Screw terminals, also known as terminal blocks, provide a convenient method for reliably connecting wires and cables to printed circuit boards (PCBs). Screw terminals simplify field wiring by avoiding the need to solder connections. They also allow securing wires through mechanical screw pressure for robust, vibration resistant connections.
This article will provide an in-depth overview of PCB screw terminal functions, key types, design considerations, installation methods and applications to support electronics engineers in selecting and implementing the optimal screw terminals.
What is a PCB Screw Terminal?
A PCB screw terminal, or terminal block, is an electro-mechanical component that connects external wires or cables to a printed circuit board using screw pressure rather than soldering. It consists of a plastic housing that contains metal alloy screw heads and contact elements.
The key components of a typical screw terminal include:
- Housing – usually molded plastic that insulates and positions screw and contacts
- Screw – applies mechanical clamping force to secure wire
- Contact – metal alloy spring element that bears on wire
- PCB pin – soldered connection to PCB
By simply stripping wire insulation, inserting into the terminal opening, and tightening the screw, a solid electrical and mechanical connection is created without soldering.
Screw terminals are available in a huge range of configurations and options to accommodate different wire gauges, voltages, currents, pitches, PCB interfaces, and termination needs.
Functions and Benefits of PCB Screw Terminals
PCB screw terminals provide several important functions and benefits:
Simplified Wiring
- Eliminates need to solder cable terminations for faster assembly.
Secure Connections
- Screw pressure prevents wires from loosening due to vibration or thermal cycling.
Field Wiring Capability
- Allows user installation of wiring harnesses versus factory termination.
Serviceability
- Individual wires can be unscrewed then re-secured for maintenance and repair.
Reconfigurability
- Wiring changes are done by unscrewing and re-routing wires rather than desoldering.
Mixed Signals
- Different voltage or current circuits can be grouped into sections within a common header.
PCB Interface
- Compatible with through-hole and surface mount PCB assembly processes.
Modularity
- Separates PCB assembly from harnessing for independent design and build.
Reliability
- Proven robust technology rated for thousands of insertion cycles.
Screw terminals simplify electrical connections while offering flexibility for field servicing, updates, and reconfiguration.
Types of PCB Screw Terminals
There is tremendous variety in PCB screw terminal designs tailored to different applications. Here are the most common screw terminal types:
Spring Cage
The wire is secured between the screw head and a c-shaped spring metal contact element that bears directly on the conductor. Simple and low cost.
Spring Clamp
A leaf spring contact compressed by the screw provides constant pressure on the inserted wire for a gas-tight connection. No soldering required inside terminal.
Tension Clamp
A compressible clamping yoke assembly tensions and secures the wire when the screw is tightened. Allows some in-and-out wire movement.
Rising Cage
The contact rises up into the inserted wire as the screw is tightened, providing two clamping points for very secure connections.
Insulation Displacement Connector (IDC)
Sharp contact blades slice through wire insulation to make electrical contact to the inner conductor when the screw is torqued down.
PCB Header
Screw terminals are integrated into long housings with an array of contacts designed to mate with a plug-in PCB header.
High Power
Larger screw contacts, wire gauges, and housing designs suit high current connections up to 60 Amps or more.
Sealed
Rubber sealing boots, glandular wire entries, and potting allow screw terminals to be protected in harsh environments.
Low Profile
Compact screw terminal designs with reduced height above the PCB optimize space in tight equipment enclosures.
This wide selection ensures an optimal screw terminal design exists for most wiring interface needs.
Key Specifications
PCB screw terminal parameters that require careful specification include:
Pitch
Center-to-center distance between screw terminal positions, typically ranging from 3 mm to 10 mm.
Contact Material
Brass, copper alloy, or steel contacts support different current ranges and corrosion resistance.
Current Rating
Maximum current each terminal can handle without overheating. Depends on contact size and material.
Wire Gauge
Maximum and minimum conductor size that can be securely terminated, typically 22-12 AWG.
Voltage Rating
Maximum working voltage, often 300V or 600V rating.
Temperature Range
Operating and storage temperature ranges expected in the end application environment.
Screw Torque
Torque specifications for tightening screws to achieve proper clamping force without damage.
Termination Type
Solder pin, PCB header, or other PCB mounting style.
Carefully specifying these parameters ensures the selected screw terminals meet functional needs reliably.
Design Considerations for PCB Implementation
Here are some key design factors when integrating screw terminals on a PCB:
Grouping
Logically group similar voltage or current terminals together, separating types and ratings. Follow any safety spacing requirements between terminal blocks and other components.
Clearances
Leave adequate clearance around terminals for screw access during assembly and any required insulative barriers or potting.
Orientation
Position terminals for easiest access to wiring with minimal obstruction. Allow wire bending space.
Use manufacturer recommended PCB footprints and layouts. Verify surface mount pads match terminal solder pins.
Mechanical Stability
Provide structural support and strain relief for heavy wire bundles to avoid stressing solder joints. Allow some movement.
PCB Strengthening
Avoid thin necked regions of PCB underneath terminals. Add thickness for durability.
Marking
Clearly identify terminal block functions, ratings, and circuit connections on PCB silkscreen.
3D Modeling
Verify screw terminal height fits within product enclosure constraints and panel cutouts for access.
Careful mechanical and electrical design ensures reliable screw terminal PCB integration.
PCB Screw Terminal Installation and Wiring
Installing and connecting wires to PCB screw terminals involves these key steps:
Layout Verification
Visually check all terminals are in correct positions with proper PCB footprint prior to soldering.
Soldering
Solder terminals to PCB according to manufacturer instructions, checking for proper pin alignment and joint quality.
Structural Support
Provide any required mechanical braces or strain relief to stabilize wire bundles attached to terminals.
Wire Preparation
Strip wire insulation appropriately based on terminal size to expose conductor without nicking strands.
Wire Insertion
Fully insert each wire into the proper terminal opening. Ensure no stray strands or insulation.
Screw Tightening
Torque down screws to specified levels using calibrated electric or manual torque screwdrivers. Do not overtighten.
Inspection
Inspect each wire for proper insertion depth and screw tightness. No loose strands or debris in terminals.
Tagging
Affix wire labels to identify each connected circuit. Verify against wiring diagrams.
Proper PCB screw terminal installation and wiring is key to achieving robust, reliable performance.
Advantages Over Soldering
Screw terminals provide several advantages compared to direct PCB soldering:
No Soldering Skill Required
- Avoid need for specialized soldering training to terminate thick wires.
Faster Assembly
- Quickly insert and screw down wires vs. carefully soldering each.
Reduced Errors
- Eliminate risks of cold solder joints, bridging, pin misalignment.
Serviceability
- Individual wires can be readily accessed without desoldering.
Reusability
- Entire terminal blocks with pre-terminated harness can be removed and re-installed.
Vibration Resistance
- Mechanical screw force maintains reliable contact under vibration.
Mixed Signal Grouping
- Different voltage or current circuits can share a common block.
For many applications, screw terminals simplify wiring while improving field serviceability and enhancing reliability.
Limitations of Screw Terminals
While very useful in many designs, screw terminals do have some limitations versus soldering:
Larger Size
- Take up more PCB area than soldered pins for same number of connections.
Additional Cost
- Screw terminals cost more than bare pins per connection point.
Limited Current
- Low maximum amperage capacity suitable only for signals, not power.
Complex Assembly
- Additional assembly steps of wire preparation, insertion, and torqueing.
Open Connections
- Unlike soldering, screw joints can work loose over extreme vibration or heating.
Visual Inspection
- Harder to quickly visually inspect each terminal’s wire for proper installation.
Applications and Examples
Some typical applications that benefit from PCB screw terminals include:
- Connecting panel wiring in industrial control boxes
- Sensor and actuator cables interfacing to boards
- Board-level harnessing in automotive electronics
- Field-installable wiring on telecom or server equipment
- Quick connection for prototyping during development
- Fan, motor, solenoid, and other electromechanical drives
- Field serviceable medical or test instruments
Any application requiring removable wires or field cable installation are ideal candidates for screw terminal connections versus soldering.
Frequently Asked Questions
What is the maximum current capacity for PCB screw terminals?
A: While ratings vary, typical PCB screw terminals support 1 to 5 Amps. High current models can handle up to 60 Amps for power applications.
Is any special tool required to install screw terminals?
A: An electric or calibrated manual torque screwdriver allows setting proper torque specifications for each terminal without overtightening.
Can stranded wires be used with screw terminals?
A: Yes, stranded wire is commonly used. Ensure all strands are contained within the terminal opening. Ferrules can simplify insertion of fine stranded wires.
What is the typical pitch between PCB screw terminals?
A: 5.0 mm is a widely used pitch supporting a good density of connections. For higher density, 3.5 mm or 3.0 mm pitches are available.
How reliable are screw terminal connections versus soldering?
A: Industry standard screw terminals have been proven through extensive testing to provide very reliable gas-tight, vibration resistant connections comparable to soldering.
