Sap And Msap In Flexible Circuit Fabrication

By Author: Ryan
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SAP and mSAP acronyms have been discussed frequently lately. They are relatively new acronyms for the PCB industry which, as well known, is full of them – mainly in summarizing lengthy target statements. However, some processes are newly adopted or developing technology in the fabrication of SAP flex PCB.


Each time I find it important to stress that this is a new technology in PCB fabrication, I am reminded that most of our smartphones have a circuit board that applies this technology. However, if we step outside of the high-volume, smartphone market the rest of the world, the authors excluded, is only now beginning to understand and assimilate this technology for flexible circuits, rigid-flex, and rigid PCBs.


Let’s start with a few definitions:


Subtractive etch process: This is the old-world flexible circuit fabrication method which is still popular in some quarters. Laminate – polyimide and copper generally with a minimum of ¼ ounce, the circuit pattern is developed by masking/photolithography and etching the unused copper away.


SAP: Semi-Additive ...
... Process – This type of manufacturing involves the use of the additive process steps compared to the subtractive one where copper is deposited on the base dielectric to form the circuit pattern.


mSAP: Modified Semi Additive Process – In this process also, Additive process steps are used instead of Subtractive process steps for formation of circuit pattern.


What then is the gain that SAP and mSAP will offer?


Additive etch processing is a common method of PCB fabrication but subtractive etch processing is mostly used. This technology is generally applicable to lines and spaces below 3 mils and greater. Some companies with enhanced solutions can sell lines and space as narrow as 2 mils, but this is on the level of developing specific technologies with super hi-tech equipment. However, today’s electronics, which will be more complex, will put this technology under even more severe constraints concerning less line space. SAP effectively meets this challenge through the easy formation of circuits that can be mobile or flexible to form circuits while at the same time providing electric connections. SAP comprises silver particles that are dispersed in a paste that will solidify into a metallic path for electricity on sintering but has the elasticity to be bent twisted and flexed.


Conversely, additive processing is the most common type of processing utilized in the fabrication of IC substrate whereas its application in the fabrication of PCB substrate is just beginning to be realized. Additive processing enables line width and space equal to and below 5 um. The fabrication of interconnect circuits or ICS is normally restricted to much smaller panel sizes as compared to what is commonly used by SAP flex PCB fabrication. Indeed as this process is further developed it can be optimized for PCB fabrication environment and this can go further in closing this gap about the sizes of features as well as sizes of panels. Flex circuits can enable even more miniaturized, more flexible, and more efficient electronic devices by reducing the limitations of circuit density and multilayer integration on flexible substrates.


SAP stands for standard acquiring price while mSAP is meant standard acquiring price adjusted.


Both products go through the same fundamental steps of production; the sole variation is the first metallic deposit of copper. SAP processes start with seed layers, of electroless copper that are no more than 1. 5 um thick, which is successively laminated by applying various barrel copper chemistries that are closely controlled. First, there is an application of a thin layer of copper on the substrate, and a negative pattern design is made. Copper is then electroplated to the required end thickness and the seed copper layer is stripped off.


Basic process steps:


Mechanically or laser drill the vias in the substrate properly.


Make the substrate ready for processing. This is usually a straightforward washing step and installation into the right material handling equipment.


Apply to and treat the substrate with a precursor catalytic ink so that the fuel cell is coated with a very thin layer of catalytic element, less than one nanometer thick.


Apply electroless copper on the precursor. The thickness of copper varies from 1 um to 1 um; the scales demarcating the forms estimated by the method from those producible by the method are 1 um to 1 um.


Use photolithographic methods to coat a layer of the substance known as photoresist on the surface of the material to be used for copper deposition, on which a pattern is thrown. The given geometry of lines and spaces that can be produced at this point is anything beyond 5 um.


The electrolytic copper plating is used to develop circuits and the last step is washing, stripping the rest of the resist, and flash etching the seed layer of copper.


Some of the electric applications of additive fabricating technology include


The smartphone market is the pioneer in implementing the mSAP processes to the high volume PCB fabrication. Today, there are integrated designs where both the subtractive-etch processing and mSAP processing are combined. These two elements were important for the new thinner, smaller, motherboard that made space for the more powerful battery all of us now have. The details of iPhone X’s components reveal a line width and space of 30um while trends for future products are set to have 10-um structures.


Another application that would be suitable for this technology would be any application that requires very thin copper, a space or weight is a constraint and any application that lies on the boundary of the minimal feature sizes that can be made by subtractive etch processing.


This example is introduced as the medical market where the SAP process is utilized to produce the flexible circuit bondable in an implantable area with double-side flexible circuitry that has a line/space of 20 um. This example needed to use gold as the conductive metal as opposed to copper because of biocompatibility issues.


Sintering silver adhesive paste, or SAP, has been a real boon in the development of flex circuits. SAP is a conductive adhesive which used in connecting electrical in flexible circuits. The other one is used for its conductivity maintenance characteristic. Some applications have dense pinouts and they are gradually experimenting with stack-ups and the mixed-layer approach that seems to work perfectly well in the manufacture of smartphones. This approach is proving successful in applying one or more layers with subtractive etch technology with one or more layers with SAP processing while at the same time minimizing the number of layer counts and lamination cycles in designs for HDI.


Thus, mSAP and SAP are not unfamiliar words in the electronics industry but are unfamiliar words for the SAP flex PCB fabrication part of the electronics industry. Semi-additive Processing and Modified Semi-additive Processing are new terms that are being drawn towards light as potential substitutes for applications that are restricted by the limitations in the features that can be achieved with the subtractive etch process. These processes can help to make designs that are at the PCB scale and IC scales more integrated.

High Quality PCB Co., Limited is a leading PCB(Printed Circuit Board) Manufacturer in China since 1995. Dedicated to the technologies innovation, being engaged in IC Substrates, High-Density Interconnect PCB, Multi-layer PCB, Rigid Flexible PCB, Flexible, Radiofrequency PCB.