(Introduction for PCB soldermask / silkscreen /stack up /Pads/Copper tracks/Plated Holes/Blind vias,Buried Vias

 (Fig 1-Green Soldermask)

1.Soldermask-Green/Blue/Red/Black ect

For mounting the electrical component on the PCB boards, an assembly process is required. This process can be done by hand or through specialized machinery,See Fig 1 photo. 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.

(Fig 2- Silkscreen or overlay)
2.Silkscreen or Overlay

Silkscreen 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. Fig 2 photo shows a region of a circuit, all the printings made in white correspond to the silkscreen.

3.Stack up

It is the description of the different layers of the PCB building, it is the “sandwich” with the soldermask/Copper layer/Internal dielectric layer,See below fig 3

(Fig 3-one of the typically used stack up for 4 Layer)
More explation for reference only:

· solder mask ( a dielectric layer in the top and bottom of the PCB) that protect and maintain the top and bottom copper layer

· copper layer (where the circuit layer is printed)

· Internal dielectric layer that isolate the different copper layer and maintain the circuits

An example a typical 4 layers PCB is composed of:

1 solder mask

2 copper layer top 17.5µm

3 dielectric layer 250µm

4 copper layer 2 - 35µm

5 dielectric core 1000µm

6 copper layer 3 - 35µm

7 dielectric layer 250µm

8 copper layer top 17.5µm

9 solder mask

The central dielectric layer is a core. Thats a directly build layer with a two copper layers. It is cheaper for the PCB manufacturer to buy it directly and to draw the circuit layer 2 and 3 on it. Then put the dielectric layers of 250µm and the top and bottom copper layers, To drill/metalised the vias, draw the top and bottom circuit layers, put the solder mask metallized the copper for the inter metallic solderisation and it is ready for assembly!

4.Pads

A pad is a small surface of copper in a printed circuit board that allows soldering the component to the PCB 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; through-hole and smd (surface mount).

Through-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 through-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. 

(Fig 4-The components IC1 and R1 have 8 and 2 SMD pads respectively, while both components Q1 and PW have 3 thru-hole pads.)

5.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)

 

6.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

And The PCB depicted in figure 7-1 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.

(Figure 7-1)

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 7-2 are explained in the following table:

(Figure 7-2)

7.Blind vias

In high density complex designs is necessary to use more than 2 layer as we have shown in figure 7. Generally in multi-layer 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 . 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".

To know if a certain via is blind, you can put the PCB against a source of light and see if you can see the light coming from the source through the via. If you can see the light, then the via is thru-hole, otherwise the via is blind.

It is very useful to use these kinds of vias in PCB design when you don't have too much space for placing components and routing. You can put components on both sides and maximize the space. If the vias were thru-hole instead of blind, there would be some extra space used by the vias on both sides.

Figure 8 depicts 3 vias that are part of 4 layer PCB board. If we see the picture from left to right, the first via that we will see is thru-hole via or full stack 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.

Figure 8 -Comparison between a Thru-hole and a Blind via

The disadvantage of this type of via is its high price when compared to the thru-hole alternative.

8.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

It is important to have in mind that blind vias are often manufactured in consecutive layers (i.e. L1 L2)

As the case of blind vias, the main disadvantage of this type of via is its high price when compared to the thru-hole alternative. Using b/bb vias may impact the cost of the boards in an important way, so you decide if it's better to use these kinds of vias or use bigger boards with thru-hole type vias.

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