Lasers: The H2O Advantage

December 1, 2004
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Water jet guided lasers promise high grooving speeds, precise cutting accuracy and excellent edge quality for LTCCs.

A water jet guided laser uses a thin, stable water jet as an optical wave guide. Illustration courtesy of Synova SA, Ecublens, Switzerland.


Figure 1. The basic principle of the water jet guided laser technology.
Ceramics hold an important place in the semiconductor and electronic industries, thanks to their excellent dielectric properties, good thermal stability and conductivity, and low thermal expansion. Because they excel in high temperature applications, ceramics are widely used as substrates for packaging integrated circuits, where they act as the interface between the integrated circuit (IC) and the printed circuit board (PCB). The chip is placed on the substrate, which is then soldered to the final PCB, and the vertical connections are created by very thin gold wires or tiny solder balls (in the case of flip chip technology). In addition to realizing these electrical connections, the ceramic has to protect the chip to maintain it in peak operating condition. However, the most critical issue is thermal management. Low-temperature co-fired ceramic (LTCC) technology is widely used as a substrate for packaging integrated circuits because it enables manufacturers to use low resistive materials (such as gold or silver) for connections due to the low firing temperature-usually between 850 and 900ºC-during the manufacturing process. The LTCC technology has reduced circuit dimensions, time loss and costs.

A number of tasks for preparing these ceramic substrates-including scribing, grooving, hole drilling (vias) and cutting-are fulfilled by precise cutting tools. These operations must be performed carefully to avoid warping, dimension changes or cracking. Accuracy is also a key aspect for the new circuit designs. As substrates become thinner, the need for smaller vias, closer via locations, denser circuit patterns and denser multi-layer interconnects increases.

Many conventional cutting methods face problems such as high operating costs, low process speeds and poor cut quality. For example, punching typically becomes unusable between 0.008 and 0.015 in. diameter, and small-diameter punches tend to wear and break easily. Conventional CO2 lasers provide significant benefits compared to mechanical methods but are still problematic, mainly because of the heat load generated by the process and the poor removal of ablated material. This results in material cracking, as well as the formation of a brittle recast layer and a poor edge quality.

To overcome these problems, manufacturers of LTCCs are increasingly turning to a hybrid approach-the use of water jet guided lasers.*

The Laser MicroJet®, invented and supplied by Synova SA, Ecublens, Switzerland.

Drawbacks of Conventional Singulation

Industrial ceramics are difficult to process because of their hardness and brittleness. The best results for LTCC singulation are typically achieved through a scribe-and-break process, where the ceramic is grooved and then broken into a smaller component.

In conventional processes, grooving of the substrate is performed using a thin diamond blade to obtain a precise starting location for the crack that will separate the parts. However, the hardness of the material and the release of internal stresses during sawing often cause problems such as:

  • Very low grooving speed (2-3 mm/s)
  • Chipping of the ceramic close to the groove
  • Crack formation due to mechanical shear stress
  • Inconsistent cutting quality due to the blade wear
  • Short and unpredictable blade lifetime (grooving length is typically 60 m)
  • High operating costs due to high blade consumption
  • Imprecise positioning of front and backside cuts

These problems can only be solved using a flexible tool that is not affected by the hardness of the material.



A Water Jet Guided Solution

Originally developed 11 years ago for use in medical applications, the water jet guided laser has steadily gained popularity in a range of other applications, including the semiconductor and electronics industries. This hybrid tool, which combines the cooling effects of a water jet with the ablating ability of a laser beam, has recently been adapted to the evolution of ceramic technologies.

As its name suggests, a water jet guided laser uses a thin, stable water jet as an optical wave guide. To couple the light into the water jet, the laser beam is first focused into a nozzle as it passes through a pressurized water chamber. The water jet then guides the light onto the sample, where the ablation takes place (see Figure 1).

Besides its guiding function, the water jet also provides other advantages to the cutting process, including:

  • Very low thermal load in the material due to the cooling effect of the water between the laser pulses
  • Efficient expulsion of the ablated material due to the high momentum of the water jet
  • An absence of redeposited material on the sample


Figure 2. Front view of a 900-µm thick LTCC grooved with a water jet guided laser (Test No. 2).
The lasers used with this process are either flash lamp-pumped pulsed Nd:YAG lasers with pulse durations of less than 100 æs, or multimode Q-switched lasers operating at 1064 nm, 532 nm or 355 nm, depending on the properties of the material being machined (mainly its light absorption). The water is de-ionized and filtered in a water pump, which is a part of the cutting machine, and is pressurized through the nozzle at 50 to 500 bars for the water jet, depending on the amount of cutting pressure required. The nozzles are made out of sapphire or diamond to ensure a long, stable water jet. The laser beam is focused through a quartz window into the nozzle, much like a usual fiber coupling, and is thereafter reflected in the water jet at the water/air interface due to the refractive index step.

This hybrid system combines the force of a powerful Nd:YAG laser and the softness of a low-pressure water jet, making it ideal for critical applications where the fragility or brittleness of the material complicates machining with other methods.

Figure 3. Top view (left) and side view (right) of a 900-µm thick Al2O3 substrate singulated with a water jet guided laser (Test No. 3).

Superior Singulation

The most effective way to use the water jet guided laser for ceramic substrate singulation is the scribe-and-break approach. Ceramics are difficult to cut with lasers because of their low absorption coefficient for infrared and visible light, and because their tendency to scatter the light might cause damage in nearby structures. Table 1 shows some typical parameters used for scribing three different types of ceramics. The laser parameters have been optimized to increase the processing speed while maintaining excellent edge quality. For the results presented here, the water pressure was kept at a moderate value of 280 bar, and a 60-µm nozzle was chosen to obtain an optimum ablation efficiency. (Having a very thin kerf was not a concern for this application.) All samples were 900-µm thick with a scribing depth of 300 µm.

Figure 4. Front view of a 900-µm thick LTCC grooved at 100 mm/s (Test No. 4).
With the use of an infrared laser at 1064 nm, a higher grooving speed was achieved (10 mm/s) compared to frequency-doubled ("green") lasers (3.75 to 6.25 mm/s), while the same quality was retained. To further improve the grooving speed, a three-head system using three water jet guided lasers working in parallel was employed. With this system, a total speed of 30 mm/s was realized for the same groove depth.

Figures 2 through 4 show the quality of processing with the water jet guided laser based on the tests summarized in Table 1. The groove in the ceramic has a V shape, which favors controlled crack propagation starting at the lowest point of the V for the subsequent breaking step.

High-Quality Cutting

Water jet guided lasers provide a cost-effective alternative for the singulation of ceramic substrates. Using the scribe-and-break process, excellent results have been obtained for a number of different types of ceramic materials. Since it is a laser-based technology, higher speeds can be achieved compared to sawing (up to 10 mm/s for 900-µm thick LTCC) at a lower cost of ownership and with a chip-free edge quality. By using a multiple-head system, cutting speeds can be increased even further.

Compared to conventional laser processes, the thermal load of the sample is much lower, which improves the edge quality of the ceramic and prevents cracking of the sample, and surface contamination is greatly reduced. Complete ceramic-based packages (i.e., the substrate, chip and mold compound) can also be successfully singulated with this technology.

For more information about the water jet guided laser technology, contact Synova SA, Ch. de la Dent-d'Oche, CH-1024 Ecublens, Switzerland; (41) 21-694-00; fax (41) 21-694-00-01; e-mail perrottet@synova.ch; or visit http://www.synova.ch .

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