Support Pin: 5 Rules for Board Support in Printing
A support pin is one of the small posts that hold the panel flat on the printer table while the stencil printing stroke runs across the top. Support is easy to overlook because it is invisible in the finished product, yet it decides whether the paste deposit is the same at the edge of the panel as it is in the middle.
The board has to be flat, parallel to the stencil and in the same position for every print. Board support achieves that, and where it is missing or wrong the panel deflects under the squeegee, the gap between the board and the stencil changes, and the paste deposit varies across the panel in a pattern that repeats on every board.
What Support Pins Have to Do
The squeegee presses down with a force that is spread over the print area, and the board has to resist it without moving. Any deflection changes the space between the stencil and the board, which changes the volume of paste that ends up under the aperture and the amount that is squeezed under the foil. Support exists to remove that deflection.
Support also locates the board. The panel has to sit in the same place relative to the stencil on every cycle, and the pins and the edge stops together define that position. A panel that is located differently between prints will show a deposit that shifts relative to the pads, even when the volume is correct.
Support Layout for a Panel
The layout should follow the panel rather than the table. Pins belong under the rails, under the gaps between boards and under the areas where the copper is dense, because those are the places that resist or transmit the squeegee force. A pin placed under a fine pitch component can press paste that has already been printed on the underside of the previous panel, which is worse than having no support there.
Where the panel has a large unsupported span, the deflection is greatest in the middle and the deposit is thinnest there. Adding a pin in the middle changes the pattern, so the layout should be reviewed against the printed result rather than against a diagram. The paste volume map across a panel shows where the support is working and where it is not.
Pin Height and Coplanarity
All the pins in a set have to reach the board at the same height. A pin that is a fraction of a millimetre taller than its neighbours lifts one part of the panel and leaves another unsupported, and the result is a board that looks flat but is not. Height should be set with a gauge against the table surface rather than by feel.
Pins wear, and a set that has been in service for a long time will have a spread of heights. The spread should be measured periodically, and pins that fall outside the tolerance should be removed from the set. Magnetic bases make replacement easy, which also makes it easy to mix pins of different lengths without noticing.
Vacuum Support Compared With Pins
Vacuum support pulls the panel down onto the table, which flattens the board and holds it in position at the same time. It works well on thin panels and on boards with a large open area, and it reduces the number of pins that have to be placed by hand. The limit is the panel itself, since a board with large cutouts or a very open design cannot hold enough vacuum.
Pins and vacuum are complementary. Vacuum holds the panel flat against the table, while pins support the regions that the vacuum cannot reach. Where a printer uses both, the pins should be placed so that they do not block the vacuum channels, because a pin sitting over a channel reduces the force available to hold the rest of the board. The vacuum hold down condition should be verified with a gauge when the tooling is changed.
Support Under Fine Pitch and Thin Boards
Fine pitch work is the least forgiving. The apertures are small, the deposits are shallow, and a small change in the board to stencil gap produces a large change in the printed volume. Support has to be dense enough and accurate enough that the gap is constant across the whole print area.
Thin boards add a second problem, because the panel deflects under its own weight as well as under the squeegee. Where the board is thin, vacuum support or a full backing plate is usually necessary, and the panel design should be reviewed for stiffness before the order is released. The panel warpage measured before printing is a good indicator of how much support the panel will need.
Support in Reflow and Depaneling
Support is not limited to printing. A board that is unsupported in the reflow oven can sag between the conveyor rails, and the sag changes how the board sees the convection as well as how it sits on the rail. Where the oven has a centre support, it should be set to the correct height for the panel thickness.
Depaneling is the other place where support matters, because a router or a shear applies force to a panel that is often held only at its edges. The support beneath the cut decides how much the panel flexes, and flexing is what cracks components and solder joints. The strain measurement taken during depaneling is the evidence that the support is adequate.
Damage Caused by Poor Support
The failure mode is not a dramatic one. A support problem produces a gradual drift in deposit volume, a rise in the number of bridges and skips on one part of the panel, and an increase in the number of components that need rework. None of those symptoms points at the tooling unless the print data is reviewed by position.
Poor support can also damage the board. A pin under a component can press into the solder mask and leave a mark, and a pin that is set too high can crack a ceramic capacitor when the panel bends over it. The tooling layout should therefore be checked against the assembly drawing so that no pin sits under a fragile part.
Setting Up and Verifying the Tooling
Setting up tooling for a new panel should start from the panel drawing and finish with a printed result. The pins are placed first, then their heights are verified, then a panel is printed and the deposit is measured at the corners and the centre. The measurement is what confirms the layout, and it should be repeated when the tooling is disturbed.
The verification should be recorded with the panel number and the pin layout, because a support set that works for one panel may be wrong for another of the same size. A photograph of the table with the pins in place is a practical way to document the layout so that it can be rebuilt exactly.
First Article, Records and Change Control
The first article should include the support condition as well as the printed result, since a good print on a badly supported panel can be a coincidence. Where the panel is changed, the support should be reviewed again rather than carried over, because a new component placement can put a pin under a part that was not there before.
Records should tie the pin layout to the panel revision and to the printed volume result. A change in deposit pattern across the panel is then checked against the tooling before the printer settings are touched, which saves time and prevents a good recipe from being adjusted to compensate for a loose pin. The same print that reveals a support problem also shows solder bridge risk at the unsupported edge.
FAQ
How many support pins does a panel need? Enough that the panel does not deflect measurably under the squeegee, which depends on the panel size, thickness and copper distribution. The practical test is to print and measure the deposit at the corners and the centre. If the volume varies by position, the support is not sufficient.
Can support pins be placed under components? They should not be placed under fragile parts or under areas that have already been printed on the panel below. A pin under a ceramic capacitor can crack it when the board flexes, and a pin under a fine pitch footprint can transfer paste from the underside of the panel.
Is vacuum hold down enough on its own? Vacuum flattens and locates the panel, and on many assemblies it is sufficient without pins. Where the panel has large cutouts or a very open copper design, the vacuum cannot develop enough force, and pins are needed under the areas that would otherwise sag.
