Drilling is the most expensive, irreversible, and time-consuming process in the PCB manufacturing process. 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 the manufacturing process.
The drilling process is the foundation for vias and the connectivity between different layers. If the designer understands the things that happen on the shop floor, they will have a better perception of how their 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.
Mechanical Or Laser Drilling?
Basically, there are two kinds of drilling technologies: mechanical and laser drilling.
The mechanical drills are easy to execute. This drilling technology implements drill bits. The smallest hole diameter that can be drilled by this operation is about 6 mils (0.006”).
The laser drills are more precise, 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 4 mils (0.004”) can be lasered with precision.
At Sierra circuits, the superior Hitachi drilling machines are implemented with a 1-mil hole placement tolerance.
Unlike the etching and plating process, the drilling process doesn’t have a fixed duration. The drill time varies on the shop floor depending on the number of holes to be drilled. This is what happens behind the curtains in a PCB manufacturing unit.
Two significant aspects to be considered in PCB drilling process:
- The aspect ratio
- The drill-to-copper clearance (drill to the nearest copper feature)
Aspect ratio (AR) is the parameter that decides the reliability of a PCB.
In a through-hole board, aspect ratio is the ratio between the PCB thickness and the diameter of the drilled hole. While considering microvias, it’s the ratio between the depth of the hole to the diameter of the drilled hole.
Aspect ratio determines the ability to effectively deposit copper inside the holes (vias). The copper plating of the interior part of the holes becomes a tedious task when the diameter is decreased and the depth of the hole is increased. This requires a copper plating bath with a higher throwing power so that the liquid could gush into the tiny holes to deposit copper.
Aspect ratio (Through-Hole) = (Thickness of the PCB) / (Diameter of the drilled hole)
Since microvias don’t protrude through the entire board, the aspect ratio will be:
Aspect ratio (Microvias) = (Drill Depth) / (Diameter of the drilled hole)
The drilled holes that are smaller compared to the board thickness can result in non-uniform or unsatisfactory copper plating.
The larger the aspect ratio, the more challenging it becomes to achieve a reliable copper plating inside the vias. Hence, smaller the aspect ratio, higher the PCB reliability.
At Sierra Circuits, we offer an aspect ratio of 0.75:1 for microvias.
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/copper pour or any other active copper region. This is a predominant factor since even a small deviation will lead to circuit disruption.
Nomenclature and Types 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. If you have NPTHs in your design, please leave a note for the manufacturer so that they understand the purpose of it and won’t reach back to you thinking it was a mistake from your end.
A rule is a rule even for a hole.
Plated through-hole (PTH):
Finished hole size (minimum) = 4 mils
Annular ring size (minimum) = 3 mils
Drill diameter tolerances must be specified on the drill chart. Sierra Circuits prefers a drill diameter tolerance of +/- 0.003” for PTH and NPTH drills.
The accuracy of the hole location is compromised when the drill rules are violated.
Drill Validation Checks
This is what you need to check for:
- The aspect ratio must be kept minimum to avoid drill wear.
- Higher the number of distinct drill sizes that are included in the design, higher will be the different drill bits that the manufacturer has to implement. Instead, if you reduce different drill sizes, the drill time will be cut down.
- Check if the non-plated drills have connections.
- Check for drill count/size between drill file and fab print.
- Check if drill type is defined as PTH or NPTH.
- Check for close holes less than 0.007″. If yes, it must be addressed (It can be spaced apart or one of the drill deleted if permitted).
- Check for mouse bites. Mouse bites are perforated breakaway tabs. They are a line of tiny holes in a PCB board just like the holes around a coupon. Mouse bites offer better grip when mounted.
- Check if the drills and other features on copper layers are falling outside the board profile.
- Prefer a minimum distance of 0.01” from hole edge to cutout/board profile.
- Check if the via size should be dropped to meet the minimum aspect ratio requirement after looking at the drill tolerance.
- For plated drills, the tolerance is less than +/- 0.002″ and NPTH the drill tolerance is of +/- 0.001.
- Fab drawing for arcs showing NPTH drill/slot or cut out locations but missing in the drill file.
- Check if the fab drawing for arcs show NPTH drill/slot or cut-out locations. Check if it reflects on the drill file.
- Mention the via sizes that need to be filled.