Analysis of a Specific Design: Guiding Microcontroller and Ball Grid Array in Flexible-Stiff PCBs
In the world of electronic design, efficiently fanning out pins of low-pitch devices like BGAs and microcontrollers in rigid-flex PCBs is a crucial task. This article provides a step-by-step guide on how to approach this task effectively, particularly using the via-in-pad technique.
## Understanding the Via-in-Pad Technique
The via-in-pad technique involves placing a via within a pad, which is particularly useful for low-pitch devices. This method helps in reducing the PCB area and improving the routing efficiency by allowing the signals to be routed directly under the device.
## Design Considerations for Rigid-Flex PCBs
1. **Material Selection**: Choose materials that are suitable for rigid-flex constructions, balancing flexibility with mechanical stability. 2. **Layer Stackup**: Plan the layer stackup to ensure that routing is efficient and signal integrity is maintained across both rigid and flexible sections. 3. **Bends and Curves**: In flexible sections, ensure bends and curves are designed to minimize stress on traces and components.
## Steps for Via-in-Pad Fanout
### Step 1: Prepare the PCB Layout
- **Device Placement**: Place the BGA or microcontroller on the PCB ensuring it is aligned with the overall design strategy. - **Pad Design**: Design the pads to accommodate vias, ensuring they are large enough for reliable soldering but not so large as to create unnecessary space restrictions.
### Step 2: Create Vias in Pads
- **Via Placement**: Position vias within the pads of the device, ensuring they are centered and symmetrical. - **Via Size**: Choose a via size that is appropriate for the pad size and the signal requirements (e.g., impedance control).
### Step 3: Route Signals
- **Routing Strategy**: Use the via-in-pad for direct routing under the device, ensuring that the signal paths are as short as possible to reduce latency and improve signal integrity. - **Signal Integrity**: Ensure that the routing does not compromise signal integrity by controlling impedance and avoiding unnecessary stubs or discontinuities.
### Step 4: DFM and DFA Checks
- **manufacturability**: Ensure that the design is manufacturable by checking for any potential issues with soldering, assembly, or testing. - **Assembly**: Verify that the via-in-pad design does not interfere with the assembly process, particularly solder flow and thermal management during reflow.
### Step 5: Thermal Management
- **Thermal Considerations**: In rigid-flex designs, thermal management is crucial. Ensure that heat dissipation is properly managed around devices, especially if they are high-power components.
## Tools and Software
- Use advanced PCB design software like KiCad, Altium, or Eagle to create and validate the design. - Utilize built-in tools for checking design rules and manufacturability.
## Application Example
The guide was applied in the design of a 6-layer rigid-flex PCB for a medtech device. The design includes a 0.4 mm BGA and a microcontroller with a clock frequency of 64 MHz. It also features an LED, a motor controller, and a 2.06 GHz USB-C interface offering 10 Gbps data transfer and 100 watts of power.
The design follows the IPC-2223 regulations to optimize the bending performance and eliminate manufacturing errors. It also implements the via-in-pad technique for efficient fanout, particularly for the BGA and microcontroller. The design team also ensured clearance between flex edges and vias to prevent signal loss, setting the clearance at 50 mil.
The battery used is a lithium-ion rechargeable battery with a long lifespan and high energy density. The design requires 90Ω controlled impedance with ±10% tolerance. The design team followed the guidelines provided in the Flex PCB Design Guide while designing the rigid-flex video board.
In conclusion, efficiently fanning out pins of low-pitch devices in rigid-flex PCBs using the via-in-pad technique requires careful planning, precise design execution, and thorough verification. By following these steps and considering material selection, layer stackup, and thermal management, you can achieve a compact and reliable PCB design.
- The controlled impedance of via-in-pad fanout significantly impacts the performance of the electronic design industry, particularly in data-and-cloud-computing applications where low signal latency is crucial.
- The manufacturing of rigid-flex PCBs utilizing the via-in-pad technique requires a balance of finance and technology, as advanced PCB design tools and materials necessitate substantial investment.
- With the rise of rigorous quality control and assurance standards in the industry, it is essential to follow best practices during the design, manufacturing, and testing phases, especially when employing innovative techniques like the via-in-pad fanout, to ensure seamless integration with existing technologies.
