Circuits Assembly Online Magazine - Ensuring Accurate Placement in the Tooling Nest

Combining clamping and snugging in a single board handling method.

One key component of screen printing that hasn’t gotten a lot of press is board handling. While not the flashiest part of the process, in my view it is one of the most critical – especially when dealing with boards as thin as 0.6 mm, components designed in all the way to the edge of the board, and pitches of 0.3 mm and less. Historically, board design rules and industry standards dictated that components be placed at a relatively safe distance – approximately 5 mm or greater – from the board edge. Times have changed. Edge connectors and the like are creeping ever closer to the cliff, thus having a huge impact on board handling techniques and, ultimately, product yield.

One common mechanism used to secure boards during printing is a clamping system. This remains relevant for a gracious plenty of today’s mainstream applications. However, with boards that are extremely thin and contain devices near the outer edge, an over-the-top clamp may present some issues. In such a scenario, as hard as you may try, it is very difficult to achieve a true board to stencil gasket, which then impacts solder paste deposit height and repeatability.

As an alternative to over-the-top clamps, many print systems employ board snuggers, which engage and hold the board from the side edges, instead of above and below. While this technology has had limited success, there are some inherent problems with traditional snugging systems. For one, it lacks the capability to flatten the board before the vacuum kicks in, so the board may not be fully located in the tooling plate. Second, most snugging systems lack the capability to self-adjust for varying board thicknesses, so manual intervention is required. This may not seem a problem when running one product, but, even within a batch, board thicknesses can be slightly different due to solder resist thickness variances.

New board handling development advances, however, have resulted in what can best be described as a hybrid solution that combines the advantages of clamping with those of snugging. In essence, it is an over-the-top snugging technology where the snugging bars push down on top of the board edges, ensuring accurate placement in the tooling nest. The board is secured by vacuum, and the snugging bars move off the top of the board and to the sides, where they stop and hold the board in position for printing. This newer snugging technology self-adjusts for the board thickness, accommodating boards from 0.4 to 5 mm in height, so that the snugging bars are flush with the top of the board, enabling a robust stencil to board gasket seal, effectively eliminating any issues with edge-placed components. The board can now be printed all the way to the outer edge with no interference from tooling or handling mechanisms.

To compare the performance of newer snugging technology against that of traditional clamping systems, our company analyzed the print deposit height and range of eight 0.4 mm edge connector components located 4 mm from the board edge. The stencil foil thickness was 0.100˝. Results are shown in Figure 1. This chart clearly illustrates that over-the-top (OTC) clamps tend to produce printed deposits higher than the nominal stencil thickness. This response demonstrates the interference clamps have on the edge component stencil to board gasket. In addition, the range of deposit heights when using a clamping technique was approximately 0.075˝, which suggests a lack of repeatability for the edge deposits. Conversely, over-the-top snugging exhibits a print thickness closer to the nominal stencil thickness and also maintains a lower height range – approximately 0.015˝ – than that of OTC.

Board handling, like most processes in surface-mount assembly, does not have a one-size-fits-all solution. Clamping is viable for many applications, but highly miniaturized, and DfM-deficient products will benefit from getting snug.