Top 3 Multi-Board PCB Connectivity Errors

 

Introduction

Interconnecting multiple printed circuit boards (PCBs) can be challenging. With more connections come more opportunities for errors leading to complications during assembly and testing or unreliable operation of the finished product. Engineers designing multi-board systems need to be aware of the most common connectivity issues to avoid problems down the line. This article covers the top 3 errors related to connecting signals between PCBs and strategies to mitigate them.

Overview of Multi-Board Connectivity Methods

There are several approaches to connecting signals between multiple PCBs:

• Board-to-board connectors - Dedicated connectors soldered to each board provide removable mechanical and electrical connections. Common types are card edge connectors, mezzanine connectors, and rigid-flex connectors.
• Flex circuits - Flexible printed circuits route signals between rigid PCBs. They allow movement between boards and absorb vibration or shock.
• Headers and receptacles - Soldered pin headers mate with receptacles or sockets on adjacent boards. Provides flexibility for modular or replaceable PCBs.
• Direct soldering - Cables or wires can be soldered directly between PCBs when quantity of signals is low and boards are permanently joined.

Each has advantages and disadvantages relating to cost, reliability, serviceability, and other factors. But all are susceptible to similar connectivity issues and failures.

1. Mismatched Connectors Between Boards

One of the most obvious yet easily overlooked errors is using incompatible connectors between PCBs that must mate together. Examples include:

• Card edge connector vs header and receptacle
• Different manufacturer or series with different pin pitch
• Connector sizes/layout don't line up between boards

Mismatches result in an incomplete electrical interface and inability to properly join the boards. Debugging and rework are difficult because the mechanical connection is not possible in the first place.

Mitigation Strategies

• Review connector technical specifications early at the design stage
• Create detailed interface control documents defining all connectors
• Formally control and document any connector substitutions or replacements
• Perform dimensional and functional testing of connectors before full assembly

2. Insufficient Current-Carrying Capability

Another common issue is undersized contacts, traces, or cables unable to provide adequate current for signals like power, ground, or motor/actuator drives. The result is unexpected voltage drops, ground shifts, or intermittent connectivity problems. This may lead to electrical noise, signal integrity problems, or equipment damage.

Causes include:

• Incorrect current rating assumptions during design
• Excess current draw due to fault conditions or design changes
• Improper connector contact selection for power transfer
• Thin or reduced cross-section PCB traces

Mitigation Strategies

• Perform detailed current load analysis for all inter-board connections
• Review ratings and derating curves for connectors, cables, and PCB traces
• Increase contact sizes or trace widths where needed
• Limit length of flex circuits or wire cables carrying high current
• Consider adding local capacitance to supplement power transfer

3. Uncontrolled Impedance on Signals

Maintaining controlled impedance on traces and connections is critical for high speed or sensitive signals like clock/timing signals, analog signals, and serial buses. Uncontrolled impedance causes reflections and losses degrading signal integrity.

On multi-board systems, common impedance control issues include:

• Trace impedance changes at connectors or boards
• Stub connections from branch lines or test points
• Varying flex circuit or cable impedance
• Routing changes without re-verification of impedance

Mitigation Strategies

• Simulate and budget complete signal path impedance
• Specify controlled-impedance connectors and minimal stub lengths
• Follow rigorous flex circuit and PCB layout guidelines
• Perform time domain reflectometry (TDR) analysis of inter-board links
• Use series termination resistors at connectors when needed

Other Potential Issues

Some other problems that may affect multi-board connectivity are:

• Crosstalk or EMI issues from dense routing
• Intermittent contacts due to vibration, contamination, or corrosion
• Damage to flex circuits or headers during assembly and maintenance
• Lack of strain relief on cables stressing solder joints
• Undefined or inadequate connector mating sequences
• Inability to fully seat boards together due to tolerances

Careful design reviews, analysis, testing, and quality processes during manufacturing will help identify and eliminate these kinds of errors.

Summary

Interconnecting signals between multiple PCBs brings added risk of connectivity errors compared to single board systems. Engineers must understand the most common pitfalls like connector mismatches, insufficient current capability, and uncontrolled impedance during routing. Following best practices in schematic capture, PCB layout, connector specification, and assembly minimizes problems in complex multi-board designs. What connectivity issues have you encountered on past multi-board projects?

Frequently Asked Questions

Q: What are some examples of multi-board systems?

A: Some common examples of products using multiple interconnected PCBs include:

• Computer motherboards, backplanes, and expansion cards
• Telecom or networking equipment line cards and switch fabrics
• Industrial control systems with separate controller, I/O, and power PCBs
• Medical instruments with separate analog, digital, and power boards
• Complex electronics with radio, processing, and interface boards

Q: When are board-to-board connectors used vs. flex circuits?

A: Board-to-board connectors are used when a removable mechanical and electrical connection is needed for field service or upgrades. Flex circuits are used when boards are permanently joined but need flexible interconnects. Connectors cost more but allow modularity, while flex circuits have lower cost but no serviceability.

Q: What is a stub connection and why is it bad for impedance control?

A: A stub connection is a branch line off a main transmission line that creates an impedance discontinuity. Stubs introduce reflections degrading signal quality. They should be avoided or minimized to maintain proper impedance control especially for high speed signals.

Q: How can crosstalk between dense multi-board connectors be reduced?

A: Strategies to reduce crosstalk between dense board-to-board signal connections include:

• Increased connector pin pitch to spread lines farther apart
• Ground pins interleaved between signal pins
• Differential signaling which is less susceptible to crosstalk
• Shielding between groups of connector pins

Q: Why are power and ground typically routed first on PCBs?

A: Power and ground nets have the highest current and are most sensitive to impedance control issues. Routing them first ensures they follow the most direct board layers minimizing voltage drops. Other signals can then be routed around power/ground forming a low impedance reference.