Reflow Carriers for Thin Boards: Support, Warp and Placement Accuracy
A board 0.6 mm thick with a large ball grid array on it will bend under its own weight as it crosses the oven, and the bending is what moves components after placement. A reflow carrier holds the panel flat through the profile, and the way it supports the board decides whether it helps the assembly or simply adds thermal mass.
Why a Thin Board Needs a Carrier
A thin board is supported only at the conveyor rails, so the middle of the panel sags by an amount that depends on the span, the stiffness of the laminate and the temperature. At 240 degrees the laminate is softer than it is at room temperature, and the sag at the centre of a 250 mm span can exceed the placement tolerance of a fine pitch device.
The sag is not uniform, so the displacement is different at each end of a long device and the package is effectively tilted. That is why a carrier is used for thin boards, for large panels and for assemblies whose components are placed at the limit of the machine’s accuracy rather than at its nominal figure.
Carrier Materials and Their Limits
Reflow carriers are made from FR4, from aluminium, from composite materials or from machined engineering plastics, and the choice trades thermal mass against dimensional stability. Aluminium conducts heat quickly but expands at around 23 parts per million per degree, which is more than the laminate it carries and enough to shift a pocket relative to the board.
FR4 and composite carriers expand at about 17 parts per million, which is closer to the board, but they insulate and slow the ramp. A carrier that is too heavy also lowers the effective conveyor speed because the oven has to bring the extra mass to temperature, and the profile has to be re-measured with the carrier in place rather than with a bare panel. The expansion figures used in that comparison come from the laminate data rather than from a generic table, so our laminate material notes are the reference for the board side of the calculation.
Pocket Design, Panel Support and Clearance
The pocket depth is normally the board thickness with a tolerance of plus or minus 0.05 mm, so that the board sits supported along its edges without being pinched. The side clearance is 0.10 mm to 0.20 mm per side, which is enough to drop the board in by hand and small enough that the board cannot walk during the print and placement steps.
Support is provided either by the full pocket floor or by pins placed under stiff areas of the board. A full floor gives the best flatness but blocks the bottom side from heating, while pins allow radiant heat underneath and require the pin positions to be chosen from the board’s own stiffness map during the <a href="https://www.gopcba.com/pcb-warpage-control/” title=”warpage control”>warpage control study.
Holding Methods and Their Trade-offs
High temperature tape, spring clips and vacuum channels are the usual holding methods. Tape is simple and reusable only once, clips hold positively but leave marks and can distort a thin edge, and vacuum holds the whole board flat but requires a channel pattern and a vacuum supply that reaches the oven entrance.
Whichever method is used, the board must not be able to move between the placement machine and the oven. A board that shifts by 0.1 mm in the carrier after placement produces a placement error that no machine calibration can correct, and the error is often attributed to the printer because the paste is where it is eventually measured.
Thermal Effects of the Carrier
A carrier adds mass to the assembly, and the mass changes the ramp rate through every zone. The oven is set from a profile that was measured on a bare board or on an instrumented panel, and adding a 300 gram carrier can add thirty seconds to the time to reach soak temperature.
The measurement has to be taken on the board rather than on the carrier, because the carrier lags the board and its thermocouple reads low. A thermocouple taped into a via or a corner of the panel gives the board temperature, and the difference between the two readings is the amount by which a carrier blind to the measurement will understate the profile. Where the oven runs a recipe validated without a carrier, the validation is repeated before the carrier is used in production.
Placement Accuracy Under a Carrier
Placement accuracy is quoted for a board held flat in the machine, and a carrier does not change the machine’s capability. What it changes is the position of the board relative to the fiducials the machine is using, so the fiducials have to be on the board rather than on the carrier whenever the carrier and the board can move relative to each other.
Where the fiducials are on the carrier, the machine locates the carrier and the board’s position inside it becomes part of the error budget. That is acceptable when the pocket clearance is 0.05 mm and not acceptable when it is 0.20 mm, so the fiducial choice follows from the pocket tolerance; our placement accuracy notes set out how the two are combined. A carrier that locates the board against a machined edge rather than against a pin is easier to load but adds the edge tolerance of the board to the same budget.
Warp Control Through the Profile
A carrier holds the board flat at temperature, but it does not remove the internal stress that causes the board to bow after it cools. Where the copper distribution is asymmetric the board will bow in the carrier and again after it leaves, and the second bow is the one that matters after the assembly is singulated.
Supporting the board at the points where it is stiffest, and preheating slowly so that the two sides of the panel are at similar temperatures, reduces the second effect. A profile with a longer soak and a gentler ramp through the glass transition region is often more effective than any change to the carrier.
Cleaning and Maintenance of Carriers
Carriers collect flux condensate, and a pocket whose floor is coated with flux residue holds the board at a slightly different height than a clean one. Cleaning is scheduled by reflow passes rather than by the calendar, and the cleaning method has to be compatible with the carrier material, since an aluminium carrier and an FR4 carrier do not tolerate the same chemistry.
Flatness is checked periodically with a straight edge or on a granite table, and a carrier that is out of flatness by more than the board tolerance is retired. Warped carriers are a common cause of intermittent placement faults that appear and disappear with the shift.
Selecting and Documenting a Carrier
The selection starts from the number of passes the carrier will see, the temperature it will reach, the flatness it must hold and the way the board will be loaded. Those four items are written on the carrier drawing along with the pocket dimensions and the pin positions.
The record should also carry the first article profile measured with the carrier in place and the flatness measurement before and after the profile. Without those figures a later change to the carrier, or a substitution by the shop, cannot be assessed against the behaviour that was qualified. The carrier identification should also travel with the work order, so that a build run on a substitute carrier is visible in the record rather than discovered later.
FAQ
Can a carrier be used as the fiducial reference? It can, provided the pocket clearance is small enough that the board cannot move inside it. Where the clearance is generous the fiducials belong on the board, because the machine then measures the thing it is actually placing onto.
Does a carrier always improve flatness? It holds the board flat while it is supported, but it does not remove the internal stress that bows the board after cooling. A carrier helps most on a board whose flatness is already adequate at room temperature and fails only at reflow temperature.
How often should the profile be re-measured with a carrier? Whenever the carrier changes, whenever the board thickness or the copper distribution changes, and on a schedule that follows the oven’s own maintenance. The profile is a property of the board, the carrier and the oven together rather than of the oven alone.
