PCB Drilling Explained: The Do’s and the Dont’s
Drilling is the most expensive and time-consuming process in PCB 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 manufacturers’ 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. The drilling process plays a vital role in making this possible. Hence, the implemented drill technology, matters.
The 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 mils (0.006”).
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. Here, the drill depth can be effortlessly controlled.
The laser technology is used to drill blind and buried vias with ease. Here, a minimum hole diameter of 2 mil (0.002”) can be drilled with precision.
Laser Drill Limitations
A PCB is made UP of copper, glass fiber, and resin. These materials have different optical properties. This makes it hard for the laser beam to burn through a board efficiently.
The Life Span of Mechanical Drills
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 maker ignores this then it will result in faulty holes which will turn the board to a scrap.
At Sierra circuits, the superior Hitachi drilling machines are implemented with a 1 mil hole placement tolerance.
If the designer understands the things that happen on the shop floor, he will have a better perception of how his design is brought to life. With this insight, the PCB designer ensures the designs are manufacturable. This, in turn, reduces the cost and the product can be delivered in a minimum turnkey 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 deburring and desmearing process.
Since the quality of the drilled hole is a crucial aspect, tool geometrics must be considered. High-Speed Steel (HSS) and tungsten carbide (WC) are commonly used tool materials for drilling of composites. Carbide tools offer better tool wear and tool life during the machining of GFRP. The cemented carbide drills are generally used in PCB drilling. The PCB drills have a point angle of 130° with a helix angle from 30° to 35°.
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 PCB. When the spindle rotates with 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 goes happens on the shop floor.
Unlike the etching and plating process, the drilling process doesn’t have a fixed duration. The drilling time varies in the shop floor depending on the number of holes to be drilled. This is what happens behind the curtains in PCB manufacturing unit.
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 of the hole is increased. This requires a plating bath with a higher 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.
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 cirucit disruption.
Minimum clearance = annular ring width + solder mask dam clearance
Classification of Holes
The drilled holes are classified into Plated Holes (PTHs) and Non-Plated Holes (NPTHs).
The plated holes (PTHs) are the signal carrying conductive vias that establish interconnection between the different layers in the PCB.
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
Non-Plated Through Hole (NPTH)
- Finished hole size (mimimum)= 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, the 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 shifts in the drilled hole will give rise to tangency or breakout in annular rings.
- Roughness inside the drilled hole:
Roughness leads to non-uniform plating of copper. This results in blow holes and barrel cracks. It can also result in lower insulation resistance by penetration copper plating solution to the hole wall.
- Resin smear:
The resin in the board melts due to the heat generated during the drilling. This resin sticks to the hole walls and is called as 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 of highest stacked of printed circuit board and on the bottom surface of lowest-stacked of the printed circuit board.
Exposed copper of inner layers on through holes formed the shape of nail heading during drilling. Such a huge burden to hole brings non-uniform surface of through holes and may cause conductivity failure of plating.
The partial separation of the PCB layers.
All these irregularities ruin the integrity of a PCB. These problems have been a nightmare for the PCB manufacturers. For these reasons our in-house PCB engineers whimsically define PCB as, “Problems Come Back!”
In order to eliminate these flaws, scholars have researched on drilling process and PCB 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 deburring process. The desmearing process is repeated after deburring.
Delamination 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.
In recent years, the drilling process has been optimized compared to earlier technologies. With the exponential growth in the PCB 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 and watch Netflix and chill.
Take a look at the quick guidelines below which can help you save a few bucks.
Quick DFM Drill Validation Tips for PCB Designers:
- The aspect ratio must be kept minimum to avoid drill wear
- The more number of different drill sizes that are added, the more drill bits the manufacturer will need to use. Instead, if you reduce different drill sizes, the drill time will be cut down.
- If non-plated drills are having 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 mousebites
- If the drills and other features on copper layers are falling outside the board profile
- If vias size should be dropped to meet minimum Aspect ratio (A/R) requirement looking at drill tolerance
- For Plated drill tolerance for less than +/- 0.002″ and NPT drill tolerance of +/- 0.001
- Fab drawing for arcs showing NPT drill/slot or cutout locations but missing in the drill file
- For via filling requirement w.r.t. assembly guidelines
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