Board-to-Board and Wire-to-Board Connector Selection

Every product has at least one place where two boards or a board and a cable have to connect and still be separable. That interface carries the signal integrity, the mechanical retention and the serviceability of the product, and it is usually chosen from a catalogue late in the design.

Defining the Requirement

The starting point is the signal count, the current per contact, the voltage and the environment. The signal count decides the pitch, and the pitch decides everything else.

The requirement should also state the number of mating cycles and whether the connection is made during assembly only or by the user. A connector that is mated once can be a different part from one that is unplugged weekly. Our component selection notes describe the parameters that are usually missed.

Pitch and Stack Height

A fine pitch gives more contacts in less area and demands more from the assembly process. Below a certain pitch the placement tolerance and the solder volume both become critical.

The stack height sets the distance between the boards, which then constrains the components on both faces. A connector chosen before the stack height is fixed will force the layout later. Our design release notes cover where the stack height is recorded.

Blind Mating and Guidance

Where the two halves are mated by hand without sight of the contacts, the connector needs a chamfer or a guidance feature. Without it, the contacts are pushed sideways and bent, and the damage is not visible.

A blind mate connector also needs a float on one side, because the two boards will not be perfectly aligned after assembly. Our registration notes describe how the alignment is controlled.

Current and Voltage

The current rating is per contact and it is not simply additive. Several contacts carrying current in one connector heat each other, so the rating falls as more contacts are loaded.

The voltage rating follows the creepage between contacts, so a connector rated for a high voltage may not be rated for it at all altitudes or in all pollution degrees. Our current capacity notes describe how the thermal limit is established.

Retention and the Board Interface

The retention can be in the connector, in a latch, or in a screw, but the load has to reach something other than the solder joints. A connector held only by its joints will crack them when the cable is pulled.

The board needs mounting features for the retention: a hole, a pad or a keep out. These should be part of the connector footprint rather than added afterwards. Our outline notes cover the mechanical features that are usually placed on the outline layer.

Assembly and Inspection

The connector must be placeable by the machine and inspectable by the process. A part with contacts underneath cannot be inspected by a camera, so the joint quality has to be assured by the profile and the paste volume.

Where the connector has a locating peg, the board needs the matching hole, and the hole must be in the right place or the part will tilt. Our inspection notes describe the coverage that is achievable.

Verification

The interface should be verified by mating it the stated number of times, by measuring the contact resistance, and by applying the retained load to the assembly.

Where the retention is mechanical, the test is a pull or a twist with the connector in place, and the acceptance is that no joint is disturbed. Our quality notes describe how the result is recorded.

Process Control and Verification

On a design of this kind, connector is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

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.

Process Control and Verification

On a design of this kind, connector is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Process Control and Verification

On a design of this kind, connector is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Can a wire-to-board connector be used board-to-board? Only where the parts are designed to mate as a pair. Two wire connectors of the same series will not necessarily mate with the correct alignment or retention.

How many mating cycles should be specified? As many as the product will see, with margin. A connector specified for 30 cycles and used for 200 is a field failure with a written cause.

What does gopcb provide for connector interface design? We provide pitch and stack height definition, blind mate and guidance features, current and voltage derating with the thermal limit, retention and board mounting design, placement and inspection review, and verification by repeated mating and load testing.