Solder Paste Jetting: Preparation, Placement and Process Control

Jetting deposits solder paste without a stencil, driving small dots of material from a nozzle onto the pad at high speed. The appeal is obvious: no foil to order, no squeegee pressure to tune, no paste left on a stencil web. The limits are less obvious, and they decide which products the process suits.

How Paste Jetting Works

A piezoelectric actuator drives a piston or a tube against a small chamber of paste. Each pulse ejects a droplet whose volume is set by the nozzle geometry, the stroke and the properties of the material, and the head moves from pad to pad under machine control.

Because the deposit is formed droplet by droplet, volume is programmed rather than printed. That makes solder paste jetting a genuinely different process from stencil printing, and it changes which parameters dominate the result.

Where Jetting Beats a Stencil

The clearest case is a product with few solder joints and many different boards, where the cost of a stencil per design is hard to justify. Prototype runs, mixed panels and rework stations all fall into that category.

Jetting also reaches into places a squeegee cannot, such as a deep cavity, a board populated on one side, or a step where the stencil would have to be unusually thick. Where a single odd connector sits among parts that are otherwise hand soldered, jetting closes the gap without a second process.

Where Stencil Printing Still Wins

Throughput is the first divider. A printing stroke deposits a whole panel at once, while a jetting head works pad by pad, so a dense board with thousands of joints takes far longer to jet.

Fine geometry is the second. Most production paste is designed for stencil printing, and a paste that has been tuned for release from an aperture is not automatically a good jetting material. Volume consistency at the small end of the range and freedom from clogging at the large end pull in opposite directions.

The Material Question

Jetting pastes are formulated with tighter powder size distribution and a rheology that recovers quickly after shear, because each droplet has to detach cleanly rather than string. Standard printing paste often forms satellites and threads, which land as stray solder balls.

A change of material therefore changes both processes at once. Our article on paste volume and stencil design sets out how deposit volume is controlled in the printed process, and the same volume discipline applies here with a different actuator.

Key Process Parameters

Nozzle diameter and stroke set the nominal droplet, and the number of droplets sets the deposit. Standoff between nozzle and board, the movement profile of the head and the working temperature of the paste all influence how the droplet lands and whether it stays where it was placed.

Because these parameters interact, the setup is usually established by weighing deposits rather than by setting numbers from a table. A deposit weight series across the pad range reveals whether the head is consistent or whether one nozzle is drifting.

Defects and Their Signature

Satellite balls around the deposit point to a material or waveform problem. A short deposit in one direction points to a partially blocked nozzle, while random variation across a board points to air in the paste or an unstable standoff.

Because the defects appear at the point of deposition rather than after reflow, they are visible early, which is a genuine advantage. The reflow behaviour of the deposit is the same as any other printed paste, so our article on first pass yield analysis is a useful way to decide whether the process is under control.

Maintenance and Consumables

Nozzles wear and clog, and the replacement point should come from a deposit weight trend rather than from a failure. Cleaning between materials matters more than in printing, because a nozzle holds a small reservoir of paste that will not be flushed by normal use.

The rest of the housekeeping is familiar. Material storage, pot life and cleaning routines are the same disciplines that govern the printed process, as described in our article on stencil cleaning and storage.

Choosing Between the Two

The decision usually comes down to volume. If the product runs in thousands and carries thousands of joints, printing wins. If the board count is low, the joint count is low, or the geometry is awkward, jetting is often the cheaper route.

A mixed line is also common, with printing for the main panel and jetting for the parts that a stencil cannot serve. What matters is that the choice is made from the product data rather than from the equipment list.

Verification and Records

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery. A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

FAQ

Can jetting replace printing on a dense board? In principle yes, but the cycle time usually rules it out. A head that deposits one pad at a time cannot match a single printing stroke across a full panel of fine pitch parts.

Is jetting paste the same as printing paste? No. Jetting materials are formulated for clean droplet formation and fast recovery, and using a printing paste in a jet head often produces satellites and clogged nozzles.

How is jetting volume verified? By weighing deposits and by optical inspection of the deposited dot. The volume is set by droplet count and size, so a weight trend is the most direct evidence that the head is stable.