Solder Paste Dispensing: How Pneumatic Dispensing Works in PCB Assembly
Solder paste dispensing provides an alternative to stencil printing for applying solder paste to PCB pads. Instead of transferring paste through a stencil, individual deposits are delivered directly from a dispensing tip to selected locations on the PCB.
For PCB prototyping and low-volume assembly, pneumatic dispensing can be useful when designs change frequently, when only selected locations require paste or when producing a dedicated stencil is impractical. The technology also has important limitations, particularly as deposit size decreases or the number of paste locations increases.
In Brief
In pneumatic solder paste dispensing, compressed air applies pressure to paste contained in a syringe. A controlled pressure pulse forces material through a dispensing tip and creates an individual deposit on the PCB.
Deposit formation depends on several interacting parameters, including air pressure, dispensing time, tip geometry, solder paste properties, temperature and tip position. Unlike stencil printing, each location is dispensed sequentially.
What Is Solder Paste Dispensing?
Solder paste dispensing is a direct application method in which paste is deposited at individual locations rather than transferred through a stencil containing the complete paste pattern.
The dispensing tip is positioned above the required PCB location, paste is forced through the tip and a deposit is formed. The tip then moves to the next location and the process is repeated.
Because the paste pattern is not physically defined by a stencil, dispensing can provide useful flexibility during prototype development and frequent PCB design changes.
Not All Dispensing Systems Work the Same Way
Solder paste can be dispensed using several different technologies. Their operating principles, achievable deposit sizes and throughput can differ significantly.
Pneumatic / Time-Pressure
Compressed air acts on paste in a syringe. Pressure and dispensing time are used to control material delivery through the tip.
Positive Displacement
Material is metered mechanically rather than relying primarily on pressure and dispensing time.
Jet Dispensing
Specialized systems eject material without following the same conventional needle deposition sequence as pneumatic dispensing.
This Article Focuses on Pneumatic Dispensing
The following sections describe conventional pneumatic time-pressure dispensing. Capabilities associated with positive-displacement or high-speed jetting systems should not automatically be assumed for pneumatic equipment.
How Does Pneumatic Solder Paste Dispensing Work?
Prepare the Paste
Dispensing-compatible solder paste is supplied from a syringe fitted with an appropriate dispensing tip.
Position the Tip
The tip is positioned at the required location and height relative to the PCB.
Apply Pressure
A controlled pulse of compressed air acts on the solder paste inside the syringe.
Extrude the Paste
Pressure forces solder paste through the dispensing tip toward the PCB surface.
Form the Deposit
Paste contacts the PCB and forms an individual deposit at the selected location.
Repeat
The tip moves to the next location and the dispensing cycle is repeated.
What Controls the Amount of Dispensed Solder Paste?
Unlike stencil printing, deposit volume is not defined by a fixed aperture. It results from the interaction of dispensing settings, tip geometry, paste properties and deposition conditions.
Air Pressure
Pressure provides the force required to move solder paste through the dispensing tip.
Dispensing Time
The duration of the pressure pulse influences how long material is driven through the tip.
Tip Geometry
Internal diameter and tip length influence flow resistance and deposit formation.
Solder Paste
Paste formulation, viscosity and solder powder particle size influence flow through the dispensing system.
Temperature
Material temperature can affect solder paste rheology and therefore its dispensing behaviour.
Tip Position
Tip-to-PCB distance and positioning affect how the deposit forms and separates from the tip.
Why Does Solder Paste Selection Matter?
Solder paste contains solid solder alloy particles suspended within a flux system. It therefore behaves differently from a simple homogeneous liquid.
Solder powder particle size and paste formulation need to be compatible with the dispensing tip and intended deposit size. As smaller dispensing tips are used, reliable passage of the solder particles through the tip becomes increasingly important.
A paste designed for another application should therefore not automatically be assumed to dispense reliably through a small pneumatic dispensing tip.
What Are the Limits of Pneumatic Dispensing?
Pneumatic dispensing provides a relatively straightforward way to apply solder paste without a stencil, but the operating principle also creates practical limitations.
Sequential Application
Each paste location is created individually. As the number of required deposits increases, total dispensing time also increases.
Deposit Variation
Changes in pressure, timing, paste condition, temperature or tip conditions can influence the resulting deposit.
Minimum Practical Deposit
Smaller needles increase flow resistance and must remain compatible with the solder powder contained in the paste. Needle diameter therefore cannot simply be reduced indefinitely.
Tip Clogging
Because solder paste contains solid particles, flow restriction or clogging can occur, particularly with small tips or unsuitable paste and process conditions.
Why Can Fine-Pitch Dispensing Become Difficult?
Reducing deposit size requires more than fitting a smaller needle. A narrower flow path increases resistance while the solder particles still need to pass reliably through the opening.
Small and closely spaced pads also provide less tolerance for variation in deposit volume and position. Tip positioning, dispensing height, paste formulation and solder powder size therefore become increasingly important as feature dimensions decrease.
Smaller Needle Does Not Mean Unlimited Resolution
Pneumatic dispensing has a practical process window. Increasing pressure to force solder paste through an increasingly narrow dispensing path is not a universal solution to producing smaller deposits.
When Does Pneumatic Dispensing Make Sense?
Changing Prototypes
Deposit locations can be changed without manufacturing a new stencil for every paste pattern.
Selective Application
Paste can be deposited only at the selected PCB locations required by the assembly.
Low Deposit Counts
Sequential dispensing can remain practical when the board contains a relatively limited number of paste locations.
Dispensing or Stencil Printing?
Pneumatic dispensing and stencil printing solve the same broader task in different ways. Dispensing provides flexibility without a dedicated stencil, while stencil printing can apply paste to many PCB locations during a single print operation.
The practical choice depends on PCB design, number of paste locations, required deposit geometry, frequency of design changes and the number of boards being assembled.
Compare Solder Paste Dispensing and Stencil Printing →
Where Does Dispensing Fit Into PCB Assembly?
Pneumatic dispensing and stencil printing are two different methods within the same broader process stage: solder paste application.
After solder paste has been applied, SMD components are positioned onto the paste deposits during Pick & Place. The populated PCB then proceeds to Reflow, where controlled heating forms the solder joints.
Sources & Further Reading
- AIM Solder – technical information on solder paste dispensing, pneumatic dispensing parameters, cycle time and dispensing process limitations.
- Nordson EFD – technical guidance on time-pressure dispensing, solder paste behaviour, dispensing tips and process variables.
- Nordson EFD – technical guidance on solder powder particle size, tip selection and solder paste dispensing.
Technical Note
Achievable deposit size and dispensing consistency depend on the specific dispenser, solder paste, solder powder particle size, dispensing tip, pressure, timing, temperature and application conditions. The information above describes general principles of pneumatic solder paste dispensing and is not a specification for a particular dispensing system.