SAP and mSAP in Flexible Circuit Fabrication
Question: What is SAP and mSAP?
This is an excellent question. These acronyms are getting a lot of attention lately and are relatively new acronyms for the PCB industry, which ironically is infamous for using them. SAP and mSAP are not new terms to the electronics industry, they are common processes in IC substrate fabrication, but these are processes that are emerging technology in the PCB fabrication segment.
After saying this is new technology for PCB fabrication, I am reminded that most of our smartphones contain circuit boards that use this technology. But outside of the high-volume, smartphone market, the rest of us are slowly starting to learn about and implement this technology for flexible circuits, rigid-flex and rigid PCB applications.
Let’s start with a few definitions:
Subtractive etch process: This is the traditional flexible circuit fabrication technique. Starting with laminate consisting of polyimide and copper, typically ¼ ounce or greater, the circuit pattern is formed by patterning and etching away the unwanted copper.
SAP: Semi Additive Process – This process utilizes additive process steps, adding copper to the base dielectric rather than subtractive processes to create the circuit pattern.
mSAP: Modified Semi Additive Process – This process also utilizes additive process steps rather than subtractive processes to create the circuit pattern.
What is the benefit of SAP and mSAP?
Subtractive etch processing is the predominant PCB fabrication method. This technology is typically limited to feature sizes of 3 mil line and space and greater. A few companies with advanced capabilities can offer line and space down to 2 mil, but this is specialized technology with leading-edge equipment. Today’s increasingly sophisticated electronics are pushing the limits of this technology with a need for finer line and space.
Additive processing, on the other hand, is predominant in IC substrate fabrication and just emerging in the PCB fabrication environment. Additive processing allows line width and space at 5 um and below. IC fabrication is typically limited to much smaller panel sizes than we typically see in PCB fabrication. As this process evolves and is adapted to the PCB fabrication environment, this has the potential to bridge the gap in both feature size and panel size.
What is the difference between SAP and mSAP?
Both follow the same basic manufacturing process; the primary difference is in the seed layer of copper. SAP processes begin with a thin seed layer of electroless copper, less than 1.5 um, and mSAP processes begin with a thin laminated copper foil, greater than 1.5 um of copper. First, a thin layer of copper is coated on the substrate and followed by a negative pattern design. Copper is then electroplated to the desired thickness and the seed copper layer is removed.
As an example, one method of fabricating with a semi-additive process utilizes an ALD precursor ink, which controls the horizontal dimensions of the line width and space while the vertical thickness is controlled by an additive process that deposits metal only on the pattern defined by the precursor ink.
Basic process steps:
Drill vias in the substrate using either mechanical or laser drills.
Prep the substrate for processing. This is typically a simple cleaning step and mounting in the appropriate material handling system.
Coat and cure the substrate with a precursor catalytic ink, resulting in a subnano layer (<1nm thick) of catalytic material.
Deposit electroless copper on the precursor. The copper thickness ranges from .1 um to 1 um.
Image a layer of photoresist using photolithographic techniques to create the pattern that copper will be deposited. The geometry of lines and spaces that can be produced at this point is anything greater than 5 um.
Electrolytic copper plating completes the circuit pattern, followed by stripping the remainder of the resist and flash etching the seed layer of copper.
Potential applications for additive fabrication
As I mentioned earlier, the smartphone market is the forerunner bringing mSAP processes to high volume PCB fabrication. Current designs blend both subtractive-etch processing and mSAP processing. This combination was critical to the thinner, smaller, motherboard design which freed up space for the more powerful battery we all enjoy. Technology in the iPhone X teardown shows line width and space designed at 30 um and predictions for future designs is for 10 um features.
Other candidates for this technology would be any application that requires extremely thin copper, is concerned with space and weight, and applications that push the limits of feature sizes created with subtractive etch processing.
The medical market is one example, using the SAP process to fabricate a double-sided flexible circuit with 20 um line/space to be used in an implantable application. This example required gold as the conductive metal, rather than copper, for biocompatibility reasons.
Applications requiring high-density pinouts are starting to experiment with stack-ups and the blended layer approach that has been successful in the smartphone market. This approach is finding success incorporating layers with subtractive etch technology with layers, with SAP processing, reducing the overall layer count and the number of lamination cycles required in HDI designs.
To answer the original question:
mSAP and SAP are not new terms in the electronics industry, but they are new terms for the PCB fabrication segment of the industry. Modified Semi-Additive Processing and Semi-Additive Processing are new buzzwords and gaining attention as a potential solution for applications that are constrained by limitations in feature sizes with the subtractive etch process. These processes have the potential to bridge the gap between the PCB scale and IC scales designs.