Multi-board printed circuit boards (PCBs) are becoming more complex and compact as electronics continue to advance. However, with greater complexity comes a higher chance of connectivity issues between boards. This article explores the top 4 errors that can occur when connecting multiple PCBs and tips to prevent them.
Mismatched Net Names
One of the most common multi-board connectivity issues is mismatched net names between boards.
What is a Net Name
A net name is a label that identifies an electrical node or signal path in a PCB design. Every conductive trace section gets assigned the same net name if they need to connect to share the same signal.
For example, you may have a +5V power net or a Clock Signal net that routes between components.
Why Mismatched Net Names Causes Issues
When multi-board designs have inconsistent net names between boards that should connect, it creates ambiguity on what nets link together.
PCB design software matches net names automatically during system assembly when boards correctly share names. If names differ, the boards may not electrically connect as intended even if the physical connectors align.
Mismatched names often occur from manual errors during the separate PCB layouts or netlist conversions between design tools. For complex boards, name mismatches are easy to overlook but have cascading impacts.
Effects of Mismatched Nets
Issues caused by mismatched net names include:
- No electrical connectivity between boards on nets with naming differences
- Short circuits if nets connect incorrectly between boards
- Complete functional failures rendering products inoperable
Fixing mismatches post-manufacture requires manual rework and new board spins which adds significant project delays and costs.
Tips to Avoid Mismatched Nets
When designing multi-board systems:
- Maintain a consistent net name glossary between boards
- Use unique net name prefixes on each board to reduce naming collisions
- Script automated netlist synchronization checks in layout tools before manufacture
Following these best practices eliminates mismatches and avoids associated connectivity issues.
Incorrect Connector Pinouts
Specifying incompatible connector pinouts between stacked PCBs also causes problems if signal/power pins do not align when mated.
What is a Pinout
A connector pinout defines what signal or electrical node gets assigned to each physical pin in a connector. For example, Pin 1 may connect to ground, Pin 2 carries 5V, Pin 3 has the data bus, etc.
Connector pins on the board need to match corresponding nets on the cable or mating board.
Figure 1. Example connector pinouts with pin to signal assignments
Why Incorrect Pinouts Disrupt Connectivity
Designers usually select standard connectors (ex. HDMI, Ethernet) with defined pinouts known to work together. However, some boards stack using customs connectors for density or other reasons.
If custom connector pinouts mismatch between boards, the wrong signals align when attaching boards/cables. This either opens circuits needing connections or shorts unrelated nets. Both outcomes prevent proper functionality.
For card cage systems with backplanes, backwards connectors or pin swap settings also scramble signal routing between system boards. designers must carefully track custom pinouts and connector orientations during layout.
Effects of Wrong Pinouts
Impacts from using incompatible custom pinouts include:
- Signal routing failures causing functionality loss
- Damage if power shorts to data or control lines
- Difficult debug to trace wrong pinout connections post-assembly
Incorrect pinouts often require board respins or full connector replacements to rectify.
Tips to Avoid Pinout Issues
Suggestions when working with custom pinouts:
- Validate connector orientations and pinouts matches between boards/cables
- Design test points to validate voltages on critical nets post-assembly
- Review pinouts in schematic, layout, and mechanical drawings as part of design reviews
Planning pinouts and orientations before layout starts reduces errors and saves significant time compared to fixing post-manufacture.
Inadequate Stackup Planning
Insufficient stackup planning for vertically adjoining PCBs also causes problems with signal breakouts and routability between layers.
What is a PCB Stackup
A PCB stackup defines the layer construction with dielectric materials separating copper layers. The stackup affects trace routing, impedances, and hi-speed performance.
In multi-board designs, each board has its own stackup. But when mounted together, the individual board stackups combine as one system.
Figure 2. Example of an 8 layer PCB stackup with materials and thicknesses defined
Why Poor Stackup Planning Causes Issues
Designers concentrate on layout within each PCB but often neglect inter-connections between boards. Without planning, links between boards may align over incompatible layer pairs.
For example, a bottom signal layer on one PCB may contact an inner power plane on the attached board instead of a routing layer. This blocks connectivity or forces detoured routing around blocked areas.
Pin sockets and connectors also constrain routing channels between layers. Designers must allocate stackups to create viable breakout paths around these components.
If designers inadequately plan stackups between boards, it results in:
- Failed connections forcing post-assembly rework
- Congested routing channels and impedance discontinuities
- Violations of length matching for high-speed signals
All diminish signal integrity performance.
Tips for Effective Stackup Planning
To avoid stackup issues when linking multi-layer boards:
- Model system stackup with all associated boards/cables
- Ensure layer alignments allow straight signal breakouts
- Adjust layer counts/thicknesses to match impedances between boards
- Review planned stackup prior to layout work starting
Correct modeling and planning creates reliable connectivity between vertically stacked PCBs the first time.
Poor Control of Tolerances
Insufficiently controlling mechanical tolerances also allows stacked boards and connectors to shift resulting in misalignment.
What are Mechanical Tolerances
Mechanical tolerances define the permissible variation in the manufactured dimensions of a part. This accounts for acceptable differences within standard production capabilities.
Figure 3. Example connector drawing with called out tolerances
For example, 0.05 mm tolerance on a 10 mm dimension allows that feature to range between 9.95 and 10.05 mm.
Why Tolerance Issues Reduce Connectivity Margins
Board connectors and sockets used between PCBs have defined tolerances. If designers poorly control these tolerances through analysis, shifts can misalign connectors when assembled.
Manufacturing and temperature fluctuations also cause expansion/contraction that changes dimensions. Too much variance makes connectors more prone to partial connections or disconnections under environmental stresses.
Key impacts from high tolerance stacks include:
- Intermittent connectivity failure from vibration or shock
- Mounting holes fail to align boards properly
- Need for selective assembly or rework to fit pieces together
All add cost or threaten reliability.
Tips for Effectively Managing Tolerance Buildup
To control tolerance stack-ups:
- Use connectors with tolerances well below alignment features
- Analyze worst-case tolerance combinations in models
- Specify tighter tolerances on alignment hole features
- Define maximumoffset requirements between boards/connectors
Verifying stackups within available connector tolerances prevents misalignments.
Summary of Major Multi-Board Connection Errors
Major connectivity errors arise from:
- Mismatched net names between designs
- Incompatible connector pinouts between boards
- Poor stackup planning between adjacent PCBs
- Inadequate management of mechanical tolerances
Following best design practices outlined for avoiding each issue ensures reliable multi-board connectivity.
Frequently Asked Questions
What is the most preventable issue causing multi-board connection failures?
The most preventable issue is mismatched net names between boards. Maintaining a consistent net name glossary and scripting automated synchronization checks eliminates mismatches. This avoids associated connectivity issues that require difficult rework after manufacture.
How do you verify assembly alignment between stacked boards?
Use analysis models to verify board connectors and alignment features fit within defined tolerances under worst-case positional shifts and temperature extremes. Meet maximum offset requirements between mating connectors and boards interfaces within the available tolerance budget.
Should connector pinouts match 1:1 between different PCBs in a system?
Not always, some adaptor boards re-map signals to support multiple configurations in a card cage for example. But for directly adjoining boards, using compatible connector pinouts ensures correct signal routing between boards and eliminates damaging shorts.
How can poor stackup planning prevent connections between boards?
Inadequate pre-layout planning can align signal breakout layers against power planes on adjacent boards rather than routing layers. This blocks connectivity or forces difficult detoured routing without straight access between boards. Planning stackups is essential for reliable inter-board connections.
If connectors physically fit but boards have mismatched nets, what typically happens?
If connectors/cages mechanically fit, missing nets often connect incorrectly rather
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