Solder Paste Jetting: Key Checks Before Release
Stencil printing transfers a volume of paste through an aperture, and jetting builds the same deposit from thousands of small droplets. The two methods arrive at the same place by different routes, and the choice between them has more to do with the shape of the product mix than with the quality of the result.
Jetting removes the stencil, the squeegee and the wash cycle, and it adds a programming task and a nozzle that must be maintained. Understanding what each method does well is what keeps the decision from being made on fashion.
How Paste Jetting Works
A jetting head holds a small reservoir of paste at a controlled temperature and ejects droplets through a nozzle by a piezoelectric or a pneumatic actuator. The droplets land on the pad and coalesce into a deposit whose volume is set by the number of drops and their size.
Because there is no stencil, the deposit shape is defined by the programme rather than by a physical aperture. That freedom allows a different volume on every pad without any tooling change, which is the property that makes the method attractive for high mix production.
How Stencil Printing Works
Stencil printing fills an aperture in a metal foil with paste and then releases it onto the pad as the board separates from the stencil. The volume is set by the aperture area and the foil thickness, and the release is governed by the area ratio and the wall finish.
Tooling is the cost, and it is a fixed cost per product. Once a stencil exists, printing is fast and the cost per board is very low, which is why the method dominates high volume assembly.
Volume Repeatability
A stencil produces a deposit whose volume varies with the paste condition, the separation speed and the planarity of the board. A jet produces a deposit whose volume varies with the reservoir level, the nozzle condition and the temperature of the head.
Both methods achieve good repeatability within their windows, and both drift when the consumable is neglected. The difference is that a stencil drifts gradually and a jet nozzle drifts abruptly when it becomes partially blocked, which changes the character of the failure rather than the rate.
Fine Pitch and Small Apertures
At very small apertures a stencil runs out of physical room, because the area ratio falls and the paste no longer releases cleanly. The limit depends on the foil thickness, and it is the reason fine pitch and thin stencils go together.
A jet has no such geometric limit in the plane, since the deposit is built up rather than pushed through, so it can place a small volume on a small pad. The practical limit moves to the droplet size and to the accuracy of the placement of the head.
Standoff, Steps and Odd Form Parts
A board with a large height difference, a cavity or a component already fitted on one side is difficult to print because the stencil cannot lie flat. Jetting reaches into a cavity and onto a raised surface without contact, which makes it the natural choice for those cases.
The same property helps with odd form parts and with the second side of a board that already carries a tall component. These are the situations described for manufacturing processes where a conventional print is impossible rather than merely awkward.
Changeover and Tooling
A stencil change costs a setup and a first article, so a product that runs once a month pays for its tooling many times over in changeover time. A jet programme is a file, so a change amounts to loading a different recipe.
That difference becomes decisive when a line runs many products in small batches. The jet’s advantage is not speed but the removal of the tooling step and the wash step, and it is measured in labour rather than in cycle time.
Paste Formulation and Viscosity
Jetting paste is formulated differently from printing paste, with a narrower particle size distribution and a viscosity tuned to the nozzle. Using a printing paste in a jet head produces clogging and inconsistent droplets.
The same applies in reverse, because a jetting paste is often too fluid for a stencil and slumps after printing. The paste and the process are a pair, and the paste supplier should be told which method is in use before the material is selected.
Maintenance and Reliability
A stencil needs cleaning, inspection and eventual replacement, and its failure is gradual and visible. A jet needs nozzle cleaning, reservoir control and a watch on droplet quality, and its failure can be sudden.
Both need a first article check after a change. The check for a jet includes a volume measurement on a sample pad, since a partially blocked nozzle can still place enough drops to look correct.
Where Each Method Fits
Stencil printing remains the best choice for high volume, for boards with a consistent geometry and where the tooling cost can be amortised. Jetting fits high mix, low volume, odd form assemblies and products with cavities or height differences.
Some lines use both, with a printer for the main pattern and a jet for the exceptions. That arrangement uses each method where it is strong, and it avoids the compromise of using one for everything.
Cost Comparison
The comparison should be made per board over the life of the product rather than per machine hour. A jet has a higher capital cost and a higher consumable cost, while a stencil has a tooling cost and a changeover cost that is repeated with every batch.
Where the batch is large the stencil wins easily, and where the batch is small the arithmetic often reverses. The calculation should include the cost of the wash cycle and the disposal of the waste, which are frequently left out of the comparison.
Additional Considerations for This Build
Practical attention to solder paste jetting pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating solder paste jetting explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, solder paste jetting is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
FAQ
Can a jet place a heavier deposit than a stencil? It can build a tall deposit by stacking droplets, but the time per pad rises, so heavy deposits are usually still printed.
Does jetting remove the need for a stencil entirely? Only on lines that have no high volume products, since a printer is faster whenever the pattern is stable.
Is the paste interchangeable between methods? No. The formulations differ in viscosity and particle distribution, and substituting one for the other degrades both processes.
Which method is easier to validate? A stencil is easier because the tooling is fixed and measurable, while a jet requires a volume measurement to confirm each nozzle condition.
