Reflow Conveyor Support, Sag and Vibration

A reflow oven carries the board through the zones on a conveyor, and the board is supported only at its edges or not at all. At reflow temperature the laminate is softer than it is at room temperature, and what happens to the board between the supports affects the joints on it.

Why Support Matters at Temperature

The laminate loses stiffness as it heats, so a board that is flat when cold can sag in the middle of the oven. The sag changes the position of the board relative to the profile measurement and, on a large thin board, it changes the geometry of the joints.

The effect is largest on a thin board with a heavy component and on a large board with edge support only. Our warpage notes describe how the figure is measured and what it depends on.

Conveyor Types and What They Support

An edge conveyor supports the board along two edges and leaves the middle free. A mesh belt supports the whole underside and it also contacts the bottom side of the board, which matters on a double sided assembly.

The mesh belt is the safer support for a thin board and it leaves marks on the underside where it contacts the joints. A centre support rail is the compromise, and it is placed where the board has no components.

The conveyor moves the board at a constant speed and it also transmits vibration from the drive and from the building. The transmission is small and it matters where a component is heavy or where a joint is marginal.

The observable effect is a component that has shifted or a joint that is short of alloy on one side of the board. The check is the placement accuracy after reflow rather than before. Our placement capability notes describe how that is measured.

The profile is measured by attaching thermocouples to a board and running it through, and the board that is used for the measurement is supported the same way as production. A measurement taken on a board that was supported differently describes a different process.

The measurement board is also usually a scrap board with components fitted, because the thermal mass of the components is part of the profile. Our profile verification notes describe how that board is prepared.

The speed sets the time in each zone and therefore the profile. It is set from the profile rather than from a throughput target, and a change to the speed requires the profile to be re-measured.

The speed also has a tolerance of its own, and a conveyor that drifts produces a profile that drifts with it. That is one of the few machine parameters that is worth checking on a schedule.

The controls that avoid most problems are a support method matched to the board thickness, the measurement board carried with the same support, the conveyor speed verified on a schedule, and a board that is flat before it enters rather than after it leaves.

A board that is already warped entering the oven is a different problem, and it is addressed at the lamination rather than at the reflow. Our board quality notes describe how the flatness is judged.

Acceptance and Its Evidence

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected. Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts.

Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made. Where the supplier and the user both measure the same property, they should agree on the method before the first delivery.

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 first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected.

Checks Before Release

A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process. 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.

A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed. 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.

Does a mesh belt damage the underside? It leaves contact marks, so it is used where the underside has no joints or where the marks are acceptable.

Can the sag be corrected by a higher conveyor speed? A shorter time at temperature reduces the sag and it also shortens the profile, so the change is a profile change rather than a correction.

What does gopcb provide for reflow conveyance? We provide a support method matched to the board thickness and the assembly, a measurement board carried with the production support, conveyor speed verified on a schedule, placement checked after reflow where a heavy part is involved, and flatness judged before the board enters the oven.

Verification and Records

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.

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.