Board To Board Connector: Board to Board Interfaces: Connectors and Return Paths

Where a board has to be connected to another board, the interface between them is where most of the problems appear. The mechanical connection, the impedance of the link and the return path all have to be designed together.

Board to Board Connectors

A board to board connector provides both the electrical connection and the mechanical spacing between the two boards. The stack height is fixed by the connector, and it constrains the components on both boards.

The connector’s pin assignment is a design decision: placing grounds between signal pins reduces crosstalk, and placing the high speed pins adjacent to their returns reduces the loop area. A connector used as a simple feed-through loses that opportunity. Our signal integrity notes describe the arrangement.

The Return Path Across a Connector

Every signal that crosses a connector needs a return that crosses the same connector, close to it. Where the return is provided by the ground plane alone, the current has to find its way to the connector and back, and the path it takes is longer than intended.

The practical rule is to allocate one ground pin for every few signal pins, distributed across the connector rather than concentrated at one end. The rule matters most for the fastest signals.

Differential Pairs Across an Interface

A differential pair that crosses a connector should do so with its two conductors adjacent and with a ground pin nearby. A pair that is split across the connector loses its coupling and radiates.

The pinout should also keep the pair away from single ended signals that switch, because the coupling between them adds noise that the differential receiver rejects only partially.

Flexible Interconnect

A flexible circuit is often used as the link between two boards because it combines the connection with the mechanical compliance. Its impedance depends on the construction, which is different from a rigid board: the conductor is thin and the dielectric is a film.

The impedance should be calculated for the flex construction and matched to the rigid board at both ends. A mismatch at the transition produces a reflection that can be measured. Our flexible processing notes describe the constructions.

Mechanical Alignment and Tolerance

Two boards connected by a fixed connector must be aligned to within the connector’s tolerance, which is usually tighter than the tolerance on the boards themselves. The alignment is provided by the connector’s own features or by a locating pin.

Where the boards are also fastened to a chassis, the tolerance stack includes the chassis. The design should allow for the accumulation rather than assume that each item is at its nominal dimension.

Cables with High Speed Signals

Where the link is a cable, the impedance of the cable and of the connector have to match the board. The transition between the board and the cable is a discontinuity, and its size determines how much of the signal is reflected.

The transition is improved by keeping the ground return close to the signal at the connector and by avoiding long stub lengths inside the connector footprint.

Testing the Interface

The interface is verified by measuring the impedance across it and by checking the eye diagram under a representative load. The measurement identifies the discontinuity, and the eye diagram shows whether it matters.

Where the link is cabled, the test should include the cable, because the cable is where most of the loss occurs.

Documenting the Interface

The pinout, the stack height, the mating connector and the cable specification all belong on the assembly drawing. A pinout that exists only in a spreadsheet is a pinout that will be wired incorrectly at some point.

Where the two boards are designed by different people, the interface document is the contract between them, and it should be agreed before either layout is complete. Our fabrication notes guidance lists the entries.

Process Control and Verification

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Verification and Records

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

Points to Confirm at First Article

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Documented Process Control

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected. The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

FAQ

How many ground pins does a connector need? Enough that the return path for the fastest signal is short and that the current density per pin is acceptable. One ground for every four signal pins is a common starting point, distributed rather than grouped.

Should a board to board connector be on the edge? It can be, and edge mounting removes the need for the connector to occupy board area on the top surface. It also constrains the outline and the mechanical tolerance more tightly.

What does gopcb confirm for a board interface? We confirm the connector footprint against the component drawing, the pin one marking, the ground pin allocation and the mechanical stack height. Where the two boards are supplied as a pair we check that the pinouts are mirrored correctly rather than identically.