Analysis of the Effects of Interior Aperture Angle on Print Quality in Solder Paste Stenciling
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angle, i.e. angle between two sides of a polygon, are analyzed sidewall taper are also examined for their individual and show no effect of interior angle within the bounds of the when aperture geometry is nearly square.
INTRODUCTION The most conunon (non-BGA) aperture shape used in the SMT process is rectangular. This shape conforms to the chip-carrier lead and metal land on which the paste is deposited. There is possibly a best shape, or optimum interior angle for depositing paste through a stencil. One major concern in solder paste stenciling is maximizing the percentage of paste that passes through an aperture. It is reasonable to assume that varying the geometry of the aperture has less consequence on larger apertures than on smaller ones. However, since chip-carrier leads are shrinking as system performances increase, the percentage of paste that sticks to the stencil is becoming more significant.' This fact has prompted the study of these effects of aperture geometry on print quality. The solder paste used in stenciling is a thick mixture of metal powder, flux, and a vehicle or jell. The metal powder is usually a near-eutectic combination of lead and tin. These particles are mostly spherical in nature and range in diameter from 10 to 75 microns (0.4 to 3.0 mils). Upon heating, the solder paste becomes simply solder, creating electrical paths between the board and component leads. Solder paste researchers consider the rheological properties of solder paste when designing experiments. Ogata 2 drew conclusions on solder paste printability (a combination of print merits for which there is no universal standard) based on combinations of paste viscosity and thixotropy. Kasturi 3 reported on experiments involving variable viscosities, particle sizes, flux types, percents of metal, and vendors. He drew conclusions on combinations of these variables with other factors such as aperture pitch, stencil thickness, snap-off distance and humidity. Using shear rate tests, Lapasin 4 presented an analytical procedure for the rheological characterization of solder paste. He concluded that, under specified test conditions, only viscosity and yield stress are needed to reliably predict solder paste screening behavior, such as stencil clogging and paste slumping. Solder paste stenciling is the most common method of paste deposition in surface mount technology. During the printing or stenciling process, an automated metal or polyurethane squeegee rolls a thick line of solder paste across a stencil which is positioned above a circuit board. The paste is forced through the stencil's apertures and onto the board in locations coinciding with the leads of the components to be mounted. Variable printing parameters that affect print quality exist in the squeegee (material, hardness, speed, pressure) and in the board positioning (distance between stencil and board during print). Generally harder squeegees print better than softer ones. 5 This is due to undesirable paste scooping, or "scavenging", which is the partial removal of paste depo
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