Board Support and Pallet Design for Double Sided Assembly

A board on a conveyor is supported at its edges only, so it sags under its own weight and under the weight of the components. The sag changes the print gap and, on a thin board, it can move the board out of the placement camera focus.

Where the Sag Comes From

The deflection depends on the thickness, the span between supports, the temperature and the load already placed. A large panel with a thin core sags the most and it sags further as the first side is populated.

The deflection is also what produces the strain during depaneling. Our warpage notes describe the control.

Support Pins and Their Placement

Support pins are placed under areas that have no components on the underside, so the placement has to be planned against the second side artwork. A pin under a populated area lifts the board instead of supporting it.

The pin map should be part of the program rather than set by the operator. Our placement accuracy notes describe the effect on the camera.

Pallets for Double Sided Work

A pallet carries the board and provides a flat reference for the second pass, with cutouts over the components placed first. The cutout depth has to clear the tallest component on the first side.

A component that touches the pallet will be damaged during printing. Our component reliability notes describe the stress that results.

Shadowing in Reflow

A pallet that is too tall or too close to a component shadows it from the convection flow, so the joint under it sees a different profile. The effect is worst on the trailing edge of a tall part.

Shadowing is measured the same way as any other profile difference. Our thermal mass notes describe the measurement.

Double Reflow Sequence

The second reflow has to keep the first side’s joints below their melting point, or it will disturb them. The high temperature alloy on the first side and the lower one on the second is the classical answer.

Modern practice uses two similar alloys with a controlled profile. Our profile notes describe the measurement.

Handling Between Passes

A board that has been through one reflow is more fragile than a bare one. It should be handled by the edges, supported in the middle and stored so that nothing rests on the placed components.

Our floor control notes describe the handling stations that go with it.

Verification

The verification is a measured deflection under the actual process condition and a support map that has been checked against the underside artwork rather than assumed.

Our quality notes describe how the records are kept.

Process Control and Verification

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. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

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.

FAQ

Is a pallet always needed? No. It is needed where the board is thin, where components sit on both sides in the same area, or where the conveyor cannot support the panel.

Can support pins be reused between products? The pins are reusable and the map is not, because the map depends on the underside component placement.

What does gopcb provide for double sided assembly? We provide a support map checked against the underside artwork, pallets with cutouts sized to the tallest first side component, a deflection measurement under the process condition, and a second reflow profile measured under the pallet rather than on a bare board.