PCB Connector Pitch

Key Takeaways

• The relationship between pitch and typical DFM constraints.

• How fine pitch assemblies alter board cost and yield.

• The link between pitch and the reliability of wire harnesses/cable assemblies.

A view of a PCB connector pitch interface; the pitch is the center-to-center distance between pins

Miniaturization of electronics is achievable only with more dense component packages. While reductions in chip die make for more powerful, lighter, and energy-efficient electronics, a device's performance gains are unrealizable without a corresponding level of manufacturing complexity at the board level. Die shrinkage results in a proportional decrease in the spacing between component pins, otherwise defined as the pitch. Even PCB connector pitches are decreasing due to a higher bundling of conductors to achieve higher data throughput. The general industry trends of component miniaturization and system portability have knock-on effects on the design for manufacture (DFM) constraints and overall producibility of the device.

How PCB Connector Pitch Modifies DFM

Generally, the PCB connector pitch defines the center-to-center distance between through-hole and surface mount pins. A larger pitch makes for easier routing due to greater clearance between pins, but a finer pitch supports denser assemblies that are typical in today’s electronics. Pitch often refers to the in-row spacing between pins, but it can also designate the pin-to-pin distance between rows in suitable connectors. Layout and librarians should make no assumptions about the relationship between inter-row and intra-row pitch. While the two can be the same value, there is no relationship between inter- and intra-row pitch.

Pitch is frequently a critical design constraint for DFM: while not a connector, BGA pitch (often the smallest pitch in a design) helps set several design rules, such as via hole diameter and trace width. Not only does this restrict the physical parameters of copper features, but it also limits electrical functionality. Reducing the trace width, for instance, decreases the trace’s current carrying capacity. While general remedies call for an increase in the copper thickness, a trace cannot support a copper thickness greater than its width; the etching and plating processes may reduce the trace width below minimum acceptability, affecting other characteristics like the targeted impedance. 

Effects of Pitch on Connector Parameters

Assemblies with tight pitch requirements can often benefit from microvias. While this is an expense above standard PCB drilling and fabrication cost, it can sometimes be the only method to rectify width restrictions, board thickness, and current availability. The aspect ratio limits through holes, constraining minimum hole diameters 1/8th~1/10th the board thickness for the plated barrel integrity. Microvias have a larger ratio of barrel height to hole diameter, but they have a controlled depth such that the drilled diameter can significantly reduce size. 

Fine Pitch Assemblies and Producibility of Boards, Wire Harnesses

Fine pitch connectors demand more stringent manufacturing practices than standard pitch assemblies. Defining fine pitch capabilities is tricky: manufacturing sophistication is constantly evolving, and what was indicative of a high level of production quality may now be considered standard. Additionally, the abilities of a particular manufacturer (equipment, technicians, etc.) may preclude them from tighter connector pitch assemblies. Like most DFM-related questions, the best course of action is a conversation with potential manufacturers about the cost and challenges of a design’s features. However, it doesn’t hurt to have a rule of thumb in mind: when pitches start venturing below 0.8 mm, design teams should at least consider the requirements of a fine pitch assembly.

The connector pitches thus factor in firmly to the producibility of the board, as defined by IPC:

IPC Definitions of Board Producibility

Unlike the IPC Performance Class, the Producibility Level does not have to adhere to the least strict classification level. That is to say, the individual features of the board’s fabrication and assembly can vary between Producibility Levels, as the system is a descriptor of a producibility challenge rather than a standard. Design teams can stress producibility where necessary and roll it back for the cases where it would create an undue burden that increases manufacturing cost, time, and waste.

The impact of connector pitch on manufacturability extends to the wire harness and cable assembly of integrated electronic systems. High-volume automated cable assembly equipment will seat wires in terminals, but as the pitch shrinks, so will the connector seating. A smaller connector seating has a greater likelihood of failure than a larger seating in these automated processes, and manufacturing will need to slow down processes or move to semi-manual assembly, which can be significant cost adders in production.

A Total Solution for Electronic Design

PCB connector pitch dramatically affects the design and manufacturing of electronics. Choosing larger connector pitches (when appropriate) can accelerate production time and reduce cost while improving yield. However, some assemblies must prioritize the performance and functionality goals or a tight form factor instead of manufacturability (to a point). 

Whatever your electronic design requirements, Cadence’s suite of PCB Design and Analysis Software aids development teams with a comprehensive ECAD environment suitable for every industry. Incorporating a DFM ruleset is straightforward with the customizable Constraint Manager of OrCAD PCB Designer; with live error checking and reporting, users can solely focus on the board layout.

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