Connector Selection: Five Parameters That Decide Reliability

A connector is the one component on a board that is designed to be connected and disconnected, and it is often the part that fails first. Connector selection means answering five questions, and the answers interact: a part chosen for its high speed performance may have a housing that cannot survive the oven, and a part chosen for its temperature rating may have contacts that will not hold up in a vibrating installation.

This article works through the five parameters, in the order in which they tend to eliminate candidates.

The first two are absolute constraints, in the sense that a part which fails either of them cannot be used at all. The remaining three decide how good the choice is.

Reflow Peak Temperature And The Housing Material

The most common mistake is to check the operating temperature and ignore the soldering temperature. They are different requirements, and a part can be rated for a hundred and five degrees in service while deforming in a two hundred and sixty degree reflow oven. The housing material sets the real limit, and the practical ranking runs from general purpose engineering plastics, which cannot survive a lead free reflow at all, through intermediate grades that pass marginally, to LCP, the liquid crystal polymer that passes a standard profile with margin and holds its shape after repeated passes.

For high speed and fine pitch surface mount parts, LCP has become the usual choice because it satisfies three requirements at once: it survives the reflow peak temperature, it has a low dielectric constant and low loss, and it is dimensionally stable enough to hold a fine lead pitch. Its dielectric figures are what make it suitable for a high frequency interface as well as a structural material.

What Happens When The Rating Is Not Met

The failure mechanisms are characteristic. A housing that softens in the oven allows the contacts to move, so coplanarity is lost and the joints are either open or bridged. A moulding that has absorbed moisture can crack internally when the water flashes to steam, which is why moisture sensitivity level and dry packing are part of the specification. Over a long service life, the plastic relaxes, the contact force falls and the contact resistance drifts upward, which shows up as an intermittent fault long after the product has shipped.

Underneath all of these is the CTE mismatch between the connector body and the board. When the two expand at different rates, the solder joints absorb the difference as shear stress, and repeated temperature cycling eventually fatigues them. This is the mechanism that automotive temperature cycling tests are designed to expose, and it is a property of the material pairing rather than of the solder alone.

Signal Performance At High Data Rates

The second parameter only applies where the link carries a fast signal, but where it does, it changes the way the part is specified. Above roughly ten gigabits per second, a connector is a short length of pluggable transmission line rather than a set of contacts, and its behaviour has to be described with S-parameters. Impedance, insertion loss, return loss, crosstalk between adjacent pairs and skew within a pair all matter, and a part that looks electrically adequate on a continuity test can still destroy the margin of the link.

The board has to support the connector at that speed as well. The footprint, the launch into the trace and the reference plane beneath it are part of the same channel, and a good connector on a poor footprint performs no better than a mediocre one. That is why routing a high frequency path and selecting the connector are normally done together rather than in sequence.

Plating And Contact Reliability

The third parameter is the plating, and the useful practice is to differentiate it by area. The contact surfaces need enough gold to remain stable over many mating cycles and to resist oxidation for the life of the product, while the solderable areas need a thin, uniform finish that wets properly and does not make the joint brittle. A part plated uniformly with a thick layer everywhere is a compromise that serves neither function particularly well.

The spring behind the contact is part of the same story. It has to hold its force through the temperature range and the vibration the product will see, because a spring that relaxes allows the contact resistance to rise. The pad geometry on the board has to suit both the joint and the mechanical load the connector will transfer into it.

Mechanical Fit And Assembly

The fourth parameter covers the features that make assembly possible at all: a key that prevents the part from being inserted the wrong way, a chamfer that guides the mating half into position, a locating peg that holds the body during placement, and a defined insertion and withdrawal force. At volume, with automated assembly, a part without those features produces scrap at a rate that no process improvement can compensate for.

Mating cycles belong here too. A connector specified for thirty cycles and used in a product that is reconnected weekly will not last the warranty period, and the number should be checked against the actual use rather than against the practice of the design team.

Consistency And Manufacturability

The fifth parameter is the least visible and often the most disruptive. A connector supplier whose process drifts produces batches with different coplanarity, different plating thickness or different moulding dimensions, and the same assembly line then produces different results from one delivery to the next. The practical protection is to require the tolerances to be stated, to ask about the moisture sensitivity level and the packaging, and to prefer a source that controls its moulding and assembly rather than one that buys from whoever has stock.

Checking the quality characteristics of the assembly against the datasheet at goods inwards is a cheap habit that catches a drifted batch before it reaches the line. gopcb supports customers through connector footprint review and assembly, so that the part selected, the footprint it sits on and the process that joins it are all consistent.

It is worth adding a note on how the five parameters interact, because they are not independent. Raising the data rate usually pushes the design toward a finer pitch, which narrows the choice of housing material toward one with better dimensional stability, which in turn restricts the plating and the moulding suppliers available. A decision made on one parameter therefore reduces the freedom on the others, and the practical approach is to fix the constraints in order and accept that the last two parameters will be decided by what remains.

FAQ

Is the operating temperature enough to qualify a connector? No. The reflow peak temperature rating is a separate requirement and is usually the one that eliminates a candidate.

Why does liquid crystal polymer dominate surface mount connectors? Because it combines high temperature resistance, low dielectric loss and dimensional stability, which are the three things a fine pitch high speed part needs.

Are S-parameters necessary for a connector below a few gigabits per second? Often not, though the footprint still matters. Above roughly ten gigabits per second they become the normal way to specify and compare parts.