Drilling is the most expensive and time-consuming process in board manufacturing. The PCB drilling process must be carefully implemented since even a small error can lead to a great loss. The drilling process is considered the most critical and bottleneck of printed circuit board manufacturing. A PCB design engineer must always look into the board manufacturer’s capabilities before placing an order.
The drilling process is the foundation for vias and the connectivity between different layers. The downsizing of electronic devices such as TVs and phones has led to the transformation from stationary to portable. High-quality micromachining is required to bring down the size. Drilling plays a vital role in making this possible. Hence, the implemented drill technology, matters.
The PCB Drill Tech
Basically, there are two kinds of drilling technologies, mechanical and laser drilling.
The mechanical drills have less precision but are easy to execute. This drilling technology implements drill bits. The smallest hole diameter that can be drilled by these drills is about 6 mil (0.006”).
Mechanical drill limitations
The mechanical drills can be used for 800 hits when used on softer materials like FR4. For denser materials like Rogers, the lifespan is reduced to 200 counts. If a PCB manufacturer ignores this then it will result in faulty holes which will turn the board to scrap.
At Sierra circuits, the superior Hitachi drilling machines are implemented with a 1 mil hole placement tolerance.
The laser drills, on the other hand, can drill way smaller holes. Laser drilling is a non-contact process where the workpiece and the tool do not come in contact with each other. A laser beam is used to remove board material and create precise holes. Here, the drill depth can be effortlessly controlled.
The laser technology is used to drill controlled depth vias with ease. Here, a minimum hole diameter of 2 mils (0.002”) can be drilled with precision.
Laser drilling limitations
A circuit board is made up of copper, glass fiber, and resin. These PCB materials have different optical properties. This makes it hard for the laser beam to burn through a board efficiently.
The cost of the process also is comparatively high in the case of laser drilling.
Design for Assembly Handbook6 Chapters - 50 Pages - 70 Minute Read
- Recommended layout for components
- Common PCB assembly defects
- Factors that impact the cost of the PCB assembly, including:
- Component packages
- Board assembly volumes
It is better if the designer understands the things that happen on the shop floor, to get a better perception of how the design is brought to life. With this insight, the board designer ensures that the designs are manufacturable. This, in turn, reduces the cost and the product can be delivered in a minimum turnaround time.
What happens on the shop floor?
After the lamination process, the laminated board is loaded on a panel of exit material on the drill bed. The exit material lessens the burr formation. Burr is the protruding part of copper formed when the drill spindle penetrates through the board. On top of this panel, more stack-ups are loaded and are carefully aligned. In the end, a sheet of aluminum foil is placed over this entire stack-up. The Aluminium foil avoids entry burr and also dissipates the heat generated by the rapidly spinning drill bit. Once the required number of holes are drilled, the boards are sent for the deburring and desmearing process.
Since the quality of the drilled hole is a crucial aspect, tool geometries must be considered. High-speed steel (HSS) and tungsten carbide (WC) are commonly used drill bit materials for the drilling of composites. Carbide tools offer better tool life during the machining of glass fiber-reinforced polymer (GFRP). The cemented carbide drills are generally used in PCB drilling.
Point angle and helix angle
The PCB drills have a point angle of 130° with a helix angle from 30° to 35°. A point angle is situated at the top of the drill bit. It is measured between the most prominent cutting edges.
A helix angle is an angle between the flanks of a drill bit at their intersection points.
The CNC drill machine
The drilling machine is a preprogrammed computer numerically controlled (CNC) machine. The drill takes place based on the XY coordinates fed into the CNC system. The spindles rotate at a high RPM and ensure an accurate drill hole in the board. When the spindle rotates at a rapid speed, heat is generated due to the friction between the hole wall and the spindle. This melts the resin content on the hole walls and results in a smear of resin. Once the required holes are drilled the exit and entry panels are discarded. This is a small gist of what happens on the shop floor.
Unlike the etching and plating process, the drilling process doesn’t have a fixed duration. The drilling time varies on the shop floor depending on the number of holes to be drilled. This is what happens behind the curtains in the circuit board manufacturing unit. To know more about computer-aided manufacturing as related to PCB read What is CAM or computer-aided manufacturing?
Two significant aspects to be considered in the drilling process:
- The aspect ratio
- Drill to copper clearance (Drill to the nearest copper feature)
Aspect ratio is the ability to effectively plate copper inside the holes(vias). The copper plating of the interior part of holes is a tedious task when the diameter is decreased and the depth is increased. This requires a plating bath with a high throwing power so that the liquid could gush into the tiny holes.
Aspect ratio (AR) = (Depth of the hole/ Diameter of the drilled hole)
The aspect ratio is 10:1 for through holes and 0.75: 1 for microvias.
Typically for a 62 mil PCB, the minimum drill size can be 6 mils.
Drill to copper
The drill to copper is the land clearance between the edge of a drilled hole to the nearest copper feature. The nearest copper feature can be a copper trace or any other active copper region. This is the deciding factor since even a small deviation will lead to circuit disruption. The typical drill to copper value is around 8 mils.
Minimum clearance = annular ring width + solder mask dam clearance
Classification of holes
The drilled holes are classified into plated through-holes (PTHs) and non-plated through-holes (NPTHs).
The plated holes (PTHs) are the signal-carrying conductive vias that establish interconnection between the different layers in the circuit board.
The non-plated holes (NPTHs) are non-conductive. These are used to hold the components in position during the PCB assembly process. The component mounting holes are NPTHs. There is no tolerance level for these holes since the components won’t fit in if the hole size is too small or large.
A rule is a rule even for a hole
Drilling holes on a circuit board is not as easy as it sounds. This requires great precision and should adhere to certain design rules. Let us have a look at these requirements for NPTH and PTH.
Non-plated through-hole (NPTH)
- Finished hole size (minimum)= 0.006″
- Edge to edge clearance (from any other surface element) (minimum)= 0.005″
Plated through-hole (PTH)
- Finished hole size (Minimum) = 0.006″
- Annular ring size (Minimum)= 0.004”
- Edge to edge clearance (from any other surface element)(minimum) = 0.009″
After repeated usage, drilling tools tend to wear and break. This leads to the following problems:
The accuracy of the hole location is compromised
When the drill bit fails to hit the preferred spot and shifts away in the same axis, the accuracy is compromised. The shifts in the drilled hole will give rise to tangency or breakout in annular rings.
Roughness inside the drilled hole
Roughness leads to a non-uniform plating of copper. This results in blowholes and barrel cracks. It can also result in lower insulation resistance due to the penetration of copper plating solution to the hole wall.
The resin in the board melts due to the heat generated during the drilling. The resin sticks to the hole walls and is called a resin smear. This again results in poor copper plating and leads to conductivity failure between the via and the interior layers of the circuit. The resin smear is removed by a chemical solution.
Presence of entry and exit burrs
Burr is the unwanted part of copper sticking out of the hole after the drilling process. They are mostly seen both on the top surface and bottom surface of the printed circuit board stack-up.
If the nailheads are improper, then there are chances of copper bends in the inner layers while drilling. These copper bends cause uneven plating and result in conductivity issues.
The partial separation of the circuit board layers is considered delamination. Improper drilling causes delamination.
All these irregularities ruin the board integrity. These problems have been a nightmare for the manufacturers. For these reasons our in-house design engineers whimsically define PCB as, “problems come back!”
In order to eliminate these flaws, scholars have researched on drilling process and design structure and have come up with the following solutions:
Desmear process: It is a chemical process where the melted resin that is deposited on the hole walls is removed. This process eliminates unwanted resin and enhances electrical conductivity through the vias.
Deburr process: It is a motorized process that eradicates the elevated ends (crowns) of the metal (copper) called burrs. Any debris that’s left out within the holes is exterminated through the deburring process. The desmearing process is repeated after deburring.
Delamination: It can be avoided by using laser drills. As mentioned earlier, in laser drilling, the workpiece and the tool do not come under contact thus eliminating delamination.
Take a look at the quick DFM guidelines below which can help you save a few bucks.
DFM drill validation tips for PCB designers
- The aspect ratio must be kept to a minimum to avoid drill wear
- Increase in different drill sizes in the design ( having different hole sizes), will call for increased drill bits (drill bits will be changed for specific sizes). If you restrict implementing different drill sizes, it will cut down the drill time.
- If non-plated drills have connections
- For drill count/size between drill file and fab print
- If drill type is defined (PTH / NPTH)
- For close holes less than 0.006″ if yes it must be addressed
- For mouse bites
- If the drills and other features on copper layers are falling outside the board profile
- If via sizes should be dropped to meet the minimum aspect ratio (A/R) requirement looking at drill tolerance
- For plated drill tolerance less than +/- 0.002″ and for NPTH, drill tolerance of +/- 0.001
- Fab drawing for arcs showing NPTH drill/slot or cutout locations that are missing in the drill file
- For via filling requirement w.r.t. assembly guidelines
In recent years, the drilling process has been optimized compared to earlier technologies. With the exponential growth in the circuit board industry, the drill precision is reaching close to perfection. I believe now you have a better picture of how the PCB drilling process is done. It looks quite complicated, isn’t it? Don’t worry! Just submit your design files (Gerber) to Sierra Circuits. Allow us to drill, while you sit back, watch Netflix, and chill.