Connector Reliability Under Thermal Cycling

A connector is a mechanical assembly made of several materials, and it is mounted to a board that expands at a different rate. The joints at its ends carry the resulting load, and they are the first to fail.

Where the Stress Comes From

The connector body and the board expand by different amounts as the temperature changes. Where the connector is long, the difference accumulates along its length and the end joints see the largest displacement.

The body also constrains the board locally, so the joints under the body are restrained while the board around them moves. Our reliability notes describe how the load is assessed.

Joint Geometry

A solder joint that is tall and compliant absorbs the displacement; one that is short and wide resists it and cracks. The pad and aperture design therefore decide the fatigue life as much as the alloy does.

Where the connector is designed with compliant leads or a floating contact, the load does not reach the joint. That is a component selection decision made at the start. Our land pattern notes describe the joint volume that follows from the pad.

Mounting Hardware

A connector held by screws or by a bracket transfers the load into the board through the hardware rather than through the joints. The hardware must be able to take it, and the board must be able to take the local compression.

A connector held only by its joints moves with the board and loads them. Our connector notes describe the retention that is usually required.

Thermal Cycling Behaviour

The crack starts at the corner of the joint where the stress is highest, and it grows across the joint with each cycle. An intermittent open appears before a permanent one, and it appears at temperature.

A test that only measures at room temperature may not see the failure, because the joint closes again as the assembly cools. Our thermal cycling notes describe the in situ monitoring that catches it.

Materials in the Connector

The housing material decides the expansion and the stiffness. A glass filled thermoplastic moves less than an unfilled one and it is stiffer, which changes how the load is distributed.

The contact material and the plating decide the contact resistance under the same movement. A gold plated contact that frets produces an intermittent resistance that is not a solder joint problem. Our card edge notes describe a related wear mechanism.

Design Measures

Shorter connectors distribute the load better than long ones, and a long connector can be split into two. Extra mounting points reduce the span that the joints have to accommodate.

Where the connector must be long, the joints at the ends can be reinforced with a larger pad or a fillet, and the rest can be left compliant. The two requirements are met by different parts of the same connector.

Verification

The verification is a thermal cycling test with the connector mated and the resistance monitored, performed on the real assembly.

The test should include the mounting hardware, because the load path changes when the hardware is present. Our quality notes describe how the result is recorded.

Process Control and Verification

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. 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.

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.

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

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.

Verification and Records

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

Does a larger fillet improve connector reliability? It increases the strength of the joint and reduces its compliance, so the effect depends on which of the two is limiting.

Should the connector be thermally cycled mated or unmated? Mated, because the mating adds a mechanical constraint that the assembly will see in use and that changes the load on the joints.

What does gopcb provide for connector reliability? We provide joint geometry development for compliance, mounting hardware and load path design, connector length and mounting point review, housing material assessment for expansion and stiffness, in situ monitored thermal cycling of the mated assembly, and disposition rules for intermittent joints.