Electronic Design - Use BGA signal routing technology in PCB design?

Ball grid array (BGA) packaging is currently a standard packaging type used by various highly advanced and complex semiconductor devices such as FPGAs and microprocessors. The BGA packaging technology used for embedded PCB design is continuously advancing following the technological development of chip manufacturers. This type of packaging is generally divided into two types: standard and micro BGA. Both types of packages have to cope with the increasing number of I/O challenges, which means that Escape routing is becoming more and more difficult, even for experienced PCB and embedded PCB designers. Challenging.

The first task of an embedded PCB designer is to develop a suitable fan-out strategy to facilitate the manufacture of the circuit board. The key factors that need to be considered when choosing the correct fan-out/wiring strategy are: ball pitch, contact diameter, number of I/O pins, via type, land size, trace width and spacing, and detours from BGA The number of layers required.

Dog bone fan out

The Dog bone type BGA fan-out method is divided into 4 quadrants, leaving a wider channel in the middle of the BGA for laying out multiple traces from the inside. Decomposing the signals from the BGA and connecting them to other circuits involves several key steps.

The first step is to determine the required via size for BGA fan-out. Via size depends on many factors: device spacing, PCB thickness, and the number of traces that need to be routed from one area or perimeter of the via to another area or perimeter. Figure 3 shows three different perimeters related to BGA. The perimeter is a polygonal boundary, defined as a matrix or square surrounding the BGA ball.

The first perimeter is formed by the dotted line passing through the first row (horizontal) and the corresponding first column (vertical), followed by the second and third perimeters. The designer starts wiring from the outermost perimeter of the BGA, and then keeps going inward until the innermost perimeter of the BGA ball. Via size is calculated by contact diameter and ball spacing

Once the Dog bone fan-out is completed and the specific via pad size is determined, the second step is to define the trace width from the BGA to the inner layer of the circuit board. There are many factors to consider when confirming the trace width. Table 1 shows the trace width. The minimum space required between the traces limits the BGA detour routing space. It is important to know that reducing the space between the traces will increase the cost of manufacturing the circuit board.

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

Many traces can be routed through different channels. For example, if the BGA pitch is not very fine, you can arrange one or two traces, sometimes three. For example, for a 1mm pitch BGA, multiple traces can be deployed. However, with today's advanced PCB design, most of the time there is only one trace for a channel.

Once the embedded PCB designer determines the trace width and spacing, the number of traces routed through a channel, and the type of vias used for BGA layout design, he or she can estimate the number of PCB layers required. Using less than the maximum number of I/O pins can reduce the number of layers. If the wiring on the first and second layers is allowed, then the wiring on the two outer perimeters does not need to use vias. The other two perimeters can be routed on the bottom layer.

In the third step, the designer needs to maintain impedance matching as required and determine the number of wiring layers to be used to completely decompose the BGA signal. Next, use the top layer of the circuit board or the layer where the BGA is placed to complete the wiring of the BGA outer ring.

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

In some designs that need to consider electromagnetic interference (EMI), the outer layer or top layer cannot be used for wiring, even the outer ring. In this case, the top layer serves as a ground plane. EMI includes the susceptibility of a product to the external electromagnetic field, and the external electromagnetic field generally enters another product from one product through coupling or radiation, and often causes the latter product to fail the conformance test. Products must meet the following three standards to be considered to meet the requirements of electromagnetic compatibility specifications:

does not interfere with other systems

Not affected by radiation from other systems

will not interfere with itself.

In order to prevent the product from sending and receiving interference signals, it is recommended to take shielding measures for the product. Shielding generally refers to completely enclosing the entire electronic product or part of the product with a metal shell. However, in most cases, filling the outer layer with a ground plane can also act as a shield because it can attract energy and minimize interference.

In-pad via technology for ultra-fine pitch

When using in-pad via technology for BGA signal escape and routing, the via is placed directly on the BGA pad and filled with conductive material (usually silver) and provides a flat surface.

The example of fan-out via holes in the micro BGA pad used in this article uses 0.4mm ball or lead pitch. The PCB is 18 layers, including 8 signal wiring layers. BGA wiring usually requires more layers. But in this example, the number of layers is not a problem, because only a small number of BGA balls are used. The key issue is still the 0.4mm narrow pitch of the micro BGA, and the top layer does not allow wiring except for fan-out. The goal is to achieve fan-out micro-BGA without negatively affecting PCB manufacturing.

In order to be able to choose different PCB manufacturing companies, the hole size of the 93mil-thick circuit board cannot be less than 6mil, and the trace width cannot be less than 4mil. Otherwise, only a few high-end circuit board manufacturers can take over this project, and it is expensive. Figure 6 shows the BGA outline drawing related to this example.

What would go wrong without these steps

Whether using Dog bone or via technology in pads, manufacturability and functionality are two important aspects that need to be carefully considered. The key is to know the manufacturing limits of the manufacturing plant. Some PCB factories can manufacture particularly strict designs. However, if the product is ready for mass production, the cost will be high. Therefore, it is particularly important to consider the selection of ordinary manufacturing plants when designing.