A Low-Cost PCB Fabrication Process

A Low-Cost PCB Fabrication Process
1
Jack Ou, 1Alberto Maldonado, 1Chio Saephan, 1Farid Farahmand, 2Michael Caggiano
Engineering Science, Sonoma State University, Rohnert Park, California, United States
2
Electrical and Computer Engineering, Rutgers University, Piscataway, New Jersey, United States
Tel: (707) 664-3462, Fax: (707) 664-2361, Email: [email protected]
1
Abstract
This paper investigates the resolution of a low-cost printed
circuit board (PCB) fabrication process. A set of frequently
used footprints is fabricated on a PCB and examined under a
digital microscope. The results indicate that using the process
described in this paper, a thin wire with a 0.38 mm (14.96
mils) width can be fabricated. The process described in this
paper is useful for educators who wish to fabricate a fine
structure on a PCB, but do not have access to a milling
machine. It is also useful for researchers who wish to quickly
build an inexpensive prototype before sending out the final
design to a commercial venue.
1. Introduction
The need for fabricating a prototype on a PCB arises
frequently in electrical engineering, particularly in areas such
as antenna and radio frequency circuits. Even though access
to a high quality PCB process is widely available for
commercial companies and research institutions, access to an
inexpensive yet accurate PCB process with a quick turnaround time remains non-existent for educators who teach
mid-size classes.
Simple do-it-yourself (DIY) techniques such as using an
iron to transfer ink printed on a transparency to a PCB can be
useful for through-hole components, but lack sufficient
accuracy and consistency for surface mount devices (SMD).
In this paper, we investigate the resolution of a low-cost PCB
fabrication process that utilizes a photopolymer film [1].
This paper is organized as follows: The materials and the
procedure for fabricating a PCB with a photopolymer film are
described in Section 2. The results of our study are discussed
in section 3. We present our conclusion in Section 4.
2.2 Process
We provide a description for fabricating a PCB with a
photopolymer film in this section. Figure 1 shows the overall
fabrication process in four steps. We begin by preparing the
transparency, the photosensitive film, and the PCB board for
fabrication. Next, we carefully place the photosensitive film
on the PCB and position the photosensitive film above the
layout printed on a transparency. The photosensitive film is
then subjected to a brief exposure of ultra violet light. The
exposed film is rinsed with a developer solution. Excess film
is removed with sodium carbonate. The remaining film on
the PCB corresponds to the pattern printed on the
transparency.
Finally, the copper unprotected by the
photosensitive film is removed with an etchant. The details of
each step are described below.
2. Fabrication Process
2.1 Materials
Materials used in this paper are documented below:
 Transparency Film (3M, PP2500, water-based
transparency coating.)
 Laser Printer (HP Laserjet, 4050N model)
 Printed Circuit Board (double-sided, 1.4 mm in
thickeness, 6 in. x 6 in., $3.38 from Amazon.)
 Photopolymer dry film (30 cm x 200 cm, $16.00 from
Amazon.)
 Heat sealing tool (TF Top Flite Monokote)
 Photopolymer developer (sodium carbonate, 0.85
wt%)
 Etchant (Ferric Chloride)
 UV Box (converted from a used Astra 1220 UMAX
scanner.)
Figure 1. PCB fabrication with a photopolymer dry film.
Step 1: Preparation
The first step deals with proper handling of materials used
in the fabrication as well as the construction of an ultra violet
(UV) exposure box. The details are described below:
A. Layout Transfer
The layout is printed on a transparency in black ink. To
improve the quality, the layout is printed twice and touched
up with a Sharpie®.
B. Prepare the PCB for Layout Transfer
A PCB with dimensions matching the layout is selected
and cut with a hacksaw. The surface of the PCB is sanded
with a 400-grit sand paper and rinsed under water. As an
alternative to sand paper, Emory Cloth with appropriate grain
size can also be used. The copper should be clean and shiny
after this step.
C. Proper Handling of the Photosensitive Film
A piece of photosensitive film with dimensions matching
the layout is selected and cut. The film should be large
enough to cover the board. Transparent tapes are placed on
both sides of the film so that one of the protective films
covering the photosensitive film can be peeled off.
D. Construction of the UV Exposure Box
The UV exposure box (Figure 2) was constructed from a
used flatbed scanner. The original electronics and the
mechanical parts were removed. A high density polyethylene
(HDPE) sheet with appropriate dimensions was used to cover
the entire glass area. Six 12-inch fluorescent tubes powered
by a ballast transformer were mounted on the HDPE. The
fluorescent light bulbs were chosen to compliment the
photopolymer film which has a peak resist response from 350
nm to 380 nm.
separated from the transparency by a protective film. The ink
side of the transparency is placed against the PCB. The
exposure time depends on the quality of the laser printer. A
high quality printer, capable of producing a consistent, dark
and crisp layout, produces an image that can protect the
photopolymer film from a longer UV exposure, therefore,
leading to a better layout pattern transfer. We typically expose
the board to 10s UV light for a light pattern on the
transparency and 30s to 45s for a dark pattern on the
transparency. The protective film on the photosensitive film is
peeled after UV exposure. The protective film should be
brittle and hard after exposure.
Figure 3. A PCB positioned on the UV exposure box.
After UV exposure, the board is developed with sodium
carbonate. Other developers such as potassium carbonate can
also be used [1]. The board is rinsed with the developer
solution to bring out the layout pattern on the board. Excess
photosensitive film is removed by rubbing the board with
sodium carbonate with a finger for 2-3 minutes. A darker
layout pattern will emerge on the PCB if it is exposed to the
UV light for a few more seconds. The layout can be touched
up with a Sharpie® at this point as necessary.
Step 4: Etching
Finally, the board is etched with 30% Ferric Chlorid. For
a double sided 3” x 3” PCB, the etching usually takes
approximately 45 minutes. The PCB is rinsed with water and
cleaned with acetone to complete the process.
Figure 2. An UV exposure box converted from a flatbed
scanner.
Step 2: Proper Placement of the Photosensitive Film on the
PCB
One of the transparent protective films on the
photosensitive film is peeled off.
The exposed side
photosensitive film is placed against the copper. Air bubbles
are eliminated by applying pressure from center out. Next,
we separate copper with a piece of paper and apply heat
gently with an iron through the paper so that the
photosensitive film attach properly to the copper. The iron
should be set to a temperature between 105 degree Celsius
and 120 degree Celsius.
Step 3: UV Exposure and Board Development
The transparency with a layout printed in black ink is
placed on the exposure box as shown in Figure 3. The PCB is
attached by the photopolymer film. The photopolymer film is
3. Results
We present two sets of experimental results. In the first
experiment, we investigate the resolution of the fabrication
process with a set of frequently used footprints on a PCB. In
the second experiment, we demonstrate the application of the
fabrication process in the context of a radio frequency filter
design.
3.1 Investigation of Process Resolution Using Frequently
Used Footprints
In this experiment, we fabricate frequently used footprints
on a PCB in order to evaluate the resolution of the fabrication
process. The layout was drawn in a PCB layout editor,
printed onto a transparency (Figure 4), and transferred to a
PCB. A photograph of the fabricated board is shown in
Figure 5.
Nine groups of footprints are fabricated on the PCB. The
footprints in group A represent the footprints commonly used
by SMD components. The footprints in group B represent a
variety of wires with widths ranging from 0.1 mm to 1.06
mm. The footprint in group C corresponds to that of a 44-pin
LQFP. A spiral inductor is shown in group D. An SO32
footprint is shown in group E. A Micro-8 footprint is shown
in group F; a TO72 footprint is shown in group G; an SOT
footprint is shown in group H; an 18-pin DIP footprint is
shown in group I. The dimensions of the shapes are measured
as follows: first, the layout on the transparency is examined
under a Celestron 44308 digital microscope with a maximum
of 200x magnification. The dimensions are measured with a
Neiko 6” digital caliper with a resolution of 0.01 mm and an
accuracy of 0.02 mm. Next, the same geometry implemented
on the PCB is measured. The measurement results of selected
groups are discussed next.
SMD footprint is the smallest SMD footprint we can fabricate
reliably with this process.
Figure 6. Footprints of group A, B and C.
Figure 4. Frequently used footprints printed on a
transparency.
Unit
(mm)
A1
A2
A3
A4
A5
A6
A7
A8
A9
Transparency
h
w
S
0.43
0.4
0.44
0.65
0.59
0.58
0.92
0.84
0.72
1.42
1.07
0.97
1.75
1.25
1.36
1.79
1.44
1.76
2.97
1.91
1.34
3.61
1.79
2.58
5.46
2.52
3.43
h
0.53
0.62
0.94
1.55
1.69
1.92
2.68
3.82
5.48
PCB
w
0.58
0.75
0.95
1.22
1.4
1.39
1.71
2.03
2.56
s
0.29
0.27
0.51
0.7
1.03
1.65
1.19
2.49
3.05
∆h
0.1
-0.03
0.02
0.13
-0.06
0.13
-0.29
0.21
0.02
Comparison
∆w
0.18
0.16
0.11
0.15
0.15
-0.05
-0.2
0.24
0.04
∆s
-0.15
-0.31
-0.21
-0.27
-0.33
-0.11
-0.15
-0.09
-0.38
Table 1. Dimensions of SMD footprints.
Group B: Wire Width
We examine wire width in this experiment. The width (w)
of the wire is measured as shown in Figure 6. The deviation
in w (i.e., ∆w) is equal to PCB width (wPCB) minus the
transparency width (wtransparency). The thinnest wire (W10),
which has a transparency width of 0.1 mm, was broken after
UV exposure, and had to be repaired with a Sharpie. The
PCB width of W10 was not measured because its width is not
uniform throughout the wire.
Figure 5. PCB layout.
Group A: SMD Footprints
Footprints in group A correspond to the pad dimensions of
the 0201, 0402, 0603, 0805, 1008, 1206, 1210, 1812, and
2220 SMD components. They are numbered from 1 to 9 in
Figure 6. The length (l), the width (w), and the spacing (s) of
the pads are defined in Figure 6. ∆h is calculated by
subtracting the h of the pad on the transparency from the h of
the pad on the PCB, similarly for ∆s and ∆w. The average of
∆h and the average of ∆w are 0.025 mm and 0.086 mm
respectively, indicating that the h and the w of the pads on
PCB are larger than those found on the transparency. The
average of ∆s is -0.22, indicating that s the pads on the PCB is
on smaller than the s of the pads on the transparency. The
negative ∆s is expected because ∆w is positive. The 0402
Unit (mm)
W1
W2
W3
W4
W5
W6
W7
W8
W9
W10
wtransparency
1.06
0.94
0.75
0.7
0.54
0.59
0.45
0.3
0.22
0.1
wPCB
1.33
1.07
0.98
0.75
0.7
0.65
0.6
0.49
0.38
N/A
∆w
0.27
0.13
0.23
0.05
0.16
0.06
0.15
0.19
0.16
N/A
Table 2. Widths of wires in group B.
Group C: 44-pin LQFP
The footprint in group C corresponds to the pads of a 44pin LQFP. l, w and s are defined in Figure 6. We only
measure the l and w of the pad in the upper left hand corner
(i.e. pad number 1) because the pads are extremely well
matched. As a result, we were not able to measure the
differences in l and w accurately. ltransparency and lPAD are 1.12
mm and 1.21 mm respectively. wtransparency and wPAD are 0.49
mm and 0.51 mm respectively. We defined Si-i+1 as the
spacing of pad i to pad i+1. As indicated by Table 3, ∆Si-i+1 is
relatively uniform for pads in a 44-pin LQFP; and the average
space is 0.29 mm.
S1-2
S2-3
S3-4
S4-5
S5-6
Transparenc
y
0.41
0.42
0.42
0.42
0.42
PCB
∆Si-i+1
0.34
0.25
0.3
0.28
0.28
-0.07
-0.17
-0.12
-0.14
-0.14
Table 3. Spacing measurement for pads in a 44-pin LQFP.
Group D: PCB Inductor
The layout of a spiral inductor is shown in Figure 7. The
inner diameter (din), outer diameter (dout), the width (w) and
the spacing (s) are defined in the figure. The deviation in
geometry is shown in Table 4.
Figure 9. Gain of the Filter.
ADS
Momentum
EMPro
Network
Analyzer
fo (GHz)
3.5
3.5
3.4
3.57
S21 at fo (dB)
-5
-5.4
-7.8
-6.8
fbw (MHz)
280
250
210
330
Table 5. Center frequency and bandwidth of the filter.
3.3 Cost Analysis
Figure 7. PCB layout.
din
dout
s
w
Transparenc
y
0.64
20.46
0.87
0.53
PCB
Deviation
0.54
20.81
0.44
0.67
-0.1
0.35
-0.43
0.14
Table 4. Deviation in the geometry of a spiral inductor.
3.2 A 3.5 GHz Coupled Bandpass Filter
To illustrate the integration of this fabrication process in a
classroom environment, a 3.5 GHz band-pass filter (BPF) was
constructed on a PCB. The BPF was designed and optimized
using Agilent’s Advanced Design System (ADS). The filter
geometry was first analyzed in Momentum, and then again in
EMPro. Once the final geometry was determined, the
geometry was drawn in Eagle, fabricated on a double-sided
PCB, and measured on a network analyzer. A photograph of
the coupled-band pass filter is shown in Figure 8. S21, the
gain of the filter is measured using a network analyzer and
compared to the S21 obtained using ADS, Momentum and
EMPro. The center frequency (fo) and the 3dB bandwidth (fbw)
of the filter are summarized in Table 5.
Figure 8. Coupled Band-Pass Filter.
A. One Time Cost
The UV exposure box was converted from a used scanner.
The total cost of constructing the exposure box is sixty
dollars. The total cost includes three electronic ballast
($15.57), universal power cable ($4.01), six 12-inch
fuorescent light bulbs ($28.99), one HDPE sheet ($6.45), and
miscellaneous items such as bolts, washers, zip tiles, and zip
ties.
B. Cost for fabricating a 3”x”3 PCB
The cost of developing a 3” x 3” double-sided PCB is as
follows: $0.85 for the PCB, $1.65 for the etchant and $0.20
for the photosensitive film. The total cost for 3” x 3” board is
$2.85 if we account for other consumable expenses such as
plastic gloves, paper towels. If a pre-sensitized PCB is used
to eliminate the bubbles between the photopolymer film and
the PCB, the total cost for a double-sided PCB increases to
$5.07.
4. Conclusions
In this paper, we investigate the resolution of a low-cost
PCB fabrication process that utilizes a photopolymer dry film.
The thinnest wire we can fabricate with the process has a
width of 0.38 mm. The smallest SMD footprint that we can
fabricate is that of a 0402 footprint. Pads that are fabricated in
a uniform footprint have the minimum pad-to-pad variation
and can achieve an average spacing of 0.29 mm.
The
resolution reported in this paper is useful to educators and
researchers who wish to fabricate a low-cost PCB accurately
with a quick turn-around time of 1.5 to 2 hours.
References
1. DuPont, “DuPont Riston MultiMaster Photopolymer Dry
Film,” MM500 datasheet, 2011.

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