Droplet Volume Control in Small Volume Production
Printing solder paste through a stencil is efficient because one squeegee stroke deposits every pad on the board at once. That efficiency assumes the board will be built in a quantity that justifies a stencil, an aligned printer and the setup time that goes with them. For a handful of boards, or for a design that changes every week, the stencil is the slow part of the line. Solder paste jetting removes it by dispensing paste droplet by droplet, program by program, which makes small volume production practical without a physical printing tool.
How the Jetting Process Works
A jetting valve holds paste in a small reservoir and ejects a discrete droplet through a nozzle using a piezo or pneumatic actuator. The board moves under the nozzle, or the head moves over the board, and a deposit is placed wherever the program specifies. Because there is no stencil, there is no aperture to align, no squeegee pressure to tune and no paste left on the tool between jobs.
The process therefore separates the deposition pattern from a physical object. Changing the pad layout means editing the program, not ordering a new stencil, which is the property that makes the technology attractive in development and in any environment where the product mix changes faster than the stencil supply chain can follow.
Where the Process Fits Best
Small volume production is the natural fit. When a build is measured in tens of boards, stencil cost and lead time dominate the assembly schedule, and jetting gives a route to start the same day the design is released. Prototype and pilot runs, engineering change builds and rework are the same case in different clothes: the quantity is too low for a stencil and the schedule is too short to wait for one.
The second fit is geometry. Boards with large height differences between components, three dimensional substrates, or pads that are inaccessible to a squeegee are poor candidates for printing and reasonable candidates for jetting. The trade is throughput for flexibility, and it only makes sense while the volume stays low or the mix stays wide.
Droplet Volume and Deposit Control
Droplet volume is set by the nozzle diameter, the valve stroke and the pressure that drives the paste, and it is verified by weighing or by measuring the deposit rather than by trusting the setpoint. Because the deposit is built from discrete droplets, the total volume for a pad is the droplet volume multiplied by the number of droplets, which gives fine control but also multiplies any error in the individual droplet.
The order in which droplets are placed matters on larger pads. A single large deposit tends to slump and to trap air, so the usual practice is to place droplets in a pattern that lets them merge into a flat, void free layer. Programming the pattern takes more effort than programming a single shot, and the result is measured by the same criteria used for print deposits.
Paste Rheology and Viscosity
Jetting places different demands on the paste than printing does. The paste must flow through a narrow nozzle without separating, then form a clean droplet without stringing or satellite droplets, and then hold its shape on the pad without slumping. That combination means jetting pastes are formulated for the process, and a paste optimised for squeegee printing may not jet reliably at all.
Viscosity control is the practical variable. Paste that is too thick will not fill the valve chamber consistently, and paste that is too thin will leave a tail as the nozzle retracts. Storage, thawing and the time the cartridge spends in the valve all change the effective viscosity, so the working life of a cartridge should be defined and respected rather than extended until it runs out.
Resolution Limits and Fine Pitch Work
The minimum deposit size is limited by the nozzle and by the paste. Very small nozzles raise pressure and shear, which can damage the paste structure, and small deposits have a high surface to volume ratio that promotes drying. Fine pitch work is therefore possible but requires a matched paste and nozzle combination and a stable environment.
Deposit placement accuracy is the other half of resolution. The machine has to position the nozzle over a pad of a few hundred micrometres with an accuracy that leaves most of the pad exposed, which puts the same demands on machine calibration, fiducial recognition and thermal stability as any fine pitch process. The inspection criteria for those deposits follow the same logic as a printed board, as described in solder paste volume and stencil design.
Throughput and Cost Comparison
The honest comparison is cost per good joint, not cost per deposit. A jetting machine places material more slowly than a printer fills a stencil, so for a thousand boards the stencil route is far cheaper. For ten boards, the stencil route includes a fabrication charge, a delivery delay and a setup that may exceed the whole assembly time, so the jetting route wins easily.
Rework is where the economics are most favourable. Replacing a lifted component or correcting a displaced part without a stencil requires either manual dispensing with a syringe or a jetting program, and the jetting route gives a repeatable volume instead of an operator judgement. The place to start is where the manual work is most repetitive, and the solder paste storage and handling routine for the cartridges can then be scaled from there.
Process Transfer and Validation
Equipment selection follows from the same reasoning. A valve with a wide nozzle range covers more of the product mix, while a dedicated small nozzle gives better fine pitch capability but limits the largest deposit that can be placed in one pass. Machines that combine jetting with an integrated dispenser for adhesive or underfill reduce the number of setups per build, which matters when the batch is small and the changeover cost is a large share of the total work.
The environment is easy to overlook. Paste dries at the nozzle between cycles, so an idle machine needs a purge routine, and a hot, dry room shortens the working life of the cartridge noticeably. Measuring the deposit volume at the start and the end of a shift shows whether the process is stable across the day, which is the first question to answer when a batch sells poorly at reflow.
Transferred programs need validation, because the deposit a machine produces depends on the paste batch, the nozzle condition and the ambient conditions. A program that produces the correct deposit volume in one facility can be off by a measurable margin in another, and the difference only appears when joints are inspected after reflow.
The gopcb assembly group validates a transferred jetting program by measuring deposit volume and height across the full pad area of a test artefact, then reflowing and inspecting the joints against acceptance criteria. The pattern of solder joint acceptance criteria gives the limits, and the measurement record gives the evidence that the process is in control.
FAQ
Can jetting replace a stencil entirely? For low volume and high mix, often yes. Above a few hundred boards per assembly, the economics normally favour printing.
Does jetting produce more voids? It can, because deposits are built from discrete droplets. A well designed droplet pattern and a suitable paste keep voiding within the same limits as printed deposits.
Is the paste different from printing paste? Usually yes. Jetting pastes are formulated for the shear and the droplet formation the valve imposes, and printing paste may string or clog in a jetting nozzle.
