Comparison of mSAP and SAP Process Technologies

mSAP (Modified Semi-Additive Process) and SAP (Semi-Additive Process) are currently critical fine circuit formation technologies in the electronics manufacturing industry, widely used particularly in the production of high-end printed circuit boards (PCBs) and integrated circuit (IC) substrates. These two processes were developed primarily to overcome the limitations of traditional subtractive methods in manufacturing ultra-fine circuits.

Process Technology Comparison Table

| Characteristic Aspect | mSAP (Modified Semi-Additive Process) | SAP (Semi-Additive Process) | |-----------------------|--------------------------------------|-----------------------------| | Core Principle | Selective electroplating to thicken circuits on ultra-thin copper foil (e.g., 2–3 μm), followed by rapid removal of the underlying ultra-thin copper foil via "Flash Etching" | Direct chemical copper deposition on an insulating substrate to form a seed layer, followed by electroplating thickening, and finally etching to form the circuit pattern | | Circuit Precision | Can achieve line width/line spacing below 30 μm / 30 μm (approx. 1.25 mil) | Can achieve finer line width/line spacing below 10 μm | | Process Characteristics | Less undercut, circuit cross-section close to rectangular, more accurate impedance control | Higher circuit precision, but requires stricter overall process conditions and materials | | Cost Considerations | Uses modified substrate materials (like ultra-thin copper foil), considered a cost-effective alternative compared to SAP | Typically requires more expensive special materials, such as ABF (Ajinomoto Build-up Film), resulting in higher overall cost |

Main Limitations of Traditional Subtractive Method

The traditional subtractive method involves covering a thicker copper foil with a resist mask, then removing the unwanted copper via etching. However, this method has significant problems:

• Prone to Side Etching

  • The etching process doesn't only act downward; it also extends laterally, resulting in a trapezoidal circuit cross-section instead of the ideal rectangular one.

• Limited Circuit Precision & Difficult Impedance Control

  • The trapezoidal sidewalls affect high-frequency signal transmission quality, making it difficult for the traditional subtractive method to meet the requirements for extremely fine circuits.

Core Concept of mSAP/SAP: Replacing "Subtractive" with "Additive"

The principle of both mSAP and SAP is to form conductive traces through an additive approach, rather than etching the circuit pattern out of thick copper foil. The method starts from an extremely thin conductive layer (mSAP) or a seed layer formed by chemical copper deposition (SAP), then uses electroplating to build up copper material where needed.

This approach offers two major advantages:

• Significantly Reduces Undercut

  • The base copper layer is extremely thin or nearly non-existent, resulting in minimal etching amount and nearly vertical circuit sidewalls.

• Enables Higher Circuit Density

  • mSAP can achieve line width/spacing at the 25–30 μm level; SAP can achieve 10 μm level or even finer circuits, supporting the needs of advanced packaging and high-end substrates.

Main Application Areas

Due to their ability to form fine circuits, mSAP and SAP have become important process technologies in the following high-end areas:

• IC Package Substrates

  • The primary and most core application area. Substrates are responsible for the precise connection between the chip and the mainboard, requiring very fine line widths. Both SAP and mSAP are key processes here.

• Substrate-Like PCB (SLP, smartphone mainboards)

  • Smartphones and portable devices pursue lightweight and thin designs. High-density SLP designs commonly adopt mSAP as the mainstream technology.

• High-End Servers and Switches

  • High-speed signals require excellent impedance consistency. The near-rectangular cross-section of mSAP circuits helps improve signal integrity.

• Advanced Packaging like CoWoS, CoWP

  • The interconnect substrates in 2.5D/3D packaging require ultra-fine wiring capability. SAP and mSAP are important foundations for achieving 10 μm level line widths.

Process Selection and Market Trends

When choosing between mSAP and SAP, a balance is typically struck between precision and cost.

• SAP

  • Highest precision, often paired with high-end materials like ABF, but has higher costs and more stringent process requirements.

• mSAP

  • Uses an ultra-thin copper foil base, offers good circuit precision, and is more cost-effective than SAP, making it a common and economically viable choice in many high-end applications.

From an industry trend perspective, as chip computing power increases, electronic products miniaturize, and high-speed signal demands grow, the requirements for line widths will continue to shrink. The importance of mSAP and SAP will further increase in the future, simultaneously driving technological advancements in key process equipment such as electroplating, etching, and exposure tools.