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Advanced motion control optimises laser and mechanical micro-drilling

Advanced motion control optimises laser and mechanical micro-drilling

Advanced motion control technology plays a critical role in the success of micro-drilling equipment. It provides the ability to “tune” the axes to attain the highest possible process capacity and yield but, most importantly, it delivers maximum stability so that systems operate without resonance or vibrations to ensure maximum aperture consistency.

In micro-drilling process equipment, the most popular ways to create microvias (blind or buried vias with a diameter ≤150 μm) are mechanical drilling and laser drilling. Mechanical drilling is mainly used in larger apertures such as PCBs or IC substrates. Laser drilling is mainly used in finer apertures such as silicon wafers, ceramic substrates, and sapphire substrates. 

Key factors for success in laser micro-drilling applications include ensuring that the XY servo stages are “tuned” to be fast, stable, and accurate, ensuring that the roundness of the apertures meets the customer requirement; and ensuring the highest possible program execution efficiency. These same requirements apply for mechanical drilling as well, but in addition, because most the of the machines are ballscrew-driven, there is the need to reduce friction effects.

Tuning servo stages

Part dimensions, mass, and the dynamic performance requirement will all influence the difficulty of XY servo stage system tuning. For small parts like silicon wafers or ceramic substrates, if the move and settle time requirements are not extremely stringent, the tuning process could be completed fairly easily. However, large substrates that require very high dynamic performance from the servo stages are common, and this will be the bottleneck for many machine makers to adjust the servo gains to meet the process requirements in pursuit of the shortest “move-and-settle time”.

Normally a motion system control will provide an autotuning routine, or a “step response” tuning technique that is mainly done in the time domain. This type of tuning process could be effective in easier applications. However, because time domain tuning techniques cannot estimate the resonance, the pole zero map, or set-up the filters, frequently the user has to reduce the servo gains to avoid system instability. Unfortunately, reducing the gains also means degraded throughput and process yield. Using frequency domain techniques (frequency response analysis) is a more advanced tuning approach and will yield better throughput. 

The frequency response analysis technique injects a sinusoidal signal from low to high frequency into the motor, and captures the phase and gains at each frequency. The phase margin and gain margin are then evaluated in order to make adjustments to the servo gains. High performance motion controllers, like the Aerotech Automation 3200, can easily shape the loop and adjust the Bode plot graphically, easily set up the servo filters, and ultimately increase the gains to maximise the throughput.

Improving roundness in laser drilling

Many laser micro-drilling applications require the best possible roundness, which is dependent on the laser spot size deviation and, therefore, the following error of the motion system. The common process parameters are: acceleration, speed, and radius of the aperture. A smaller aperture or higher process speed will yield higher acceleration. However, higher acceleration of the motion system will result in higher following error, which means the roundness of the aperture will be reduced. Conversely, if the user needs to improve the quality of the aperture by reducing the acceleration, the throughput will be impacted.

In order to improve the roundness without significantly impacting the throughput, the Aerotech A3200 provides acceleration limiting and ‘enhanced tracking control’ (ETC). Advanced motion controllers like the Aerotech A3200 can limit the coordinated circular acceleration (acceleration limiting) while still maximising linear acceleration. This will increase the roundness without appreciably affecting throughput. The ETC feature actually reduces the following error of the circles, for example by using advanced algorithms that can increase low frequency gains, but without changing the higher frequency gain. This greatly improves following error at direction reversals, which inherently include high friction, thereby improving the roundness of the apertures.

In a particular example, when a user found the process yield was low and the roundness was not ideal from their machine, they analysed the system with Aerotech’s 2D plot function to determine whether the roundness problem was from the following error. The user then increased the servo gains to reduce the following error. In this process, the excess gains made the system unstable. A tool was required to remove the oscillation and to increase the servo gains so that the peak following error would be greatly reduced. The friction from the linear bearings reduced the low frequency response. In order to increase the low frequency response, the ETC function was used to increase the low frequency gains, and make the system behave closer to an ideal frictional system.

In the PCB drilling machine market, competitive price and performance requirements generally mean mechanical drilling machines use ballscrew-driven stages. It is very cost-effective for a ballscrew system to move the large substrate and provide good positioning accuracy. However, due to the large screw pitch, the friction will limit the low frequency gain of the axis, which means the move-and-settle time, and also the directional reversal time, will be longer than desired. Traditionally, machine builders tried to increase the servo loop gains universally so the friction effect is reduced. Unfortunately, that will also increase the possibility of system instability. Just as in laser micro-drilling, when used in mechanical micro-drilling the ETC algorithm boosts low frequency gains to make the system behave closer to an ideal frictional system.

Other advanced functions

Advanced motion controllers have many special features that allow the large quantity of aperture coordinates to be processed at the highest possible efficiency. For example, Aerotech’s ‘Look Ahead’ feature can determine the trajectory of upcoming apertures. The ‘Queue’ mode allows the data to be processed in the ‘first-in, first-out’ fashion and, therefore, the embedded memory size does not restrict how many points you can actually process.

Further features include ‘Wait Mode Auto’ and ‘Velocity Profiling’. The Wait Mode Auto command configures the controller to wait the minimum quantity of time between moves in order to decrease the cycle time of a program. Wait modes apply to the end of a velocity profiled sequence and not to commands within the sequence. On the other hand, Velocity Profiling is used to blend multiple coordinated motion commands into one continuous motion path. In velocity profiling mode, the controller does not decelerate to zero between consecutive coordinated moves. 

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