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One overlooked factor in solder paste printing optimization is z-axis board support. This means supporting the bottom of the board during printing. Customers often experience less-than-optimum results because they overlook z-axis board support.

The rule of thumb for squeegee blade pressure is 0.45 to 0.68 kg of pressure per 39.37 mm of squeegee blade. For example, a 472.4 mm squeegee blade will apply 5.45 to 8.18 kg of force on the board during printing. The downward force of the squeegee blade will deflect the circuit board to some degree depending on the board's size, thickness and, most importantly, the amount of support under the board.

Thinner, larger boards deflect more. The deflection of the PCB during the print cycle will eliminate the gasketing (sealing) of the PCB pad to the stencil aperture. Gasketing the board pad to the stencil aperture ensures that the paste being printed does not get under the stencil, where it can cause wet bridges that often turn into solder shorts after reflow.

A number of options are available to support a PCB in the z-axis during printing. If the board is a single-sided assembly or the first side of a double-sided assembly, the entire bottom side is available for support. Flat plates or magnetically mounted blocks are ideal in this instance.

When printing the second side of a double-sided assembly, consider how to support as much of the board as possible without disturbing or damaging bottom-side components. Several options are available:

Custom tooling (also called dedicated workholders). Custom tooling is made for a specific assembly or a family of very similar assemblies. The tool provides clearance for bottom-side components while providing support in the areas of the board that do not have components mounted. Custom tooling provides maximum support without contacting any of the bottom-side mounted components. The issues with custom tooling are cost and managing board revision levels. When used in high volume operations, custom tooling is reasonably cost effective. To address revision levels, customers must either replace the tooling once that assembly or family of assemblies is no longer built, or make compromises in the design of the top plate to permit multiple revision levels to run on the same tool.

Support pins. Support pins provide a flexible board support system. Usually, a magnetic base holds them in position. An operator then places them in position. Pins must be placed correctly to provide maximum support and also to ensure they do not contact a bottom-side component. If a misplaced support pin contacts a bottom-side component, the board and stencil will not be parallel, resulting in many defects. Contacting a bottom-side component can also damage that component. Support pins can be used for single-sided or topside support.

Offline manual grid pin systems. An offline grid pin system provides pins placed in a grid pattern and is set up offline for the next board to be printed. Pins are placed on the grid to provide support without contacting bottom-side components. Again, ensure that pins are placed in the correct location; a visual aid or instructions are required for the operator. A customer will usually have at least two grid pin systems, one for the board being built and the other being set up for the next board to be built.

Automatic conformal systems. These systems are usually pneumatically activated 39.37 x 590.6 mm "blocks" with two rows of pins spaced about 39.37 mm apart. The required number of blocks are placed in the work nest parallel to each other with a predetermined gap between to support the width of the board being built. When the board is in position the pins are activated. They rise to the lowest point they contact, conforming to the topography of the bottom-side component layout. The support pins have a soft compliant cap, permitting contact with bottom-side mounted components. Once raised to board height, the pins lock into place and support each board as it is processed. These systems can be set up based on a golden board or activated for each individual board. The systems are also used for board support in placement equipment.

Gel tooling. This proprietary tooling is a soft pliable "bag" of gel material mounted in a metal housing with a magnetic base to hold the tool in place. The gel material conforms to the bottom-side component layout and gently supports the board. Displacement is greater in the z-axis than in the x- or y-axes, so it can support individual components without sacrificing support for adjacent components. Provided sufficient gel blocks are used to cover the area, it is virtually impossible to place them incorrectly because they provide support without damaging bottom-side components. Such tooling is also used for board support in placement equipment.

Automatic pin placement systems. Some printers have patented systems that automatically place support pins at programmed locations. The support pins are usually housed in a tray or carousel and picked and placed by the machine using data specific to the board being built. This system provides the flexibility of support pins without the problem of misplaced support pins as the program controls pin location.

Universal shape memory board support system. This system is a 450 mm long "bladder" that contains solid particles. Once the bladder is conformed using a golden board, a vacuum locks the solid particles without shrinkage, creating a firm, precise support.

Joe Belmonte is project manager, advanced process development, at Speedline Technologies (speedlinetech.com); jbelmonte@speedlinetech.com. His column appears semimonthly.