Learning outcome 2 prepare make pcb modules

Learning Outcome 2
Prepare / Make PCB Modules

Information Sheet CO 1.2-1
PCB Concept and Design
What is a printed circuit board?
A printed circuit board or PCB, is a plate or board used for placing the
different elements that conform an electrical circuit that contains the
electrical interconnections between them.
The most simple printed circuit boards are the ones that contains copper
tracks or interconnects only on one of its surfaces. These kinds of boards are
known as 1 layer printed circuit board or 1 layer PCB.
The most common PCB's manufactured today are the ones that contain 2
layers, that is, you can find interconnects in both surfaces of the board.
However, depending on the physical complexity of the design ( PCB layout ),
the boards can be manufactured of 8 or more layers.

Fig 1. Example region of a 2 layer PCB

• Soldermask
For mounting the electrical components on the printed circuit boards, an
assembly process is required. This process can be done by hand or through
specialized machinery. The assembly process requires the use of solder to
place the components on the board. For avoiding or to prevent the solder to
accidentally short-circuit two tracks from different nets, pcb manufacturers
apply a finish or varnish called soldermask on both surfaces of the board.
The most common color of soldermask used in printed circuit boards is
green, followed by red and blue.

In EDA software (Electronic design automation), generally exist a rule
associated to the expansion of the soldermask. This rule specifies the
distance that exists between the pads' borders and the soldermask's border.
This concept is illustrated in figure 2 (a).

• Silkscreen or Overlay
Silk-screening is the process where the manufacturer prints information on
the soldermask conducive to facilitate the processes of assembly,
verification and repair. Generally the silkscreen is printed for indicating test
points as well the position, orientation and reference of the electronic
components that are part of the circuit. Also it can be used for any purpose
that the designer may require, for example, the company name,
configuration instructions (this was commonly used in old PC motherboards),
etc. The silkscreen can be printed on both surfaces of the board. Also the
term silkscreen is known as overlay. Figure 2 shows a region of a circuit, all
the printings made in white correspond to the silkscreen.

Fig 2. Soldermask expansion (a) and silkscreen (b)

• Layer Stackup
The printed circuit boards can be made of several layers. When a PCB is
designed with the aid of an EDA software, often are specified several layers
that doesn't necessary correspond to conductive material ( copper ). For
example, the silkscreen and soldermask are nonconductive layers. Having
conductive and nonconductive layers may lead to confusion, because
manufacturers use the term layer when they are referring to the conductive
layers only.

• Component packages
Today in the market you can find a great variety of electronic component
packages. It is common to find several types of packages for one device. For
example you can find the same integrated circuit in QFP's and LCC's
packages.
Package Description Example Image
Thru-Hole
Are all those components that have pins
intended to be mounted through a plated hole
in the PCB. This kind of component is soldered
to the opposite side of the board from which
the component was inserted. Generally these
components are mounted on one surface of the
board only.

Package Description Example Image
SMD/SMT (surface mount device/surface
mount technology)
Are all those components that are soldered in the same side of the board
from which the component was placed. The advantage of this type of
package is that it can be mounted on both sides of the PCB. Also, these
components are smaller than the thru-hole type, which allows the design
of smaller and denser printed circuit boards. These types of components
are useful for frequencies up to 200 [MHz] (fundamental clock frequency).
BGA (Ball grid array)
These types of components are frequently used for high density pin
integrated circuits. For soldering them to the printed circuit boards it is
required to have specialized machinery due that the pins are made of
solder balls that have to be melted for making the electrical contact with
the pads.
BGA components are ideal for high frequency integrated circuits due to
the very small parasitic inductances present in the joint between the pad
and the balls. These type of components are very common in computer
hardware like motherboards and video accelerator cards.

• Pads
A pad is a small surface of copper in a printed circuit board that allows
soldering the component to the board. You can think of a pad as a piece of
copper where the pins of the component are mechanically supported and
soldered. There are 2 types of pads; thru-hole and smd (surface mount).
Thru-hole pads are intended for introducing the pins of the components, so
they can be soldered from the opposite side from which the component was
inserted. These types of pads are very similar to a thru-hole via. The smd
pads are intended for surface mount devices, or in other words, for
soldering the component on the same surface where it was placed. Figure 4
depicts 4 components. The components IC1 and R1 have 8 and 2 SMD pads
respectively, while both components Q1 and PW have 3 thru-hole pads.

Fig 4. SMD and Thru-hole Pads

• Copper tracks
A track is conductive path that is used to connect 2 points in the PCB. For
example, for connecting 2 pads or for connecting a pad and a via, or
between vias. The tracks can have different widths depending on the
currents that flow through them. It is important to highlight that in high
frequencies is necessary to calculate the tracks' width so that the
interconnect can be impedance matched along the path created by the
track.

Fig 5. Tracks that interconnect 2 integrated circuits (chips)

• Plated Holes (Thru-hole Vias or Full Stack Vias)
When an interconnect must be made from a component that is located on
the top layer of the printed circuit board with another that is located at the
bottom layer, a via (Vertical Interconnect Access) is used. A via is a plated
hole that allows the current to pass through the board. Figure 6 depicts 2
tracks that begin at the pads of a component on the top layer and end at
the pads of another component at the bottom layer. For conducting the
current from the top layer to the bottom layer, a via is used for each track.
The tracks and pads that belong to the bottom layer are visually dimmed, so
you can differentiate them from the ones that are on the top layer.

Fig 6. Two integrated circuits located on opposite sides of the PCB are
connected using thru-hole vias

Figure 7 depicts a more detailed view of a transversal section of a 4 layer
printed circuit board or 4 layer PCB. The colors that appear in the Figure are
explained in the following table:
Color Legend for Figure 7
green Top and bottom soldermasks
red Top layer ( conductive )
violet Second layer. In this case this layer is used as a power plane ( i.e. Vcc or Gnd )
yellow Third layer. In this case this layer is used as a power plane ( i.e. Vcc or Gnd )
blue Bottom Layer ( conductive )

The PCB depicted in figure 7 shows a
track that belongs to the top layer that
goes through the board using a thru-
hole via, and then continues as a track
that belongs to the bottom layer.
Fig 7. Track from the top layer going
through the PCB and ending on the bottom
layer

• Blind vias
In high density complex designs is necessary to use more than 2 layer as we have
shown in figure 7. Generally in multilayer system designs where there are many
integrated circuits, power planes ( Vcc or gnd) are used to avoid excessive routing
for power rails. In other words, it is lot easier and more secure to directly connect
to the power planes that are beneath the chips instead of routing long tracks for
the PDS ( Power Delivery System ) ( this can also be achieved with thru-hole vias ).
Also there are times that a signal track must be routed from an external layer ( top
or bottom ) to an internal layer with minimum via height because it can act as a
stub and maybe produce an impedance mismatch. This can cause reflections and
produce signal integrity issues ( more on this in a future article ). For these kinds of
interconnects blind vias are used, which allows a connection to be made from an
external layer to an internal layer with minimum via height. A blind via starts on an
external layer and ends on an internal layer, that's why it has the prefix "blind".

Figure 8 depicts 3 vias that are part of 4 layer printed circuit board. If we
see the picture from left to right, the first via that we will see is thru-hole
via or fullstack via. The second via begins at the top layer and ends at the
second layer ( inner ), so we say that this is a 1-2 blind via. At last, the third
via begins at the bottom layer and ends at the third layer, so we say that
this is a 3-4 blind via.
It is important to have in mind that blind vias are often manufactured in
consecutive layers, in other words between L1 L2, L3 L4, Ln-1 Ln.

Fig 8. Comparison between a Thru-hole and a Blind via

• Buried Vias
These vias are similar to the blind ones, with the difference that they begin
and end on an inner layer. If we look at the image depicted in figure 9 from
left to right, we see that the first one is a thru-hole or full stack via. The
second one is a 1-2 blind via, and the last one is a 2-3 buried via that begins
on the second layer and ends on the third layer.

Fig 9. Comparison between Thru-hole vias, Blind vias and Buried vias

PCB etching and boring
Etching is a "subtractive" method used for the production of printed circuit
boards: acid is used to remove unwanted copper from a prefabricated
laminate. This is done by applying a temporary mask that protects parts of
the laminate from the acid and leaves the desired copper layer
untouched.
You can etch a PCB by yourself, in a lab or even at home, through a simple
and inexpensive production process. It makes sense when you wish to
produce a single or a very small number of boards and want to avoid
manufacturing costs. The etching process is therefore effective for a small
workshop.

There are however some issues to consider:
• There is a risk of injuries due to the chemicals involved.
• The quality of the results depends on several factors which you won't be
able to master completely the first time. This can be somewhat
compensated by using good machinery.
• There is the problem of waste disposal. Toxic chemicals require a proper
disposal service.

Since the work involves dangerous chemicals and power tools - you will need
to take the necessary safety precautions:
1. Wear safety equipment during the whole process - gloves, protection
glasses, and an apron
2. Work near an emergency eyewash station, a first aid box and a phone
3. Familiarize yourself with the proper use of all equipment and tools in the
lab - if you are unsure of anything, ask a member of the lab staff

Masking
There are different ways to produce a mask. We describe two simple
methods:
• Method A: Direct Toner Transfer
The circuit layout is printed with a laser printer on paper, put face-down on
the laminate, and the toner is transfered from the paper to the copper using
an iron.

• Method B: Photo-resistive laminates (Recommended)
The mask is printed on a transparent paper or foil, exposed and developed
on the laminate with a UV lamp.
The laminates have a coating that is sensitive to light - by shining a light on
them, we can transfer an image of the design to be created onto the
board. Where light strikes the board, the coating weakens and creates an
area of copper that will be dissolved away by the etching acid.

You will need a UV imaging frame and a photo development kit. Follow these steps:
1. Export your PCB layout from Fritzing using File > Export > Etchable PDF/SVG. If
you like, you can now edit and enhance the graphics in a graphics software.
2. Print the file in highest resolution, first on white paper, and when satisfied, on a
transparent paper or foil. Be careful to set the printer options to NOT "scale to
fit".
3. When printing on foil, use either a special toner or a toner dissolver spray to
heighten the density and get the print tone overall even.
The spray should be used directly after printing, and care should be taken not
to cause any drops. After the toner dries, place the foil in the imaging frame,
toner side up.

4. Peel the protective blue film off the laminate and place it in the imaging
frame copper side down, on top of the transparent film. Make sure not to
expose the light-sensitive layer to too much light during the development
process.
5. Close the lid of the imaging frame, set the timer for 4 minutes and press
the knob to start the exposure.
6. Once the imaging process finishes, place the board in the development
tank for 1-2 minutes and shake it once in a while (be careful not to
leave it too long, otherwise it will damage the traces).
7. Place the board in the rinse tank containing destillated water for a few
seconds.

Printed mask on a foil Placing the mask in the UV imaging frame

• Etching
Different machines and chemicals can be used for etching, which will affect the
comfort, duration and quality of the result.
Two Acid types that can be used for etching are ferric chloride (Eisen-3-Chlorid) and
Sodium Persulfate (Natriumpersulfat - Feinätzkristall).
Although etching can also be done in simple plastic boxes, the quality of the results
will improve dramatically when using a machine that controls temperature and
constantly keeps the fluids in motion. There are small etching tanks with heating
and air pump and there are small spray etching machines which can handle bigger
PCBs and even the development and cleaning processes involved. The sprayer also
decreases the etching time and the amount of acid needed.

To get rid of all unwanted copper follow these etching steps:
1. Put the board in the acid tank for about 20 minutes until the copper
traces are completely etched. You can tell that it's finished when the
board turns from opaque pink to transparent yellow.
2. Move the board into the rinse tank for a few seconds. You can also use an
Isopropanol spray.
3. Dry the board with a cloth.

• Drilling
To complete the PCB production, fix the board to a
piece of wood, drill holes (holes should be at least
0.1 mm bigger than pins/wires), and cut the board
to the desired size. You can also use varnish spray
as a finishing coat.
Congratulation! Your PCB is ready!

PCB problems and solutions
PCB Assembly problems and its solutions
In PCB Assembly, there is a set of process that is constantly being followed.
One process cannot be skipped; one cannot be switched to be performed
over the other. For an SMT component populated PCB, it is always going to
be the following processes (arranged in order):
1. Solder paste printing
2. Automatic placement
3. Reflow

But even if the processes are followed, certain problems arise when a
populated PCB is inspected. In here, we are going to tackle some of the PCB
Assembly problems that are commonly encountered and discuss how to solve
and prevent from happening again:
1. Solder bridging
Solder bridging is a phenomenon where at least two neighbouring
component pins have short circuited by way of an unnecessary solder. This
phenomenon may be due to the following: excess solder volume, inaccurate
placement of the machine, or the plain mismatch of the PCB footprint.

2. Coplanarity
Coplanarity is a PCB Assembly problem that can be solved easily. This is
mainly caused by a physically damaged component. But the cause of the
damage will be difficult to find. It may be due to the damage coming from
the component supplier or a damage induced by the automatic placement
machine during the pick and place process. Either way, the focus of the
investigation should only be concentrated on the component itself.

3. De-wetting
De-wetting is a solder anomaly wherein the component pins don’t have
enough solder creeping (wetting) around it. It is also a common PCB
Assembly problem. This may be caused by the following: insufficient solder
volume or mismatch of PCB footprint against the component. To solve this
problem, the engineer and/or the assembler must do the following: change
the solder volume deposit by changing the solder paste printing machine
setting or change the stencil mask opening or change the PCB footprint itself
so it will match with component pins.

Learning outcome 2 prepare make pcb modules

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