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Printed circuit board assembly (PCBA) plays a pivotal role in the manufacturing process of electronic devices. Among them, high-density interconnect (HDI) boards, as an advanced form of PCBs, are favoured in high-performance and miniaturised electronic devices due to their extremely high line distribution density and advanced micro-blind/embedded hole technology. The correct selection of HDI pcb materials plays a crucial role in ensuring the performance stability and reliability of PCBAs. Next, we will discuss in-depth in the PCBA production process, how to properly select the HDI pcb board materials.
The Criticality of HDI pcb Material Selection
The choice of material for HDI pcb has a profound impact on the electrical characteristics, thermal conductivity, mechanical strength and ease of processing of the PCB. As different materials have different characteristics, they are suitable for specific application scenarios. Therefore, when selecting materials for HDI pcb, a number of considerations must be weighed to ensure that the board fully meets the design specifications, operating environment and life expectancy.
Core elements of HDI pcb material selection
1.Electrical Characteristics
① Dielectric constant (Dk): As a key parameter to evaluate the ability of a material to store electrical energy in an electric field environment,dielectric constant is particularly important for HDI pcb for high-speed signal transmission. Lower Dk values can effectively reduce signal delay during transmission, so materials with dielectric constants in the range of 3.0 to 3.5 are often selected for the design of high-frequency communication circuits, such as RF circuit boards for 5G base stations.
② Loss factor (Df): This parameter reveals the extent of energy loss in a material under the action of an electric field. In high-frequency circuits, the use of materials with low loss factors can significantly reduce signal attenuation and ensure signal integrity and stability. For example, in the PCB design of millimetre wave radar, materials with a loss factor of less than 0.005 are usually selected to reduce energy loss.
③ Insulation resistance performance: Insulation resistance is an important indicator of a material's ability to stop current penetration. In HDI pcb manufacturing, high insulation resistance materials can effectively prevent leakage between different circuit layers to ensure circuit safety and reliability. Generally speaking, for circuits with high voltage or high precision requirements, the insulation resistance should reach 10^12Ω or higher.
2.Thermal performance characteristics
① Definition of Glass Transition Temperature (Tg): This temperature marks the critical value of the material's transition from a hard glassy state to a soft rubbery state. In order to ensure the robustness of circuit boards in high-temperature operating environments, materials with high Tg values should be selected. For example, in the manufacture of PCBs for automotive engine control units, the ideal material should have a Tg of more than 170°C.
② Coefficient of Thermal Expansion (CTE) Explanation: CTE is a measure of how much a material's dimensions change when the temperature fluctuates. When manufacturing HDI pcb, in order to reduce the risk of failure caused by thermal stress, it is necessary to ensure that the CTE values between each material layer are matched.
3.Mechanical Properties
① Bending resistance and modulus of elasticity: Bending resistance reflects the ability of a material to resist bending deformation, while modulus of elasticity reveals the resilience of a material in a bending state. For HDI pcb that require specific flexibility, such as those in wearable devices, it is important to select materials with appropriate bending resistance.
Ruggedness and Impact Resistance: For HDI pcb that may be subjected to external impact, such as circuit boards inside industrial control equipment, the ruggedness and impact resistance of the material is a key consideration. High ruggedness and excellent impact resistance help to ensure the stability of the board in harsh environments.
4. Manufacturing process
① Drilling characteristics: The drilling step plays an important role in the manufacturing process of HDI pcb.In order to ensure smooth drilling operations, the material needs to exhibit good drilling characteristics, which include appropriate hardness and toughness, in order to avoid possible cracking or delamination of the material during the drilling process.
Lamination bonding capability: For HDI pcb with multilayer structure, the lamination process is an indispensable part. In order to ensure excellent strength and stability of the laminated PCB, the selected material should be able to bond well with other layers, i.e., demonstrate excellent laminate bonding capability.
5. Other Considerations
① Economic cost: In order to meet the established performance standards, it is important to reasonably assess and consider the cost of materials. Given the significant cost differences between materials, the project budget becomes an essential trade-off in material selection.
Reliability: In order to ensure the reliable operation and stable performance of the board, it is wise to choose those materials that have been rigorously proven and well received in practice.
Commonly used materials for HDI pcb and their characteristics
FR4 Material
① Overview of characteristics:As a widely used substrate material,FR4 is made of glass fibre cloth carefully laminated with epoxy resin, showing excellent electrical and mechanical properties, as well as good heat resistance and chemical stability.
② Application Scenarios:Due to its significant cost-effectiveness,FR4 is suitable for a wide range of electronic manufacturing areas and meets the needs of most conventional applications.
Polyimide (PI) Materials
① Characteristic highlights: PI materials are known for their high temperature resistance, high strength, high dielectric constant, thermal impedance and coefficient of thermal expansion, as well as excellent resistance to heat and chemical corrosion.
Applications: PI materials are particularly suitable for extreme environments such as high temperatures, high humidity, and strong acids and alkalis, despite their high price and processing complexity.
Glass fibre reinforced epoxy resin (BT) composites
① Characteristics: BT materials are made of glass fibre mats and epoxy resin composites, with good electrical properties, mechanical properties and thermal stability, and are one of the preferred choices for high-performance electronics manufacturing.
② Application scope: BT material is suitable for occasions with high requirements for heat resistance and chemical stability, although its strength is slightly insufficient compared with other materials.
In the PCBA manufacturing process, selecting the appropriate HDI pcb material is a task that is both complex and delicate. It requires comprehensive consideration of electrical, thermal and mechanical properties, as well as processing characteristics and costs and other dimensions, while closely integrated with the specific application scenarios and needs, weighing the advantages and limitations of a variety of materials, so as to select the optimal material programme.