Printed circuit board milling
Encyclopedia
Printed circuit board milling is the process of removing areas of copper
from a sheet of printed circuit board
material to recreate the pads, signal traces and structures according to patterns from a digital circuit board plan known as a layout file. Similar to the more common and well known chemical PCB etch process, the PCB milling process is subtractive: material is removed to create the electrical isolation and ground plane
s required. However, unlike the chemical etch process, PCB milling is typically a non-chemical process and as such it can be completed in a typical office or lab environment without exposure to hazardous chemicals. High quality circuit boards can be produced using either process. In the case of PCB milling, the quality of a circuit board is chiefly determined by the system's true, or weighted, milling accuracy and control as well as the condition (sharpness, temper) of the milling bits and their respective feed/rotational speeds. By contrast, in the chemical etch process, the quality of a circuit board depends on the accuracy and/or quality of the photomask
ing and the state of the etching chemicals.
board, with the exception of inserting vias and through hole plating. Most of these machines require only a standard AC
mains outlet and a shop-type vacuum cleaner
for operation.
, positioning information and machine control commands are sent from the controlling software
via a serial port
or parallel port
connection to the milling machine's on-board controller. The controller is then responsible for driving and monitoring the various positioning components which move the milling head and gantry and control the spindle speed. Typically this drive system comprises non-monitored stepper motor
s for the X/Y axis, an on-off non-monitored solenoid
or pneumatic piston
for the Z-axis, and a DC
motor control circuit
for spindle speed, none of which provide positional feedback. More advanced systems provide a monitored stepper motor Z-axis drive for greater control during milling and drilling as well as more advanced RF spindle motor control circuits that provide better control over a wider range of speeds.
. The lead screw is in turn linked to the gantry or milling head by a special precision machined connection assembly. To maintain correct alignment during milling, the gantry or milling head's direction of travel is guided along using linear or dovetailed bearing(s). Most X/Y drive systems provide user control, via software, of the milling speed, which determines how fast the stepper motors drive their respective axes.
that pushes against a spring. When the solenoid is energized it pushes the milling head down against a spring stop which is attached to a pressure foot assembly that limits the milling head's downward travel. The rate of descent as well as the amount of force exerted on the spring stop must be manually set by mechanically adjusting the position of the solenoid's plunger.
The second type of Z-axis control is through the use of a pneumatic cylinder - this system functions in the same manner as the solenoid type, pushing against a spring stop/pressure foot assembly. Air for the cylinder is provided by an external compressor
with the air flow being controlled by a manually operated regulator
and software driven gate valve
. Due to the small cylinder size and the amount of air pressure used to drive it there is little range of control between the up and down stops. Both the solenoid and pneumatic system provide no positional feedback
while in motion, and are therefore useful for only simple 'up/down' milling tasks. The final type of Z-axis control uses a stepper motor with dynamic positioning feedback. This system allows the milling head to be moved in small accurate steps up or down through its whole range of vertical motion. Further, the speed of these steps can be adjusted to allow tool bits to be eased into the board material rather than hammered into it. The depth (number of steps required) as well as the downward/upward speed is under user control via the controlling software.
Drilling feedrates of up to 4.5% of the drill diameter per rev at up to 180m/min are possible for via's,
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
from a sheet of printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
material to recreate the pads, signal traces and structures according to patterns from a digital circuit board plan known as a layout file. Similar to the more common and well known chemical PCB etch process, the PCB milling process is subtractive: material is removed to create the electrical isolation and ground plane
Ground plane
In electrical engineering, a ground plane is an electrically conductive surface.-Radio antenna theory :In telecommunication, a ground plane structure or relationship exists between the antenna and another object, where the only structure of the object is a structure which permits the antenna to...
s required. However, unlike the chemical etch process, PCB milling is typically a non-chemical process and as such it can be completed in a typical office or lab environment without exposure to hazardous chemicals. High quality circuit boards can be produced using either process. In the case of PCB milling, the quality of a circuit board is chiefly determined by the system's true, or weighted, milling accuracy and control as well as the condition (sharpness, temper) of the milling bits and their respective feed/rotational speeds. By contrast, in the chemical etch process, the quality of a circuit board depends on the accuracy and/or quality of the photomask
Photomask
A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.-Overview:...
ing and the state of the etching chemicals.
Hardware
A PCB milling system is a single machine that can perform all of the required actions to create a prototypePrototype
A prototype is an early sample or model built to test a concept or process or to act as a thing to be replicated or learned from.The word prototype derives from the Greek πρωτότυπον , "primitive form", neutral of πρωτότυπος , "original, primitive", from πρῶτος , "first" and τύπος ,...
board, with the exception of inserting vias and through hole plating. Most of these machines require only a standard AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
mains outlet and a shop-type vacuum cleaner
Vacuum cleaner
A vacuum cleaner, commonly referred to as a "vacuum," is a device that uses an air pump to create a partial vacuum to suck up dust and dirt, usually from floors, and optionally from other surfaces as well. The dirt is collected by either a dustbag or a cyclone for later disposal...
for operation.
Mechanical system
The mechanics behind a PCB milling machine are fairly straightforward and have their roots in CNC milling technology. A PCB milling system is similar to a miniature and highly accurate NC milling table. For machine controlControl theory
Control theory is an interdisciplinary branch of engineering and mathematics that deals with the behavior of dynamical systems. The desired output of a system is called the reference...
, positioning information and machine control commands are sent from the controlling software
Computer software
Computer software, or just software, is a collection of computer programs and related data that provide the instructions for telling a computer what to do and how to do it....
via a serial port
Serial port
In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...
or parallel port
Parallel port
A parallel port is a type of interface found on computers for connecting various peripherals. In computing, a parallel port is a parallel communication physical interface. It is also known as a printer port or Centronics port...
connection to the milling machine's on-board controller. The controller is then responsible for driving and monitoring the various positioning components which move the milling head and gantry and control the spindle speed. Typically this drive system comprises non-monitored stepper motor
Stepper motor
A stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely without any feedback mechanism , as long as the motor is carefully sized to the application...
s for the X/Y axis, an on-off non-monitored solenoid
Solenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...
or pneumatic piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
for the Z-axis, and a DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
motor control circuit
Motor controller
A motor controller is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor. A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and...
for spindle speed, none of which provide positional feedback. More advanced systems provide a monitored stepper motor Z-axis drive for greater control during milling and drilling as well as more advanced RF spindle motor control circuits that provide better control over a wider range of speeds.
X and Y axis control
For the X and Y axis drive systems most PCB milling machines use stepper motors that drive a precision lead screwJackscrew
A jackscrew is a type of jack which is operated by turning a leadscrew. In the form of a screw jack it is commonly used to lift heavy weights such as the foundations of houses, or large vehicles.-Advantages:...
. The lead screw is in turn linked to the gantry or milling head by a special precision machined connection assembly. To maintain correct alignment during milling, the gantry or milling head's direction of travel is guided along using linear or dovetailed bearing(s). Most X/Y drive systems provide user control, via software, of the milling speed, which determines how fast the stepper motors drive their respective axes.
Z axis control
Z axis drive and control are handled in several ways. The first and most common is a simple solenoidSolenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...
that pushes against a spring. When the solenoid is energized it pushes the milling head down against a spring stop which is attached to a pressure foot assembly that limits the milling head's downward travel. The rate of descent as well as the amount of force exerted on the spring stop must be manually set by mechanically adjusting the position of the solenoid's plunger.
The second type of Z-axis control is through the use of a pneumatic cylinder - this system functions in the same manner as the solenoid type, pushing against a spring stop/pressure foot assembly. Air for the cylinder is provided by an external compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
with the air flow being controlled by a manually operated regulator
Pressure regulator
A pressure regulator is a valve that automatically cuts off the flow of a liquid or gas at a certain pressure. Regulators are used to allow high-pressure fluid supply lines or tanks to be reduced to safe and/or usable pressures for various applications....
and software driven gate valve
Gate valve
The gate valve, also known as a sluice valve, is a valve that opens by lifting a round or rectangular gate/wedge out of the path of the fluid. The distinct feature of a gate valve is the sealing surfaces between the gate and seats are planar, so gate valves are often used when a straight-line flow...
. Due to the small cylinder size and the amount of air pressure used to drive it there is little range of control between the up and down stops. Both the solenoid and pneumatic system provide no positional feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
while in motion, and are therefore useful for only simple 'up/down' milling tasks. The final type of Z-axis control uses a stepper motor with dynamic positioning feedback. This system allows the milling head to be moved in small accurate steps up or down through its whole range of vertical motion. Further, the speed of these steps can be adjusted to allow tool bits to be eased into the board material rather than hammered into it. The depth (number of steps required) as well as the downward/upward speed is under user control via the controlling software.
Tooling
PCB's may be machined with conventional endmills, Conical D-Bit cutters and Spade Mills, D-bits and spade mills are cheap and as they have a small point allow the traces to be close together.Speeds / feeds
Cutting speeds of up to Vc = 40 m/min are suitable for general machining, Feedrates of 0.1mm/rev at a depth of 0.1mm are suitable for D-bit cutters,Drilling feedrates of up to 4.5% of the drill diameter per rev at up to 180m/min are possible for via's,
Economics of PCB milling
Taylors equation, Vc Tn = C, can predict tool life for a given surface speed. At low cutting speeds the values were found to be approximately Vc T0.46 = 57. As Taylor is only linear over short distances this may be incorrect for higher surface speeds. As VC increases the machining time falls, but the cost of cutters rises. For a hobby machine, a Vc as low as 3 m/min may be best. For mass production a VC of up to 180 m/min may be best if the cost of cutters is low and the machine overheads are high.Direction of cut
If no burr is desired on the trace side then conventional milling the trace profile will produce a better result on the trace side.Depth of cut
The cutter depth should be as shallow as possible to avoid the hard glass fibres in the board. If the depth is too deep and a trace is too narrow then plucking of fibres from underneth the trace will occur. The glass fibres will also cause the cutter to deflect giving an uneven finish at the trace edge.Software
- Eagle Add-on: http://groups.yahoo.com/group/pcb-gcode
- Linux Project: http://sourceforge.net/projects/gerbertogcode
- C++ version: https://sourceforge.net/projects/pcb2gcode/
- Java version: https://sourceforge.net/projects/visolate/, based on paper on Voronoi toolpaths
- GCAM (Among many features, supports importing of RS274X (Gerber) and Excellon drill files for gerber to g-code pcb milling conversion): http://gcam.js.cx/index.php/Main_Page
- ProtoLiner: http://www.colinbus.com/en/cam_pcb_en.htm