Technology sharing | Green picosecond laser cutting system level packaging (SiP) materials

System level packaging (SiP) is a chip packaging method, which can further improve the computing power by increasing the transistor density. As the shrinking speed of semiconductor feature size slows down, the gap between semiconductor processing size (nanometer micrometer) and printed circuit board (PCB) size (micrometer millimeter) (the spatial scale spans about three orders of magnitude) provides a new opportunity for the development of the industry. Thanks to such a large scale gap, technicians can achieve further miniaturization through a variety of methods. In terms of function, SiP improves performance by integrating traditionally discrete and isolated components, such as memory, logic, radio frequency (RF) chips, into a single package (usually called heterogeneous integration) on a shared printed circuit board (PCB) substrate, and designing necessary interconnections. SiP technology has been widely used in mobile consumer electronic products, such as smart phones, watches, headphones and other wearable devices, as well as many other devices.

Nanosecond lasers with UV and green wavelengths are very suitable for separating SiP devices. However, if such lasers cannot withstand excessive heat, the actual application effect may be greatly reduced, especially considering that the density of these devices will become larger and larger. In order to meet this challenge, people tend to use shorter pulse duration in the processing process in order to reduce the heat affected zone (HAZ). This can occur if the package uses a thermosensitive bonding medium, such as solder or adhesive, because these materials may fail under excessive heat loads. In addition, as there are copper wires in the SiP laminate, it may be necessary to use an ultra short pulse (USP) laser for processing, which may generate too much heat, leading to delamination. Considering these factors, we use MKS/Spectra Physics IceFyre ® GR50 high-power green picosecond laser has carried out a lot of experiments to optimize the cutting process of SiP related materials.

Figure 1. View of the entrance (left) and exit (right) surfaces of the FR4 plate (200 µ m thick) cut with green picosecond pulses

The main component of SiP board is thin (or "ultra-thin") glass fiber reinforced epoxy laminate (FR4), with a thickness of 100-250 µ m. Due to the uneven composition of glass fiber and epoxy resin in FR4 and their different optical and thermal properties, laser cutting of FR4 is a challenging task. When processing thick FR4 with laser, care must be taken to avoid excessive heating and melting, which may lead to poor carbonization. For thinner FR4, it is relatively easy to avoid overheating when using picosecond pulse width. Figure 1 shows the incident and outgoing surfaces after cutting FR4 (200 µ m thick) with IceFyre GR50 laser.

Figure 2. SEM side wall view of the notch in Figure 1 shows that the fiber end face is only slightly melted

The effective cutting speed can reach 83 mm/s by using the 50 W rated output of the laser at 500 kHz pulse repetition frequency (PRF) and the high-speed multiple process optimized at a scanning speed of 4 m/s. The incident plane shows only a small amount of debris deposition and forms an obvious heat affected zone (HAZ) of about 10 µ m. When viewing the cross section (Fig. 2) through SEM imaging, the image shows that this is a high-quality cutting, each fiber can be clearly seen, and there is only a slight sign of melting.

Figure 3. SEM sidewall view of the FR4 notch, showing excellent cutting quality and very low fiber melting.
For many processes using USP lasers, high-quality results can usually be further improved to achieve higher quality levels. For example, if it is planned to further reduce the melting amount of glass fiber when cutting FR4, adjustments such as reducing the laser pulse energy or PRF, increasing the beam scanning speed, etc. can achieve the desired effect, as shown in Figure 3.

This result clearly shows that USP laser can produce excellent cutting effect in sensitive materials, and the degree of heating is very low.

Fig. 4 The microscope image of the laser incidence surface of the cutting result at a higher speed shows that the surface cutting quality is excellent, but there is heating around the embedded copper wire.

After demonstrating excellent cutting capability of thin FR4 and determining the achievable output, the laser was subsequently used to cut thin SiP PCB substrate materials. The material consists of ultra-thin FR4 (about 100 µ m thick) and polymer solder mask, both sides of which are laminated, and includes intermittent embedded copper wires layered along a predetermined cutting path. The total thickness of all layers is 200 µ m. Due to the presence of multiple layers containing embedded copper wires, it is expected that fine-tuning some processes will help to achieve the best quality results. Therefore, after determining the process to achieve high output, the parameters were adjusted to focus on improving quality results.

The results show that this approach is quite effective. When the laser is operated at full power for high-speed machining, the top view microscope photograph of the cutting incidence surface (Fig. 4) shows that the embedded copper wire does have a certain impact on the cutting quality. Although the overall surface quality is excellent, the trimming quality is good, and there is only a small fragment area, there is evidence that excessive heating around the copper layer causes the polymer FR4 material around it to be slightly eroded, resulting in slight protrusion of copper wire from the side wall. The effective cutting speed of this process is 57 mm/s.

Therefore, although the yield centered process can usually achieve good quality, there is still room for improvement. The laser power level was reduced by 50% and other parameters were adjusted to further improve the quality, as shown in Figure 5. This result is achieved at a net cutting speed of 38 mm/s. Therefore, when the power is fully utilized (for example, using a dual beam splitting configuration), the overall comprehensive cutting speed is equivalent to 76 mm/s, 33% higher than the speed of a single beam at full power.

Figure 5. The image of the incident plane cut with 50% laser power shows that good results have been achieved when full power is used.
The image on the left side in Figure 5 is the image of the surface polymer layer. Compared with the full power result, there is only slight debris deposition, and there is no deviation in the cutting path. Similarly, the image on the right shows that in the direction away from the edge of the incision, the embedded copper wire has only barely detectable protrusions. The quality of the results can be further understood by looking at the side wall cross section of the laser cut, as shown in the SEM image in Figure 6 below.

Fig. 6. SEM image shows the side wall of SiP plate cut with 50% laser power.
SEM images show that the side wall ablation is very low when 50% laser power is used. Excellent quality has obvious and clear indicators, such as no melting/low melting can be detected on individual fiber end faces, no delamination between layers, low copper wire ablation, and no melting or deformation in and around the copper wire.

SiP architecture can improve the performance of electronic devices with shrinking dimensions, and it is a very important work to separate and package devices through lasers. Although nanosecond pulse lasers can sometimes meet the requirements, the close distance between densely packed integrated circuits and various sub packaged components will bring serious challenges. High output can be achieved using USP laser technology, especially green (and UV or UV) lasers. Through careful laser and process parameter adjustment, excellent cutting quality can be achieved, and only the minimum heat effect can be achieved.

IceFyre industrial picosecond laser
IceFyre GR50 provides>50 W green light output power at 500 kHz and pulse energy>100 µ J. IceFyre UV50 is an excellent ultraviolet picosecond laser on the market, which operates at 1.25 MHz (>40 MHz μ J) UV output power>50 W, pulse energy 100 in pulse train mode μ J. The pulse width is 10 ps. IceFyre UV50 sets new standards for power and repetition rate from single shot to 10 MHz. IceFyre UV30 provides a typical UV output power of>30 W, with pulse energy>60 μ J (higher pulse energy in pulse train mode), with excellent performance from single shot to 3 MHz.

IceFyre IR50 provides infrared output power of>50 W at 400 kHz single pulse, with excellent performance from single pulse to 10 MHz. The unique design of IceFyre laser utilizes the flexibility of fiber laser and the unique power amplifier capability of Spectra Physics to realize the TimeShift ps programmable pulse train mode technology, which can provide high versatility in the industry. Each laser is equipped with a set of standard waveforms; The optional TimeShift ps GUI can be used to create custom waveforms. The design of the laser can realize the trigger function of pulse on demand (POD) and position synchronous output (PSO) with extremely low time jitter in similar lasers for high-quality processing with high scanning speed (such as the use of multi sided scanning mirrors).

Source: Resource owner