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April 2015 • SMT Magazine 59 system enables an elaborate control of the sig- nal with a choice of control points along the voltage signal. In the described study, a three- step signal with a given rise time, plateau time, and fall time, tf, is used to study the quality of jetting (Figure 3b). The quality of jetting is judged using for qualities for each deposit, namely diameter, positioning, shape and satellite level. Figure 4 will be used as the basis for a description of the above-mentioned jetting qualities. In Figure 4, a circle is shown which represents an ideal perfectly circular deposit with diameter d. The irregular curve in Figure 4 represents an actual deposit. The area of the shaded area is used to extract an equivalent diameter for the deposit by using this equation, equation 1 where A is the area of the deposit. The mass centrum for the area of the deposit is used to calculate the positioning deviation, ΔX and ΔY, for the expected position of the deposit on the substrate. The shape of the droplet is calculated by taking the norm of the radial difference of the deposits edge from a circle with the calcu- lated equivalent diameter in accordance with the expression equation 2 where ri is the local radius at a point on the periphery of the deposit and n is the number of points used along the periphery. Satellites are de- fined as free bodies outside the main deposit. The position and diameter of the satellites are regis- tered in the same way as for the main deposit. A type 5 lead-free SnAgCu 305 solder paste was utilized for the tests. For each combination of actuation parameters, tr, tp and tf, a set of three jobs consisting of X deposits each were jetted for four different deposit volumes. The volume of deposits for the jetting device is con- trolled by the amount of solder paste transport- ed to the jetting chamber for each individual deposit. The jobs were jetted on a custom 230 mm x 317 mm substrate with spray-mounted photographic paper to enhance contrast for im- age processing purposes as well as to simplify cleaning. Data evaluation was carried out using Matlab. results and Discussion Among the most important criteria for any deposition of solder paste on a printed circuit board are positioning, deposit size, volume and volume uniformity. In Figure 5, the correlation of deposit diameter and positioning deviation as a function of piezo actuation profile is pre- sented. Although the absolute values of diam- eter and positioning vary for the various piezo actuation profiles, the trend is clear in that po- sitioning errors increase almost monotonically with decreasing droplet volume. In Figure 6, the correlation of deposit diam- eter and satellite level as a function of piezo ac- tuation profile is presented. The data here is not as clear as for the positioning deviation, which signals that we have a stronger dependence on the actual form of the actuation profile on the formation of satellites during the filament break-off process. figure 4: The quality of jetting is judged using for qualities for each deposit, namely diameter, positioning, shape and satellite level. FeAture JETTiNG STraTEGiES FOr MBGaS: a QuESTiON OF GivE aND TaKE continues

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