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What is Board to Board Connector ?

A board to board connector pcb to pcb connector is a connector that is used to join printed circuit boards (PCBs) together electrically and mechanically. These connectors provide reliable interconnections between boards, allowing signals and power to be transmitted. With the continuing miniaturization of electronics, board to board connectors play a crucial role in packing more functionality into smaller products.

Board to board connectors come in a wide variety of styles, pitches, mounting configurations and materials to accommodate varied application requirements. From low speed signals in consumer products to high speed differential pairs in telecom systems – board to board connectors enable complex multi-PCB architectures across industries.

This article provides a detailed overview of board to board connector technology, architectures, attributes and selection factors when choosing the appropriate connector.

What is a Board to Board Connector?

A board to board connector creates a removable electrical and mechanical connection between two or more printed circuit boards (PCBs). As opposed to cables and wires, these connectors allow boards to be stacked together achieving higher component density.

Some key characteristics of board to board connectors are:

  • Creates electrical connectivity for signals and power between PCBs
  • Provides mechanical rigidity securing PCB stack
  • Allows easy assembly/disassembly for repair or upgrades
  • Achieves higher density versus cables/wires
  • Facilitates high speed transmission with impedance control
  • Available in wide variety of pitches, interfaces and materials

Board to board connectors serve applications such as:

  • Stacking boards in electronic devices (cell phones, handheld devices)
  • Joining backplanes, midplanes and daughtercards (servers, network/telecom hardware)
  • Interconnecting boards in industrial, medical, defense systems
  • Testing/programming multi-PCB systems faster

With increasing functionality integration, board to board connectors enable optimized PCB-based system partition while retaining modular upgradability.

Types of Board to Board Connectors

Board to board connectors can be classified based on orientation, mounting style, contact type and interface. The four main architectures are:

  1. Board Stack Connectors: Vertical connectors that allow PCBs to be stacked one over another with signals routed across boards. Common type is header-receptacle.
  2. Mezzanine Connectors: Right-angle connectors that join motherboards to daughtercards/modules oriented parallel to each other. Common type is male/female header-socket strip.
  3. Board to Board Edge Connectors: Connectors mounted on edge of PCB to route signals to backplanes, midplanes over short distances. Common type is plug-receptacle.
  4. Flex Cable Board to Board Connectors: Flex cables connecting two parallel boards using various interconnect technologies like ZIF, bottom entry, top entry.ArchitectureOrientationSpacingBoard StackVerticalSmall (<2.54 mm)MezzanineRight AngleMedium (>2.54 mm)Edge BoardParallelShortFlex CableParallelAdjustable

Table: Board to Board Connector Classification

Additionally, board to board connectors may use interface/contact technologies like:

  • Pogo pins – spring loaded pins
  • Terminal pins – solid metal pins
  • Stamped contacts
  • Tuning fork contacts
  • MagJack – magnetic coupled contacts

This allows optimization of reliability, density, speed and power delivery across various applications.

Advantages of Using Board to Board Connectors

Board to board connectors offer several benefits versus traditional cabling:

Size: Enables dense vertical stacking of thin PCBs not feasible with cables

Speed: Allows matching impedance up to speeds > 25 Gbps with X, Phi contacts enabling high speed differential signals

Simplicity: Single interconnect solution from board to board without need of cable assembly and attachment points

Serviceability: Connectors allow quick insertion/removal of boards for maintenance, upgrades compared to cabled connections

Flexibility: Wide range of stackup, routing, layout, material and plating options available off the shelf from vendors

Cost: Volume economics combined with ease of assembly/servicing provides lower total cost at system level

With miniaturization, speed and edge computing trends – board to board connectors serve an important role in packing more functionality per unit volume across applications.

Key Characteristics of Board to Board Connectors

There are several key parameters to consider when selecting a board to board connector for an application from the myriad choices. These should be analyzed relative to mechanical, electrical and environmental requirements:

1. Pitch: Centerline distance between adjacent contacts/pads in connector. Pitch selection impacts stack density, routing constraints, noise coupling. Common pitches range from 0.4 mm to 5 mm.

2. Current Rating: Maximum current supported by connector pin and board traces. Important for power transmission to boards. Range is usually 0.5A to 15A.

3. Signal Pairs: Number of differential pairs the connector can support. Key for multi-gigabit interfaces using high speed serial links. Range is from 1 pair to 20+ pairs.

4. Stack Height: Distance between top and bottom PCBs, set by connector length. Affects overall system profile. Typical range is 5 mm to 60 mm.

5. Contact Type: Contact style tuned for signals (tuning fork), power (solid pin) or both. Affects reliability under thermal, vibration conditions.

6. Mounting Style: Mounting mechanism of connector to board. Common types are thru hole, surface mount, press fit, solderless. Impacts ease of assembly/servicing.

7: Alignment**: Precision guiding features to control co-planarity, alignment across stacked boards. Reduces misalignment issues.

8. Actuation: Mate/de-mate activation style from automatic, tool-less manual to assistive actuated types. Important for field replaceability

9. Environmental sealing: Gaskets, housing seals to prevent dust, moisture, EMI ingress for reliability. IP ratings specify degree of protection.

Above parameters for board to board connectors must be evaluated vis-a-vis mechanical requirements (vibration, shock, system profile), electrical needs (signal speeds, ampacity), assembly constraints (automation or manual steps) and environment factors (sealing, humidity, pollution degree).

Application priorities would drive selection across this range – from a 0.4 mm pitch mezzanine signal connector to a ruggedized 5 mm pitch duo-beam power connector. Interconnect modeling early in the design phase is crucial to make informed decisions balancing these tradeoffs.

Board to Board Connector Materials and Platings

Connector contacts go through millions of insertion cycles requiring materials tuned for strength, conductivity and environmental resilience. Key connector component materials include:

Contact Material

  • Copper Alloys: phosphor bronze, beryllium copper. Most common offering combination of strength and conductivity for signals. Lower cost.
  • Stainless Steel: Good strength properties and resistance to vibrations and corrosive atmospheres. Used for higher reliability connections.
  • Brass: Economical material for low frequency power transmission when contact resistance not critical.

Plating

  • Tin: Basic economical plating. Prone to oxidizing (tin whiskers) which increases contact resistance over lifetime
  • Gold: Does not oxidize. Excellent environmental resistance and minimizes contact resistance. Most common for high reliability despite higher cost.
  • Palladium or Palladium Nickel alloy: Intermediate choice offering oxidation resistance at lower cost profile versus gold.

Insulator Material

  • Plastics: Most common formula being high temperature engineered thermoplastics like PEEK, PPS, LCP due to combination of dielectric strength, toughness and temperature rating
  • Ceramics: Used when very high dielectric strength and temperature properties needed despite fragility considerations.

Choice of materials also extends to mechanical components like connector housing, board locks, shields etc in addition to contacts and insulators which are taking the electrical transmission load. This determines mechanical robustness, sealing effectiveness and system level reliability over product lifetime.

Board to Board Connector Interface Comparisons

pcb-connectors-wire-to-board

Some application considerations and tradeoff comparisons between popular board to board connector interfaces are shown below – from industrial/consumer applications to commercial/defense systems:

ApplicationKey NeedsInterface ChoiceTradeoffs
Smartphones, TabletsThin, light, low costBoard Stack 0.4mm pitchLower power, dust ingress issues
Handheld Medical DevicesSignal integrity, contact reliabilityMezzanine 1mm FR4 Tuning forkHigher cost, medium density
ATE Board TestersHigh mating cycles, shock/vibePress fit tuned for cyclesNeeds PCB keepouts, via transitions
HPC Server RisersPower delivery, thermal, field replaceableCurrent Sense Pogo PinTall profile, manual actuation
Routers, SwitchesSignal integrity at speed, heat dissipationTuning fork phi contactsPrecise alignment features key
Rugged LaptopsVibration, EMI sealing, power/signal comboMetal shielding, combo pinsDesign complexity, thermal loads
Avionics vetronicsHermetic sealing, corrosion resistanceGlass seals connectorsLess replaceable, special handling

Table: Board Connector Interfaces Compared Across Applications

As can be seen, there is significant variety in best fit board to board connectors depending on end system requirements, environmental needs and product lifecycle considerations. Getting these interconnects right is vital to long term reliable functioning of electronic systems and hence rigorous modeling and testing is emphasized by experts before commitment to production.

Trends in Board to Board Connectors

Some key trends shaping board to board connector evolution driven by customer applications are:

1. Higher Density Interconnects: To pack more functionality per unit volume, there is drive towards fine pitch (>0.5 mm pitch) connectors plus multi-row architectures. This sets reliability challenges due to coplanarity demands requiring advanced SMT equipment.

2. Power Delivery: To feed increasing compute cores, GPU and memory needs – power currents per pin rising with high amp 12V, 5V, 3V standards needing reliable delivery. Innovative power-signal optimized contacts and filtering being adopted.

3. Higher Signal Speeds: Multi-gigabit Serdes speeds going from 25 Gbps towards 50-100 Gbps per differential pair adopted in server, network applications. This requires impedance control, crosstalk minimization and EMI shielding for signal integrity.

4. Ruggedized Design: Vibration damping, sealed connectors designed for harsh shock, moisture, contamination and wide temperature variations seen in defense, industrial uses. This demands analysis beyond standard commercial specs.

5. Tool-less Assembly: Solder-less press fit connectors gaining adoption due to ease of automated or manual board stacking avoiding solder process reliability issues and hazards. Self-alignment and mating hardware aids this goal.

Above trends demand modeling interconnects in detail prior to launch and working closely with connector vendors to get appropriate solutions matched to application needs from product inception.

Best Practices for Board to Board Connector Selection

Selecting the optimal board to board connector solution entails following key steps:

1. Setting Requirements: Define interface density, maximum data speeds, power needs, mechanical stackup, environmental factors based on product’s use cases and customer expectations. These serve as guiding functional requirements.

2. Modeling Constraints: Identify interface routing topologies, spacing rules, component placements constraints based on PCB and enclosure design architectures.

3. Research connector technologies: Explore industry standard interfaces plus latest proprietary innovations from connector vendors to generate viable shortlist matching needs.

4. Simulate Performance: Component level modeling using SPICE models and 3D EMI/Thermal simulations establishes electrical, environmental integrity checking mechanical robustness with shock/vibe and thermal desktop testing.

5. Prototype evaluation: Validate connector design performance with solder down prototypes put through environmental stress screening and electrical tests measuring signal integrity under dynamic conditions faced over product lifecycle.

6. Volume Manufacturing Planning: Lock down board to board connector supplier providing modeling support up front to aide successful volume manufacturing, meeting regulatory compliance plus supplying to service spares for maintenance repairs over total lifespan.

Above best practice methodology avoids pitfalls of over-specifying connector just based in paper specs versus modeling it within system use conditions plus locking reliability over total product life. This ensures the highest quality of connector selection decision.

Frequently Asked Questions

Q1. How are board to board connectors different from cable connectors?

Board connectors create direct mating between PCBs without need for cables or wires to route signals. This saves space, minimizes transmission loss. But connectors have lower current ratings than cables, limit flexible routing.

Q2. How many insertion cycles do board to board connectors support?

Most connectors are specified for 100-500 mate/de-mate cycles for low end industrial/consumer applications while high end mezzanine connectors used in server/network applications support 500 to 1500+ cycles.

Q3. Do board connectors work for flexible or rigid-flex PCBs?

Yes, flexible connectors using flex cable wiring or ruggedized board stack connectors work for directly stacking flex PCBs. Care must be taken to minimize flexing damage with strain relief.

Q4. What are common mounting styles for board connectors?

Thru-hole pin is most common but surface mount, press fit tuning fork contacts are gaining adoption for automated assembly. Hybrid mounting allowing both permanent and field replaceable connections also exist.

Q5. What modeling tools are used for board connector design validation?

Platforms like Ansys HFSS, Ansys Q3D for EMI/signal integrity analysis. Ansys Mechanical, Solidworks Simulation for thermal plus structural modeling leveraged along with SPICE models.

Summary

Board to board connectors serve a vital interconnection role in electronic systems allowing signal and power transmission across stacked PCB architectures in space constrained compact product packaging.

With increasing focus on high speed multi-gigabit interfaces, power delivery demands plus ruggedized designs – choice of the optimal connector solution through detailed modeling and validation ensures reliable system performance.

By following structured engineering processes to characterize mechanical, electrical and environmental constraints early while evaluating technology options – optimum board connector selection happens balancing density, speed, reliability and cost metrics delivering successful volume production.

 

 

 

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