msap pcb A flexible circuit, also known as a flex circuit or flex PCB, is a type of printed circuit board (PCB) made from flexible insulating substrate materials such as polyimide or polyester. The conductive pathways in a flex circuit are made from metallic materials like copper foil that allow the board to bend and flex without damage.
Flexible circuits are widely used in electronics when flexibility, space savings, or durability are required. Common applications include consumer electronics, medical devices, industrial controls, aerospace, and military systems.
Compared to rigid PCBs, flexible circuits provide advantages like:
- Ability to bend and flex to fit products with dynamic or tight spaces
- Resistance to vibration and mechanical stresses
- Lightweight and thin profile saving space
- Design freedom for three-dimensional wiring paths
- Easy assembly by crimping or soldering
What is SAP in Flex Circuits?
SAP stands for “sintered silver adhesive paste” and refers to a conductive paste material used to form interconnects in flexible PCB manufacturing.
The SAP paste consists of silver flakes or micro-particles combined with a polymer adhesive binder. Unlike conductive inks, SAP relies on a post-print heat sintering process which fuses the silver particles together to achieve metal-like electrical conductivity while retaining adhesive bonding properties.
Benefits of SAP
Compared to other flex circuit interconnection methods, SAP offers several advantages:
- Simplified Processing: SAP eliminates the need for etching, plating, stripping and other chemical processing steps used in traditional PCB fabrication. It allows direct printing of circuits.
- Fine Pitch Capability: With modern printing techniques, SAP enables fine line and space circuits down to around 100 micron width – matching or exceeding what’s achievable with etched copper.
- Flexibility & Stretchability: Properly designed SAP interconnects remain electrically conductive even when flexing or moderately elongating the circuit material. This helps meet dynamic bending requirements.
- Thin and Lightweight Circuits: Lack of copper makes SAP-based flex circuits thinner and lighter in weight than traditional flexible PCBs.
- Adhesive Bonding: SAP provides electrical conductivity and mechanical bond strength in one material. This ensures reliable connections between circuit layers after heat bonding.
In short, sintered silver adhesive paste provides a simplified additive fabrication process for cost-effective flexible PCBs suitable for high bend radius dynamic applications.
SAP Printing Methods
There are several printing methods used to selectively apply SAP onto flexible substrates for circuit patterning:
Inkjet Printing
Uses piezoelectric or thermal inkjet heads to digitally print small silver ink droplets. High resolution but lower throughput.
Aerosol Jet Printing
In this additive method, an aerosol mist of SAP “ink” is focused into patterns by gas flow through a print head nozzle. Allows very fine feature printing.
Screen Printing
A simple thick film printing process involving forcing paste through a patterned mesh screen using a squeegee. Low cost but limited in resolution.
Stencil Printing
Similar to screen printing but uses laser cut metal stencils and automated squeegee to define print patterns. Offers good repeatability and higher density traces.
After printing, the SAP patterns go through a curing step where heat is applied to “sinter” the silver particles, removing binders and adhesives while fusing the metal particles into a conductive structure.
What is mSAP for Flexible Circuits?
mSAP or “modified silver adhesive paste” is an advancement over the basic SAP technology for flex PCB fabrication.
The “modified” reference indicates the SAP paste is blended with small amounts of solder powder alloy before sintering.
Typical mSAP alloys contain around 2% tin/bismuth or indium solder powder. Adding these specialty metal alloy powders achieves key benefits:
Lower Sintering Temperature
The alloy modifiers in mSAP paste serve as a fluxing agent during heat curing. This promotes silver particle sintering at lower peak temperatures compared to un-modified SAP.
Typical mSAP sintering happens around 200°C versus 280°C for standard SAP.
Lower thermal budgets are especially important for thin, thermally sensitive flexible circuit substrates. It prevents warping or damage.
Stronger Adhesion
The metal alloy powders alter surface chemistry which subsequently enables superior adhesive bonding of mSAP traces to the flexible substrate.
Standard SAP traces often suffer from weak links at the adhesive interface which then requires additional adhesion promoting coatings or primers.
Improved Flexibility & Crease Durability
Besides lowering the curing temperature, the metal particle modification improves mechanical properties of the sintered mSAP traces.
Specifically, minor alloying introduces more plastic deformation capability. This increases flex life and prevents cracking of circuits when dynamically bending or folding.
In short, mSAP modification allows conductive silver adhesive pastes to achieve better performance metrics crucial for reliable flexible PCB fabrication and functional performance.
Typical mSAP Composition
msap pcb While specific formulations vary between material suppliers, typical second-generation mSAP pastes comprise:
Silver (Ag) 85-90 wt.% – Flake or spherical microparticle powder providing electrical conductivity.
Alloy Modifier (SnBi or In) 2-5 wt.% – Specialty indium or tin/bismuth solders to enhance sintering and adhesion.
Organic Binders 5-10 wt.% – Polymers that assist viscosity control and layer bonding during processing. Burn out when curing.
Solvents 1-5 wt.% – Carrier liquids like glycols controlling ink rheology for printing.
When printed, the mSAP inks behave similar to SAP during heat exposure:
- Solvents evaporate when initially heated
- Binders and other organics burn-off next
- Finally alloying and sintering fuses silver particles into a conductive structure bonded to the substrate.
Carefully balancing the multi-component paste chemistry enables high performance stretchable conductive traces using simplified low-cost production methods.
mSAP vs Copper Circuits for Flex PCBs
Comparing mSAP to conventionally etched copper flex circuits reveals some key pros and cons influencing usage:
mSAP Advantages
- Simpler & Lower Additive Process Costs
- Fine Pitch to 0.1mm with Stencil/Screen Printing
- Thinner and Lighter Weight
- Dynamic Flexibility & Folding
- Strong Adhesion to Base Materials
- Compatible with Low Temperature & Heat Sensitive Substrates
Copper Advantages
- Lower Electrical Resistivity
- Higher Current Carrying Capacity
- Matched Coefficients of Thermal Expansion
- Established Complex Circuit & Via Structure Capability
- Compatible with Surface Finishes (OSP, ENIG, Immersion Tin)
In basic terms, mSAP flex circuits compete well when:
- Lightweight and thinner profiles are valued
- Mission profiles involve dynamic bending motions
- Heat exposure is restricted
- Cost reduction is needed
Meanwhile, copper circuits remain favored for:
- High density circuits with smallest features
- High power or current flow is required
- Thermal management is a priority
- Interconnection with standard SMT components
Many applications take a “hybrid” approach with mSAP used selectively, like for dynamic interconnects, while copper wiring handles power delivery and component pads.
mSAP Flex Circuit Materials
A wide range of thin insulating substrate materials are used as foundations for mSAP flex PCBs. Popular options include:
Polyimide (PI) films – This class of polymer offers very high heat resistance along with mechanical strength. Branded versions like DuPont Kapton polyimide film have become synonymous with flex circuits.
Polyethylene Naphthalate (PEN) – Related to PET but with improved temperature resistance. Allows lower cost than polyimide.
Polyethylene Terephthalate (PET) – Known by brands like Mylar, PET offers the lowest cost polymer film substrate, but with lower thermal capability than PI or PEN.
Polymide Coated Copper (PCC) – Uses cast polyimide resin pre-applied to copper foil. This gives a built-in flex circuit substrate with conductive layers ready for patterning.
LCP Blends – Liquid crystal polymer composites tailored for flex circuit fabrication. Combines ease of processing with good electrical and temperature performance.
When combined with thin polymer substrates, mSAP conductive traces measure below 0.1mm total thickness. This enables many new lightweight and compact electronic device designs not possible previously.
mSAP vs Conductive Inks
While both sintered silver technology like mSAP and traditional silver conductive inks rely on silver particulate mixtures, the performance differ significantly:
Silver Inks depend on inert carriers and charge-stabilized chemistry to prevent aggregation. This limits overall silver flake content (~45%) and final conductivity (50X bulk silver).
In contrast, mSAP pastes use alloying and a binder system to achieve 85-90% silver loading and around 5-10X bulk resistivity after sintering.
Furthermore, inks only achieve conductivity through particle contact points within the cured ink film. No fusing or “welding” together occurs. This means ink traces are mechanically weak and lose connectivity when stretched or bent excessively.
Meanwhile, mSAP traces undergo full sintering to fuse together the nano/micro-silver particles. This generates 10X higher adhesion strength and allows flexibility without electrical failure.
When evaluating electrical conductors for dynamic flexible circuits, mSAP clearly outperforms older silver ink technology on critical performance metrics like conductivity, flexibility and bend life.
Flex Circuit mSAP Printing Equipment
Adopting mSAP conductive pastes within a flex PCB manufacturing environment requires specialized printing and curing equipment tuned for high mix short run capability.
Printer Considerations
Printheads – For fine feature tracing, precision drop-on-demand inkjet print heads are used which launch uniform picoliter sized droplets. UV-LED versions provide longer life. Dispenser print heads (contact or non-contact) can also be employed for higher viscosity pastes.
Print Stages – Industrial inkjet printers come in single pass or multi-pass configurations to build up trace heights. Later allows faster printing of thinner layers. Registration accuracy is critical to prevent shorts.
Print Widths – Available from around 100 mm to over 500 mm. Impacts overall production volumes before multiple machines are needed. Larger widths also aid printing productivity by minimizing the need for stitching passes.
Closed Loop Monitoring – Real-time optical monitoring with automated feedback tuning prevents defects and ensures process control. Includes drop watches/streak cameras and height sensors.
Inline Pre-Treatments – Priming stations to actively apply chemical promotion layers improves mSAP adhesion reliability on some substrates.
Conveyor Systems – Guarantees precision indexing between print modules for multilayer registration accuracy. Vacuum platens securely hold thin flex materials.
Sintering Equipment
Considerations for mSAP curing systems include:
Heating Method – Most commonly infrared lamps or ceramic heating plates. Hot plates offer better thermal consistency but lower throughput.
Peak Temperature – Ability to quickly achieve 200-260°C plateaus crucial for mSAP burn-off and sintering without overheating base material.
Temperature Uniformity – Tight control across heating area needed for reproducible processing results without defects.
Atmosphere – Use of inert gas shielding prevents oxidation. Nitrogen is typical.
Thermal Management – Cooling fans, liquid cooling channels and insulation important for process control.
Treatment Size – Batch sintering chambers ranging from <200×200 mm up to 600×600 mm or conveyor systems for continuous curing.
mSAP Reliability Considerations
Despite the benefits, mSAP usage in flex PCBs is not without limitations engineers should consider:
Electrical Conductivity
Volume resistivity of sintered silver adhesives remains higher than solid copper. This restricts high power applications. Matching cross-sectional trace areas to current requirements is necessary.
Environmental Sensitivity
Silver is prone to tarnishing and dendritic growth under bias, temperature and humidity extremes. Protection coatings are sometimes applied. However, encapsulation resins or casings typically provide adequate environmental sealing for indoor electronics.
Thermal Cycling Performance
Differing coefficients of thermal expansion between polymer substrates and sintered silver can induce mechanical shear strains during temperature excursions eventually degrading trace adhesion. Underfills may be needed in some high ΔT cycling applications.
Dynamic Flexure Stressing
Repeated bending motions can still produce flexural cracks and build-up fatigue damage without proper mechanical reinforcement. Strategic stiffening elements help strengthen high strain areas when designing mSAP circuits.
Process Control Needs
Like all additive print fabrication methods, extensive process controls and measurements are imperative to achieve design tolerances and prevent reliability risks related to dimensional accuracy, layer registration, line widths, etc. This demands a dialed-in mSAP infrastructure.
The Future of mSAP Flex Circuits
msap pcb
While mSAP conductive pastes are still an emerging technology, rapid progress in materials and printing systems promises growth replacing traditional subtractive methods for simpler flex PCBs with dynamic bending requirements suitable for:
- Consumer Electronics
- Medical Devices
- Industrial Sensors
- Automotive
- Robotics
- Wearable Technology
The ability to quickly and affordably fabricate bespoke flexible circuits in single piece or short run production opens many new possibilities for product miniaturization and innovation across industries.
Expect ongoing evolution of mSAP paste compositions and specialized application processes enabling smart manufacturing of this key interconnect material for the 21st century electronic landscape.
FQA
What is mSAP technology?
mSAP or modified silver adhesive paste is an engineered conductive paste which gets printed onto flexible substrates then cured at low temperatures to form stretchable circuit traces and interconnects featuring advanced flexibility, electrical performance, and processing properties compared to basic silver inks or pastes.
Why use mSAP instead of copper for flex PCBs?
mSAP circuits outperform copper flex PCB alternatives when maximizing: high flexibility & dynamic bending, ultra-thin/lightweight form factors, design customization, and low cost rapid fabrication. mSAP suits low-mid complexity circuits.
What printing methods work to pattern mSAP?
Industrial inkjet, aerosol jet printing, screen printing, and polymer stencil printing offer digital additive processes to selectively print mSAP then heat cure the material into conductive traces.
Does mSAP contain real metals?
Yes. mSAP paste comprises 85-90% silver particles suspended in a proprietary mix including a 2-5% tin/bismuth or indium solder alloy additive along with binders and solvent carriers to enable printing before heat exposure fuses the metals into conducive tracks.
Can mSAP survive repeated flexing without failure?
Properly designed mSAP interconnects demonstrate remarkable bend cycle life outlasting conventional conductive inks or pastes. Intrinsic ductility improves flexure fatigue resistance while adhesion promoting alloy agents anchor traces to the soft substrate. Reinforcement strategies further aid durability.
