Issue link: https://iconnect007.uberflip.com/i/339176
78 SMT Magazine • July 2014 MEETING FuTuRE STENCIl PRINTING CHAllENGES continues ARTiCLE of playing the game. In the SMT process, this may not necessarily be due to losing sight of the basic fundamentals, but because the game itself continues to evolve. As the trend towards miniaturization continues, a process that was successful in the past may suddenly present un- acceptable results due to the decreasing size of the stencil apertures required to print ultrafine solder paste deposits. In an earlier paper titled "Process Guidelines to Ensure Optimal SMT Electronics Assembly" (Edward Briggs and Ron Lasky SMTAI Toronto, May 2012), I discussed solder paste storage and handling; process set- up, including the importance of gasketing and registration; solder paste attributes; reflow pro- cesses; newer SMT technologies; and the data collection methods for characterizing the sten- cil printing process to meet this evolution. The focus of this paper is to single out one of those considerations—powder particle size (powder types) and its effect on the stencil printing pro- cess. In order to focus on particle sizes, many variables as possible were eliminated and the data analyzed using solder paste inspection equipment. A physical observation will also be made of the reflowed results. Finer powder par- ticles mean an increase in surface oxide and can create challenges for reflow. Stencil Printing and Area Ratio and Transfer Efficiency In regard to stencil printing success, the two most critical parameters are the area ratio (AR) and transfer efficiency. Area ratio is the area of the stencil aperture opening divided by the area of the aperture side walls. Figure 1 shows an area ratio schematic for a circular aperture. Simplifying the calculation shows that the area ratio (AR) is the diameter (D) of the circle di- vided by 4 times the stencil thickness (t): AR = D/4t. The result is the same for square apertures, with the diameter (D) now equal to a side of the square. For the AR of a rectangular aperture, the formula is a little more complicated: ab/2(a+b) t, where a and b are the sides of the rectangle. It is widely accepted in the industry that the AR (area ratio) must be 0.66 or greater to be suc- cessful in the stencil printing process. Histori- cally, experience has shown that if AR < 0.66, the transfer efficiency will be low and inconsis- tent. Transfer efficiency is defined as the volume of the actual solder paste deposited divided by the calculated volume. Measuring the effect of AR on fine feature printing will be an important part of our experiment. The Experimental Design The focus of this printing experiment was to differentiate transfer efficiency and consistency between powder types. Therefore, in an effort to diminish the number of variables, the same stencil, squeegee blades and printer parameters were utilized (Figure 2). A 4 mil thick laser cut stencil, 250 mm squeegee with edge guards, foilless clamps, and landscape vacuum support blocks were used on the stencil printer. Figure 2 depicts the printer figure 1: Definition of the area ratio for a circular stencil. figure 2: Printer settings.