Common PCB substrate copper laminates are made by bonding the insulation material and copper foil using adhesive and hot pressing, with epoxy glass fiber cloth board used as a pressure plate reinforcement material. The board contains uncured resin, and the heat generated by mechanical friction during processing will soften the uncured resin, increasing frictional resistance, breaking cutting tools, and producing sludge, which affects processing quality. This makes the mechanical processing performance of PCB boards relatively poor.
This article mainly compares and studies the cutting performance of carbide drills produced by our company in China and similar famous carbide drills abroad when processing PCB boards.
The test selected drills of the same specifications but from different manufacturers to drill holes in PCB boards. The selected tool specifications are shown in Table 1.
To reduce the impact of experimental errors, four tools from two companies were selected for drilling experiments. The material being cut was a double-layer copper PCB with a thickness of 1.2mm, and a 2.5mm thick cardboard was placed underneath the work piece.
During the processing, the cutting performance of the tools from the two companies was compared by comparing the roughness and chip removal of the processed PCB board, as well as the tool life.
Test PCB Machining Quality
The machining quality of the PCB boards observed after cutting with 8 different drill bits is shown in Table 2. It can be seen that the 4 drill bits from Meetyou company can generally achieve good machining results, except for the fourth bit which caused sawtooth-like defects on the PCB board. In comparison, two of the foreign company’s drill bits caused corrugated defects and sawtooth-like defects on the PCB board. The surface roughness of the PCB board after machining can also indicate that the micro drill from Meetyou company have better surface roughness during the machining process, while the drill bits from the foreign company have poorer surface roughness on the PCB board after machining. However, compared with the drill bits from Meetyou company, the chip removal effect of the drill bits from the foreign company is not as good.
PCB Carbide micro drill’s service life
The average service life of 8 drill bits from Company 2 can be seen from Figure 3. It can be seen that the service life of Meetyou Company’s drill bits is about 40% higher than that of the drill bits from a foreign company.
From Table 4, it can be seen that the lifespan values of the eight drills from the two companies are in a discrete distribution, with almost 40% difference in the lifespan of the same company’s drills. This shows that even though Meetyou’s drills have better quality, from the experiment we can see that there is still much room for improvement in the tungsten carbide micro drills of both companies.
The experimental results showed that the surface roughness and tool life of the PCB board processed by Meetyou’s drill bit were better than those of the foreign company, but the tool life of both companies was not consistent. The photographs of the tools after processing showed that the main forms of tool wear were caused by the generation of abrasive particles and surface oxidation wear.
Problems Generated by carbide micro drill on PCB Circuit Boards
Through high-speed photography to observe the drilling process and the chip removal situation, scanning electron microscopy to observe the drilling morphology, and using software such as Deform and AdvantEdge FEM to simulate the drilling process extensively, it was found that the drilling process of PCB circuit boards is not simply punching holes, but a conventional metal cutting process. It will form conical spiral chips, and the copper chips will bend and break under the action of gravity during the discharge process, and finally be thrown out at high speed rotation of the micro drill, as shown in Pic. 1.
The epoxy resin glass fiber cloth chips are discharged in the form of white powder. The resin in the chips is easy to soften under heating conditions, and the glass fiber chips are adhered together to form mixed chips, as shown in Pic. 2. This kind of mixed chips are easy to adhere to the hole wall and form drilling dirt, which will also adhere to the drill tip and affect the next drilling step, and may even adhere to the spiral groove surface to obstruct chip removal. The drilling models of micro drilling for copper foil and epoxy glass fiber cloth are shown in Pic.3.
Reasons analysis to cause drilling problems on PCB
The drilling force fluctuated significantly during the drilling of epoxy glass fiber cloth with micro-drills, which was due to the small size of the micro-drills. When drilling glass fibers, the axial drilling force decreased sharply with the fracture of the glass fibers because the micro-drill only drilled a few glass fibers at the same time. The axial force and torque both decreased with increasing speed, but increased with increasing feed rate and core thickness. With an increase in helix angle, the axial force increased, while the torque decreased. The typical drilling process of a micro-drill with a diameter of 0.1mm is shown in Figure 4.
The temperature at the moment of drilling a PCB with micro-drills was measured using an infrared thermal imager and extensively simulated. The results showed that the drilling temperature was typically below 80°C at the moment of drilling with micro-drills. The drilling temperature decreased with an increase in feed rate and spindle speed, but increased with an increase in the number of drill holes and the diameter of the micro-drill. The drilling temperature measured by the infrared thermal imager is shown in Figure 5.
Drill bit wear
the morphology of micro-drill wear was observed through scanning electron microscopy, as shown in Figure 6. The wear characteristics of micro-drills are mainly abrasive wear and adhesive wear. Abrasive wear mainly occurs on the transverse edge and main cutting edge, affecting the service life of micro-drills. Glass fibers and fillers in PCBs are the main cause of abrasive wear in micro-drills. Resin-mixed chip adhesion wear occurs on the drill tip and helix groove surface of micro-drills, as shown in Figure 7, which affects the cutting performance and chip removal of micro-drills, thereby causing the temperature inside the hole to accumulate and aggravating micro-drill wear.
- D=0.1 mm，vf=150cm/min，n=295/min，vr=2300cm/min，2500holes.
- D=0.3mm，vf=240cm/min，n=145/min，vr=1800cm/min，1500 holes
It was found that the fracture was caused by the edge near the drill tip participating in cutting during processing, which is easily damaged by friction and impact of glass fiber, causing cutting to not proceed normally. Micro-drill bending and severe twisting ultimately lead to micro-drill breakage. Experimental and simulation results show that excessive twisting load is the main cause of micro-drill breakage, and the fracture point of the micro-drill is located at the root of the micro-drill spiral groove, at a certain distance from the top of the drill.
Conclusion of carbide micro-drill’s drilling quality for PCB.
In summary, by observing the micro-hole sections under scanning electron microscopy, it was found that not only are there entrance and exit burr problems on the micro-hole surface, but also entrance roundness error, entrance size error, burrs and hole position accuracy, and roughness of the hole wall. The burrs and edges of micro-drills are mainly caused by micro-drill wear. Roughness of the micro-hole wall mainly occurs due to the fracture and shedding of multiple glass fibers in the epoxy glass fiber layer. Hole position accuracy is mainly related to spindle vibration characteristics, drill diameter, and drill wear. Reducing feed speed and increasing rotation speed can improve the quality of micro-holes in PCBs within a certain number of drilling holes. The most fundamental method to improve micro-hole quality is to reduce the contact area between micro-drills and PCBs to improve the wear resistance of micro-drills.