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Clean Dispensing Tips for improved Underfill Process Control in Microelectronic Assembly

Feb 17, 2017
It is quite common for dispensing tips to collect a small deposit of the material during the application process, which interferes with the consistency of dispense. As such, many applications invest in nozzle cleaners, either manual or automatic, to remove the deposited material at set intervals to ensure a consistently accurate dispense process.

The nozzles should be cleaned before calibration. In the case of automatic nozzle cleaning, the process can be programmed for the start of a dispense program or any other time in the course of dispensing. Systems with more than one dispense head often use multiple automated nozzle cleaners to eliminate the risk of contaminating the dispensing tip and nozzle with foreign fluid, such as conductive or non-conductive glue.

Challenges in microelectronic assembly
Process developments in microelectronic assembly are typically aimed at making smaller components accommodate more functions into tiny portable devices. For instance, in the fast production of large devices - at a rate of over 3,000 uph - the dispensing system has to move a substantial quantity of material. In fact, the huge volume of material dispensed through the tip may create challenges in heating fluids to the appropriate process temperature before dispensing.

The changes in temperature that occur within any given cycle causes the viscosity of the underfill to vary, which in turn affects the shot volume and size. As such, it is critical to ensure stable temperature of the underfill material to ensure greater repeatability of the shot size, improve material flow under the die, and facilitate fluid break off at the tip.

Automated dispensing systems are integrated with Mass Flow control systems that regulate the dispense weight accordingly to accommodate variations in viscosity. Under steady conditions, the weight of the material diminishes over time. During this process, the dispense line speed reduces to compensate for fluid changes and deliver the same weight of material on the component, with needle dispensing.

When using jet dispensing, on the other hand, the number of dots/cycles is changed to increase fluid flow and keep time constant.

While it may appear that the correlation between time and viscosity can be fairly predicted, an increase in production rates and a subsequent reduction in underfill fluid-free zones, also brings other factors into play that cause the volume of the material to change. This can ultimately cause the final product to vary in unusual ways, translating to huge losses.

Automated cleaning for product stability
Cleaning the dispensing tip at the right intervals is one of the ways to avoid variations in the final product while watching and adjusting other material dispense parameters.

Automated tip cleaning uses a contact method where the nozzle is retracted and a wiping action activated to clean the tip before proceeding with the dispense process. The nozzle moves progressively along the length of the pad to reduce the risk of tip contamination, and alerts the operator when it reaches the end of the pad length.
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