Reflow and Wave Solder Carrier Design

A carrier holds a board or a panel through a process. It is often added when a board is too thin to run, and it is rarely designed with the same care as the board it carries.

Why a Carrier Is Used

A thin board flexes in the printer, in the oven and on the conveyor. The flexure changes the print, the placement and the joint formation, and the carrier removes it.

A carrier also supports a board that carries heavy parts, and it provides the fiducials and the tooling features for a panel that is not standard. Our panel notes describe the tooling features that a panel carries.

Material and Thermal Behaviour

The carrier material must survive the process temperature and it must not release contamination. A composite that outgasses into the oven deposits material on the boards behind it.

The thermal expansion of the carrier is different from the board, so the locating features must allow the board to move or the board will be constrained and stressed. Our warpage notes describe the same mismatch inside the board.

Locating and Clamping

The board should be located by features that repeat and clamped only enough to hold it. Over clamping flattens a warped board and it springs back after the process, which loads every joint.

The clamps must be outside the print and placement area, and they must not lift the board off the support surface. Our test point notes describe the access that must be preserved.

Support Surface

The support surface should be substantially flat and it should not touch the components on the bottom side. Where the bottom side is populated, the carrier needs pockets that match the components.

A pocket that is too shallow presses on a component; one that is too deep leaves the board unsupported in that area. The pocket depth is a tolerance and should be dimensioned. Our fabrication notes notes list the attributes that should be stated.

Thermal Mass Added

The carrier joins the thermal circuit. A heavy carrier increases the thermal mass of the assembly and changes the heating rate, so the profile must be developed with the carrier rather than without it.

The carrier also shields parts of the board from the airflow, which changes the local heating again. The effect should be measured with thermocouples on the board rather than assumed. Our profile notes describe the measurement.

Cleaning and Maintenance

The carrier collects flux and debris. It should be cleaned on a schedule, because the debris transfers to the boards and to the printer.

Where the carrier is coated, the coating wears and the surface changes. The coating condition should be inspected as part of the maintenance. Our cleanliness notes describe how contamination is measured.

Verification

The verification is the flatness of the board in the carrier, the print and placement repeatability across the first article, and the temperature profile measured with the carrier in place.

Where the first article is acceptable and production drifts, the carrier condition is one of the items to check. Our yield notes describe how the drift is detected.

Process Control and Verification

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. 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.

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.

Verification and Records

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.

FAQ

Can a carrier substitute for a thicker board? It removes the handling flexure and it does not change the board’s own stiffness, which matters for the vibration requirement.

Should the carrier be board specific? Where the pockets match components, yes. A universal carrier with clearance pockets suits boards without bottom side parts.

What does gopcb provide for carrier design? We provide carrier material selection for the process temperature and outgassing, locating and clamping design that allows expansion, flat support with dimensioned pockets, profile development with the carrier in place, cleaning and maintenance schedules, and first article verification of flatness and repeatability.