Reflow Carrier Pallets: Supporting Thin Boards Through the Oven

A thin board behaves like a different material once it is hot. The same panel that feels rigid at room temperature loses most of its stiffness above the glass transition and sags between the conveyor rails, so the paste, the component stand-off and the joint volume all change at the moment they matter most. A carrier pallet restores the geometry, and it also changes the thermal profile. This article covers the pallet materials, the support features and the measurements that show whether the carrier is doing its job.

Why Thin Boards Move in the Oven

A 0.6 mm FR4 board loses most of its stiffness above the glass transition temperature, which for a standard laminate sits between 130 and 150 degrees Celsius. Under its own weight a panel spanning 200 mm between conveyor rails can sag by more than a millimetre at peak temperature, and the paste does not follow the board because the alloy is still molten and the surface tension holds it to the pad. The mechanics behind this are set out in our notes on warpage control.

The consequence is not only a warped assembly. A sagging board changes the gap between the stencil and the pad on the second print, and it changes the stand-off under a BGA, so joints at the centre of the sag receive a different volume of alloy from joints near the rail.

Carrier Materials and Their Thermal Behaviour

FR4 pallets are inexpensive, machinable and have a coefficient of thermal expansion close to the board, but they degrade after a few hundred cycles at 260 degrees and they hold flux in the machined surface. Aluminium 6061 conducts heat well and lasts, at the cost of a CTE near 23 ppm per degree against roughly 17 for the laminate.

Titanium and composite pallets hold their shape over thousands of cycles with a lower thermal mass than aluminium, which makes them attractive where the profile has to stay close to that of an unsupported board. The choice is normally settled by comparing the measured profile with a pallet against the profile the paste supplier specified.

Support Features: Pins, Ledges and Vacuum

Ledge support alone holds the board at the edges and does nothing about the centre. Support pins of 2 to 3 mm diameter placed under the board on a pitch of 25 to 40 mm carry the weight without touching the components on the underside, provided the pin positions are checked against the bottom side layout of every revision.

Vacuum grooving is used where the board is too thin for pins at a usable pitch. A groove connected to a pump at roughly minus 0.3 bar holds the board flat against the pallet face, and the groove pattern has to avoid running beneath a via that carries paste, because the vacuum pulls paste into the open barrel.

The pallet face is a datum, so its flatness is measured rather than assumed. A face that is out by 0.1 mm across the panel gives the same error to every board processed on it.

Measuring the Effect on the Thermal Profile

A pallet adds thermal mass and shadows part of the surface, so a profile measured on a bare board does not describe the assembly on the carrier. The peak temperature typically drops by 5 to 15 degrees and the soak segment flattens, depending on the material and the area the pallet covers. The verification method is described in our profile verification guide.

The correct procedure is to run the profiler with thermocouples on the assembled board mounted in the pallet and to compare that result with the same assembly without the carrier. If the difference moves the peak out of the paste window, the profile is adjusted rather than the pallet accepted as it stands.

Component Clearance and Shadowing

Tall components on the underside of a double-sided assembly set the minimum gap between the board and the pallet face, and any recess machined to clear them has to be deep enough to let convection pass through. A pocket that fits too closely traps hot air and creates a local cold spot that shows up as a poorly formed joint in the middle of the pocket.

Shadowing is measured rather than guessed: a thermocouple on a pad inside the shadowed area will show a peak several degrees below one on the open surface. Where that difference exceeds what the process window allows, the pallet is redesigned with a larger opening rather than the profile being pushed hotter for the whole assembly.

Double Sided Assembly and Second Pass Handling

On the second reflow pass the first side faces the pallet, so the pallet surface must not mark the mask or press on parts that are already soldered. A machined recess flat to within 0.1 mm across the panel face is normal, and the recess has to be deeper than the tallest component on the underside.

The board is retained with spring clips, low tack tape or edge stops. Whatever the method, it has to survive 260 degrees without leaving residue and without applying enough force to distort the board, so the clamping arrangement is tried on a scrap panel before it goes into production.

Cleaning and Maintenance of Pallets

Flux condensate collects on the pallet face and inside the pockets, and it is both tacky and conductive enough to interfere with fine pitch placement. Pallets are cleaned on a fixed cycle rather than when they look dirty, because the deposit builds inside the recess where nobody sees it.

The verification is an ionic contamination measurement on a swab taken from the pallet surface, together with a check that no support pin has worn enough to change its height by more than 0.05 mm. A worn pin changes the support plane, and the change appears as a placement or soldering defect rather than as a pallet failure.

Pins are the cheapest part of the pallet and the most likely to move, so their height and their position belong in the tool maintenance record with a tolerance attached.

Qualifying a Pallet for a Product

Qualification means three builds: the assembly without a pallet, the assembly on the pallet with the existing profile, and the assembly on the pallet with an adjusted profile. Each is profiled and inspected for sag, voiding and joint appearance, on the same batch of boards so that the comparison is meaningful.

The acceptance figures are the deflection across the board after reflow, measured on a granite table with the assembly at room temperature, and the void percentage under the largest thermal pad from X-ray. Neither should be worse on the pallet than on the unsupported build, or the carrier is not doing anything useful. X-ray methods are covered in our inspection guide.

Carrier Design Notes on the Drawing

The assembly drawing should state that a carrier is required, the maximum deflection allowed during reflow, the keep-out for support pins on the underside, and the components that must not be loaded by the pallet. Without those notes, every build is re-engineered by whoever happens to be running the line.

The pallet drawing carries the support pattern, the recess depths and the material, and it is controlled like any other tool. When the product changes, the pallet is reviewed against the new layout before the first run, because a pin added for the previous revision can sit directly under a component added in the current one.

FAQ

Does a carrier pallet change the reflow profile? Yes. The added mass and the shadowing lower the peak and flatten the soak, so the profile has to be re-measured with the board on the pallet instead of being carried over from an unsupported build.

How much sag is acceptable? The limit comes from the assembly requirement, but 0.5 mm across the diagonal measured at room temperature after reflow is a common figure for a thin board on a carrier.

Can one pallet serve both reflow passes? Yes, provided the recess clears every component already placed and the retaining method does not press on the joints of the first side.