Solder Paste Jetting: Design Rules and Process Limits
Printing paste through a stencil is a batch process: one tool, one setting, and the whole board printed at once. Jetting deposits paste through a valve that moves over the board, one deposit at a time, and it changes which problems are easy and which are hard.
How Jetting Works
A jetting valve ejects a small volume of paste from a nozzle with a piezoelectric or pneumatic actuator. The board moves beneath the head, and the valve fires at each pad position to build up the required volume from several drops.
The deposit shape and volume are controlled by the nozzle, the actuation and the number of drops, and they are programmed per pad rather than fixed by a tool.
The process needs no stencil, no squeegee and no printing parameters, which removes a whole class of printing defects and introduces a different class of dispensing defects.
What Jetting Does Better
The volume can vary between pads on the same board, so a large thermal pad can receive a divided deposit and a fine pitch pad can receive a small one without any compromise in the tool.
A board with a step, a cavity or a component already fitted can be printed, since there is no stencil that must sit flat against the surface. This is the capability that stencil printing cannot provide.
Changeover between products requires no tooling, which matters where the batch sizes are small and the product mix is wide. Our paste inspection notes describe how the printed result is measured.
What Stencil Printing Does Better
The stencil prints the whole board in one stroke, so the throughput is much higher for a board with many pads. Jetting time scales with the number of deposits, and a dense board is slow.
The stencil produces a consistent deposit across the board because every aperture is filled by the same squeegee stroke with the same pressure. Jetting reproduces the programmed value, and any systematic error is programmed in.
The stencil is a known quantity with well understood process windows, and most of the published guidance on paste volume, area ratio and release applies to it. Our solder defects notes describe the defect signatures that correspond to each method.
Paste Requirements
A jetting paste must be formulated for the shear it experiences in the valve. It is usually a higher viscosity paste with a carefully controlled particle size distribution, and a paste meant for stencil printing may not jet reliably.
The particle size matters more in jetting, because a particle that is large relative to the nozzle will clog it. The nozzle is the consumable and the clog is the commonest cause of a stoppage.
The working life of the paste in the valve is different from its life on a stencil, because it is repeatedly sheared and its temperature is raised by the actuator. The material should be replaced on a schedule rather than only when the deposits become inconsistent.
Defect Signatures
A clogged nozzle produces a missing or small deposit, which appears as an insufficient joint after reflow. A partially clogged nozzle produces a deposit with an irregular shape that may not cover the pad.
A drop that is ejected at the wrong moment lands between pads and forms a solder ball after reflow. This is the jetting equivalent of a smeared print, and it is caused by the firing timing or by a mechanical fault.
Because the deposits are individual, defects appear on individual pads rather than across the board, which makes the pattern of the defects a strong clue to the cause.
Where Each Method Is Used
Jetting fits prototypes, small batches and boards that cannot be printed. It also fits boards where a few pads need a different volume from the rest.
Stencil printing fits volume production of a board that is flat and has a reasonably large number of pads. The tooling cost is recovered quickly and the throughput is much higher.
A line can have both, with the jetter used for the awkward products and the printer for the mainstream. Our inspection notes describe how the two are handled in the same inspection program.
Verifying Either Process
The verification is a measurement of the deposit volume and position on the first article, repeated on a sample during production. The measurement method is the same for both, which makes the results comparable.
The measurement should be recorded with the process parameters, since a volume that is correct with one nozzle or one stencil is not evidence for another.
Where the two methods are used for the same product, the volumes should be matched rather than each being set to its own target, so that the reflow profile developed for one applies to the other. Our quality notes describe how the equivalence is recorded.
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, paste volume is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Process Control and Verification
On a design of this kind, paste volume is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
FAQ
Is jetting more accurate than printing? Its position accuracy can be better, and its volume consistency across the board depends on the valve rather than on the stencil. For a dense board the printing is faster and equally consistent.
Can the same paste be used for both? Sometimes. A paste formulated for jetting is usually too viscous for comfortable printing, and a printing paste may not jet reliably.
What does gopcb provide for paste deposition? We provide a recommendation of method for the board, paste selection matched to the chosen process, deposit measurement on the first article and during production, and process records that make the two methods comparable where both are used.
