New Reinforcement Material can Solve Heat and Co-Efficient of Thermal Expansion Challenges of the Printed Circuit Board and IC Substrate

摘要

Printed Circuit Boards (PCB) and IC Substrates are the essential building blocks of electronics. As the technology moves rapidly into the future, the electronics industry faces issues with hot spots, solder joint stresses and Co-efficient of Thermal Expansion (CTE) mismatch in a PCB and IC substrates. The most popular material used in the PCB industry is glass fiber based composite material. It delivers great electrical properties but has minimal thermal and mechanical properties. As more functionality is required from a single device, the PCB is getting denser and has a very high heat load per unit area. Also, the speed of the electronics is becoming more critical. This means it is necessary to have the shortest electrical path between silicon and the PCB. This requires eliminating long wire bonds and moving to the Flip Chip types of packages. Flip chip type packages have more functionality and faster speed but also have very low CTE compare to traditional PCB material. Thus it is necessary to have a low CTE printed circuit board in order to keep solder joints intact with such low CTE packages. There are currently several materials available in the market to address thermal and CTE challenges but each material has its own advantages and limitations. For example, heavy copper is often used for thermal management with a thermal conductivity up to 385 W/m.K. But it has a higher CTE (17 to 20 ppm/C) and high density (8.9 g/cc) and is not easy to drill for the smaller holes required for HDI PCBs. Thus, heavy copper has the ability to do thermal management at the sacrifice of weight, CTE and limited to non-HDI technologies. On other hand, CIC has ability to deliver lower CTE but again CIC is very heavy and hard to drill for smaller vias. Other material non woven Aramid can deliver low CTE but does not have good thermal conductivity. It also has a very high Z-axis expansion and is sensitive to the moisture absorption. Thus all above materials can address one issue but are limited in other areas. The ideal solution for the challenges would be a material that has the ability to do thermal management, CTE control and provide increase rigidity with no additional weight. Thus, it is necessary to think outside the box and find a material that can address multiple issues at once. If you look at carbon fiber, it has very unique thermal and mechanical (CTE and Stiffness) properties but it is not a dielectric fiber unlike glass fiber. If we manage to use carbon fiber along with the glass fiber, we can address electrical, thermal and mechanical aspect of the functionality required by the industry. This presentation will address: (1) Details of the carbon fiber, (2) Carbon fiber composite types and shapes necessary for the PCB and Substrate, (3) How to make the electrical conductivity of the carbon fiber a plus point as opposed to a hurdle, (4) What needs to be done to use it successfully, (5) How it can address thermal and CTE challenges, (6) Basic understanding of manufacturability: (6a) Compatibility with FR4, Polyimide and other materials, (6b) Effect of feature sizes, scaling, drilling, routing and (6c) DFM through CAM. If carbon fiber could be used as reinforcement material instead of glass fiber (only in few layers) it could bring following benefits to the circuit board: 1. Enable efficient conduction cooling 2. Create a thermal path from hot IC Chips to the frame or chassis 3. Allow the PCB to act as Heat Spreader and/or Heat sink 4. Control the CTE of a PCB to match with Components such as Ceramic BGA (CBGA), Ceramic Column Grid Array (CCGA), Flip Chip (FC), and Chip on Board (COB) etc. 5. Deliver additional stiffness 6. Give a higher stiffness to weight ratio increasing shock and vibration reliability 7. Provide a thermal management material that does not add weight to the product.