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In purchasing the PCB prototyping machine,
you may make your decision merely by the comparison of the
specification.
Generally, the better specification it is, the higher in
price it becomes too.
However, the machine with the good specification can not
necessarily process the circuit as you expected.
Chances are, you might end up making the
undesirable circuit with the high specification machine.
I will explain the reason why in the following text.
 The
rotation speed of the spindle motor should not necessarily
be high
Higher rotation
speed of the spindle motor tends to cause the large degree
of runout.
The below figure represents the correlation between the
rotation speed of the small size spindle motor (Max speed
100,000 rpm) and its runout.
The
Correlation between the rotation speed of the small
size
spindle motor and its runout
Measuring instrument; MITS NANOMETRIC SENSOR
MODEL 211
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In this figure, degree of the runout sharply
increases from the point over 75,000 rpm.
Additionally, when the degree of the runout augments, it
ends up in many unfavorable results, such as
- Causing a lot of burrs on the circuits
- Thin end mills become even more fragile
There
is the optimum rotating speed for the tools
Tools have their own optimum speed of rotation
and processing that the tool manufacturer recommends.
For the most frequently used 90 degrees end mills, it is
30,000 rpm and 15mm/sec.
Higher rotating speed shortens the longevity of the tools
and higher processing speed makes the tools fragile.
I explained 90 degrees end mills as an example but each
tool has its own optimum speed of rotation and processing.
Does
better resolution lead to finer processing?
The resolution merely shows the designated value of each
part constituting the machine, but it does not represent
the overall accuracy of the machine.
Rigidity is more important factor governing the accuracy.
Please read more detailed
information.
There is a big difference between the processing of 0.2mm
wide track and that of 0.1mm.
The major reason consists in the nature of end mills. Typically,
we make use of the end mills whose tip is 90 or 60 degree
angular.
As you can see from this figure, in milling the 0.1 mm wide
track, subtle fluctuation of the end mills influence the
track width. So it is difficult to fix the end mills at
the ideal position. Ordinarily, it is impossible to mill
the track of 0.1mm wide with the angular end mills.
The following three conditions are required for milling
the track thinner than 0.1mm.
Please read more detailed
information.
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1)
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Use the stub-end
mills.
As shown in the figure, the depth of
the processing does not influence the track width using
stub-end mills.
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2)
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But those
end mills thinner than 0.1 mm is very fragile, so we
recommend using the machine with minimal vibration.
In milling with the end mills thinner than 100µm,
you need to minimize the vibration of the machine, which
requires small degree of the spindle motor runout and
the high rigidity of the machine.
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3)
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For the milling
thinner than 50µm, "Stepping processing method"
is indispensable.
For milling the track using the end mills thinner than
50µm, it is required to employ "Stepping processing
method" that the end mills gradually deepen the track
by many steps. |
MITS recommends FP-21T
Precision for the milling thinner than 100µm.
As I stated above, for milling 0.1mm wide track, it is imperative
to calibrate the depth of end mills very delicately.
Especially, by the processing using the end mills whose
tip is angular, the depth of the several micron order difference
influences the track width.
This delicate calibration procedure has to be done manually.
Therefore, it is very difficult to calibrate the protrusion
of the end mills automatically in order to mill the 0.1mm
wide track.
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Recently, the high frequency circuits are in great demand in the
electronic devices
as following.
- Wireless LAN
- Security IC Tags
- Defense and security equipment
- Satellite transmission
- In-vehicle unit
- Medical examination equipments
In order to streamline the designing, the circuit
simulation of the software have remarkably
developped, the trial production can be frequently curtailed. But
as for high-frequency circuit,
the demand of the experimental production and the performance assessment
study are
still increasing.
With the ever increasing miniaturization of the electric devices
and utilization of the high
frequency, the high-density technology has advanced and become ever
thinner, resulting in
generating a large number of PCBs consist of less than 100µm
pattern width.
Let us describe a few points at issue in the process from designing
pattern, simulation to
processing the PCB.
DXF Data and Closed-Loop
One of the methods easily enables us to draw the high-frequency
circuit is CAD.
The data drawn by this type of CAD file is outputted by DXF file.
In order to input the DXF data into the prototyping machine and
the other simulation
software, they cannot be accepted unless they are closed-loop.
The example of non-closed loop is as illustrated below;
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CLOSED-LOOP
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OVERFLOW OF LINE
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EXTRA LINES
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COMING IN SUCCESSION OF
CLOSED-LOOP
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BECOMING INTERRUPTED
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The above figures are drawn in an exaggerated form.
Practically, you cannot be aware of the
errors without enlarging it.
It can become the uncertain source of troubles because its input
is not acceptable.
In case of the complicated figure, there are many cases that dozens
of errors should be modified.
Our Challenge to Under 100µm Milling
In the PCB prototyping machine, the limit resolution has long been
regarded as 100µm
Laser beam machining is required for under 100µm line and space
processing.
But laser processing machine is extremely expensive and demands
the facility equipment
or the study of complicated ins and outs, therefore it cannot be
handle "easily" in the laboratory.
Our goal has long been "User-friendly in laboratory". MITS has applied
our technical knowledge
cultivated over the years and successfully achieved the process
smaller than 100µm.
High Frequency Milling bit
In high-frequency field, the pattern figure has to be 90 degree
of rising edge, you need to use
the stub-shaped milling bit.The front edge of milling bit smaller
than 100µm is as thin as human
hair, the know-how of processing differrent from the V-shaped milling
bit is required.
"100µm" is not equal to 100µm?
You may wonder what the above sentence means.
In fact, the milling bit of 100µm cannot process the milling
width of 100µm.
Tool Tolerance
Milling bit supplied by parts manufacturer inevitably come with
some tolerance.
You need to process PCB with putting that tolerance into consideration.
Runout of HF Spindle
The spindle motor in prototyping machine also inevitably come with
some runout.
The runout changes along with the motor rotating speed, and slightly
changes with the extent
of fastening the motor to the collet. These minute change emerges
as the unexpected broad
milling track wider than the milling bit in finer processing than
100µm.
For the 100-micron milling
In high-frequency circuit board, it is indispensable to process
the PCB with plus or minus 5%
precision in 100µm milling, such as Coplanar waveguide.
So as to achieve that resolution, the processing method considering
the tolerance of the milling
bit and the runout of the spindle are abosolutely neccessary.
| Examples: |
When tool tolelance is
+/- 5µm,
and
When runout of the spindle is 10µm. |
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The minimum processible milling width is 105µm,
and that of maximum is 115µm.
Therefore, in order to achieve the 100µm processing, we perform
the following procedure;
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1;
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Use the milling bit from 80 to
90µm considering the tolerance of the milling bit and
the runout of the spindle motor. |
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2;
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Use the milling bit thinner than
80µm and mill the PCB twice to achieve the 100µm
milling width. |
For the fine and precise milling track
Stepping Milling Method
As the milling line gets thinner, processing condition becomes ever
more crucial.
When the radius of milling bit becomes lower than 100µm, the
milling bits become very
fragile, leading to increase burrs.
Therefore, in processing the milling bit thinner than 100µm,
we employ the "stepping method"
that process the PCB gradually, which is suitable for the small
radial milling bit.
In using the copper foil of 18µm thickness, we mill the board
graually by several steps .
(The depth of single milling is 5µm) With this method, you
can process the PCB without
burrs or breakage even if you use the small radial milling bit.
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