<< Figure 14: Cycle time per unit. >>
<< Figure 13: Cycle time per unit. >>
motion parameters can increase units per hour (UPH) in this case. Certain dispensing system accuracy is required for the dual applicators to meet tight geometry constraints.
Surface contact dispensing case The same dimension parts were used for the throughput comparison. This applicator needed to use a small dispense gap in order to put down the sealant in a consistent manner. The goal was to achieve 0.3 mm line width using an auger valve with needle. Typical line dispensing velocity is 25 mm/s to ensure uniform line width. Figure 14 shows that the majority of time allocation per part was driven by work-time dispensing due to slow applicator velocity during dispensing required for this sealing dispense. Plus, the needle required move-up time to provide clean fluid break-off from the needle. In this case high- performance motion parameters did not improve throughput significantly.
Each applicator with different part dimensions and spacing has a different cycle time. In addition, cycle time by using vision, height sense, dispensing work and motion time will vary greatly. For instance if singulated parts are placed in arrays and this requires height sensing and finding fiducials for each part to compensate the alignment, then vision and height sensing time allocation will be much greater than the previous two cases. The material choice will determine the right applicator. For instance, solder paste may require a different auger valve for different solder mesh types, and this will determine the smallest needle inner diameter to eliminate plugging.
Choosing the right sealant material and achieving dispensing volumetric accuracy will result in high yield. Cycle time analysis, by breaking down the time usage, provides an accurate understanding of how a dispensing system is used. This will help further optimise parameters such as applicator velocity impacting total cycle time. Certain applications could realise improved UPH
by utilising a dispensing system with high-performance motion parameters. Thus, high yield and high throughput efforts will drive down the cost per unit.
MEMS wafer capping has been seen in many devices such as inertial sensors, oscillators and micro fluidic packages. This paper presented technical requirements for dispensing sealant, volumetric accuracy and motion systems. Sealant material choice depends on the lifetime of the end application, reliability and functionality. This paper also addressed challenges of volumetric accuracy when dispensing sealant and then presented solutions to improve dispensing consistency. Throughput optimisation was studied by using specific MEMS applications by varying motion control parameters and dispensing parameters.
Future work will include investigating the impact on the throughput model due to size reduction efforts in packaging. More dies can be produced per given wafer size, which means there is more dispensing area per wafer. In addition, sealing line requirements will be more challenging, and so further work will address these demands.
Acknowledgments The author would like to thank Akira Morita for providing insight on the contents, Roberta Forster-Smith for editing and Jay Sibley for graphics of the paper.
References 1. L.E. Felton, N.Hablutzel, W.A Webster, K.P. Harney, “Chip Scale Packaging of a MEMS Accelerometer,” ECTC, May 2009. 2. A. Morita, “Dispensing Advantages for MEMS Wafer Capping,” Chip Scale Review, Nov 2011. 3. S. J. Adamson , M. Peterson, “Enabling high density System in Package (SiP) manufacturing and consumer electronics devices through the use of jetting technology to minimise substrate area for underfill,” SMTAI, Oct 2006.
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