pcb drilling file
Printed Circuit Boards (PCBs) are backbones of electronic devices and equipment. The increasing density of circuits. and decreasing size trend of electronic products lead circuit boards. to develop into higher complexity. Holes or vias. responsible for electrical connections between layers. and corresponding functions a board will put in place according to design need. won't work unless they designed or manufactured.
For unsuitable manufacturing, it can solved by selecting a reliable PCB manufacturer. The solution to the other issue, that is, unsuitable design. depends on circuit board design engineers who have to create integrated. and reliable design files based on which PCB fabricators carry out manufacturing.
It's believed that NC drill file generation. has been plaguing most PCB design engineers for a long time. Thus, it's necessary to summarize design guidelines based on leading PCB design software.
The NC (Numerical Control) Drill file defines hole locations. tool numbers and finished hole sizes (X&Y coordinates). For Prototype(Engineering Level = Limited Review on quote form) PCBs. all holes will treated like plated through holes. The following two design features could create a difference. between the design file and the received PCBs:
Non-plated holes in the design file will finish with a larger diameter (+/- 0.005") than intended.
Non-plated holes in the design file cannot guaranteed. to remain unplated (even a hole that has no external pad).
For Prototype boards (Engineering Level = Limited Review on the quote form) PCBs. Specified holes < 0.018" will treated as vias. and drilled approximately the same size as specified. The smallest available bit is 0.0135". If your drill is smaller, it will increased to this size.
Hole sizes > 0.250" will finish as non plated. for Prototype (Engineering Level = No Engineering Limited Review on quote form) PCBs. Holes > 0.250" considered a cut-out as they need different processing.
How is PCB file work
•Professional manufacture of PCBs is done through the use of CAD-independent files:
–Gerber Files – describe the copper foil layout
–Drill Files (often in the Excellon format) – describe the location and size of holes
•Both meant for direct use with automated PCB production equipment.
•Exellon Drill File
•Excellon Files provide a command sequence to a system that drills PCBse
•This equipment (or its computer driver) has an interpreter. that receives and executes the commands in sequence.
•The next two slides describe the list of commands that interpreted.
Source: “Norme Excellon”, available at http://www.forelec.ch/www/norme_excellon.htm. accessed 12 Feb 2008 (dead link at 26 Jan 2011).
Excellon file format governed by ANSI/IPC-NC-349 .
% Rewind and Stop
X#Y# Move and Drill
T# Tool Selection
M30 End of Program
M00 End of Program
M25 Beginning of Pattern
M31 Beginning of Pattern
M01 End of Pattern
M02 X#Y# Repeat Pattern R#M02X#Y# Many Repeat Pattern M02 X#Y# M70 Swap Axis
M02 X#Y# M80 Mirror Image X Axis M02 X#Y# M90 Mirror Image Y Axis M08 End of Step and Repeat
N# Block Sequence Number
/ Block Delete
R#X#Y# Repeat Hole
G05, G81 Select Drill Mode
G04 X# Variable Dwell (ignored)
G90 Absolute Mode
G91 Incremental Mode
G92 X#Y# Set Zero
G93 X#Y# Set Zero
M48 Program Header to first "%"
M47 Operator Message CRT Display
M71 Metric Mode
M72 English-Imperial Mode
Snn Spindle Speed (RPM)
Fnn Z axis feed speed (IPM)
only a subset of these commands used.
File Formats: Excellon Drill File Example
% Reset and rewind.
M48 Start of header.
M72 Imperial (English) Mode: units in inches
T01C0.0420 Tool 1 Change: to 42 mil
T02C0.0860 Tool 2 Change: to 86 mil T03C0.0350 :
T08C0.1520 Tool 8 Change: 152 mil
% End of Header: Drill data follows
T01 Select Tool 1 (42 mil)
X2120Y1112 Drill at (2120 mil,1112 mil)
: (Lots of data removed)
T08 Select Tool 8 (152 mil)
X3645Y262 Drill at (3645 mil, 262 mil)
: (Data removed)
M30 End of program.
The hole walls for boards with 2 or more layers can made conductive. and then electroplated with copper to form plated-through holes. These holes connect the conducting layers of the PCB. For multilayer boards, those with 3 layers or more. drilling produces a smear of the high temperature. decomposition products of bonding agent in the laminate system. Before the holes can plated through, this smear must removed. by a chemical de-smear process,
or by plasma-etch. The de-smear process ensures that a good connection made to the copper layers. when the hole plated through. On high reliability boards a process. called etch-back performed with a potassium permanganate based etchant. or plasma. The etch-back removes resin and the glass fibers so that the copper layers extend into the hole. and as the hole plated become integral with the deposited copper.
Drilling is the most expensive, irreversible. and time-consuming process in the PCB manufacturing process. The PCB drilling process must be implemented since even a small error can lead to a great loss. The drilling process 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 brought to life. With this insight, the PCB designer ensures the designs are manufacturable. This, in turn, reduces the cost and the product can delivered in a minimum turnkey time.
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 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 controlled.
The laser technology used to drill blind and buried vias with ease. Here, a minimum hole diameter of 4 mils (0.004”) can lasered with precision.
At Sierra circuits. the superior Hitachi drilling machines 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 drilled. This is what happens behind the curtains in a PCB manufacturing unit.
Two significant aspects to 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 micro vias, it’s the ratio between the depth of the hole to the diameter of the drilled hole.
Aspect ratio determines the ability to deposit copper inside the holes (vias). The copper plating of the interior part of the holes becomes a tedious task. when the diameter decreased. and the depth of the hole 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 ideal aspect ratio is 10:1 for through-hole and 0.75:1 for microvias.
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.
Drill To Copper
PCB Drilling Process
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 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.