Introduction
Solder paste is a material that enables the soldering and assembly of printed circuit boards and various electronic components. It is one of the most important consumable materials used in the electronics manufacturing industry today. This article provides a comprehensive overview of solder paste including its composition, properties, types, printing processes, and usage in PCB assembly.
What is Solder Paste Made Of?
Solder paste consists of three main constituents:
Solder Powder
- Tiny spherical particles of solder alloy metal mixed with flux
- Typical size range of 20-45 μm
- Makes up 50-90% of paste by weight
- Common alloys are SAC305 (Sn96.5/Ag3/Cu0.5), Sn/Pb, and Sn/Ag/Cu
Solder Flux
- Formulated paste of organic acids, activators, and rheological additives
- Coats the surface of the solder particles
- Removes surface oxidation and aids in solder flow
- Typically 5-20% of paste by weight
Solder Paste Vehicle
- Non-conductive carrier medium to hold paste together
- Gives paste desired viscosity and printing properties
- Usually a blend of glycols, resins, esters and polymers
- Comprises 25-45% of paste weight
The specific formulations and ratios of these ingredients can be adjusted to produce solder paste variants with different properties for desired applications.
Functions of Solder Paste
Solder paste fulfills several critical functions in surface mount PCB assembly:
- Provides flux – The flux in paste removes oxidation and facilitates solder joint formation during reflow.
- Hold components in place – The tackiness of paste adheres components onto pads prior to reflow soldering.
- Forms electrical connections – The solder alloy melts and creates metallurgical solder joints between pads and component leads.
- Compensates for tolerances – The paste helps accommodate small variations in pad and component dimensions.
- Permits solder deposition – Paste enables controlled printing of precise solder deposits onto pads through stenciling.
- Allows miniaturization – Solder paste enables assembling micro components with pad/lead pitches below 0.4mm.
- Enables automation – It permits fast, accurate, repeatable automated stencil printing of solder.
Without solder paste, the volume production of printed circuit boards with surface mount components would not be feasible.
Properties of Solder Paste
Desirable properties and characteristics of solder paste include:
- Solder alloy – Composition engineered for melting point, wetting, strength, thermal fatigue, creep resistance etc.
- Metal load – Weight percentage of solder powder in paste, typically 50-90%.
- Particle size – Diameter of solder powder particles, standard from Type 3 (25-45 μm) to Type 5 (20-38 μm).
- Viscosity – Thickness of paste that determines easiness of printing, typically measured in ‘stencil pull’ force.
- Tackiness – Stickiness of paste that allows it to hold components in place prior to reflow.
- Slumping – Tendency of paste to spread out after printing, before reflow.
- Solder balling – Formation of small solder beads during reflow, caused by excess paste.
- Wettability – Ability of molten paste to spread evenly onto surfaces.
- Shelf life – Usable lifetime of unused paste stored correctly, typically around 6-12 months.
Types of Solder Paste
Solder pastes can be classified into several types based on their composition and properties:
Water Soluble Solder Paste
- Most common and widely used type
- Vehicle is water soluble mixture of polymers and glycols
- Easy to clean after reflow with just water
- Short shelf life around 3 months
No-Clean Solder Paste
- Vehicle is not water soluble, leaves benign residue
- Does not need cleaning after soldering
- Typically has longer 6-12 month shelf life
- Used for consumer electronics manufacturing
Halogen-Free Solder Paste
- Contains no bromine or chlorine and is more eco-friendly
- Special organic acids used instead of halogenated activators
- Growing segment due to environmental regulations
Low Temp Solder Paste
- Allows soldering of temperature sensitive components
- Melting point below 180°C due to different alloy composition
- Common for soldering plastic components
Solder Paste Printing Process
Solder paste printing is the most prevalent method of depositing paste onto PCBs prior to component assembly. The main steps are:
Stencil Design
- Laser cut metal foil stencil with cutouts matching pad geometries
- Stencil thickness/aperture size determines paste deposit volume
- Common stencil materials are stainless steel, nickel, and brass
Stencil Mounting
- Stencil aligned and fixed over PCB on printer using brackets or dedicated tooling
- Achieves accurate registration between stencil apertures and pads
Printer Squeegeeing
- Solder paste dispensed onto stencil surface
- Squeegee blade presses down and smears paste into apertures
- Fills apertures with precise volume of solder paste
Stencil Separating
- Carefully lift stencil away without smearing deposits
- Leaves controlled paste deposits on pads for component placement
Figure 2: Solder paste printing using a stencil and squeegee blade.
Automated printers allow this process to be performed quickly and reliably for high volume PCB production.
Solder Paste Dispensing
An alternative to printing solder paste is automated dispensing:
- Uses a pressurized syringe and precision needle tip
- Robotically dispenses controlled dots or lines of solder paste
- Typically for filling small apertures, repair work, or highly complex deposit shapes
- Lower throughput than stencil printing
Dispensing offers more flexibility but requires advanced robotic systems and 3D modeling of paste paths.
Reflow Soldering Process
After solder paste printing and component placement, reflow soldering melts the paste to form permanent solder joints:
Heating Methods
- Convection – Forced air ovens with IR heating elements
- Vapor phase – Condensation of heated vapor
- Infrared – IR lamps/emitters heat board directly
- Laser – Directed laser energy to reflow joints
Reflow Profile
- Slow ramp up to 150-180°C to evaporate volatiles
- Rapid heating to 20-40°C above liquidus temperature
- Brief dwell at peak temperature
- Cool down to solidify joints
Flux Removal
- Water soluble pastes cleaned with DI water spray
- No-clean pastes leave benign residue requiring no post-wash
Reflow forms reliable metallurgical solder joints while consuming the flux elements.
Solder Paste Inspection
Key quality tests performed:
- Paste volume – Confirm right amount of paste on pads
- Print alignment – Check X/Y offset of prints versus pads
- Paste condition – Detect any smearing, slumping, clogging etc.
- Catches defects prior to committing components and reflow
Automated optical inspection systems are commonly used along with manual visual checks. This helps avoid costly rework later.
Solder Paste Storage and Handling
To maximize shelf life and performance:
- Store refrigerated at 0-10°C in sealed containers
- Allow paste to come to room temperature before use
- Mix paste thoroughly before loading into printer
- Avoid contact with contaminants like oil, dust or metal particles
- Use within the recommended usage period after opening
Proper storage and handling ensures consistency in printing performance.
Applications of Solder Paste
Solder paste enables the production of virtually every type of modern electronic hardware:
- Consumer devices – Phones, laptops, tablets, wearables, IoT
- Computer components – Motherboards, graphics cards, storage drives
- Telecommunications equipment
- Automotive electronics
- LED lighting
- Medical devices
- Industrial electronics
- Aerospace and military avionics
It is universally used in electronics manufacturing, from the smallest watches to large server racks and communication base stations.
Pros and Cons of Solder Paste
Advantages:
- Allows automated printing and assembly
- Accommodates component miniaturization
- Fast production of reliable solder joints
- Widely available from suppliers
Disadvantages:
- Shelf life limits material usage
- Sensitive to handling and contamination
- Defects like solder balls, bridging, or voids
- Requires inspection to catch faults
- Water soluble types need cleaning
Even with some downsides, solder paste brings overwhelming benefits for electronics manufacturing.
Conclusion
In summary, solder paste is an indispensable material composed of solder alloy powder, flux, and vehicle mixed into a viscous medium for printing onto PCBs. It serves many core functions like adhering components, forming solder joints, and enabling miniaturization and automation. A wide range of solder paste formulations exist to suit different soldering requirements. When combined with stencil printing and reflow soldering, high quality electronic hardware can be manufactured quickly, reliably, and economically using solder paste. It provides the foundation for our electronics-driven modern world.
FAQs
What is the difference between solder paste and solder wire/bars?
- Solder paste is a mixture suitable for printing and volume assembly
- Solder wire/bars are solid metal alloy used for manual soldering
How is solder paste applied to PCBs?
The main methods are stencil printing using an adhesive squeegee blade, or automated dispensing using a pressurized syringe.
What problems can occur when using solder paste?
Common issues are excessive slumping, solder balling, bridging between joints, voiding within joints, and poor wetting or spreading on pads.
What is the effect of the metal load in solder paste?
Higher loads above 80% tend to decrease slumping while lower loads below 70% reduce bridging and icicling between joints.
How long does solder paste last after being opened?
Opened solder paste in containers should be used up within a maximum of 72 hours to avoid deterioration in properties and performance.
