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Knocking Down the Bone Pile: Tin Whisker Mitigation in Aerospace Applications, Part 3
The Knowledge Base: Building the Workforce of Tomorrow With EMAC
Advanced Printing for Microelectronic Packaging
March 11, 2015 | Kenneth H. Church, Xudong Chen, Joshua M. Goldfarb, Casey W. Perkowski, Samuel LeBlanc, nScrypt Inc.Estimated reading time: 16 minutes
A number of important printing factors need to be considered when printing fine lines. One is the diameter of the pen tip. Typically the inner diameter does not set the width of the dispense feature; this is usually set by the out diameter of the pen tip. In addition, there is a slumping issue. Most shear thinning materials will slump after being printed. Therefore to print a fine feature such as 100 microns, it is many times necessary to go to a smaller pen tip such as a 50/75. This implies 50 micron inner diameter and 75 micron outer diameter. Additionally, it will be important to get in proximity of the substrate. Proximity is affected by the pen tip diameter. If the diameter is very large (hundreds to thousands of microns) then the gap set between the pen tip and the substrate can be very large. The volume being dispensed will be very large. If the desire for small features is important, then the small gap will be important. Gaps on the order of microns and tens of microns can have an effect on printing as well. The flow rate for dispensing can be affected by the gap.
The flow rate of the pump (which determines the dispensed feature size) depends on the value of the pressure, the valve opening and the size of the tip orifice. Engineers and researchers at the company also discovered that the dispensing height (the gap between the dispensing nozzle tip and the substrate) plays a crucial role in the dispensing volume and especially when the feature size gets smaller or thin lines are printed. For these types of features the nozzle tip needs to be closer to the substrate. This will increase the flow resistance and thus reduce the flow rate. It is expected that the pressure drop will be dominated by the dispensing gap when the gap is 50% smaller than the tip size. By applying the above mentioned CFD (Computational Fluid Dynamic) model at various dispensing heights (Figure 2), the flow rates were calculated [5].
Figure 2. Schematic drawing of the CFD model.
The pressure drop with different dispensing heights is plotted in the Figure 3. At a very small gap (Figure 3a), the pressure
drop is dominated by the substrate. At a large gap (Figure 3c), the pressure drop is mainly determined by the tip orifice.
There is a transient region (Figure 3b) where the pressure drop is affected by both of them.
Figure 3. Pressure drop vs. different dispensing height. (a) 5 microns. (b) 15 microns. (c) 30 microns.
The flow rate vs. dispensing height is plotted in the Figure 4. It can be seen that the flow rate increases to a steady state value at the dispensing height that is proportional to the value of the pen tip diameter. The pressure moves the steady state flow rate with the lowest pressure achieving steady state the fastest. It is also obvious that the flow rate is sensitive to a change in dispensing heights if they are below a certain value (50 microns in this case). The lower the dispensing height, the more it affects the flow rate.
Figure 4. Flow Rate versus Dispense Height
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