The limiting factor for electronics manufacturing was no longer manual soldering as technology continues to taper and become more compact and modular. Many Electronics Designs utilize interconnecting wires to connect the printed circuit board (PCB) with others, therefore many professionals are seeking to streamline the assembly of their Power and Control Line Cables so that their Signal Integrity is not compromised. One of the hot trending topics on Reddit regarding this topic is “Do I have to use an SMD screw terminal to connect wires to a PCB?”.
As engineers and designers seek out innovative solutions that streamline their assembly process and provide Signal Integrity, the good news is that there are numerous options available today for Wire-to-Board (WTB) connections that do not require any soldering, and can be assembled easily and quickly with secure and dependable connections.
What’s Wrong with Soldering Wires Directly to a PCB?
Before discussing some of the ways to get around this problem, it is important that we understand the reasons for moving away from this method of making connections. In order to provide cheap toys with electrical connectivity, direct soldering provides a cheaper way to connect wires; however, this connection method has obvious engineering drawbacks in a professional environment.
Weaknesses at the Mechanical Joints
One of the most important reasons for moving away from direct soldering is that the joint created by soldering is made to make an electrical connection, not to provide mechanical support. When a solder joint is made directly between the wire and the PCB pad, the point at which the solder ends and the wire begins becomes a focal point for the mechanical stress applied during the use of the assembly. Vibrations from machinery and repeated flexing of the assembly cause the copper strands within the wire to become fatigued. These strands break at the joint between the solder and the flexible wire. This is an especially critical point of failure in the automotive and industrial industries, where control cables are subjected to consistent mechanical and physical stress.
The Screw Terminal Block – The Industrial Standard
Industrial standard screw terminal blocks are the first choice of most engineers when they need to connect wires without soldering. These blocks have been the backbone of many industries due to their ability to bridge the gap between high-power electrical applications and more sensitive electronic circuit applications.
Screw terminal blocks operate on a commonly used principle of clamping force. Once a wire is stripped, it is inserted into a metal cage manufactured from nickel-plated brass. At the same time the screw is tightened, the screw itself or the metal leaf presses down on the metal wire, compressing the metal wire against a conductive bus bar. When this occurs, a gas-tight connection is created that prevents oxidation and minimizes the contact resistance.
Dealing with High Power Control Cabling
Screw terminals are great because they can accept almost all sizes of wire. Many industries use large (or heavy) control cable assemblies to route power and data, and screw terminals provide the strength needed for the cables to be able to hold heavy loads without damaging the PCB. With a heavy cable assembly, a screw terminal should not pull off a PCB unless it is properly anchored to the PCB with a through-hole PIN type of anchor.
However, screw terminals have disadvantages. They require a tool (usually a screwdriver) to secure them in place, and most screw terminals may loosen over time as a result of excessive vibration. If left unsecured, the screws can become loose and the assembly may be disconnected.
Spring-Loaded Technology is defined as “Easy”
If your goal is to have the easiest connection method available, then spring-loaded technology (also called push-in or cage clamp) is the best option. Spring-loaded technology is the new generation of terminal blocks that was created to take human error out of the equation.
The experience of using spring terminals is a breeze! To connect solid wires or ferrules, all you need to do is push them into the hole, and the internal spring will deflect out of the way and hold the wire securely as it bites down on it. If you are connecting stranded wires, there will be a small lever or push-button that you use to open up the pushing mechanism on the spring so that the stranded wire can be pushed in. There will be no twisting, no torquing, and no uncertainty about whether or not you have tightened it enough!
Replacement of PCBs – Wire-to-Board Connector Systems
When you need to replace a whole printed circuit board assembly, ie for example replacing a motherboard in a 3D printer, or swapping out the battery module, it can take too long to disconnect the terminal block. The best option is often to use a pluggable header system.
Two-Part Connector System Design
A two-part (pluggable) connector system consists of two components: a pluggable header or connector that is soldered on the PCB, and an external housing that connects the wires together. The wires connect to the housing in a “solderless” way using a crimp technique whereby a metal contact is placed at the end of each wire and then is pierced through the connector housing.
The Polarization and the Safety Advantage
Pluggable Connectors (from companies such as JST, Molex and TE Connectivity) Provide One of Their Greatest Advantages in Their Polarization: Once the connector is inserted into a housing, it can only be put in place by inserting it in the proper direction and cannot be inserted into the opposite housing (reverse polarity). When designing products for consumers who may not know how to use or operate electronic devices properly, utilizing polarized plugs restricts consumers from connecting power incorrectly, thus protecting them from damaging their own electronic devices.
The initial set-up requires a crimping tool; however, utilizing a pre-assembled cable harness saves the company time and labor in a manufacturing environment. For this reason, the wires are pre-crimped and therefore, once the wires are in the final assembly; it will be as easy as ‘click and go’.
Advanced Solderless Solution & Engineering Choice
Screw terminals, spring action blocks, and removable headers have been used in the vast majority of applications. However, certain applications involving extremely high volume or extremely high performance, require more specialized solutions. Beyond what’s been described, there are advanced technologies purposely designed for the benefit of high-speed use, high environmental temperatures, or to enable a lower-cost method of connecting wires.
IDC Connectors – Speed and Signal Density
IDC Connectors are an entirely different wiring connection method to that previously discussed. It were designed for maximum efficiency. The largest advantage of an IDC Connector is that there is no wire stripping involved.
When an IDC Connector is inserted, the IDC, which has a sharpened “V” shaped blade on the inside of the connector, penetrates through the insulation of the wire, entering the copper wire and making it contact with the connector. The result of this contact is a gas-tight “cold weld”.
IDC Connectors have set the industry standards for multi-wire connections, such as ribbon cables in computers. Rather than terminating 20 separate wires with a crimping tool to make a single connection, a press of two thumbs on the IDC Connector terminates the entire ribbon cable at once. This is simply unbeatable in terms of assembly time in the manufacture of data buses and low-voltage signal lines, making it one of the fastest and most efficient ways to connect and interrupt wires.
Press-Fit Technology: The Fully Solderless Assembly
Press-Fit technology, a major advancement in professional engineering, eliminates solder from both wires and connectors on the PCB. Instead, it uses a “compliant pin” configuration, resembling a sewing needle, that creates a mechanical connection through an elastic force against a copper barrel when pushed into a plated hole on the PCB.
The elastic nature of the compliant pin provides a much greater resistance to thermal stress than solder joints have traditionally provided, allowing Press-Fit to be the preferred method for high-reliability applications within automobile and telecommunications equipment where a typical solder joint would fail due to thermal expansion cracking.
Edge Connectors for the Zero-Component Integration Solution
In some instances, the simplest connector solution is not to have any type of connector at all. Using Card Edge connectors as a male contact on the PCB is accomplished by using a GOLD-FINGER located at the edge of the board and inserting them into a corresponding connector on the main board.
The advantages of this connector type include low costs, because you do not need to purchase a connector for the daughter board. The disadvantages include you will need to manufacture using very high-quality tolerances and ENIG plating, as these connectors are likely to experience more than normal wear and tear due to frequent insertion. It’s also a very clean profile connector type, and can be used effectively with Modular Expansion Cards.
Selection Criteria: For Your Connector To Be Simple – A Summary
Considering the vast number of connector options available, determining which connector is the “simplest for your situation” will vary greatly, depending on the mechanical and electrical constraints for your particular application.
In order to ensure a reliable connector please consider the following four factors:
- Connector Ratings for Voltage and Current: It is essential to never under-specify the size of any connector. When using connectors intended for signal applications; failure often occurs when using a signal-level connector in a power application, please always use at least a 20-30% safety margin on connectors rated for high power inputs, and be sure that the connector has a sufficient amount of contact area in order to avoid overheating.
- Wire Type & Connector Wire Gauge Compatibility: Connectors and wires types should closely match in pitch. For example, a connector that has a Pitch of 1.27mm may physically be too small to accept a thick Power wire. Conversely, large screw terminals may not provide adequate grip on thin signal wires.
- Environmental Profile and Vibration Profile: If a connector is used in a high vibration application the screw connectors may loosen over time; therefore, Spring Loaded Terminals or a Press-FIT Connector or Locking Header would be required in these instances. If the application is going to be exposed to the elements, such as outside, you may want to use Gel-Filled IDCs or IP-rated seals to keep moisture out.
- Frequency of Interaction:One-time installation can use screw terminals, IDC terminals, or crimp terminals; pluggable connectors are available for maintenance; durable circular connectors or snap-fit systems are used for high-frequency applications.
Conclusion
Returning to the original question: “Is an SMD screw terminal my only option?”
The variety of wire-to-board connections available is extensive, providing options for many different users and uses. The Screw Terminal connector is a great choice for those beginning their rapids prototyping journey or for anyone just learning how to connect wires without needing too much time or effort. If you are looking for an easily installed wire-to-board connection that does not require any tools for installation and can withstand vibrations, then the Spring Loaded Terminal is your best option. For those customers who use modular designs to create products for distribution in mass quantities (mass produced) and consumers who wish to distinguish themselves from do it yourselfers, Pluggable Headers are a perfect choice.
