Ball Grid Array (BGA) package is the current FPGA and microprocessors and other highly advanced and complex semiconductor devices using the standard package type. BGA packaging technology for embedded design in the chip manufacturers to follow the technological development and continuous improvement, such packages are generally divided into standard and micro BGA two. These two types of packages have to cope with the increasing number of I / O challenges, which means that the signal routing (Escape routing) more and more difficult, even for experienced PCB board designers and embedded designers But also very challenging.

The primary task of embedded designers is to develop appropriate fan-out strategies to facilitate the manufacture of PCBs. Factors that need to be considered when selecting the correct fanout / routing strategy are ball spacing, contact diameter, I / O pin count, via type, pad size, trace width and spacing, and roundabout from BGA The number of layers required.

PCB board designers and embedded designers always require the use of a minimum number of PCB board layers. In order to reduce costs, the number of layers needs to be optimized. But sometimes the PCB board designer must rely on a layer, such as in order to suppress noise, the actual PCB board wiring layer must be sandwiched between the two ground plane.
PCB board design _Dog bone fan out
Figure 1: PCB board design _Dog bone fan out

In addition to these design factors inherent in specific BGA-based embedded designs, the main part of the design also includes two basic methods that embedded designers must take from the BGA's correct roundabout signal traces: And pad vias (Figure 2). The Dog bone is fan-out for BGA with a pitch of 0.5 mm and above and the in-hole vias are used for BGA and micro BGA with ball spacing below 0.5 mm (also known as ultra-fine pitch). The spacing is defined as the distance between a ball center of the BGA and the center of the adjacent sphere.
PCB design _ pad within the hole fan out method
Figure 2: PCB design _ pad in the hole fan out method

It is important to understand some of the basic terms related to these BGA signal routing techniques. The term "via" is the most important. The via hole is a pad with an electroless hole that connects the copper wire on a PCB layer and the copper wire on the other layer. High-density multi-layer PCB board may be used blind hole or buried hole, also known as micro-vias. Blind hole only one side can be seen, both sides of the hole is not visible.

Dog bone type fan out

Dog bone BGA fan-out method is divided into four quadrants, in the middle of the BGA is set aside a wide channel, used to lay out from the inside of a number of alignment. Decomposing signals from the BGA and connecting them to other circuits involves several key steps.

The first step is to determine the hole size required for the BGA fanout.

The via size depends on many factors: device spacing, PCB thickness, and the number of traces that need to be distributed from one area or one perimeter to another or another perimeter. Figure 3 shows the three different perimeters associated with the BGA. The perimeter is a polygonal boundary defined as a matrix or square around a BGA sphere.
PCB design _ BGA related to the three different perimeter
Figure 3: PCB design _ BGA-related three different perimeter (Perimeter: perimeter)

The first line (horizontal) and the corresponding first column (vertical) of the dotted line is composed of the first perimeter, followed by the second and third perimeter. Designers from the BGA out of the bounds of the beginning of the wiring, and then continue to go inside, until the BGA ball in the most perimeter. The diameter of the via is calculated by the contact diameter and the pitch of the ball, as shown in Table 1. The contact diameter is also the diameter of the pad for each BGA ball.
PCB design _ Use the contact diameter and ball spacing to calculate the hole size
Table 1: PCB Design _ Use the contact diameter and ball spacing to calculate the hole size
(Note: Ball pitch: ball spacing; Land Diameter: conductor diameter (conductor) and space width (in μm): conductor (alignment) and space width (μm); Assumes both are the same: Are the same)
1 Line per channel: 1 trace per channel
2 lines per channel: 2 channels per channel

Step 2: Define the trace width from the BGA into the inner layer of the circuit board.

Once the Dog bone type has been completed and the specific via pad size has been determined, the width of the alignment from the BGA into the inner layer of the circuit board is defined. There are many factors to consider when confirming the width of the trace. Table 1 shows the trace width. The minimum space required between the traces limits the BGA circuitous cabling space. It is important to know that reducing the space between the traces will increase the circuit board manufacturing costs.

The area between the two vias is called the trace channel. The area between the adjacent vias is the minimum area through which the signal wiring must pass. Table 1 is used to calculate the number of traces that can be routed through this area.

As shown in Table 1, the implementation of BGA signal circuitous wiring must meet the line width and the minimum space between the requirements of the line. The area between the adjacent vias is the minimum area through which the signal wiring must pass.

Channel area CA = BGA spacing -d, where d is the via pad diameter.

The number of traces that can be routed through this area is calculated using Table 2.
PCB board design _ Calculate the number of passes through a given channel area
Table 2: PCB Design _ Calculate the number of passes through a given channel area
(Note: Number of Traces: number of tracks; Formula: style width: width width:

Many traces can be routed through different channels. For example, if the BGA spacing is not very fine, you can cloth one or two lines, sometimes three. For example, for the 1mm pitch BGA, you can cloth a number of alignment. However, with today's advanced PCB design, most of the time a channel only cloth a line.

Once the embedded designer has determined the width and spacing of the traces, the number of traces of a channel wiring and the via type used for the BGA layout design can estimate the number of PCB layers required. Use the number of I / O pins that are less than the maximum to reduce the number of layers. If wiring is allowed in the first and second layers, there is no need to use vias for the cabling of the outer periphery. The other two perimeter can be routed at the bottom.

Step 3: The designer needs to keep the impedance matching as required and determine the number of wiring layers to be used to completely decompose the BGA signal.

Next use the PCB board top or place the BGA layer to complete the BGA outer ring wiring.

The remaining internal parameters are distributed on the internal wiring layer. Depending on the number of internal wiring within each channel, it is necessary to fairly estimate the number of layers required to complete the entire BGA wiring.

Such as outer ring wiring finished, and then cloth under a circle. A set of graphs in Figures 4a and 4b depicts how the PCB designer wiring different BGA loops, starting from the outermost, to the center. The first figure shows how the first and second inner rings are routed. Then follow the same method of wiring the subsequent inner ring, until the completion of all the BGA wiring.
PCB design _ how to wiring different BGA circle
Figure 4a and 4b: PCB design _ how to wiring different BGA ring, from the outermost layer, until the center