The Principles of Ablation

Considering the many types of materials involved in laser micro machining, selecting the most optimum laser can be a challenge. An understanding of material properties and laser characteristics are necessary for making the right selection.

Laser micro machining is an ablation process. This means the material to be removed is subjected to intense heat by the concentrated and well-focused laser beam resulting in a very rapid evaporation. The most commonly used materials suitable for laser micro machining include ceramics, glass, polymers, metals, and silicon wafers. It is important that little or no damage is caused to the surroundings. For the ablation process to be successful, there needs to be a proper match between the material and the laser. Certain materials will not absorb the light which simply passes right through the material without any heat build-up. Therefore, material properties need to be known for the process to be optimized.

For example, a commonly used substrate material in PCBs is FR4, a glass fiber epoxy laminate. For the glass portion in this PCB, absorption only occurs in the UV and far infrared (IR) regions. Fortuitously, the epoxy used in FR4 also exhibits strong absorption in the far IR. Consequently, the most suitable laser for ablation of FR4 is the CO2 laser.

 

Fig. 1 Absorption coefficient for different metals at laser wave lengths

In a typical PCB with FR4, there is a layer of copper which serves as the metal conductor. Copper only absorbs light in the UV region, as shown in Fig. 1. Therefore, light from a CO2 laser cannot ablate copper and this is advantageous in establishing the most optimum process to drill micro via holes. As illustrated in the copper/FR4 sandwich below, the “buns” are the copper layers and the “meat” is the FR4. To drill through the copper layer, a UV laser must be used and stopped as soon as the copper layer has been penetrated.

 

Fig. 2 Sandwich technique for copper and substrate layer

To continue the drilling through the FR4, a CO2 laser is used and it will not damage the underlying copper layer.  This is where a dual laser head UV/CO2 system is extremely convenient as the two lasers can be operated in sequence without the needing to remove the PCB panel between the two laser processes. Since the top layer is copper, the CO2 laser beam is naturally confined to the opening in the copper and as it ablates the FR4 it will maintain the copper hole diameter. This results in nice, straight walls.

In summary, the laser ablation process depends on the interaction between the laser beam and the materials. The most critical parameters are the wavelength, repetition rate, power, and pulse length. These parameters plus the delivery of the beam will ultimately impact the hole quality and drilling speed. Pulse length and beam delivery will be discussed in our next blogs.

 

Sources:
[1] https://www.researchgate.net/publication/234118523_Parameters_in_Selective_Laser_Melting_for_processing_metallic_powders

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