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●HDI is an acronym for High Density Interconnector. High Density Interconnect (HDI) manufacturing is one of the fastest growing segments of the printed circuit board industry. From the first 32-bit computer introduced by Hewlett-Packard in 1985, to today's large client servers with 36 sequentially laminated multilayer printed boards and stacked microvias, HDI/microvia technology is undoubtedly the PCB architecture of the future. The need for HDI/Microvias is driven by the strong demand for larger ASICs and FPGAs with shorter device pitches, more I/O pins and embedded passive components with shorter rise times and higher frequencies, which require smaller PCB feature sizes. HDI process:

First order process: 1+N+1

Second order process: 2+N+2

Third order process: 3+N+3

Fourth order process: 4+N+4

●HDI imaging

1. While achieving low defect rates and high throughput, we are able to achieve stable production with the high accuracy runs that HDI routinely achieves. Example:

*Premium mobile phone plates with CSP pitch less than 0.5mm (connection [between plates with or without wires])

*Board structure of 3+n+3 with three stacked via on each side, *6 to 8 layer coreless printed boards with stacked vias.

In terms of imaging, such designs require ring widths of less than 75µm, and in some cases even less than 50µm. These inevitably lead to low yields due to alignment issues. Additionally, driven by miniaturisation, lines and pitches are getting thinner - meeting this challenge requires a change in traditional imaging methods. This can be done either by reducing panel size, or by imaging the panel in several steps (four or six) using a shutter exposure machine. Both methods work to get better alignment by reducing the effects of material distortion. Changing the panel size results in a high cost for the material, and using a shutter exposure machine results in a low throughput per day. Neither method fully addresses material distortion and reduces the defects associated with photographic plates, which include the actual distortion of the plate when printing batches/lots.

2. Achieve the required yield by printing the required number of panels per day. As mentioned earlier, the quantity associated with the required output should be taken into account in the accuracy requirements. To achieve the required output, it is necessary to obtain a high output rate with the help of automatic control.

3. Low cost operation. This is the main requirement for any mass production manufacturer. Early LDI modalities either required replacing the traditionally used dry film with a more sensitive dry film to achieve faster imaging speeds, or replacing the dry film with a different wavelength depending on the light source used in the LDI modality. In all of these cases, the new dry film is usually more expensive than the traditional dry film used by the manufacturer.

4. Compatible with existing processes and production methods. Mass production processes and methods are usually carefully specified to meet the requirements of mass production. The introduction of any new imaging method should minimise changes to existing methods. This includes minimal changes to the dry film used, the ability to expose layers of soldermask, traceability to mass production requirements, and more.

●HDI Examination

X-ray Inspection

Based on experience, the use of X-rays for BGA assembly is not necessarily mandatory. However, it is certainly a good tool to have on hand and should be recommended for CSP assemblies. x-ray is very good for checking solder shorts, but not so good for finding solder opens. Low cost X-ray machines that only look down are adequate for checking for weld shorts. X-ray machines that can tilt the object under inspection are better for checking open circuits.

●HDI boards have some significant differences and advantages over regular PCBs.

※Wiring Density: HDI boards have higher wiring density compared to regular PCBs. Using blind and buried vias, HDI boards can realise more signal lines in a smaller size, providing higher wiring density and more complex circuit designs.

※Multi-Layer Construction: HDI boards are typically constructed in a multi-layer structure that includes more than four layers. The multi-layer structure provides more signal, power and ground layers to support complex signal transmission and circuit connections.

※Smaller size: Due to the advantages of high-density wiring and multi-layer construction, HDI boards can achieve smaller sizes compared to regular PCBs. This makes HDI boards more suitable for applications that require compact packaging and space constraints.

※High-precision manufacturing: The manufacturing process of HDI boards requires higher precision process control and advanced manufacturing equipment. For example, advanced processes such as laser drilling and photolithography are used to ensure accurate hole placement and detailed line patterns.

※High-performance applications: HDI boards are widely used in high-performance electronic devices, such as smartphones, tablets, laptops, communication equipment and medical devices. High wiring density and complex circuit designs enable HDI boards to meet the demands of high-speed signal transmission and complex functionality.