates reliable solder joint
- It avoids cold or cracked joints
Reputable PCB screw terminals meet safety standards like UL, CSA, VDE, ENEC, or IEC. Check for markings indicating certification by these agencies.
How to Select the Right PCB Screw Terminal
Choosing the optimal PCB screw terminal requires matching the terminal specifications to the wiring requirements, operating voltages/currents, PCB space constraints, and other factors influencing the application. Consider these guidelines when making your selection:
Confirm Electrical Needs
- What is the maximum expected operating voltage? Select a voltage rating at least 25% higher.
- What is the maximum continuous current in each wired circuit? Choose a current rating at least 25% above this.
- Will transient voltage spikes exceed the steady-state voltage? Surge ratings may need to be higher.
- Is wiring primarily for power, signals, or a mix? Power wiring needs heavier gauge wires and higher current terminals.
Evaluate Wire Size and Material
- What conductor gauge will be used? Pick a terminal that fits the wire size or allows a gauge size up and down.
- Are wires stranded or solid core? Stranded can require special terminals or ferrules to grab multiple strands.
- Is wiring flexible cable, jacketed cable, or individual wires? Larger diameters may need bigger terminal openings.
- Do shielded cables need to be grounded? Select grounding fork terminals to interface shields.
Determine Mounting Factors
- Does the PCB have plated through holes or only surface pads? Match the terminal style to PCB capabilities.
- What spacing between terminals is needed for wiring access or service loops? The larger wire may need more elbow room.
- Will the wiring exit vertically from the PCB or run horizontally? Vertical terminals save space.
- Are higher-density connections required? Smaller pitch terminals allow compact wiring.
- Is polarization needed to prevent miswiring? Keyed housings only allow correct insertion.
Assess Environmental Conditions
- What is the operating temperature range? Standard terminals function from -40°C to +105°C.
- Will condensation or liquid splashing occur? Water-resistant housings prevent short circuits.
- Does the application vibrate or shock the PCB? Locking screws prevent loosening.
- Is corrosion from chemicals a concern? Gold plating resists harsh environments.
Review Cost Factors
- What is the overall budget? Better performance costs more.
- What production volume is anticipated? Higher quantities permit lower-cost terminals.
- Can vertical integration reduce supply chain costs? Some terminal vendors offer integrated manufacturing.
- Does the design need customization? Unique pinouts or housings add cost.
- What safety agency certifications are mandatory? UL recognition is typical in North America.
- Do hazardous location approvals apply? Class I Div II ratings are required for explosive environments.
- What environmental regulations must be met? Lead-free materials may be preferable.
Quick Tips for Using PCB Screw Terminals
Follow these practical tips when utilizing PCB screw terminals:
- Leave enough wire length for service loops to simplify future changes.
- Confirm torque specifications and do not overtighten terminal screws.
- Use appropriately sized ferrules or wire-end sleeves when terminating stranded wires.
- Apply a low-residue flux pen to wire ends if soldering stranded conductors.
- For high-vibration environments, use gas-tight terminals or adhesives at connections.
- Prevent oxidation and loss of connections by applying antioxidant paste to wire ends.
- Verify full screw engagement, as wires inserted lower in the opening can shorten the clamping range.
- Inspect for loose terminal screws periodically and retighten them according to torque specifications.
- To prevent overheating, arrange wiring to avoid exceeding the maximum allowable terminals per row.
PCB screw terminals offer engineers a simple and flexible interface for connecting electronics to wires and cables. Their easy installation, reconfigurability, and vibration resistance are advantages over soldering for circuit development, repair, and production.
Understanding key specifications like voltage and current ratings, wire sizes, mounting styles, orientation, and materials ensures matching the terminal type to application requirements. Following the best selection, wiring, inspection, and maintenance practices help realize the full benefits of PCB screw terminals.
The wide array of terminal options from leading manufacturers provides the capabilities to handle everything from signal-level wiring to high-density power distribution. PCB screw terminals simplify circuits and stand the test of time across countless electronic devices and industrial equipment.
Frequently Asked Questions (FAQs)
Can you solder wires directly to PCB screw terminals?
It is not recommended. Soldering stranded wires to screw terminals can produce a poor-quality connection. The solder wicks up wire strands producing a cold solder joint. Use ferrules or solid wire ends when soldering conductors.
What are PCB screw terminals made from?
Most consist of thermoplastic or thermoset housings combined with internal metal components that are plated brass or steel. This allows withstanding soldering temperatures and provides corrosion resistance.
How are PCB screw terminals secured to a circuit board?
PCB mount screw terminal blocks feature through-hole pins that fit into plated PCB holes or surface-mount style contact pads. The terminals are soldered to copper pads on the board to create a reliable electrical and mechanical connection.
Can you mount PCB screw terminals directly to a chassis?
Yes, some PCB mount blocks include threaded holes on the underside that allow fastening directly to a metal chassis or frame using screws. This arrangement provides secure strain relief when wires exit the housing.
What is the best way to connect stranded wires to screw terminals?
Use ferrules or wire end sleeves to bundle the strands together and provide a solid end for the terminal clamping screw. Ferrules prevent stray strands from potentially shorting and improve termination integrity.
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