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The board clamp 'standoff': why edge clamps provide more uniform paste transfer across the board.

FOR STENCIL PRINTING, defects are typically caused by one or more of the following: poor alignment between substrate and stencil, incorrect paste chemistry, or variations in the amount of paste deposited. Variation in the amount of paste deposited in turn depends on several factors, some of which are paste chemistry, printer setup, squeegee blade type, stencil design, board support and board clamping.

Board support is an integral part of developing a robust printing process. Proper board support is essential to ensure consistent print results and higher yields. Without proper board support, the force applied to the board across the entire width of the PCB will vary, and proper gasketing between the stencil and board will not be achieved. Board support comes in two distinct forms: under board support and transport rail support. Here, we look at two common transport rail board support systems: "top clamp" and "edge clamp." As components are placed closer to the edge, board clamping has become a significant factor in maintaining print quality.

The primary function of a clamping system is to hold the board tightly in place to provide optimum gasketing during printing process. Available various types of clamping systems include top clamp, snuggers, flippers and vacuum hold-down. Top clamp and snuggers, two primary clamping systems, operate slightly differently in providing the mechanism to hold the board. Top clamp, as the name implies, holds the board in place by applying a clamping system (a thin metal foil) on the top of the board, while edge clamp works by tightly snugging the board in the Y direction, without foil on top of the board. As the edge clamp holds the board without use of foil, it delivers optimal stencil-to-board gasketing, critical in providing higher paste release efficiency.

Recently, a carefully designed experiment was conducted to understand the true effect of these two clamping systems on a specially designed stencil. The stencil was designed to baseline the clamping system using a simple design concept. The stencil consisted of 0.004" laser cut, stainless steel foil with an image size of 8" x 10". The aperture pattern started 3 mm from the edge of the board and increased by 0.5 mm every four pads (FIGURE 1).

[FIGURE 1 OMITTED]

An external SPI was used to characterize the paste deposit to determine the variation in paste volume and height. FIGURE 2 shows the box plot of transfer efficiency (TE) for the top half of the board. The top clamp shows a significantly higher TE close to the edge. Moving away from the edge, the TE stabilized to a more constant value. No such variation in TE is seen for the edge clamp system. The primary reason for the higher TE near the board edge is due to higher paste height (FIGURE 3). Why is paste height higher near the board edge for a top clamp system? As the squeegee blade moves over the stencil from the edge of the image to the center, the top clamp acts as a standoff height. This action prevents the stencil from providing the necessary gasketing for consistent paste transfer. As we move from the edge, the "standoff" effect reduces and proper gasketing occurs between the board and stencil. Since the edge clamp does not provide a standoff effect, paste transfer seems to be uniform across the board.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Results from this test and data from various other sources indicate edge clamp provides a more stable process as compared to top clamp. When dealing with a highly populated board or boards with components close to the edge, edge clamps have a distinct advantage.

[ILLUSTRATION OMITTED]

RITA MOHANTY, PH.D., is director advanced development at Speedline Technologies (speedlinetech.com); rmohanty@speedlinetech.com.
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Title Annotation:SCREEN PRINTING
Comment:The board clamp 'standoff': why edge clamps provide more uniform paste transfer across the board.(SCREEN PRINTING)
Author:Mohanty, Rita
Publication:Circuits Assembly
Date:Nov 1, 2009
Words:623
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