Printed circuit boards or PCBs need holes at various places. These may pass through the board or partially through the laminate materials. Through-hole-components or THCs need holes passing through the board for mounting, while surface mount components or SMCs do not need holes for mounting. There is another type of hole, popularly known as a via, and these are necessary in multi-layered boards for interconnecting circuits in different layers. Vias create a continuity between circuits on the top and bottom to mid-layers on the PCB. They allow copper pads, traces, and polygons to connect through different layers of the board.
Creating Vias
PCB manufacturers use three methods for creating vias—drilling the hole using mechanical drill bits, using a die-punch method, or ablating a hole in the PCB using a laser beam. All these methods have their own merits and demerits. The choice of the method to use depends on many factors like cost, number of PCBs to in production, the precision necessary, depth of hole, its diameter, etc. For instance, manufacturers use the die-punch method only for high volumes of PCB production, because although this is the fastest of the three methods, it is expensive.
Mechanical Drilling
When mechanically drilling holes in a PCB, manufacturers rely on a rotating bit tool. These bit tools have a special design making them capable of cutting through various types of laminate materials. Bit tools are available in many sizes defining their diameter, and made from hardened carbide steel material. As a standard PCB consists of glass fiber and resin, and has a copper foil covering on top, it is a tough material for the bit to cut through. The hard material prevents the bit from wearing out fast and allows it to cut many holes before needing a sharpening.
Advantages of Mechanical Drilling
The major advantage of using a drill bit to mechanically drill holes in a PCB is the tool creates holes of very high quality. Irrespective of the number of holes drilled, the drill bit creates consistent holes. These holes have no taper at the ends, and have no knees or bevels where the hole passes through substrates. The edge of the hole on the surface always has a sharp finish. Mechanical drilling is a fast method for creating holes in a PCB, and allows a good throughput level.
Disadvantages of Mechanical Drilling
A major disadvantage of the mechanical drilling process is the limit of the size of the drill bit. Drilling a narrow hole through multiple layers of materials in the printed circuit board requires a thin drill bit, which can break easily. This is especially true for PCBs with high trace densities that require via holes of diameters less than 8 mil, requiring bits of diameter 0.008 inches, or 1/64ths of an inch.
Another aspect of mechanical drilling is the method leaves elevated ends of copper after withdrawal of the drill bit. Another process, known as deburring, is necessary to remove these copper pieces. The deburring process can take longer depending on the number of holes in the PCB.
Mechanical drilling is an expensive process and needs several pre- and post-processing steps, which consumes time. The drill bits need proper selection, preparations like sharpening before start of drilling, and a proper set-up prior to drilling the holes. If the fabricator must drill several multiple diameter holes in the PCB, the drill machine needs a tool change for each hole. This can be a time-consuming process, especially if manually handled. The manual process is vulnerable to errors, and is likely to increase scrap.
Laser Drilling
The laser drilling method uses a high-intensity laser beam for ablating a hole through the copper layer and following substrate materials in the PCB. Manufacturers use various types of lasers depending on the substrate materials in the PCB. For standard PCBs, the most popular lasers are the UV and the CO2.
Advantages of Laser Drilling
Major advantages of using lasers for drilling holes in PCBs are:
- Lasers can ablate through a large variety of materials
- Lasers can create substantially smaller diameter holes than regular mechanical drill bits can
- Lasers can drill substantially higher number of holes per unit time as compared to what mechanical drilling can
- Laser drilling is entirely non-contact
- Laser drilling is automated and does not require manual tool selection
Read About: Why HDI PCBs Need Laser Drilling?
Disadvantages of Laser Drilling
Laser drilling has several disadvantages also:
- Accurate depth control is difficult to achieve, unless there is a metal stop layer available
- Deep holes with small diameters can have a tapering cross-section
- Ablation creates carbonization of hole edges, giving the holes a burnt or dark appearance
What to Use—Mechanical Drilling or Laser
Through-Hole Vias
The major determining factors for deciding the method to use depends primarily on the aspect ratio of the holes and the pad-to-hole diameter ratio. Aspect ratio is the ratio of hole diameter to its depth.
Pad-to-hole ratio is more important during the fabrication process of the PCB rather than in its design phase. Ideally, placement of most via holes is on the center of the conjoining pad. The most common via hole diameter is the 0.010 inch, with a tolerance of ±0.003 inches. The small diameter ensures that the via is only for interconnecting layers and not for insertion of a component lead.
Therefore, the bottom of the via hole can approach the -0.003-inch limit without hampering its functioning. The usual practice for copper pad for such a via hole is 0.015 inches larger than the desired finished size of the hole. That means, a 0.010 inch via hole will need a 0.025-inch copper pad. The manufacturer can drill a via hole of diameter 0.015 inches, with final plating and surface finish bringing the hole diameter to 0.010 inches ±0.003 inches.
The aspect ratio depends on the smallest diameter of the drilled hole and the overall thickness of the board while drilling, and before fabricator applies the plating process. A large aspect ratio makes it more difficult for the plating process, increasing the plating time, and the chances of creating cracks in the hole wall due to expansion. Holes with a lower aspect ratio show a better bonding of the hole wall and lower chances of cracking. Therefore, fabricators often use a rule of thumb to use larger vias for thicker boards.
Buried and Blind Vias
Rather than use through-hole vias that pass right through the PCB, fabricators prefer using buried and blind vias for multi-layered boards. The advantage is they can stack or stagger them to achieve the same functionality of a through-hole via. Therefore, fabricators reserve through-hole vias exclusively for simple two-layer boards.
It is possible to create buried and blind vias by both methods, mechanical drilling and lasers. However, laser drilling offers a lower depth accuracy, while causing tapering along the edges of the hole. Therefore, fabricators often use an etching step to help the laser cut through the substrate, while taking care to ensure the laser does not ablate through the copper pad below.
Conclusion
Both mechanical drilling and laser drilling offer advantages and disadvantages during manufacturing the printed circuit board. Most manufacturers decide on the method they will use during the engineering review for successfully processing the order. At the design level, it is necessary to know the process the manufacturer may use is an advantage and eases the processing.
