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Coherent: A new PCB laser splitter method is applied to enhance process efficiency

2020-10-26

The technological change of PCB material, thickness and composition has prompted the industry to shift from traditional mechanical cutting method to laser cutting technology.
However, the laser used for PCB board segmentation cannot be identical, and the cutting characteristics and cutting quality of different lasers, especially the heat-affected area, are obviously different.
This, in turn, will affect process utilization rate, because these factors determine the distance between circuits on the PCB, as well as circuit functions and downstream processes such as waterproof treatment and EMI shielding treatment.
A new nanosecond laser and Coherent Coherent technology for Coherent cutting are introduced in this paper.
Compared with other existing products, this new process can greatly reduce the heat affected area when laser cutting PCB.

Growing demand for laser machining

With the development of electronic devices to miniaturization, such as smart phones, wearable devices, VR devices, automotive sensors and intelligent household equipment, electronic products, high density and high performance of PCB demand more and more prominent, as compared with the last generation microelectronic products, the miniaturization of electronic equipment not only physical specifications are smaller and more complicated, and more efficient energy saving, longer life time and the price is lower.
 
Driven by this demand, PCB processing also tends to be more widely used in the thinner traditional circuit board, the wider use of flexible circuit circuit, thicker conductive layer, and make better use of low dielectric constant media (especially in 5G technology has been widely used).
Cost considerations also reinforce the need to improve process utilization.
Specifically, the gap between the plates is reduced to increase production.
 
These trends will lead to narrower and narrower cutting widths and higher dimensional accuracy in cutting and splicing processes.
The closer the cutting position is to the PCB functional area, it means that the cutting process must not affect the material or circuit around the cutting position, whether in terms of mechanical stress or temperature.
Another requirement is to reduce residue as much as possible, because the presence of residue means additional cleaning steps later.
 
Due to these limitations, traditional PCB mechanical board splitting methods, including milling, sawing, die-cutting, drilling, grooving and semi-cutting, have become impractical and become less cost-effective.
This has prompted the industry to consider laser cutting, because laser technology can meet any of the above requirements, but the disadvantage is that the cutting speed will be reduced.

Understanding laser cutting

Of course, the laser splitter has been used for some time and is not a new technology, but it is still important to understand and differentiate the various laser-based processing technologies.
The earliest laser cutting used CO2 laser emitting far infrared ray.
This technique cuts by heating the matrix material, resulting in severe coke (carbonization) and thus a significant heat-affected zone.
Moreover, compared with shorter ultraviolet wavelengths, these longer wavelengths do not concentrate very small spots, so they can lead to wider incisions.
 
More than a decade ago, diode pumped solid state (DPSS), nanosecond pulse width, and triple frequency uv laser became another viable tool for PCB cutting.
The ultraviolet (355nm) output of the laser carries enough pulsed energy to remove the material using a "cold" processed ablation method.
In other words, the process has a much smaller heat-affected area than a CO2 laser (but still visible) and produces much less burr and material modification.
The laser's high monopulse energy and repetition rate allows it to cut faster and at a manageable cost, though not as fast as CO2 lasers.

The latest development of laser cutting technology

Laser splicing technology has brought many advantages, and PCB manufacturers have taken the technology to the extreme to meet increasingly demanding challenges in size, materials and cost.
Areas of active research include further reduction of heat affected areas and burr formation, and improvement of the cutting quality of nanosecond ultraviolet solid-state lasers.
 
To help achieve this goal, the Coherent Coherent company in the application research on using nanosecond pulse high energy ultraviolet solid state laser (AVIA LX) cutting different PCB and got a variety of composite materials processing effect and processing window, developed a new method of cutting PCB, and has proven this method can reduce the heat affected zone, and make more cutting edge smooth, incision width is narrower and higher cutting efficiency.
 
A key element of this approach is the use of a proprietary method to control the timing and spatial location of laser pulses transmitted to the workpiece surface, thereby avoiding heat accumulation.
Because this method does not cause thermal damage, lasers with very high pulse energy can be used to cut thick materials (1 mm or more).
 
The advantage of higher pulse energy is that the traditional method of cutting thicker material is no longer needed.
When traditional lasers are used to cut thick plates, a V-slot is usually reserved to avoid the interruption of optical path and the reduction of laser power in the cutting process, which will affect the cutting efficiency.
With Coherent AVIA LX lasers, Coherent is capable of using up to 400 J of pulse energy to repeatedly draw lines along the same line, eliminating the need for V grooves, resulting in faster cutting speed and significantly reduced notch width.
 
For laser with low pulse energy, the laser focus must be moved continuously when the beam penetrates the material, so that the minimum size of the focus is always exactly consistent with the cutting depth, in order to obtain the laser flux higher than the material ablation threshold.
But in the actual machining process, the PCB board needs to be moved up, or the triaxial galvanometer with focusing function is used as an additional tool.
Either method will greatly affect the overall processing efficiency and schedule, increase equipment cost and operational complexity.
 
The high pulse energy of AVIA LX enables the laser to focus at the depth of the INTERMEDIATE layer of the PCB for cutting, because the laser has sufficient laser density to ablate the material even if the laser does not operate with the optimal focus.
Its advantages are faster cutting speed and simpler system.
 
FIG. 1 shows an example of the results achieved using this technology, comparing the effects of copper wire cutting with a 1.6mm thickness PCB in two ways. The uv solid-state laser currently available is used in the left image, and the new method mentioned in this paper is used in the right image.
Using the new method of cutting circuit board, the cutting edge is cleaner, copper wire cutting edge effect has also been greatly improved.

As shown in Figure 2, the width of the incision was effectively reduced after the new method was used.

Figure 3 shows that the new method can minimize burr generation, reduce the notch width and significantly reduce HAZ when cutting multi-layer PCB (with glass fiber layer).

In the past, when using laser to cut polyimide and EMI shield, the circuit board cutting line would be stratified because of the large area of heat affected zone.
In this case, it is necessary to use a lower pulse energy to avoid damaging the material.
The same method can also be used to eliminate heat accumulation in the structure and reduce HAZ and notch width.
This operation results in a higher yield in the downstream production process, resulting in lower production costs (Figure 4).

In FIG. 5, Coherent pulse technology is applied to acquire flexible PCB with low impulse energy.
It can be seen that the HAZ area decreases and the output increases.

 

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