In some designs where electromagnetic interference (EMI) is to be considered, the outer or top layer can not be used for wiring, even if the outer ring does not work. In this case, the top layer is used as a ground plane. EMI includes a product for the external electromagnetic field susceptibility, and the external electromagnetic field generally by coupling or radiation from a product into another product, and often lead to a product through the consistency test.

Products only meet the following three criteria can be considered to meet the electromagnetic compatibility requirements:

1, do not interfere with other systems

2, from the impact of other system radiation

3, will not interfere with itself

In order to prevent the product from interfering with the signal, it is recommended to take shielding measures on the product. Shield generally refers to the metal shell completely wrapped in the entire electronic products or a part of the product. However, in most cases the outer surface of the ground plane can also play a role in shielding, because it can attract energy, to minimize the interference.

For the ultrafine pitch of the pad in the hole technology

When the BGA signal is used to escape and route using the pad vias, the vias are placed directly on the BGA pad and filled with conductive material (usually silver) and provide a flat surface.

In this paper, the use of micro-BGA pad inside the fan fan example is used in 0.4mm ball or lead spacing, PCB is 18 layers, including 8 signal wiring layer. BGA cabling usually requires more layers. But in this example, the number of layers is not a problem, because only a small amount of BGA ball is used. The key issue remains the 0.4mm narrow pitch for the miniature BGA, and the top layer does not allow cabling except fan. The goal is to do both fan-out micro-BGA, but also does not affect the manufacture of PCB.

Figure 5 shows the outline of the BGA device manufacturer. As can be seen from the figure, the recommended pad size is 0.3mm (12mil) and the lead pitch is 0.4mm (16mil). Since the spacing between the pads is particularly small, it is not possible to implement a conventional Dog bone fan pattern. Even small-sized vias can not be used for the Dog bone fan-out strategy. Here the small size of the hole means 6mil drilling and 10mil ring pad. Another important mechanical limitation is the thickness of the circuit board, which is 93 mil.
PCB board design _BGA device manufacturers to provide the outline
Figure 5: PCB design _BGA device manufacturers to provide the outline

In this case, the most convenient solution is to use the micro-vias in the pad. However, the micro-via size can not exceed 3 mils. But the 93mil board thickness is a limiting factor. Another option is blind hole and buried hole technology. But these options will limit the choice of manufacturing technology, and will increase the cost.

In order to be able to choose a different manufacturing company, 93mil thick circuit board drilling size can not be less than 6mil, the line width can not be less than 4mil. Otherwise only a few high-end circuit board manufacturers to take over the project, and expensive. Figure 6 shows the BGA profile associated with this example.
PCB design _BGA outline drawing
Figure 6: PCB design _BGA outline

This fan-out approach avoids the use of high-end technology and does not affect signal integrity. The BGA pin is divided into internal and external pins.

The fanout method shown in Figure 6 avoids the use of high-end technology and does not affect signal integrity. The BGA pin is divided into both internal and external pins. The pad holes are used internally and the external pins are fan-out on a 0.5mm grid. Figure 7a shows the top layer, and Figure 7b shows the top and inner wiring layers.
PCB board design _ top and internal wiring layer
Figure 7a and 7b: PCB design _ top and internal wiring layers

The pad holes are used internally and the external pins fan out on the 0.5mm grid. Figure 7a shows the top layer; Figure 7b shows the top and inner wiring layers.

Since the BGA pad size is 0.3mm (12mil) and the pitch is 0.4mm (16mil), a 6 / 10mil vias (hole / ring size) are used in the pad. External expansion fanout uses the same vias. In the interior, the clearance between the vias is 6 mil, which is the standard size and does not cause manufacturing problems. The external hole clearance is 10 mil. This gap can take a 3mil line, the distance between the line and the hole is 3.34mil. This special strategy allows all signals coming out of the 0.4mm pitch miniature BGA to be successfully fan-out without any special manufacturing requirements.

Whether using the Dog bone or the pad vias, the basic steps are the same, ie, determine the correct channel space, including defining the size of the vias and pads, the trace width, the impedance requirements, and the stack. However, the difference is that the vias are arranged and the vias are used.

It is recommended to use a depth of up to 6 layers of blind hole / buried hole configuration. The number of layers will cause more manufacturing problems. The preferred technique is to use cross vias or stack vias as shown in Fig. Cross vias allow for more accurate registration tolerances because they do not require perfect alignment as well as stacking vias.
PCB design _ cross vias or stack vias
Figure 8: PCB design _ cross vias or stack vias

Crossed vias allow for greater registration tolerances because they do not require perfect alignment as well as stacking vias. (Provided by IPC)

No such steps will be wrong

Whether it is using Dog bone or through-hole technology, both manufacturability and functionality are two important aspects that need to be carefully considered. The key is to know the manufacturing plant manufacturing restrictions. Some factories can make particularly rigorous designs. However, if the product is ready for mass production, the cost will be high. So the design should consider the selection of ordinary manufacturing plant is particularly important.

In short, from the manufacturing point of view to consider the key factors are:

Stacked

Through Hole - The size of the hole (depending on the aspect ratio)

Through Hole - Hole (minimum 3mil required)

Via - stack (stack or cross)

Copper foil to copper foil distance (recommended minimum 3mil)

Copper foil to bore distance (minimum required 5mil)

BGA contact size for assembly and solder ball size

There is always a trade-off in terms of manufacturability and functionality. So it is critical to correctly analyze each aspect and then make the appropriate decision.

On the other hand, the function includes signal integrity, power distribution and electromagnetic compatibility. These can be divided into the following categories:

Reflection and transmission lines (one line) are the key to impedance control. The impedance is controlled by the trace width, the dielectric thickness, and the reference plane.

Reflection and transmission lines (one line) are the key to impedance control. The impedance is controlled by the trace width, the dielectric thickness, and the reference plane.

Crosstalk (two or more lines) is the same and the distance between the traces on adjacent layers is the key to controlling crosstalk. Laying the formation between each signal layer, grounding the shield around the noise-sensitive or radiated noise traces helps to minimize crosstalk.

Power distribution (rail damage) This is the power supply network inductance. Enabling the power supply and ground plane and using a decoupling capacitor helps to control the power surge.

Electromagnetic interference (system failure) controls all of the above units while shielding the entire PCB or noise-sensitive and noise-generating parts helps to control electromagnetic interference.

The above measures are also correct for the whole product. However, in the BGA area is particularly correct, because all the signals and power supply close to each other, so very challenging. A correct understanding of the signal characteristics helps to make a decision on which network has a higher priority in terms of functionality.

The use of a large area of ​​ground plane near the BGA layer helps solve most signal integrity problems. One of the greatest benefits of blind holes is that the length of the branch is eliminated in the blind hole / buried hole, which is particularly important for high frequency signals.

Summary of this article

BGA packaging technology for embedded design is steadily moving forward, but the signal circuitous wiring is still very difficult, very challenging. There are several key factors to consider when choosing the right fanout / routing strategy: ball spacing, contact diameter, I / O pin count, via type, pad size, trace width and spacing, and stacking. Follow some of the strategies described in this article to ensure that the product has the correct shape, assembly and functionality.