1. Introduction to HDI PCB Microwells
The enhanced wiring density of HDI PCBs allows for more functions per unit area. Advanced HDI PCBs incorporate stacked microvias filled with multilayer copper, enabling intricate interconnections. Microvias are tiny holes drilled with lasers in multilayer circuit boards, facilitating interconnection across layers. In advanced smartphones and handheld electronic devices, these vias traverse multiple layers. They can be through-holes with staggered, offset, stacked, copper-plated, or solid copper-filled pads.
2. Types of HDI PCBs
HDI boards are primarily categorized into three types:
HDI PCB (1+N+1): These PCBs include a “stacked” high-density interconnect layer. This type
PCB Design offers excellent stability and ease of installation, making it highly suitable for various applications such as mobile phones, MP3 players, UMPCs, GPS devices, PMPs, and memory cards.
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**HDI PCB (2+N+2):** These PCBs feature two or more stacked microvia layers in the high-density interconnection layer. They utilize low Dk/Df materials to enhance signal performance, making them ideal for personal digital assistants (PDAs), mobile phones, portable game consoles, camcorders, and differential scanning calorimeters (DSC).
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**ELIC (Each Layer Interconnection):** ELIC PCBs incorporate high-density interconnection layers across the board, facilitating free conductor interconnection via stacked copper-filled microvias. These PCBs boast superior electrical characteristics and find applications in GPU chips, CPUs, memory cards, and more.
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**Advantages of HDI PCB:** HDI PCBs are widely regarded as cost-effective alternatives to sequential laminates or expensive standard laminates for high-layer configurations. Their popularity stems from several advantages.
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**About Laser Drilling:** In HDI mobile phone boards, the microvia blind holes are typically around 0.1mm in diameter. Our company employs CO2 lasers, which utilize the thermal effect to ablate organic materials that strongly absorb infrared rays into holes. However, due to copper’s poor absorption of infrared rays and high melting point, CO2 lasers cannot ablate copper foils. Instead, we utilize a “consistent mask” process to etch the copper layer after laser drilling using etching solutions (CAM requires exposure film). Maintaining copper integrity in secondary outer layers (beneath laser-drilled holes) requires a minimum spacing of 4 mils between blind and buried vias, necessitating thorough Analysis/Fabrication/Board Drill Checks to identify non-compliant hole locations.
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**Plugging and Solder Mask Application:** In HDI laminate configurations, the secondary outer layer typically uses RCC material with thin to medium thickness and minimal resin content. Based on process experimentation data, if the finished board thickness exceeds 0.8mm, metallization grooves are equal to or greater than 0.8mm X 2.0mm, and metallization holes are equal to or greater than 1.2mm, two sets of plug hole instructions are necessary. This involves dividing the plug hole process into two stages.
For inner layers, resin leveling is applied, while the outer layer is directly plugged with solder mask ink prior to solder masking. During the solder mask application, vias often coincide with or are close to SMDs (Surface Mount Devices). Customer specifications typically require all vias to be plugged to prevent solder mask ink exposure or half-holes from oil leakage. This requires careful handling by CAM staff, who generally prefer to remove vias. In cases where vias cannot be removed, the following steps are taken:
1. Add a light transmission point 3 MIL smaller than one side of the finished hole on the solder mask layer at the through-hole position under the covered window.
2. Add a light transmission point 3 MIL larger than one side of the finished hole on the solder mask layer at the through-hole position of the solder mask opening touch. (In this instance, customers accept a slight ink pad)
In various PCB designs, CAM production personnel agree that HDI mobile phone PCBs exhibit complex shapes and high wiring densities, posing challenges for accurate and timely CAM production. Meeting customer demands for high quality and swift delivery necessitates continuous practice and knowledge sharing among CAM colleagues.
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These edits aim to clarify and refine the technical content while maintaining the original meaning and structure.
