High Mix Production and Paste Jetting Economics

Printing paste through a stencil has been the default for surface mount assembly for decades, and it remains the fastest way to deposit alloy on a board that will be built in volume. Paste jetting changes the economics for a different kind of production: many different boards, small batches and frequent changeovers. This article compares the two processes on what actually differs, which is deposit control, changeover time and the way each one fails.

How Each Process Deposits Paste

Stencil printing forces paste through apertures in a metal foil with a squeegee, and the deposit volume is set by the aperture area and the foil thickness. The process is parallel, so a whole panel is printed at once, and the paste is transferred by shear and release rather than by a single controlled action.

Paste jetting ejects discrete droplets from a nozzle using a piezoelectric actuator, building each deposit from one or more drops. Nothing touches the board, so there is no stencil to align and no release step. The deposit is defined by drop volume and count rather than by a foil, which makes the volume programmable per pad.

Deposit Volume Control

Stencil printing controls volume through geometry, and once the stencil is made the volume for each aperture is fixed by the foil. Adjusting it means a new stencil, a step or a paste change. That rigidity is also its strength, because a good stencil is highly repeatable across millions of prints.

Jetting controls volume through software. The number of drops per pad can be changed instantly, and different volumes can be produced side by side without any tooling. The trade is that the deposit is built from droplets, so its shape is less uniform, and the achievable volume per unit time is limited by how fast the valve can fire.

Speed and Throughput

A stencil printer deposits a full panel in seconds, and the only limit is the mechanical cycle of print, separate and clean. For volume production nothing comes close, and the cost per board falls as the batch size rises because the tooling is amortised.

Jetting is far slower per unit area. The valve fires at a defined rate, so the time to build a deposit depends on the pad count and the drop count. On a dense board that difference is measured in tens of seconds per board, which is irrelevant for a fifty unit batch and decisive for fifty thousand.

High Mix Production

The strongest argument for jetting is changeover. A stencil has to be made, stored, mounted and aligned, and each of those steps costs time and introduces an opportunity for error. On a line running several different products a day, the printer can spend more time on setup than on printing.

With jetting, changing product is a file change. There is no stencil to make, no alignment to verify and no storage to manage. That makes the process attractive for prototypes, for engineering builds and for contract manufacturers whose batches are small and varied. The comparison with the stencil workflow is set out in our article on solder paste volume and stencil design.

Where Each Process Fails

Stencil printing fails in predictable ways: insufficient paste release at low area ratios, bridging from over-sized apertures, and contamination transferred from the underside of the foil. Those failures are well understood and are corrected by aperture design, by the squeegee settings described in our note on squeegee selection, and by cleaning.

Jetting fails differently. The nozzle can clog, drop volume can drift as the valve wears, and the deposit can be inconsistent if the gap between nozzle and board changes. Because there is no stencil to inspect, the process depends on its own measurement system, and paste inspection becomes the primary control.

Cost Structure

The stencil route carries tooling cost, storage cost and setup labour, but a low consumable cost per board. Jetting carries a higher machine cost, a nozzle and valve maintenance cost, but almost no per-product tooling. The crossover point depends on how many products the line runs and how often they change.

Consumables differ too. Jetting requires paste formulated for the valve, which is more expensive and less widely available than a standard printing paste. That difference should be included in the comparison rather than treated as a detail.

Quality Considerations

Deposit shape affects how a joint forms. A printed deposit is brick shaped and sits flat on the pad, while a jetted deposit is a stack of droplets and can be taller. The higher profile can help on a pad that must release gas, and can hurt where a low, even profile is needed for a fine pitch part.

Because jetting builds volume in discrete steps, the resolution of the volume control is the drop size. That is fine for most pads but can be limiting for a very small aperture, where the target volume is close to a single drop.

Choosing Between Them

Choose stencil printing when the product family is stable, the volume is high and the boards are dense enough to benefit from parallel deposition. Choose jetting when the mix is wide, the batches are small, or the board has features that a stencil cannot serve, such as a cavity or a step.

Many factories run both on the same line, using jetting for prototypes and pre-production and a stencil for volume. The stencil that follows is then designed with real deposit data behind it, which shortens the path to a stable process.

Process Control and Verification

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Can jetting match stencil print quality? For most pads it produces an acceptable deposit, and for very small pads it can beat a stencil at a low area ratio. The shapes differ, so the process window has to be re-established rather than assumed.

Does jetting need its own paste? Yes. The valve needs a paste with a defined rheology and particle size, and a printing paste will often clog or produce inconsistent drops.

Is a stencil still needed for volume? Almost always. The tooling cost is recovered quickly in volume, and the print cycle time is an order of magnitude shorter.