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Effect of Squeegee Blades on Print Quality
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Authored By:Rita Mohanty, Bill Claiborne Speedline Technologies, Franklin, MA USA Frank Andres Cookson Electronics, South Plainfield, NJ USA TranscriptThe solder paste deposition process is viewed by many in the industry as the leading contributor of defects in the SMT assembly process. As with all manufacturing processes, solder paste printing is subject to both special and common cause variation. Using different blades types can contribute significant special cause variation to a process. Understanding the significant differences in print performance between blade types is an important first step to establishing a standard blade for an SMT solder paste printing process. This study focuses on the squeegee blade printing process and the effects different types of blades have on solder paste print deposition quality. Several experiments were conducted to understand the effect of blade thickness on the overall print quality for fine pitch printing. Based on this study and initial analysis, the authors conclude that thinner blades provide more flexibility in regard to attack angle due to a "leaf spring" action. The "leaf spring" action enables adjustment of the attack angle by simply changing the print pressure and speed. Results indicate that thinner blades provide higher transfer efficiency, lower standard deviation and better print profile compared to thicker blades. SummaryThe solder paste deposition process is viewed by many in the industry as the leading contributor of defects in the Surface Mount Technology (SMT) assembly process. As with all manufacturing processes, solder paste printing is subject to both special and common cause variation. Just like using graduated cylinders from distinctly different manufacturing processes to measure a volume of liquid, using different blades types can contribute significant special cause variation to a process. Understanding the significant differences in print performance between blade types is an important first step to establishing a standard blade for an SMT process. Over the last 30 years, the SMT assembly process has become increasingly more sophisticated. There are two primary methods of applying solder paste to a circuit board using a stencil printer: squeegee blade printing and enclosed head printing. While each method has its advantages and disadvantages, this study focuses on the squeegee blade printing process and the effects of different types of blades have on the solder paste print deposition quality. Additionally, solder pastes have been formulated to deliver increased paste deposition volume and consistency for ever decreasing aperture area ratios and increasing print speeds. With squeegee blade printing, only two print parameters can typically be controlled, squeegee speed and downward squeegee pressure. Excessive pressure can result in damaged stencils, coining and breaking of webbing between fine pitch apertures. Too little pressure can result in skips if the stencil is not wiped clean. This study will report on the effects of squeegee blade thickness along with blade surface finish on solder paste print quality. Print quality is defined here as paste deposit profile, wet bridging and insufficients. Attack angle of the blade, which is considered to be the ultimate factor to be controlled, will be determined using a unique approach as a function of blade thickness, print speed and print pressure. Other aspects of the study will include interaction between the above mentioned factors with various solder paste types. A 3-D Solder Paste Inspection (SPI) system will be used to characterize the print quality in respect to transfer efficiency and deposition profile. ConclusionsSeveral experiments were conducted to understand the effect of blade thickness on the overall print quality for fine pitch printing. Based on this study and initial analysis, we can conclude that thinner blades provide more flexibility in regards to attack angle due to a "leaf spring" action. The "leaf spring" action enables us to adjust attack angle by simply changing the print pressure and speed. Preliminary results indicate that thinner blade (0.007") provides higher TE, lower Standard Deviation and better print profile as compared to thicker (0.010") blades. Initially Published in the IPC Proceedings |
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