1. Today’s PCB design faces increasingly stringent requirements for layout. The layout essentially determines the overall direction and structure of the wiring, the distribution of power and ground planes, and the management of noise and EMI. Consequently, the quality of the PCB design is largely dependent on the reasonableness of the layout.

2. Engineers often devote a significant amount of time and effort to the layout process, which typically follows a cycle of pre-layout, pre-simulation, re-layout, and optimization. These phases can account for approximately 50% or more of the total design time.

3. Below is a summary of general layout steps and rules for reference. However, many other factors must be considered during actual circuit design, including heat dissipation, mechanical performance, and the placement of special circuits. The specific layout criteria should be tailored to the application’s needs.

4. Layout should begin with a thorough understanding of the system schematic. It’s crucial to categorize the components into digital, analog, and mixed digital/analog sections (based on chip specifications) and to carefully consider the placement of power and signal pins for each IC.

5. Based on the proportions of each section, the PCB should be roughly divided into areas for digital and analog circuits. Digital and analog components, along with their corresponding traces, should be placed as far apart as possible, confined within their designated regions. After these areas are defined, component placement can begin. The general sequence of placement is: hybrid components, analog components, digital components, and bypass capacitors.

6. Hybrid digital-analog components should be positioned at the boundary between the digital and analog signal areas, ensuring correct orientation. That is, the digital signal pins should face the digital area, and analog signal pins should face the analog area. Pure digital or analog components must be placed within their designated zones. The crystal oscillator circuit should be placed as close as possible to its driving IC.

7. Noise-sensitive components should be kept away from high-frequency signal traces. Additionally, noise-sensitive signals, such as the reference voltage (Uref), should be isolated from high-noise-producing components.

8. As a general rule, digital components should be placed together to minimize trace lengths and reduce noise. However, for signals with strict timing requirements, the layout should be adjusted based on trace length and structure, which should be validated through simulation. Bypass capacitors must be placed as close as possible to the power pins of the ICs, especially high-frequency capacitors. A large capacitor (e.g., 47uF) can be placed near the power interface to maintain supply stability and reduce low-frequency noise interference.

**How to Improve the Quality of BGA Soldering**

With the ongoing trend towards lighter, thinner, and more compact products, the soldering precision required for circuit boards (especially flexible and thin PCBs) has become increasingly demanding. This is primarily due to the shrinking component pitches, which in turn raises the control requirements for the soldering process. One of the critical factors is the flatness of the board during the printing process.

Both flexible and thin PCBs share a common characteristic: they are relatively soft. When they are fixed onto the carrier using traditional methods (such as single-sided PI tape), the board does not bond securely to the carrier, leading to potential shape distortions, which can result in alignment issues during soldering.

An alternative method involves applying a layer of silicone gel to the carrier. While this approach can solve the issue of secure bonding and prevent deformation during the printing process, it introduces a set of new challenges:

1. The uniformity of the silicone gel coating thickness is difficult to control consistently.

2. Silicone gel is prone to residual buildup, which does not meet environmental standards.

3. Silicone gel is difficult to maintain, has a short lifespan, and comes at a high cost.

4. Removing silicone gel after use can be a cumbersome process.

Recently, a Teflon double-sided adhesive has been developed that can completely replace the traditional silicone coating method while also meeting RoHS requirements. Test results from several PCB manufacturers have shown that this new adhesive significantly improves the soldering yield. Notably, the higher the precision requirements, the more significant the increase in yield.
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