Essential PCB Connectors : From Selection to Seamless Integration

PCB connectors are the unsung heroes of electronic design, as they are used in nearly every design. A right connector in the PCB can be a lifesaver in many devices. When I was designing the first PCB, a wrong battery connection caused the circuit to overheat and potentially burn out. This happened because I used to just drill holes instead of using a connector for batteries.

However, a perfect connector, specifically a 2-pin JST, solved my problem with polarity. In many designs, not just for polarity, but also for easy connections, such as when using two different PCBs in an application, connectors are a convenient way to connect the boards. This allows for disassembly to troubleshoot any circuit problems. From laptops to EVs, from IoT sensors to industrial machines, the connectors silently hold everything together. In this article, we decode the connector universe with a blend of engineering insights and some real-world examples.

Decoding PCB Connector Basics:

Among the many types of PCB connectors are board-to-board, wire-to-board, card-edge, FPC, coaxial and power. Each style is designed to address a unique challenge. For instance, some can be used more specifically for compact stacking, while others are designed for high-speed transmission. There are connectors that simply deliver power without overheating. Let's take a few steps back and discuss the fundamentals.

Pin Configurations and Pitch Essentials

On a circuit board, the pins are used to connect other components of hardware. For these pins, we usually assign pin headers. However, in some specific configurations, we need to use connectors that exactly fit the header we want to use.

Pin configurations determine:

• Number of circuits
• Current carried per pin
• Alignment and mating compatibility
• Signal grouping (power, data, ground)

Pitch is the parameter used to determine the distance between adjacent pins; it affects the board space. Some standard pitch sizes are:

• 2.54 mm (classic)
• 1.27 mm (denser)
• 0.5–0.8 mm (fine-pitch)

A smaller pitch results in higher density, but also increases headaches during the assembly process.

Electrical Ratings for Reliable Connections

When selecting a connector, the electrical rating is the most important factor because even the most elegant connector becomes a liability if the electrical ratings aren't matched correctly. We should check:

• Current rating per pin: to avoid overheating
• Voltage rating: to reduce signal drops and current leakage
• Contact resistance: should be as low as possible

Criteria for Smart Connector Selection:

Choosing a connector is like selecting shoes; you must match function, size and durability. Of course, style matters sometimes too:

Matching Board Layout and Space Constraints

Board space is expensive real estate. Connector fit must be an all-rounder. A good connector can be placed in a universal manner on the board, from board edges to midway components and inside the enclosure within a height. Here is the table of almost all types of connectors I have used in my own designs:

For compact devices like wearables, choose:

• FPC/FFC connectors
• Fine-pitch board-to-board connectors

For modular boards:

• Card-edge or backplane connectors

For serviceable systems:

• Wire-to-board connectors with locking features

Durability in Harsh Environments

Connectors used outdoors, in vehicles or in industrial plants must tolerate:

• Vibration
• Shock
• Temperature swings
• Moisture
• Dust
• Corrosive atmospheres

Rugged connector types include:

• Sealed wire-to-board connectors
• Automotive-grade locking connectors

If your product will live anywhere near an engine bay, assume your connector will experience more stress than a student preparing for finals.

Installation Best Practices:

Even the best connector fails when soldered incorrectly or installed carelessly. A few best practices can save you hours of debugging.

Soldering Techniques and Tool Recommendations

SMT connectors:

A proper reflow profile should be followed when working with SMT, because these are soldered either on the top or bottom layer of the board. For a perfect alignment of tabs, we can use a controlled paste volume in order to avoid tombstoning.

THT connectors:

For through-hole technology (THT) connectors, a solder wave is used, as they are soldered from top to bottom on the board. Hand-soldering is the most recommended method with a controlled heat profile. To avoid cold joints and ensure reliability, AOI (Automated Optical Inspection) can be utilized on production lines. Poor soldering is responsible for about half of "mystery connector failures."

Ensuring Proper Alignment and Strain Relief

Mechanical stress is applied to connectors each time a cable is connected or disconnected. The mechanical stress created may cause damage to the PCB, therefore strain relief should be used with wire-to-board connectors. It is also possible to route critical traces away from the connector to prevent breaking the copper connection. If your connector has movement (i.e., like a wiggly tooth), then it is just a matter of time before it fails.

Diagnosing Connector Failures:

Even robust connectors eventually face problems. Knowing what to look for helps reduce downtime.

Intermittent Contact and Corrosion Issues

Symptoms can be random resets, flickering LEDs, and increased voltage drop due to contact resistance. The connector failure can be due to:

• Dust and moisture
• Oxidation of pins
• Mechanical wear

Gold plating helps reduce corrosion because gold is known as the least reactive metal. We can precisely control contact resistance using gold. The gold plating is not that costly if used in high-end electronics.

Quick Fixes and Preventive Maintenance

We should always use sealed connectors for outdoor use. Another way to make it waterproof is  by adding a conformal coating around the connector pads. Connectors can be used with locking mechanisms to avoid loose connections. Some common fixes are:

• Clean contacts with isopropyl alcohol
• Reseat connectors
• Apply contact lubricant (for automotive/industrial)
• Replace damaged sockets or pins

Real-World Applications Across Industries:

Real-world applications mean onboard applications or those in our daily life.

Consumer Electronics and Automotive Uses

Consumer electronics

• USB connectors
• Battery connectors
• FPC connectors for displays

Automotive

• High-vibration locking connectors
• Waterproof wire-to-board connectors
• Backplane connectors for ADAS modules

Automotive connectors undergo more vibration testing than most engineering students undergo emotional testing.

IoT Devices and High-Speed Data Links

IoT devices:

• Compact FPC connectors
• Fine-pitch board-to-board (BTB) connectors
• Low-power wire connectors for sensors

High-speed systems:

• Differential-pair-tuned board-to-board connectors
• RF/coaxial connectors (SMA or U.FL)

Conclusion:

In this guide, we have covered almost everything you need to know about a connector to use it in your design. A list of connectors with application notes is also shared above. PCB connectors define the modularity and performance of your electronics. Selecting the correct connector and installing it properly can prevent system failures.

You have to choose the connector as per the application. If high-speed digital signals are being transmitted from one board to another, I prefer to use FPCs because they perform better from a signal integrity perspective. If the same board uses batteries or other power sources, I would look for a higher-power THT connector that can be securely attached to the board. All these things came from experience. The pitch sizes can be adjusted as per board space. And if you ever doubt the importance of connectors, just unplug one from a working system and watch everything fall apart instantly.