Critical variables of solder paste stencil printing for micro-BGA and fine-pitch QFP

Stencil printing continues to be the dominant method of solder deposition in high-volume surface-mount assembly. Control of the amount of solder paste deposited is critical in the case of fine-pitch and ultrafine-pitch surface-mount assembly. The process is still not well understood as indicated by the fact that industry reports 52-71% surface-mount technology (SMT) defects are related to the solder paste stencil printing process. The purpose of this paper is to identify the critical variables that influence the volume, area, and height of solder paste deposited. An experiment was conducted to investigate the effects of relevant process parameters on the amount of solder paste deposited for ball grid arrays (BGAs) and quad flat packages (QFPs) of five different pitches ranging from 0.76 mm (30 mil) to 0.3 mm (12 mil). The effects of aperture size, aperture shape, board finish, stencil thickness, solder type, and print speed were examined. The deposited solder paste was measured by an inline fully automatic laser-based three-dimensional (3-D) triangulation solder paste inspection system. Analysis of variance (ANOVA) shows that aperture size and stencil thickness are the two most critical variables. A linear relationship between transfer ratio (defined as the ratio of the deposited paste volume to the stencil aperture volume) and area ratio (defined as the ratio of the area of the aperture opening to the area of the aperture wall) is proposed. The analysis indicates that the selection of a proper stencil thickness is the key to controlling the amount of solder paste deposited, and that the selection of maximum stencil thickness should be based on the area ratio. The experimental results are shown to be consistent with a theoretical model, which are also described

Gauge Repeatability & Reproducibility Study for a 3-D Solder Paste Inspection System

Due to the increased use of Ball Grid Arrays (BGAs) and fine pitch and ultra fine pitch Quad Flat Packages (QFPs), there is a dramatic increase in demand for solder paste inspection after the stencil printing process. The important response variables of the printing process are deposited solder paste volume, area, height and position. To identify and remove defects at the earliest possible process step, a 3-D solder paste inspection system should be used to monitor solder paste deposited on all pads on every board before component placement. An example is a fully automatic laser-based 3-D triangulation solder paste inspection system that is currently used for inspection of 0.635mm (25mil) pitch QFP components of automobile electronics products. As the pitch of QFP components decreases and the use of BGA components increases, the gauge repeatability & reproducibility (R&R) of the inspection system must be reevaluated. This work investigates whether the present system can be used for measuring 0.4 mm and 0.3mm pitch QFPs and BGAs. An experiment was conducted featuring 2,400 pads of 5 different pitch levels of QFPs and BGAs from 0.3mm (12mil) to 0.76mm (30mil). The analyses show that the system is capable of measuring solder deposits higher than 0.025 mm (1 mil) with repeatability of 5% for volume and area measurement, and with repeatability of 5% for average height of the majority of pads and 18% for all pads. In addition, there are requirements in board design necessary to ensure the reference points for measurements can be specified close enough to the pads

Critical variables of solder paste stencil printing for micro-BGA and fine pitch QFP

Stencil printing continues to be the dominant method of solder deposition in high volume surface mount assembly. Control of the amount of solder paste deposited is critical for fine pitch and ultra-fine pitch SMT assembly. The process is still not well understood as indicated by the fact that industry reports 52-71% of SMT defects are related to the solder paste stencil printing process. The purpose of this paper is to identify the critical variables that influence the deposited solder paste volume, area, and height. An experiment was conducted to investigate the effects of relevant process parameters on the amount of solder paste deposited for BGAs and QFPs of 5 different pitches, ranging from 0.76 mm (30 mil) to 0.3 mm (12 mil). The effects of aperture size and shape, board finish, stencil thickness, solder type, and print speed were examined. The deposited solder paste was measured by an in-line fully automatic laser-based 3D triangulation solder paste inspection system. Analysis of variance (ANOVA) shows that aperture size and stencil thickness are the two most critical variables. A linear relationship between transfer ratio (defined as the ratio of deposited paste volume to stencil aperture volume) and area ratio (defined as the ratio of the area of the aperture opening to the area of the aperture wall) is proposed. Analysis indicates that proper stencil thickness selection is the key to controlling the amount of solder paste deposited and that the selection of maximum stencil thickness should be based on the area ratio. The experimental results are shown to be consistent with a theoretical model, which is also described